Final Report for AS93-007
Many vegetable farmers use black plastic film mulch on tomatoes, cucumbers and other warm-season vegetables because plastic blocks weed growth, conserves soil moisture and warms the soil thereby promoting early crop maturity. However, plastic does not add organic matter or nutrients to the soil, its manufacture consumes considerable fossil fuel, and its disposal adds to the nation's solid waste burden. Some growers use paper mulches, including a commercially available black paper mulch, newsprint end rolls and other paper wastes as biodegradable alternatives to plastic. However, these materials do not warm the soil as effectively as plastic, and the often break down too fast, with resulting loss of weed control.
On many small, biologically managed farms, organic mulches such as hay, straw or leaves are used to provide organic matter and nutrients, protect and improve the soil, and suppress weeds. However, these materials cool the soil, which may delay ripening in tomatoes and other summer vegetables. Also, they may be too expensive and labor-intensive to apply on larger farms. Growers and researchers have experimented for the past several years with winter cover crops that are killed by mowing in spring and left in place as a mulch. Results have varied from disappointing due to inadequate weed control, to excellent with tomato or squash yields exceeding those with plastic mulch. This approach merits further development.
The overall goal of this project is to assist vegetable growers in developing optimal mulching strategies for their farms. Specific objectives include:
1. To identify advantages and problems of different mulches used by vegetable growers, establish research priorities, and engage growers in the process of developing and disseminating information on mulching systems.
2. To evaluate recycled paper, vegetable oil-impregnated paper, and organic mulches as sustainable alternatives to plastic film mulch.
3. To disseminate information on horticultural, soil, economic and ecological merits of different mulches so growers can make informed choices appropriate to their specific sites and operations.
4. To identify safe and cost-effective means to utilize waste paper as a mulching material.
Field experiments were conducted at five biologically managed working farms in Virginia in 1993-94 to evaluate recycled kraft paper, oiled paper, hay and other organic mulches as alternatives to black plastic mulch for tomatoes. None of these alternative duplicated the early yields and superior weed control obtained with plastic. Organic mulches cooled the soil and somewhat delayed maturity, although total yields equaled those for plastic-mulched tomatoes. Paper treated with waste cooking oil dramatically warmed the soil early in the season, but weeds broke through this mulch. However, results indicated that plastic may not always be the best choice for tomatoes, especially in hot dry weather, when crops growing in the cooler organic mulches suffered less stress and gave more sustained production.
During 1993-94, seventy-two vegetable growers were interviewed to learn about existing mulching practices and innovations, and to engage growers in the process of proposing, testing and implementing new mulching strategies. A majority of participants prefer hay and straw mulches because they conserve soil moisture, add organic matter, reduce weeds and support good crop yields. Although growers noted the advantages of plastic, many raised environmental and waste-disposal concerns, and some have stopped using plastic for these reasons. Some growers reported using paper or killed cover crop mulches, and others expressed interest in experimenting with these materials.
Interview participants received reports on the findings of this survey and of field trials conducted in 1993-94, and were invited to participate in designing and conducting mulch experiments during the 1995 season. Seven growers responded to this invitation, and worked with the project coordinator to design and conduct on-farm experiments to address questions of particular interest to each grower.
Several growers in the survey suggested using two to four layers of newspaper under an organic mulch to enhance weed control or soil moisture conservation, and one recommended newsprint end rolls (available free at newspaper printing presses) because they are easier to apply and are free of inks. Experiments were conducted on five farms to evaluate this strategy, and the paper substantially improved weed control at four of these sites. Five tons per acre of hay (~ one 35-lb square bale per 150 square feet) laid over two thicknesses of newsprint controlled weeds as effectively as ten tons hay per acre without paper. Heavy applications of hay or straw can be expensive, and may upset the mineral balance of certain soils. Thus the use of paper to improve weed control by light applications of organic mulch is a significant grower innovation that has now shown promise in replicated trials.
Four growers hosted on-farm experiments with oiled paper mulch as an alternative to plastic. One participant successfully laid a small roll of oiled, heavy-duty kraft paper with a tractor-drawn plastic mulch layer, a key step toward implementing paper mulching on larger farms. Oiled kraft paper and a double layer of oiled newsprint warmed the soil early in the season, whereas a commercial black paper mulch did not. Both oiled paper mulches lasted longer than the black paper, and controlled weeds adequately, though not as completely as plastic. Participating growers expressed interest in further exploring oiled paper mulch.
In the course of conducting this project, it became apparent that choosing the "best" mulch is inherently a site-specific process. Crop, soil, climate, availability of mulching materials, and the farm's scale, machinery, financial and other resources must all be taken into account. The question, "what is the best mulching practice for this crop?" can best be answered through an integration of relevant research information with the grower's own ingenuity, knowledge and experience with his/her farm. Our objective for the remainder of this project is to develop and disseminate information on various mulching materials and their interaction with crop and soil, that will assist vegetable growers in developing ecologically and economically sound, site-specific mulching strategies.
The goal of this study is to conduct a holistic evaluation of plastic, paper and organic mulches, and communicate our findings to growers so that they can make informed choices of mulching system. Four groups of factors are considered: horticultural performance, effects on soil conditions and quality, environmental impacts, and economic feasibility, especially for the small-scale grower. Research was initiated in 1993 and continued through the 1995 growing season, with the specific objectives listed below. During 1994, objectives were modified slightly (modifications in italics) to better reflect growers' research needs within the study area.
1. To identify advantages and problems of mulching systems used by vegetable growers at different scales and degrees of mechanization, identify research priorities, and engage growers in the process of developing and disseminating information on mulching systems.
2. To evaluate recycled paper film and organic mulches as sustainable alternatives to plastic film mulch, considering horticultural performance, effects on soil conditions, costs and benefits to the grower, resource consumption, solid waste reduction and other environmental impacts.
3. To disseminate information on horticultural, soil, economic and ecological merits of different mulches so growers can make informed choices appropriate to their specific sites and operations.
4. To evaluate treatment of paper mulch with waste cooking oil to increase early-season soil temperature and to delay breakdown of paper mulch under field conditions.
5. To identify safe and cost-effective means to utilize waste paper as mulch, and explore feasibility of manufacturing a recycled paper mulch using environmentally benign processes with potential as a small business venture.
Field studies during 1993-94 showed that neither paper nor organic mulches can fully duplicate the benefits of plastic film mulches (Objective 2), but also that plastic is not always the best choice for tomato production (Schonbeck, 1995b). Objective 4 was essentially completed in the 1994 season. Oiled paper provided greater early-season soil warming than plastic, but sometimes broke down too soon, resulting in a partial loss of weed control (Ibid.).
In the course of conducting this project, it became apparent that choosing an optimal mulching system is inherently a site-specific process, depending on crops grown, soil and climatic conditions, mulching materials available on-farm or nearby, and the farm's scale, level of mechanization, and financial and other resources. Also, there exists a large body of research information on mulch interactions with soil, crop, weeds and other pest and beneficial organisms, but much of this information is not accessible to most growers. The question, "what is the best mulching practice for this crop?" can best be answered through an integration of relevant research information with the grower's own ingenuity, knowledge and experience with his/her farm. The greatest service this project can do is to provide information in such a way as to help growers make such decisions. Therefore, the focus for the current stage of this study has shifted largely to Objectives 1 and 3.
No further work was done on Objective 5 since the 1994 annual report for two reasons. First, the steam explosion process for recycling mixed paper waste into a mulch paper, explored during 1993-94, appears to entail high direct and environmental costs. Second, in 1993-94 field trials, neither untreated nor oiled paper mulches showed any horticultural advantages over plastic or organic mulches for tomato production. However, a number of growers expressed interest in exploring paper mulches for specific applications on their farms. Several indicated a strong preference for not using plastic and felt that initial results with oiled paper were promising enough to merit further evaluation. Also, several others indicated that laying paper under organic mulches may enhance weed control and/or soil conditions (Schonbeck, 1995a). Therefore, practical and logistical aspects of utilizing waste paper as agricultural mulch will be explored.
During 1993-94, experiments were conducted on five Virginia farms to evaluate paper and organic mulches as alternatives to black polyethylene (PE) film mulch for tomatoes grown without drip irrigation. Mulch effects on crop yield, earliness and quality; weeds, pests and crop diseases; soil moisture, temperature, and other physical properties; and labor requirements were recorded. Black plastic warmed the soil by about 3 degrees F, blocked weeds, and enhanced early yield, but it sometimes increased crop stress during hot dry weather. Hay and other organic mulches lowered soil temperatures and early tomato yields, but total yields equaled those of plastic mulched tomatoes and greatly exceeded those of unmulched tomatoes. Paper mulches also improved yield over bare soil, but often broke down rapidly, with partial loss of weed control. Early yields reflected mulch effects on soil temperature (plastic>paper>organic), but total yields were generally: organic >/= plastic >/= paper > bare soil.
Seventy-two biological and transitional growers in Virginia and neighboring states were interviewed to identify advantages and problems with various mulching practices for warm-season vegetables. Results paralleled those of on-farm trials, but many respondents (60%) reported using hay and other organic mulches, citing benefits to the soil as well as weed suppression, while fewer (30%) reported using plastic. A similar number indicated that they had stopped using plastic because of disposal and environmental concerns. Some survey participants expressed interest in oiled paper as an alternative to plastic, and others described a number of innovations in mulching strategy that they had already tried. Some of these provided the basis for 1995 field trials on seven farms selected from the survey.
In four out of five field experiments, mulches consisting of two layers of newsprint plus an organic material (hay, straw, tree leaves, pine needles) gave significantly better weed control than the organic mulch alone. The paper provided weed suppression equivalent to an additional 5 tons per acre hay or straw. The paper did not affect soil temperature, but slightly retarded both the entry of rainfall and the loss of soil moisture through evaporation.
Four growers tried oiled papers (either heavy-duty recycled kraft paper or a double layer of newsprint, treated with waste cooking oil), and found them more durable than a commercially available black mulch paper. One grower successfully laid a 4 by 25 ft roll of oiled 65-lb kraft paper with a home-built tractor-drawn plastic mulch layer, and another used oiled newsprint to kill a heavy bermudagrass infestation by solarization. All four expressed interest in evaluating oiled paper further as an alternative to plastic film.
In the course of conducting this project, it has become apparent that choosing an optimal mulching system is inherently a site-specific process, depending on crops grown, soil and climatic conditions, mulching materials available on-farm or nearby, and the farm's scale, level of mechanization, and financial and other resources. Optimizing the mulching system for a specific crop or situation requires an integration of relevant research information with the grower's own ingenuity, knowledge and experience with his/her farm. A series of four information sheets, based on experimental results, findings of the grower survey, and a literature review, were published to help growers accomplish this goal.
Mulches are an important component of sustainable horticulture because they suppress weeds, reduce erosion, conserve soil moisture and may modify soil temperature, structure and aeration. Appropriate mulching can minimize or eliminate the need for herbicides and mechanical cultivation in vegetable crops, significantly reduce irrigation costs, and optimize growing conditions. However, the effects of mulching may be complex and choice of optimum mulching system must depend on the crops grown, prevailing soil and weather conditions, and various economic and logistical considerations.
Many vegetable growers use plastic film mulches, usually black polyethylene (PE), for warm-season vegetables in the nightshade and cucurbit families. Black PE provides excellent weed control, enhances soil warming and crop earliness (Hill et al., 1982; Miller, 1973; Schultz, 1983; Waggoner et al., 1960), and, when laid over drip irrigation, allows precise control of soil moisture levels. Compared to clean cultivation without mulch, black plastic with drip irrigation can improve nutrient uptake efficiency and/or yields in tomato (Bhella, 1988a; Sweeney, 1987), zucchini (Bhella & Kwolek, 1984) and watermelon (Bhella, 1988b). In cooler climates, tomatoes show dramatic yield responses to black PE mulch (Bhella, 1988a; Wien et al., 1993), but in warmer climates tomatoes often yield much better in an organic mulch than in plastic (Abdul-Baki & Teasdale, 1994; Tindall et al., 1991). During the first two years of the current study, tomatoes began producing a little earlier in black PE than in soil-cooling organic mulches, but total yields were not greater (Schonbeck, 1995b). Plastic-mulched tomatoes may also develop more blossom-end rot (Hill et al., 1982; Schultz, 1983). Tomatoes prefer root zone temperatures of 75 to 80 degrees F (Tindall et al., 1991), and black PE may limit growth, yield or quality in hot weather by raising soil temperature above this range, and/or by hindering penetration of rainfall or overhead irrigation.
Plastic mulch has several other disadvantages. Unlike organic mulch, it does not add organic matter or nutrients to the soil, and its manufacture represents a significant input of petroleum. The plastic must also be removed from the field for disposal at the end of the season, which adds to labor costs. The 250 million pounds of agricultural plastic waste generated annually present a serious disposal problem for municipal landfills (Hofstetter, 1991). Smaller farms with limited capital resources may not be able to afford tractor-drawn mulch layers or other technologies needed to utilize plastic to full advantage. Also, plastic may complicate moisture management for growers who do not have drip irrigation systems.
Organic mulches like hay, straw, leaves, pine needles, sawdust and livestock or poultry bedding can conserve soil moisture, reduce weed pressure and enhance vegetable yields (Carter & Johnson, 1988; Isenberg & Odland, 1950; Munn, 1992). They also add organic matter and nutrients to the soil, thus supporting soil life and sometimes contributing to crop nutrition. Organic mulches may enhance rainfall infiltration, soil conservation, soil microbial activity, and formation of large stable aggregates near the surface (Alderfer & Merkle, 1943; Boller & Stephenson, 1946; Doran, 1980; Pillsbury & Huberty, 1941, Powlson & Brooks, 1987.
Under certain conditions, a hay or straw mulch may worsen soil structure, aeration and nutrient status (Albrecht & Uhland, 1925). Straw and grass hay have a high potassium to calcium (K:Ca) ratio (Parnes, 1990), and long-term heavy mulching with these materials can substantially increase plant-available soil K while not much affecting soil Ca (Stephenson & Schuster, 1945; Tukey & Schoff, 1963; Wander & Gourley, 1943). If the soil already has a high K:Ca ratio, this could upset cation nutrient imbalance and possibly reduce aggregation of clays. A deterioration in tilth of a clay-loam soil at the principal investigator's homestead, where grass hay mulch has been used routinely, was traced to such an imbalance (Luebben, 1995 pers. commun.). An application of calcitic limestone corrected the cation balance and improved tilth. On the other hand straw or grass hay mulches could ameliorate cation balance in soils deficient in K. Organic mulching has been variously reported to acidify the soil (Beaumont et al., 1927; Goodman, 1953; Tindall et al., 1991), have no effect on pH (Doran, 1980; Tukey & Schoff, 1963), or even slightly raise soil pH (Boller & Stephenson, 1946; Stevenson & Chase, 1953). Mulch effects on soil pH probably vary with the chemical composition of the mulch, application rates and timing, tillage practices, and soil physical and chemical properties.
Organic mulches apparently enhance earthworm populations by providing food and shelter (Hopp, 1947; Schonbeck, 1995b; Slater & Hopp, 1947), and may support other beneficial organisms However, organic mulch can also support larger populations of pest slugs than clean-cultivated bare soil (South, 1992), and at one time artificial fertilizers were recommended over manures and compost for gardens as a slug control measure (Miles et al., 1931).
Another disadvantage of organic mulches is that they can lower daily maximum soil temperatures as much as 8 to 20 degrees F compared to bare soil, which can slow maturity and reduce early harvests in warm-season vegetables (Hill et al., 1982; Isenberg & Odland, 1950; Schales & Sheldrake, 1966; Schultz, 1983). Delaying mulch application until 2 to 4 weeks after planting can minimize this effect (Isenberg & Odland, 1950).
Hauling and applying organic mulches may be impractical for medium to large farms. In recent years, growers and researchers have been experimenting with winter cover crops of hairy vetch and/or rye that are mowed or "rolled-down" by disking to create an in situ mulch just before no-till vegetable planting (Morse et al., 1993). Some researchers consider this the only feasible organic mulching method for larger farms (Morse, 1993, pers. commun.). This has been done successfully with broccoli (Schonbeck et al., 1993), tomatoes and cucurbits (Luna & Rutherford, undated), although weed control is sometimes inadequate. Tomatoes grown in a mowed hairy vetch mulch have yielded much more than in black PE or bare soil (Abdul-Baki & Teasdale, 1993, 1994), although herbicides are recommended for weed control in this system. Although the mulch left by mowing a winter cover crop may be relatively thin, the input of decomposable biomass at the soil surface, combined with reduced soil disturbance, may improve soil conditions similarly to applied organic mulches (Bruce et al., 1991).
Another alternative to plastic film mulch is paper mulch, which is biodegradable and therefore does not require pickup and disposal at the end of the season. It also provides at least partial permeability to rainfall, unlike black PE. Yield improvements over clean cultivation were reported for black paper film mulch early this century (Int'l Paper Co., 1928; Thompson & Platenius, 1931), and more recently for shredded newspaper (Munn, 1992).
Deriving a mulch by recycling waste paper can reduce energy costs and save landfill space. A 100% recycled carbon black kraft paper film mulch is commercially available, though it costs about 30% more per acre than black PE (Peaceful Valley Farm Supply, 1995). Non-blackened, recycled kraft paper is available in large rolls at lower cost (Breen, 1992, pers. commun.), and has shown promise as a film mulch (Anderson et al., in press). One problem is that paper may decompose too quickly with loss of moisture conservation and weed control. Recycled kraft paper treated with waste cooking oil may last longer than untreated paper (Ibid.). Also, it initially warms the soil more than does black plastic, with a subsequent decline in soil-warming properties as dust accumulates on the surface (Schonbeck, 1995b). This may be advantageous, as soil-warming is most desirable early in the season .
Another potential disadvantage of all mulches is that they may complicate establishment of a fall cover crop. Because cover crops need to be planted at a busy time of year (early fall), labor saving methods are often used such as broadcasting alone, or broadcasting followed by disking or rolling. Seedling establishment may be spotty, and could be reduced further by the presence of straw, hay, paper or other mulch materials.
Between March 1993 and October 1994, seventy-two vegetable growers in Virginia and neighboring states participated in an informal survey to gather information about current mulching practices, perceived costs and benefits, farmers' innovations and research priorities. Information collected from this survey was analyzed to obtain a qualitative (not statistical) picture of mulching practices in this region on market gardens and vegetable farms ranging from less than one acre to over 100 acres. Grower innovations were documented, as well as perceived advantages and problems with practices in widespread use. Findings of the survey and 1993-94 field trials were summarized in reports distributed free to all participants in spring of 1995 (Schonbeck, 1995a, 1995b).
The survey process was also used to identify additional mulching strategies and to recruit participants in on-farm field experiments. In response to a cover letter sent with the research reports, seven growers indicated interest in participating in 1995 field experiments, six of whom had not done so in the two preceding seasons. During spring 1995, the principal investigator worked with each grower to plan mulching experiments that would address questions of particular interest to the grower. Experimental design, plot sizes and other methodological details were adjusted to be compatible with each farm's production system, and with existing constraints on space, labor and other resources.
Paper + organic mulches
Several growers in the survey recommended a mulch of newspaper (two to four thicknesses) plus hay or other organic materials to enhance weed control and/or soil conditions. Project participant Andy Bradshaw suggested newsprint end rolls, which are available free at newspaper printing presses, are easier to apply than old newspapers, and are free of inks.
Experiments were established at two farms in the Tidewater region and three farms in the Appalachian region to test paper + organic mulches compared to the organic mulch alone (Table 1). Experimental design varied according to available space, mulch materials and other resources, and to the crop being grown. In all cases, the paper + organic mulch treatment consisted of laying two layers of ink-free newsprint from 29-inch wide end rolls on the soil surface, followed immediately by the organic material (Table 1). Treatment effects on weed growth were evaluated by fresh weights of weeds harvested from a sample area (minimum 40 sq. ft.) centrally located within the plot. Soil temperatures were taken at a depth of 3.5 in with a digital soil thermometer. Morning and afternoon temperatures were taken at 7 to 8 am, and 3 to 6 PM, respectively, which correspond approximately to the periods of maximum and minimum temperatures at this depth (Burrows & Larson, 1962). Afternoon temperatures were taken on sunny days (less than 50% cloud cover). Soil moisture content was determined gravimetrically by pooling several cores (0-6 in) per plot, weighing, drying at 220 ±10 F, and reweighing. With the exception of Site A, crop yields from each plot were not measured because available labor was limited, and the primary criterion of evaluation was weed control in these experiments.
At Site A, mulch treatments consisted of either pine needles or deciduous tree leaves (delivered free by the nearby town of Toano, VA). with or without paper. The design was a randomized split plot in four replicates,, with organic mulch as main plot, and ± paper as subplots. Subplots were 5 ft (2 rows) wide by 16 ft long. Onions (cv. Walla Walla) were sown in a nursery bed in September 1994, and transplanted to the experimental field in rows 30 in apart, in January 1995. Plots were cultivated, and mulches were applied on April 29. Paper was rolled out between crop rows, and leaves or pine needles spread about 2 to 3 inches deep over the entire plot area. Weeds were harvested and weighed on June 4, and onions were harvested on July 8, counted, weighed and graded as marketable or unmarketable. On both dates, afternoon soil temperatures were taken in each plot and in an adjacent strip of bare cultivated soil.
At Site B, fresh market tomatoes ('Early Girl' for two replicates; 'Celebrity' for one replicate) were transplanted on April 29, with plants placed 2 ft apart in rows 5 ft apart. Plots were hoed during the first week of June, and mulch treatments were applied June 6-7. Newsprint was folded lengthwise to produce a double thickness, applied on each side of the plot row and overlapped 1-2 inches in the middle by cutting short slots in the paper so that it could be tucked around each plant stem. Straw was spread over a 3-ft wide area centered on the crop row, at rates of 0, 4.5, 9, 13.5 and 18 tons per acre (air-dry weight). Treatments were arranged in a randomized split plot with three replications, with straw rate as whole plots and ± paper as subplots. Individual subplots consisted of a single crop row 20 ft long. Unmulched alley between plots (ca. 2 ft wide) were rototilled once to control weeds.
On July 7-10, morning and afternoon soil temperature, soil moisture, and weed fresh weight were recorded. Weeds were divided into bermudagrass (a noxious, warm-season, perennial grass weed in the Tidewater region) and all other weeds (primarily annual grass and broadleaf species). Soil moisture and weed fresh weight were recorded again August 26-29.
A supplemental experiment was conducted on mulching for bermudagrass control. On July 9, a 17 ft x 18 ft area dominated by bermudagrass sod was divided into three plots measuring 6 ft by 17 ft. The grass was mowed to a height of 1 to 2 in, and half of each plot was rototilled. Then three treatments were applied: straw mulch, two layers of newsprint plus straw mulch, and soil solarization with a single layer of oiled newsprint, followed 22 days later with straw. In all cases, straw was applied at the heavy rate of 22 tons per acre (ca. 10 inches deep). Treatments were not replicated. Plots were observed until frost for regrowth of bermudagrass.
At Site C, Early Girl tomatoes were transplanted in rows ca. 4 ft apart into the experimental field on May 25. In early July, the field was cultivated by tractor, and within-row weeds were pulled manually. On July 6-7, the following mulch treatments were applied in a randomized complete block design with four replicates: no mulch (control), 5 tons hay per acre, paper + 5 tons hay, 10 tons hay, paper + 10 tons hay, and 15 tons hay. Paper was overlapped as described for Site B. Individual plots were 8 ft (2 rows) wide by 12 ft long. Soil temperatures were recorded on July 15, 27 and 30; soil moisture on July 27 and August 25; and weed fresh weight on July 30 and September 20. Percent ground cover by crop, weeds and mulch, and percent of crop foliage affected by late blight, were estimated visually on August 25.
At Site E, three mulch treatments were established in a randomized complete block with four replications: hay at 4.3 tons/acre before planting; 4.3 tons hay before planting + 4.3 tons per hay three weeks after planting; and paper + 4.3 tons hay before planting. Crops (broccoli and lettuce) were planted in a double row in mulched beds 2.5 ft wide separated by unmulched alleys 2.5 ft wide, maintained by rototilling periodically. Plots consisted of a 25 ft length of a single bed. Preplant mulches were laid April 30, and seedlings of broccoli and lettuce (ca 20 of each per plot, alternating within each row) were transplanted April 30 and May 1. Additional hay was added to appropriate plots on May 21. Afternoon soil temperatures were taken May 20, June 3, 13 and 20 in each plot and in cultivated bare soil adjacent to the experiment. Weeds were pulled and weighed on June 13.
At Site H, the experiment was conducted in 10-ft wide terraces on a sloping field created in the 1994 season by drawing a moveable pig pen ("pig plow") along the contour. Terraces were separated by ca 10 ft wide strips of permanent grass + clover. Terraces were sown with a winter rye + hairy vetch cover crop in fall 1994, and the cover crop was pig-plowed in April 1995. On May 31, fresh market tomatoes (cv. Cherokee Purple) were transplanted 4 ft apart in a single row on each terrace. This management strategy was developed by the participating grower, Bev Eggleston. The original plan was to mow the permanent grass strips to obtain mulch for the terraces. However, severe drought curtailed grass growth, and problems also occurred with haying equipment. Therefore, plots were mulched with a mixture of old hay and grain straw from the grower's hayloft.
On July 18, weeds were hoed out, and two mulch treatments were applied: 8.7 tons per acre of the hay-straw mixture, with or without paper. Experimental design was a paired comparison with five replicates, and individual plots were 5 ft x 20 ft, containing a single crop row running lengthwise. On September 14, weeds were pulled and weighed, and soil moisture was measured.
Mow-killed winter cover crops for summer vegetables
At Site F (Table 1) a winter cover crop of rye + hairy vetch was planted in September of 1994, and managed in two different ways prior to planting tomatoes, winter squash or melons. For the tilled treatment, the cover crop was mowed twice during May, then rototilled before planting. For the mowed treatment, the cover crop was allowed to grow to a height of ca. 6 ft, then cut with a sicklebar mower before planting. Adjacent crop rows (100 ft long) were planted into mowed or tilled cover crop, and observed for crop and weed growth. Cantaloupe and watermelon seedlings were transplanted May 31, one day after cover crop management was done. Tomatoes were transplanted June 11-14 after cover crops were managed June 11. For winter squash, the cover crop was mowed or tilled June 18 and the squash was direct-seeded June 30. Afternoon soil temperatures were taken June 26, and tomato plant height and melon vine length were measured June 30. Weed growth was also evaluated June 30 by the "beaded string" method, with observations taken every foot along the 100-foot crop row.
Comparison of organic mulches for blueberries
This experiment was conducted at Site G, where the growers wanted to identify the best mulch material(s) for a new planting of blueberries. Prior to planting, the soil was amended with peat moss to lower pH to ca 5.0, which is optimal for blueberries. The crop was planted on a sloping field, in 8-ft wide terraces laid out approximately on the contour, separated by 8-ft wide alleys planted to perennial grasses. Two year old blueberry plants (cvs. Patriot and Spartan) were transplanted on April 21-22 , in a double row on each terrace, spaced 4 ft apart within the row.
Four locally-available mulching materials were evaluated: sawdust, wood chips (both purchased from sawmill businesses), pine needles (collected from pine forest adjacent to the field), and old hay (purchased from a nearby farm). Mulches were applied at a depth of about 4 inches on June 22-27. Treatments were replicated four times in a randomized complete block, each plot consisting of a 20-ft length of double row, containing five Patriot and five Spartan plants. On August 14, soil temperature was taken, samples were collected for moisture determination, and weeds were pulled and weighed. Soil moisture and weed fresh weight were measured again on October 24. Soil pH and plant height were also measured at this time.
Effect of organic mulch on slug activity
At Site D (Table 1), the residence of the principal investigator, a severe outbreak of two species of slugs (tentatively identified as the dusky slug, Arion subfuscus and the gray field slug Deroceras reticulatum) occurred between May and July 1995, causing damage to lettuce and cole crops. An experiment was conducted to determine whether mulching would aggravate problems with these pests. Nine beds planted to cole crops (Brassica spp.) were divided into two subplots assigned at random to a "mulch" or "no mulch" treatment. On May 25, about three weeks after crops were transplanted, a mulch of freshly cut grass hay was applied to the appropriate half of each plot, the other half being left bare. About 5 ± 1 tons/acre (air-dry weight) of mulch were applied.
On eight occasions between May 26 and June 11, all visible slugs were counted and removed. Slugs were divided into "dusky," "gray," and "small" (juveniles under 0.5 inch extended; these were difficult to identify to species). Although this "surface searching" technique may not reflect actual slug population densities, it does give a relative index of slug activity (South, 1992). On June 26, an additional surface search was conducted, and weed fresh weight was measured.
In October, 1995, the effects of three different organic mulching materials on slug activity was evaluated. Four treatments were evaluated in a 4 x 4 Latin square: pine needles (reported by several growers in the survey as repelling slugs), old hay, dried quackgrass (reported in HortIdeas as repellent and somewhat toxic to slugs), and no mulch. Individual plots consisted of a small area (3 x 4 ft) , with a pitfall trap (4 in wide x 1 in deep) baited with about 3 oz stale beer, and covered with a roof of wood set about 1/2 inch above the rim of the trap. Five times between October 3 and October 20, the traps were baited, and slug catches were counted and removed 36 to 48 hours later.
During the 1993 and 1994 growing seasons, various paper, oiled paper and organic mulches were evaluated in comparison to black PE film mulch and, in some experiments, an unmulched control. On-farm experiments were conducted with four Virginia growers selected from the survey, and at the residence of the principal investigator. Detailed materials, methods and results for assessments of mulch effects on crop yield and quality, soil moisture, temperature and other conditions are given by Schonbeck (1994, 1995b).
Direct costs and environmental impacts of mulches
Purchase costs of various mulch materials were calculated on a per-acre basis. Labor costs were estimated by timing mulching operations in field experiments and converting to per-acre rates. Energy costs of producing, applying and removing (plastic only) each kind of mulch were estimated. Library research was conducted to obtain information on typical fossil fuel consumption associated with manufacture of plastic and paper, production of hay and straw, and other farm operations associated with mulching. Waste stream and recycling impacts of each mulching strategy were also estimated.
Cover crop establishment after different mulches
At three sites, the effects of mulching treatment on establishment of a rye cover crop was evaluated. After tomato harvest was finished, plastic mulch was removed from the plots, but all other mulches were left in place. Rye seed was broadcast at about 2 bu/acre over the plots on the dates shown in Table 2. In spring 1995, cover crop establishment, weed growth and total soil cover were evaluated by a "beaded string" assessment. A 25-ft length of window sash cord, marked at 1.0 ft intervals, was laid lengthwise along the center of the plot, and cover beneath each mark was recorded as rye, weed, residue (old mulch, etc.), or bare soil. Rye crop height was also measured at three points within each plot.
Four participating growers wanted to explore paper film alternatives to plastic films on their farms. Replicated experiments were established at sites B, F and H (Table 1), where treatments and experimental design were selected according to growers' priorities and available space for the experiments. Because of growers' time constraints, crop yields were not recorded by plot, although yields were recorded by treatment for Site F. At the fourth farm (site C), a single 25-ft roll of oiled 65-lb kraft paper was laid with a home-built, tractor-drawn mulch layer within a field in which other rows were mulched with various plastic film and mesh mulches, applied with the same machinery.
At Site B, experiments were conducted for melons and summer squash. Oiled 65-lb kraft paper was compared with a commercially available black paper (Planters' Paper(TM)) and no mulch for cantaloupe. A winter rye cover crop was tilled in on May 8, and cantaloupes were sown on May 20. Just before planting, the paper mulches (4-ft wide x 25-ft long rolls) were laid, and planting holes were cut at 4-ft intervals. On June 23, unmulched plots and alleys were hoed, and the entire planting was mulched with ca. 3 inch depth of hay. This was done to help maintain the crop during the summer drought. Soil temperature and crop establishment were evaluated on June 5, and crop cover was estimated on July 10.
Summer squash was direct-seeded on May 8, May 20 and June 3, with a single 100-ft row planted on each date. Treatments included Planters' Paper and a single layer of newsprint end roll applied just before planting, and no mulch, with one replicate established for each date. Holes (ca 4 in diam) were cut into paper mulches at 3-ft intervals along the midline of the mulch to sow seeds. Morning and afternoon soil temperatures were recorded on June 5 and July 7. Paper mulch treatments were evaluated for durability and weed control.
At Site F, cantaloupes and fresh market tomatoes (cv Oregon Spring) were grown in black PE, green PE and oiled 65-lb kraft paper, evaluated in a randomized complete block design with five replicates. Mulches were laid manually just before transplanting melons (2 replicates, established May 7) or tomatoes (3 replicates, May 20). Plots were 1 crop row x 25 ft. Soil temperatures were taken June 26. On August 14, soil samples were taken for moisture determinations from the three tomato replicates, and mulch persistence and weed control were estimated visually.
At Site H, five treatments were evaluated in a randomized complete block with four replicates: black PE; infrared transmitting PE (=IRT); Planters' Paper; two thicknesses of newsprint treated with waste cooking oil; and unmulched control. Individual plots consisted of 25 feet of a single crop row. Mulches were laid manually on May 30 (plastics) or May 31 (paper mulches). Fresh market tomato seedlings (cv Blue Ridge) were planted 2.5 ft apart down the center of each plot. Afternoon soil temperatures were taken May 31 and June 29. On June 30, crop height was measured (three plants measured for each plot), and percent cover by crop and mulch was evaluated by a "beaded string" stretched along the midline of the plot. Soil samples were collected for moisture on June 30 and July 18; unfortunately the samples for the first date were lost.
Four information sheets have been published to provide information to help growers select the best mulching systems for their farms (copies included with this report). A summary of 1995 research results will be published in the Virginia Biological Farmer, which has already carried summaries from previous years. Three sources of information were utilized in producing these publications: field experiment results obtained during 1993-95 in this study; growers' information, experience and ideas (communicated in the survey and through other discussions); and previous research findings. We gathered the latter through a literature search, with particular emphasis on effects of mulches on soil organic matter, tilth, crop-available nutrients and other soil parameters that respond slowly and therefore could not be adequately addressed in the experimental component of this project.
Information is also being gathered on two vegetable pest complexes (slugs, and root/seed maggots of the genus Delia), which have caused serious damage in this region, and which are reportedly associated with high soil organic matter levels. Possible effects of mulching systems on these pests and some of their predators will be explored, and relevant findings communicated to growers.
For randomized, replicated experiments, standard ANOVA was applied whenever data approximated the underlying assumptions of homogeneity of variance and normal distribution (Steel & Torrie, 1960). Appropriate transformations were used for percentages, and other non-normally distributed data. In these cases, treatment means were calculated from transformed data, then back-transformed (Ibid.); thus they do not represent arithmetic means of raw data.
Because of the subjective nature of visual estimates of percent cover and intensity of disease or pest infestations, the more conservative nonparametric Friedman test was used to evaluate these data (Ibid.).
Hay and grain straw were the most commonly used mulches for solanaceous and cucurbit vegetables, with 44 out of 72 vegetable farms surveyed using one or both of these materials. Seven growers with farms in the 10 to 50 acre range have developed workable methods for spreading these materials on plantings of an acre or more, and reported net labor savings through weed control and moisture conservation. Smaller-scale growers often use a variety of organic materials, including sawdust, tree leaves and livestock litter, in addition to hay and straw. The most often cited advantages of organic mulches include weed suppression, soil moisture conservation, and improved soil organic matter or physical conditions. Some growers found that hay or straw does not always control weeds adequately, may delay crop maturity by cooling the soil, or are expensive or labor intensive to apply. However, most find that the benefits outweigh the disadvantages, and few indicated that they had discontinued using these materials.
Twenty-two interviewees reported using plastic mulches, usually black polyethylene film. The main advantages reported include superior weed control, soil warming and therefore crop earliness, and cleaner, disease-free produce. However, many objected to the large amount of non-recyclable waste generated by plastic mulch, and several noted that the "degradable" plastics currently available do not break down completely, leaving litter and possibly some invisible pollutants. Others were concerned about the lack of organic matter input or other adverse effects on soil. A substantial portion of survey participants (22) have stopped using plastic mulches for these reasons.
The survey revealed considerable interest in using mow-killed winter cover crops as an in situ organic mulch, and paper (old newspapers and other waste paper, or mulch papers such as Planter's Paper) as biodegradable alternatives to plastic films. Some growers already use these materials, and others expressed interest in experimenting with them.
Survey results are reported in greater detail elsewhere (Schonbeck 1995a; copies attached to this report).
Paper + organic mulches
At Site A, two thicknesses of newsprint under organic mulches of deciduous tree leaves or pine needles did not seem to enhance weed control in onions. In all treatments, most of the weeds emerged within crop rows, which were not covered by the paper. Mulch treatment also did not affect soil temperature, total or marketable yields, or average weight per onion. Compared to an adjacent strip of cultivated bare soil, all mulches lowered afternoon soil temperatures (at 3.5 inches depth), by 12 to 14 degrees F in early June, and about 5 degrees in early July. In warm Tidewater climates, this might be beneficial for a cool-season crop like onions.
Both paper and straw mulches dramatically reduced annual weeds in tomatoes at Site B (Table 3, top). Two layers of newsprint + 4.5 or 9 tons straw per acre were as effective as 9 or 13.5 tons straw alone, respectively. Bermudagrass occurred sporadically throughout the experiment, and treatment differences for this weed were not statistically significant. However, total weed weight (annuals + bermudagrass) showed similar trends to the annuals alone, with ca. 95% weed control achieved with 4.5 tons of straw with paper, or 9 tons straw alone (Table 3, bottom).
Straw mulch reduced afternoon soil temperature by 3 to 6 degrees F depending on mulch thickness, but all straw treatments slightly increased morning soil temperature (Table 4, top). This narrowing of the daily fluctuation in soil temperature is a typical effect of organic mulches (Burrows & Larson, 1962). Paper alone slightly lowered afternoon soil temperature, but paper underneath hay did not have any soil temperature effect beyond that of the hay itself.
On July 7, at the end of a 6-week period of slightly above normal rainfall (6.2 in. total), soil moisture levels were slightly lower under paper + 4.5 to 13.5 tons straw per acre than under straw alone (Table 4, bottom). By the end of August, following six weeks of severe drought (total rainfall 1.0 in), soil moisture levels in all treatments had fallen to an extremely low level. Straw conferred a slight but statistically significant conservation of moisture, estimated at 0.15 inch moisture in the top 6 inches of soil for the highest mulch rate. Moisture levels were no longer lower under paper + straw treatments than under corresponding straw-only treatments. Thus the paper seems to retard both the entry of rainfall and the loss of soil moisture through evaporation. In contrast, straw and hay are known to enhance rain infiltration while slowing evaporation (Alderfer & Merkle, 1943; Bond & Willis, 1969). Although tomato yields were not measured, the participating farmer noted that mulched tomatoes lived longer and yielded better during the drought than unmulched tomatoes.
Within one day after application of oil-treated newsprint to mowed or tilled bermudagrass sod, afternoon soil surface temperature under the oiled newsprint reached 130 degrees F, while maximum air temperature was 85 degrees F. Daily maximum air temperatures reached 90 to 105 degrees F during the subsequent three week solarization period (soil temperature not measured). By the end of August, very little (3% cover) bermudagrass had grown through the straw applied after solarization, somewhat more (15% cover) had emerged through two layers untreated newsprint + straw, and a thick stand (about 50% cover) had re-established itself through straw applied alone. Rototilling before mulching seemed to reduce regrowth in the straw-only treatment but not in others. Most of the bermudagrass rhizomes beneath the mulch in the solarized plot appeared dead. The grower plans to experiment further with this method for reclaiming bermudagrass-infested areas during the 1996 growing season.
At Site C, under heavy weed pressure dominated by common cocklebur, barnyardgrass, pigweeds and other aggressive summer annuals, hay mulch gave substantial weed suppression, which was further enhanced by two layers of newsprint (Table 5). With paper, 5 or 10 tons per acre of hay gave weed control similar to 10 or 15 tons of hay alone, respectively. Hay reduced afternoon soil temperatures by 10 to 16 degrees F, depending on application rate, but paper again did not exert any additional cooling effect. Mulching also slightly reduced morning soil temperature. In late August, after four weeks of drought, soil moisture levels were considerably higher under hay than without mulch, but differences between hay application rates were slight. Moisture conservation was estimated at 0.38 inch for the top 6 inches of soil. On September 20 (11 weeks after application) the paper was still mostly intact beneath the hay. At this time, ground cover by hay mulch was about 90% for 5 tons per acre, and 99-100% for higher rates.
At Site E, two layers newsprint + 4.4 tons hay per acre applied just before planting broccoli and lettuce provided significantly better weed control than hay alone (Table 6). The effect was similar to a second application of 4.4 tons hay three weeks after planting. Weeds consisted of volunteer buckwheat, summer annual grasses and a few broadleaf annuals. Mulching lowered afternoon soil temperature by 7 to 9 degrees, with 8.8 tons hay application cooling the soil slightly more than 4.4 tons hay, either with or without paper. The broccoli plants thrived and produced equally well in all three mulch treatments.
At Site H, extreme drought prevailed between the time of mulch application and weed assessment (total of 0.5 inch rain in 57 days). Weed regrowth after hoeing and mulching was minimal in both treatments, but was significantly less (10 lb/acre fresh weight) in paper + hay than in hay alone (157 lb/acre). Paper did not affect soil moisture level.
No replicated experiment was conducted at Site D, but a newspaper + hay mulch was used successfully to suppress quackgrass during tomato production. Tomatoes were planted on a 5 foot row spacing, into 18-inch wide strips that were cleared of quackgrass rhizomes. Alleys were covered with several thicknesses of old newspapers, plus a light covering of hay (weight not measured; probably ca. 3 tons/acre). The mulch greatly reduced quackgrass growth, and only a few burdock and other weeds emerged and required hand pulling. Tomato production was satisfactory, and when the soil was worked up after harvest, the quackgrass rhizome infestation was considerably lighter than it had been initially.
These experiments indicate that paper + organic mulches may be a good weed control strategy, especially for annual weeds. In three experiments with widely different weed floras, two thicknesses of newsprint under a hay or straw mulch enhanced weed control similarly to an additional 4 to 5 tons hay per acre. Because hay is expensive to buy, produce, haul and apply, a strategy such as this that reduces the application rate required for adequate weed control could yield a significant economic advantage. Furthermore, since only light applications (1 to 5 tons/acre) are needed for soil protection and moisture conservation (Boller & Stephenson, 1946; Bond, 1969; Hopp, 1947; Doran, 1980), and annual applications of higher rates of grass hay could upset soil cation balance, using paper plus a lighter rate of organic mulch could maximize benefits and minimize potential problems with organic mulching.
Mow-killed winter cover crops for summer vegetables
Both mowed and tilled strips had considerable weed growth, with no significant differences in percent weed cover. The weed flora was dominated by annuals in the tilled treatment, and quackgrass, dock and other perennials in the mowed treatment. The light mulch left by the mowed rye + vetch lowered afternoon soil temperature about 3 degrees F. Tomato plant height was not affected by treatment 18 days after transplanting. However, 30 days after transplanting, both cantaloupe and watermelon vine lengths were about one-third less in the mowed than in the tilled strips.
Comparison of organic mulches for blueberries
Heavy thunderstorms during the first five days after mulch application washed one or more gullies through the mulch and the soil surface in three of the sawdust mulched plots. Little erosion occurred in hay or pine needles, and the wood chips seemed to be the most stable of all. Weed growth was light in all four mulches. Weed fresh weights averaged lower in wood chips than other mulches, but the difference was not statistically significant because of high random variability (Table 7).
High temperatures and below-normal rainfall (3.6 inches between July 1 and August 14), rendered unmulched soil quite dry by mid-August, but moisture levels remained high under all four mulches. On the hot clear afternoon of August 14, soil temperatures did not differ between mulches, but were much lower (74 to 75 degrees F) than in unmulched soil (92 to 95 degrees F). Wood chips and sawdust retained somewhat more moisture (equivalent to about 0.2 inch in the top 6 inches of soil) than hay or pine needles, which had also settled to a lesser depth (ca. 2 inches) than the two wood-derived mulches (ca. 3-4 inches). In October, after autumn rains recharged soil moisture, levels remained slightly higher under wood chip and sawdust than the other mulches, but the difference was no longer significant.
At the end of the growing season, the Spartan variety of blueberry was slightly taller in pine needles than other mulches (Table 7). Treatment differences were not significant for the Patriot variety.
Effect of organic mulch on slug activity
Brassica beds that were mulched with freshly cut hay at the end of May showed significantly higher slug activity levels than unmulched beds. The number of dusky slugs collected in the mulched plots was equivalent to 49,000 per acre, compared to only 26,000 in bare plots (paired sample t = 3.38, p ~ 0.01). Gray and juvenile slug numbers showed a similar though statistically nonsignificant trend. Total slugs were 120,000 per acre for mulched plots compared to 67,000 for unmulched (t = 5.96, p < 0.01). However, weed growth during the 32 day experiment was significantly (t = 2.81, p < 0.05) higher in bare plots (3,130 lb fresh weight per acre) than in mulched plots (1045 lb per acre). Also, the soil surface in unmulched plots became visibly sealed and compacted during a series of intense thunderstorms in June, whereas soil surface remained somewhat more open and crumbly under the mulch.
In the fall experiment, traps placed in mulched miniplots did not catch significantly more slugs (25-36,000 per acre) than traps placed in unmulched plots (25,000). Furthermore, there were no significant differences between pine needle, hay and quackgrass mulches.
In 1993, marketable yields of Roma tomato were similar for hay, black plastic or 65-lb kraft paper mulches even though soil temperatures showed considerable differences (plastic>paper>hay). All three mulches produced substantially better yields and weed control than bare soil, and the additional labor of applying the mulch provided good economic returns.
In 1994, early marketable tomato yields (first 3 weeks of harvest) were significantly higher for black PE than for organic mulches. However, total marketable yields were similar in plastic, paper, and organic mulches except at Appalachian sites where severe late blight curtailed production in August. Tomatoes grown in oiled paper (applied pre-plant) + hay (applied 4-6 weeks later) performed similarly to tomatoes in hay alone. Tomatoes grown in bare soil or mowed cover crop yielded considerably less, possibly because both treatments had lower soil moisture levels.
Black PE gave the best weed control in and near the crop rows, followed by hay or straw, then untreated or oiled paper, which broke down too soon, with partial loss of weed control. Compost at 100 tons per acre (2 inch depth) was not as effective as 7.5 tons hay per acre (ca. 2 inch depth after settling).
Mulch treatment effects on afternoon soil temperature varied as follows: oiled paper >plastic > bare soil > compost ~ untreated paper > hay ~ straw. Plastic and oiled paper raised morning temperatures as well, while other mulches had little effect. Mulch effects on soil moisture were somewhat erratic, although organic mulches usually conserved moisture compared to bare soil. Plastic sometimes enhanced and sometimes lowered soil moisture, as it reduced both penetration by rain and evaporation. Mulch treatment had no consistent effects on bulk density, water infiltration, aggregate stability, or soil nitrate during the single season of observation. However, fall earthworm populations were lower after a season under plastic (300,000/acre) than under hay or oiled paper + hay (600,000/acre). Results of 1993-94 field studies are given in greater detail in Schonbeck (1993,1994, 1995b)
Direct costs and environmental impacts of mulches
Purchase costs for recycled kraft paper were lower than for either plastic, Planter's Paper or organic mulches (Table 8). Manual application of compost, paper or plastic was more labor-intensive than for hay or straw, although mechanized application could save much labor for film mulches and possibly compost. Plastic is most costly in terms of fossil fuel consumed in producing the material, and is the only mulch that adds to solid waste streams. Recycled paper entails much lower energy cost, and converting low-grade waste papers to an agricultural mulch could save landfill space. An in situ mulch of mowed cover crops may be economical in terms of direct and energy costs, provided it gives adequate weed control and crop yields. The use of municipal leaves as agricultural compost could remove a tremendous burden from municipal landfills, but transporting enough to the farm to produce a 2-inch thick mulch could entail prohibitive fuel costs unless the distance is short and the area mulched is small.
Cover crop establishment after different mulches
At the Tidewater site, rye cover in spring 1995 was significantly lower after black PE was used in the 1994 season, than after any other mulch (Table 9). The height of the rye was greater after composted leaves than after other mulches. Although living vegetation (crops + weeds) did not completely cover the ground in any treatment, applied organic mulches (including oiled paper + hay) left sufficient residue to protect the ground effectively. In contrast, considerable bare ground was visible after black PE and mowed rye + vetch, which did not produce sufficient biomass to leave a persistent mulch. Although the composted leaves completely covered the soil surface, their soil-like texture probably offers less of a barrier to weed growth or erosive runoff than hay residues.
At the Piedmont site, rye establishment varied erratically throughout the plots, with no significant difference between treatments. At the Appalachian site, the rye cover crop showed the best height and cover after compost and hay mulches, the poorest after grain straw or no mulch, and intermediate levels for plastic and paper (Table 10). Weed growth was heavier after straw or no mulch than other treatments. Plots that had been mulched with black PE again showed much more bare soil than other treatments, but in this case it was weed growth that was suppressed.
In these experiments, the rye cover crop was established under comparatively unfavorable conditions, including late planting (beginning of November) and broadcasting without incorporation or rolling. Thus establishment of 50% rye cover in many treatments indicates that winter cover cropping without plowing after a mulched summer vegetable crop may be feasible in most situations. Furthermore, cover crop establishment was generally as good after organic paper mulches as after black PE or no mulch.
Prior to planting rye, the black PE was removed, exposing bare, weed-free soil, whereas all other mulches were left in place. Possibly, rye and/or winter weeds could germinate and emerge better in the presence of hay or other mulches (which provide moist, sheltered microsites for seeds and seedlings) than on bare ground. At the Appalachian site, grain straw had largely decomposed by November, so that both this treatment and the bare soil control presented unmulched but somewhat weedy ground, unfavorable conditions for rye establishment. Other organic mulches probably provided favorable soil conditions, and substantial amounts of nutrients in the case of compost.
At Site B, a single layer of newsprint did not adequately suppress weeds, which broke through within two or three weeks. Despite its relatively rapid breakdown, Planters Paper provided better weed control than the newsprint, partly because it is entirely opaque and thus prevented weed growth until after it decomposed sufficiently to expose the soil. Afternoon soil temperatures under mulches, compared to bare soil, were: +3 degrees F for oiled kraft paper, little effect by Planter's Paper, and - 3 degrees F by untreated newsprint.
At Site C, a 4 ft by 25 ft roll of 65-lb kraft paper was successfully laid by machine, with the paper sustaining only a few small tears in the process. The mulch held up adequately to suppress weeds and allow a good tomato crop, although weed control was not as good as the much more persistent black and IRT plastic film mulches. The grower is interested in trying paper mulch on a larger scale in the future.
At Site F, afternoon soil temperatures six weeks after application were actually lower under oiled paper than either black PE or green IRT plastic. Oiled paper still covered 80 to 90% of the soil surface 14 weeks after application, and provided adequate weed control, though again not as good as plastic films. Crop yields averaged 12 bushels per 100 ft row for IRT plastic, compared to about 8 bushels for either black PE or oiled kraft paper. However, yield measurements were done by treatment rather than by plot; thus no replication was available.
At Site H, Planter's Paper became torn in three replicates during a severe windstorm, while two layers of oiled newsprint remained intact and suppressed weeds for seven weeks after application. As a result, percent ground cover by mulch one month after application was considerably lower for Planter's Paper than for other film mulches (Table 11).
One day after application, the oiled newsprint warmed the soil significantly more than either black PE or IRT, while Planter's Paper had little effect on soil temperature (Table 11). By June 29, the soil was considerably warmer under plastic films than under either oiled newsprint or black paper, while the bare plots were coolest. At this time, weeds about 12 inches tall completely covered the ground in the "bare" plots rendering soil temperature 10 F lower than in an adjacent bed of weed-free bare soil. A few small weeds were growing through the torn and decomposing black paper, but oiled newsprint and both plastic films still maintained nearly 100% weed control within and near the crop row. However, because the newsprint mulch was relatively narrow (about 20 inches wide after edges were anchored with soil), weeds growing along the edge the soil were leaning toward the crop row and shading part of the film-mulched area, which could have contributed to the lower temperature.
In mid July, during a severe drought, soil moisture levels were significantly lower under the IRT plastic than under either black PE, black paper or the weedy unmulched control (Table 11). Oiled newsprint had intermediate moisture levels. The cause of this pattern is not known. Crop height was slightly greater in IRT and bare soil than in oiled newsprint. The greater height in bare soil may reflect slight elongation in response to weed competition, rather than increased vigor.
The tendency for oiled paper to greatly increase soil temperature during the first few weeks after application, and to then lose this effect on temperature, was also observed in 1994 (Schonbeck, 1995b). As discussed earlier, this would theoretically be an advantage for tomatoes, which prefer a moderately warm soil, and can "burn out" in plastic mulches during hot dry weather.
A series of four information sheets (four pages each) were produced summarizing study findings in a format designed to help vegetable growers choose mulching systems appropriate to their specific crops, soils, climates and production systems. First drafts of each information sheet were distributed to at least two participating growers, and two other reviewers for comments on content, presentation, clarity and practical usefulness. Valuable suggestions were offered and incorporated into the final drafts. Grower reviews indicated that the four-page information sheet format is more accessible and useful than the longer (21-24 pages) reports published in spring 1995.
Results of experiments detailed in 1993 and 1994 Annual Reports indicated that heavy-duty (65-lb) kraft paper treated with vegetable oil provided fair durability and weed control, as well as substantial early-season soil warming. A lighter weight (40 lb) kraft paper, whether untreated or oiled, decomposed too rapidly, resulting in loss of weed control and soil moisture. A commercially available 32-lb black mulch paper also broke down quickly, but it initially blocked light and thereby provided better early-season weed control than undyed 40-lb kraft paper. Black paper also did not maintain soil temperatures as high as either oiled kraft paper or black polyethylene film.
Because most paper companies now reprocess end rolls and other factory waste, the main source for obtaining rolls of paper free of charge is newspaper printing presses, which give away newsprint end rolls (25 to 300 ft by 2.5 ft), which are suitable for small scale market gardening operations. One paper reprocessing company in Suffolk, VA was identified that will sell 65-lb kraft paper end rolls on site for about $300 per ton, equivalent to ca $125 to $150 per acre mulched, plus hauling costs. This may be competitive with the cost of black plastic mulch for farms located close to the paper company.
Discussions with growers indicate that the commercially available black mulch paper is gaining popularity. Black paper does not warm the soil as effectively as black plastic, nor does it last as long or control weeds as completely. However, an increasing number of growers are using this paper mulch, as it is biodegradable, and does not need to be picked up for disposal at the end of the season.
1. None of the paper or organic mulches tested provided all of the advantages of black plastic films: soil warming, a season-long weed barrier, and adaptability to mechanized application. However, in Tidewater and Piedmont regions of Virginia, organic mulches like hay or straw enhanced total marketable tomato yields at least as much as black plastic. Organic mulches may be the better choice for small farms that lack drip irrigation and tractor-drawn film mulch applicators.
2. Black plastic increased soil temperature 2-3 degrees F and enhanced early tomato yields compared to organic mulches, but it also interfered with entry of rainfall, which can limit midseason production. Plastic also seems to create conditions unfavorable to earthworms. However, black plastic may be advantageous in cooler Appalachian regions subject to late blight, a disease that can destroy all later-ripening fruit. Earlier harvests can also fetch higher prices in some markets.
3. Organic mulches like hay and straw enhanced tomato yields by conserving soil moisture and reducing weed growth, although they reduced annual soil temperature by about 5 degrees F and slightly delayed fruit ripening. Mulching large areas can be expensive and labor-intensive, yet some less mechanized farms use hay or straw on plantings as large as one to 20 acres. Growers report that each hour spent mulching can save two to four hours on weed control.
4. Organic mulches usually help maintain or improve soil fertility, but can occasionally be detrimental. Informal observations at one site and a literature review revealed that long term heavy use of straw or grass hay mulches can aggravate certain soil mineral imbalances, particularly Ca:K ratios that are too low. This can cause a deterioration of soil tilth. Effects of different organic mulches on soil cation nutrient balance and the possibility of selecting organic mulches to match soil nutrient needs deserve further research.
5. An in situ mulch of mowed winter cover crops provided only 1.6 tons of residue per acre, and did not suppress weeds. However, winter cover crops have provided up to 4 tons of mulch per acre and given significant weed control in other studies. Composted yard waste at 60 to 85 tons per acre did not control weeds as effectively as just 7 tons per acre of hay or straw. Compost can be utilized more effectively as an incorporated soil amendment than as a surface mulch.
6. Paper mulches decomposed before the end of the season, compromising late-season weed control and moisture conservation. However, a heavy (65-lb) kraft paper significantly improved tomato yield compared to a bare soil check during a drought year. Weeds rapidly penetrated a lighter (40-lb) kraft paper, whereas a commercially available 32-lb black mulch paper provided early season weed control by excluding light. Both black paper and natural kraft paper cooled the soil slightly. Kraft paper treated with vegetable oil initially increased mean soil temperature by 5 degrees F, but this effect disappeared several weeks after application, and did not promote early fruit ripening.
7. Several growers in this region lay paper beneath an organic mulch to enhance weed control. In four out of five experiments, two layers of newsprint plus 5 tons of hay or straw per acre reduced weed competition as effectively as 10 tons of organic mulch alone.
8. Growers showed considerable ingenuity in developing and adapting mulching practices for their specific crops, management systems, climates, soil conditions and available resources. An important conclusion of the study is that grower knowledge and experience, combined with relevant research information, can lead to development of effective, site-appropriate, environmentally-sound mulching choices for tomatoes and other vegetables.
Educational & Outreach Activities
Publications arising from this study since January 1995 include the following:
Schonbeck, Mark W., 1995. Mulching Practices and Innovations for Warm Season Vegetables in Virginia and Neighboring States. 1. an informal survey of growers. Virginia Association for Biological Farming. 24 pp.
Schonbeck, Mark W., 1995. Mulching Practices and Innovations for Warm Season Vegetables in Virginia and Neighboring States. 2. results of on-farm field trials in 1993-94. Virginia Association for Biological Farming. 21 pp.
Schonbeck, Mark W., 1994. Update on SARE-funded mulch research project. Virginia Biological Farmer, Fall-winter 1994-95, p 4.
Schonbeck, Mark W., 1995. Mulching choices for warm-season vegetables. Virginia Biological Farmer, Spring 1995, pp. 16-18.
Schonbeck, Mark W., 1995. Mulching and Slugs. Virginia Biological Farmer, Fall 1995, p. 6.
Schonbeck, Mark W., 1995. Mulching with Paper. Virginia Biological Farmer, Winter 1995-96; will be reprinted in The Natural Farmer, Spring 1996.
Schonbeck, Mark W., 1995. How a Little Mulch Can Save a Lot of Water and Soil. Virginia Biological Farmer, Winter 1995-96.
Schonbeck, Mark W. and Gregory K. Evanylo. Effects of Mulches on Soil Properties and Tomato Production. I. Soil Temperature, Soil Moisture and Marketable Yield. To be submitted to Journal of Sustainable Agriculture.
Schonbeck, Mark W. and Gregory K. Evanylo. Effects of Mulches on Soil Properties and Tomato Production. II. Plant-Available Nitrogen, Organic Matter Input and Tilth-related Properties. To be submitted to Journal of Sustainable Agriculture.
Schonbeck, Mark W. Weed Suppression and Labor Requirements Associated with Different Mulching Systems in Vegetable Production. To be submitted to Journal of Sustainable Agriculture.
Schonbeck, Mark W., 1996. Mulching for Weed Control in Annual Vegetable Crops. Information Sheet, Virginia Assoc. Biological Farming. 4pp.
Schonbeck, Mark W., 1996. Matching Mulch to Crop Needs: Tomatoes and Other Vegetables. Information Sheet, Virginia Assoc. Biological Farming. 4pp.
Schonbeck, Mark W., 1996. Mulches and Soil Fertility: Nutrients, Organic Matter, Tilth and Soil Life. Information Sheet, Virginia Assoc. Biological Farming. 4pp.
Schonbeck, Mark W., 1996. Does Mulching Pay? Costs and Benefits of Different Mulching Systems for Market Gardens. Information Sheet, Virginia Assoc. Biological Farming. 4pp.
Schonbeck, Mark W., 1996. Mulching and Nutrient Balance. Virginia Biological Farmer, Spring 1996, p 10.
Schonbeck, Mark W., 1996. Hay or Plastic? Choosing a Mulch for Warm Season Vegetables. Virginia Biological Farmer, Spring 1996, p 6.
A one-hour presentation on mulching experiments at Dayspring Farm was offered as part of a field day on August 26,. 1995, jointly sponsored by Virginia Cooperative Extension Service and the Virginia Association for Biological Farming. About fifty people attended.
Presentations based on the grower survey and 1993-94 field trial results were given at four agricultural conferences in 1995: the Southern Sustainable Agriculture Working Group meeting in Gulf Shores, AL in January 13-15; the annual meeting of the Massachusetts Chapter of the Northeast Organic Farming Association in Framingham, MA on January 21; the annual Grower to Grower Conference of the New River Valley Chapter of VABF in Wytheville, VA on February 11 and the annual Virginia Sustainable Agriculture Conference at Camp Blue Ridge in central Virginia on March 17-19.
The two reports covering the grower survey and the results of 1993-94 field trials (listed under Publications) were provided free of charge to all 75 grower participants in the study (interviews and/or on-farm experiments), and are available at cost to others through VABF at conference booths and by mail. They are being advertised through the Virginia Biological Farmer, VABF quarterly journal.
VABF has published a series of four fact sheets based on the results of this study addressing optimizing mulching system for tomatoes; comparison of paper, plastic and organic mulches; effects of mulches on soil temperature and moisture; and effects of mulches on soil organic matter and soil health. These are available at cost to others through VABF at conferences and by mail. They are also being advertised through the Virginia Biological Farmer, VABF quarterly journal.
At least one presentation will be given at agricultural conferences in 1996 (VA Sustainable Agriculture Conference, Nov. '96), focusing on the conclusions of the study.
Perhaps the greatest impact of this study has been the establishment of a two-way exchange of information, ideas and innovation between growers and scientists on the project. With the grower survey in 1993-94, followed by 1995 field trials, we have begun to create an interactive network through which grower innovations can be identified, explored and refined. This has led to an understanding that selecting a mulching system is inherently a site-specific task, and the focus of the study has shifted from finding a replacement for plastic film mulch per se to developing a farm-specific process for identifying the best mulching system for the crops being grown, resources available, and prevailing soils, weather conditions and pest/beneficial biota. Our objective now is to provide growers with information on the properties, advantages and drawbacks of several mulching materials so they can make more informed choices.
The results clearly indicate certain possible advantages for plastic film mulch, most notably higher early-season yields of tomato, which may provide sufficient financial return for the additional labor of laying the film and removing it at the end of the season. However, potential drawbacks include interference with recharge of soil moisture, requiring drip irrigation for best results, reduced organic matter input with potential adverse impacts on earthworms and other beneficial soil life, high energy cost for manufacture, and disposal. Mulching alleys with hay or other organic materials could mitigate soil effects, but not other environmental costs.
Experiments described herein have not yet identified a paper film mulch that could fully replace plastic. Also, a mulching system utilizing oiled paper for early-season soil warming has not enhanced tomato yields over those obtained with organic mulches spread several weeks after planting. However, several growers who are motivated to replace plastic with a more ecologically sound alternative remain interested in exploring oiled paper further. Also, experiments in 1995 confirmed that two layers of newsprint laid beneath hay or straw can significantly enhance weed control, equivalent to an additional 4 or 5 tons per acre of hay. Given the cost of purchasing, hauling and spreading these materials, and the potential for soil nutrient imbalances with repeated heavy applications, the use of paper to enhance the weed control efficacy of lighter applications may offer several important benefits.
Where early yields are not critical (e.g. for processing tomatoes, or on a subscription farm serving a guaranteed market), there may to be little reason for smaller farms to switch from hay or straw to either oiled paper or plastic. Together with literature reports, this study suggests that plastic usually does not increase total yields over organic mulch applied several weeks after planting, when the soil has warmed up. Heat-loving crops like eggplant, cucumbers and melons may show a more dramatic positive response to soil-warming film mulches kept in place through the growing season. Another possible future direction for this study is to compare oiled paper, plastic and hay mulch on one or more of these crops.
If reliable and economically feasible paper mulches can be manufactured entirely from wastes that are otherwise difficult to recycle (low-grade paper waste, spent cooking oil), this technology could help growers maintain yields and reduce labor for weed control. It would also reduce the amount of agricultural plastic going into landfills, and divert waste paper and cooking oil out of the waste stream and into the soil organic matter cycle.
In situ cover crop mulches may be the most feasible alternative to plastic films for large-scale growers, yet results in the current study indicate that rye + vetch may not provide adequate soil coverage or weed suppression. Further refinement of this approach is needed, and has already been the subject of numerous other research efforts.
Soil scientist Greg Evanylo (Department of Crop, Soil and Environmental Sciences, Virginia Tech) has been a major participant throughout the first two years of this project. He has made a substantial in-kind funding match; conducted the bulk density, aggregate stability, infiltration, soil NO3-N and plant TKN measurements; assisted with data analysis; gave a presentation at the 1994 American Society of Agronomy meeting, and offered to write an Extension bulletin summarizing results.
Mary Ann Hansen and Diane Reaver of the Virginia Tech Plant Disease Clinic, and Extension Entomologist Eric Day have assisted with diagnosis and recommended treatment for plant disease and insect pest problems that have arisen in the course of this study.
Areas needing additional study
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- Update on SARE-funded mulch research project (Article/Newsletter/Blog)
- Mulching Practices and Innovations for Warm Season Vegetables in Virginia and Neighboring States: 1. an informal survey of growers (Book/Handbook)
- Mulching Practives and Innovations for Warm Season Vegetables in Virginia and Neighboring States: 2. results of on-farm trials in 1993-94 (Book/Handbook)
- Mulching and Slugs (Article/Newsletter/Blog)
- Mulching choices for warm season vegetables A summary of SARE funded mulching research (Article/Newsletter/Blog)