In six on-farm trials to evaluate three cover crops (oats, oats plus vetch, and phacelia plus vetch) for strip-till sweet corn production, the oat-vetch mixture increased average corn yields by an average of 11%, or one ton per acre compared to the fallow plots. After factoring costs of cover crop establishment, the oat-legume cover crop increase net profit by $50/acre compared to the fallow treatment. In a two-year experiment to evaluate nitrogen (N) contribution of cover crops in organic broccoli production, the phacelia-vetch mixture increased broccoli yield over the fallow treatment by 1.3 tons per acre, worth $2,370 per acre.
Objective 1. To enhance farmers’ ability to select and manage cover crops in conservation tillage vegetable crop production systems
Objective 2. To evaluate the nitrogen contribution of legume-based cover crops to organic vegetable production
Conserving and enhancing soil organic matter has been an historical cornerstone philosophy of the sustainable agriculture movement since the publications of J.I. Rodale, Lady Balfour, and Sir Albert Howard in the late 1940’s. In the intervening 60 years, an extensive, worldwide scientific research effort has provided clear evidence that soil organic matter directly influences soil structure, water holding capacity, plant nutrient availability and crop yield. A similarly extensive scientific documentation shows that conventional tillage operations dramatically degrade soil structure, reduce soil organic matter, and increase erosion. Conventional tillage practices have been shown to accelerate the oxidation of organic matter, destroy beneficial soil organisms, and contribute to long-term loss of soil fertility and productivity. Long-term research trials comparing conventional tillage with no-tillage have also shown the significant improvement of soil quality following no tillage. Organic farming, however, has been dependent on conventional tillage operations to incorporate cover crops and compost, as well as prepare suitable soil surface conditions for traditional mechanical weed control practices.
Nitrogen (N) is also a critical nutrient input to all agricultural production, with productive crop yields linked to availability of soil nitrogen. Much of the nitrogen fertilizer used in conventional agriculture is derived from the Haber process, an energy-intensive process involving conversion of natural gas to urea. In organic agriculture, nitrogen inputs typically come from animal manures and waste products from fish and other animal processing operations. Because animal manures also contain similar ratios of phosphorus to nitrogen, the application of manures at high rates to provide adequate nitrogen levels can create problems of “phosphorus loading” of soils. Manufactured sources of nitrogen fertilizer inputs for organic agriculture are also expensive, with prices typically ranging from two to four dollars per pound of N. In many areas, organic farms are increasingly competing for scarce sources of animal manures, and the availability of N is increasingly a concern for the expansion of organic farming to meet expanding consumer demand.
Legumes, plant species in the family Fabaceae, have evolved a synergistic relationship with Rhizobia, a naturally occurring soil bacteria. The bacteria stimulate the plant to produce nodules on the roots, where they reproduce and do the very difficult job of converting nitrogen molecules (N2) in the soil atmosphere (unusable to plants) to the plant available nitrate (NO3) form. Legume cover crops, such as vetch, clover and peas have long been using as rotational crops in agriculture to provide biologically fixed nitrogen for following cash crops (Kuo 1997; Sullivan et al. 1991). Although a rather extensive scientific literature documents the contributions of legumes to vegetable and silage crop production systems, there are relatively few growers in California and Oregon effectively using legumes in their nutrient management programs.
While moderate tillage may provide more favorable soil conditions for crop growth and development and weed control over the short-term (Carter, 1996), intensive tillage of agricultural soils has also historically led to substantial losses of soil carbon that range from 30 to 50 percent (Schlesinger 1985). Conventional tillage practices disrupt soil aggregates exposing more organic matter to microbial degradation and oxidation (Reicosky, 1997) and are one of the primary causes of tilth deterioration over the long-term (Karlen et al., 1990). Micro- and macro-channels within the soil, created by natural processes such as decaying roots and worms may also be destroyed by tillage (Carter, 1996). Deep tillage, as is customarily done as a routine “soil preparation operation,” is also costly and requires high energy and increased subsequent effort to prepare seed beds.
The term “conservation tillage” refers to a broad category of practices, including no-till, strip-till and ridge-till, which seek to conserve and improve soil quality through minimal soil disturbance. As opposed to conventional tillage, which buries and mixes crop residue into the soil to prepare a seedbed for crop planting, conservation tillage systems plant directly into crop residues (no-till, or direct seeding) or only till part of the soil area (strip-till). Benefits of conservation tillage include reduced equipment and labor costs, reduced soil erosion, improvements in soil quality, and in some situations, increased yields (Abdul-Baki and Teasdale, 1993; Blevins et al., 1983; Coolman and Hoyt, 1993; Johnson and Hoyt, 1999). Conservation tillage practices have been widely adopted for agronomic crop production, yet most vegetable growers continue to use intensive tillage for seedbed preparation (Hoyt et al., 1994). In California, conservation tillage is used on less than 2% of the annual cropland and there is an even lower percentage of organic cropland using this approach (Mitchell et al., 2007).
Strip-tillage (ST) is a form of conservation tillage that clears a narrow zones of soil, loosening subsoil layers and preparing a seedbed, while leave undisturbed areas between the tilled strips (Luna and Staben 2003; Mitchell et al. 2009; see Fig. 1E). Like the Toyota Prius, this tillage system is a hybrid, combining some of the best aspects of conventional tillage and no-till. Tillage activity is focused to a zone, which is typically 8 to 12 inches wide and 12 – 14 inches deep. In typical row crop systems with 30 to 36” spacing between rows, tillage occurs on only 35 to 40% of the soil surface. As a hybrid system, strip tillage buries crop residue in the tilled strip, accelerates a favorable release of nitrogen through microbial mineralization of organic matter. The area between the tilled strips (between row) is managed independently, depending on the nature of the crop or cover crop residue (see later discussion of beneficial role of cover crops in this system).
Strip tillage has the potential advantages of providing a suitable seedbed for vegetable crop establishment while leaving surface residues in the inter-row area to reduce soil erosion. Strip-tillage decreases both the area and volume of soil that is disturbed, reducing the amount of dust that is typically generated in intercrop tillage. Fuel, labor and equipment costs are also reduced when compared to conventional tillage. Weed emergence in the untilled area between the crop rows can be suppressed significantly, depending on weed species and soil type (Peachey, 2004)
Cover crops have long been used for the conservation and enhancement of soil structure, conservation or improvement of environmental quality, management of plant pathogens, weeds and insect pests, improvement of soil fertility. Using legume and non-legume cover crops, such as cereals, as mixtures, may offer advantages compared to growing the cover crop species as sole crops. Potential advantages include: increased biomass yields, reduced N leaching compared with legumes, and increased crop productivity compared with non-legumes (Sainju et al. 2005; Ranells and Wagger 1996). The use of cover crops with CT systems to improve production efficiencies, provide fertility, and manage pests, is not at all widely used in organic production systems in either OR or CA that this time. There is thus considerable as yet unexplored potential and benefit in coupling these technologies in organic systems.
In Oregon we have used a participatory, on-farm research model to actively involve farmers in the Willamette Valley since 1992. This work focused initially (1992-96) on cover crop evaluation for conventional tillage vegetable crop systems, however in 1997 the focus shifted to development of strip-tillage systems. In 1998 a group of ten Oregon farmers working in the project formed “The Willamette Farm Improvement Association” to pool financial resources and collaboratively build a new strip-tillage machine called the “Transtiller.” In a 4-year study, twenty on-farm trials were conducted comparing strip-tillage systems with conventional tillage systems for sweet corn production. Across an array of soils and crop residue situations, strip-tillage and conventional tillage systems produced equivalent sweet corn yields. However, machinery and labor costs were reduced by nearly 50% by strip-tillage (Luna and Staben 2002).
An experiment was conducted at Oregon State University in 1998-1999 to apply principles of strip tillage to an organic vegetable production system. A cover crop of oats and common vetch were used on both systems, and a variety of practices were used to suppress the cover crop in the strip-tilled treatments. Although the organic strip till system showed promise, the project was terminated because the difficulty of killing the oat-vetch cover crop and of mechanically cultivating and hoeing weeds in the narrow 6” wide strips. A new cover crop was needed that could easily be killed in the between-row areas.
A relatively new cover crop species in Oregon, phacelia (Phacelia tanacetafolia), may serve as a potential replacement of the cereal component (such as rye or oats) in the cereal-legume cover crop mixtures commonly grown. Although phacelia is a native of California, and is grown as a cover crop in Europe and New Zealand, few growers in California or Oregon use it as a cover crop. Cereal cover crops perform several functions quite well, including quick establishment and soil cover, scavenging of soil nitrate to prevent leaching, and in fixing atmospheric carbon dioxide into plant matter. Grasses, however, tend to produce large amounts of fibrous material in the spring, making soil incorporation difficult. Multiple passes of tillage equipment are frequently required to prepare a seedbed, and there is usually cover crop residue remaining to cause problems with seeding and mechanical cultivation. Phacelia, however, has a much more fragile stem, apparently lacking in longitudinal, cellulosic fibers that give cereal stems strength. This fragility causes the stem to shatter easily under tillage, and suitable seedbeds can be achieved with fewer tillage passes.
Abdul-Baki, A.A., and J.R. Teasdale. 1993. A no-tillage tomato production system using hairy vetch and subterranean clover mulches. HortScience 28:106-108.
Blevins, R.L., M.S. Smith, and W.W. Frye. 1983. Changes in soil properties after 10 years of no-tillage and conventional tilled corn. Soil & Tillage Res. 3:135-146.
Carter, L.M. 1996. Tillage. In Cotton Production Manual. University of California Division of Agriculture and Natural Resources Publication 3352. Pages. 175 – 186.
Coolman, R.M., and G.D. Hoyt. 1993. The effects of reduced tillage on the soil environment. HortTechnology. 3:143-145.
Garrett, A. 2009. Cover crop nitrogen contribution in organic broccoli production. M.S. Thesis, Oregon State University, Corvallis, OR.
Hoyt, G.D., D.W. Mongs, and T.J. Monaco. 1994. Conservation tillage for vegetable production. HortTechnology. 4:129-135.
Johnson, A.M., and G.D. Hoyt. 1999. Changes to the soil environment under conservation tillage. HortTechnology. 9:380-393.
Karlen, D.L., D.C., Erbach, T.C. Kaspar, T.S. Colvin, E.C. Berry and D.R.Timmons. 1990. Soil tilth: A review of past perceptions and future needs. Soil Sci. Soc. Am. J. 54:153-161.
Kuo, S. U. M. Sainjyu, and E. J. Jellum. Winter cover cropping influence on nitrogen in soil. 1997. Soil Sci.Soc. Am. J. 61:1392-1399.
Luna, J. M. and M. L. Staben. 2002. Strip tillage for sweet corn production: yield and economic return. HortScience. 37(7): 1040-1044.
Luna, J. M. and M. L. Staben. 2003. Strip tillage vegetable production systems for western Oregon. OSU Extension. Publ. EM 8824. 12 pp.
Mitchell, J. Miyao, G., and K. Klonksy 2009. Conservation tillage production tomato in the San Joaquin Valley. UC ANR publication #, available online at http://anrcatalog.ucdavis.edu/pdf/8330.pdf
Mitchell, J.P., K.Klonsky, A. Shrestha, R.Fry, A. DuSault, J. Beyer and R. Harben. 2007. Adoption of conservation tillage in California: Current Status and future perspectives. Australian Journal of Experimental Agriculture. 47(12):1382-1388.
Ranells, N.N. and M.G. Wagger. 1996. Nitrogen release from grass and legume cover crop monocultures and bicultures. Agron. J. 88:777-782.
Reicosky, D.C., W.A. Dugas and H.A. Torbert. 1997. Tillage-induced soil carbon dioxide loss from different cropping systems. Soil Tillage Res. 41:105-118.
Sainju, U.M., W.F. Whitehead, and B. P. Singh. 2005. Biculture legume-cereal cover crops for enhanced biomass yield and carbon and nitrogen.
Schlesinger, W.H. 1985. Changes in soil carbon storage and associated properties with disturbance and recovery. P. 194 – 220. In. J.R. Trabalha and D.E. Reichle (eds.). The changing carbon cycle: A global analysis. Springer-Verlag. New York.
Sullivan, P.G., D.J. Parrish, and J.M. Luna. 1991. Cover crop contributions to N supply and water conservation in corn production. Am. J. Altern. Agric. 6:106-113.
Objective 1. To enhance farmers’ ability to select and manage cover crops in conservation tillage vegetable crop production systems
On-Farm Trials, 2003 – 2005. Alternative cover crop species and mixtures were evaluated on several farms in the Willamette Valley. Cover crop treatments were identical on all farms and included (1) Oats (‘Monida’) (2) an oat-vetch mixture (oats plus common vetch (Vicia sativa), (3) a phacelia (Phacelia tanacetifolia)-vetch mixture, and (4) no cover crop (naturally occurring weeds) (Fig. 1). Cover crops were planted in six on-farm trials from early September through late October. Individual treatment blocks varied from 2-4 acres to permit the use of commercial harvesting equipment to obtain realistic assessment of crop yield. Each field represented a single replication in a randomized complete block experimental design.
Cover crop biomass was estimated (see below) in the spring within a week of the cooperating grower’s decision to kill the cover crop with Roundup (glyphosate). Cover crop sampling occurred from April 3 through April 27, depending on the grower and the year. In all fields a strip tillage system was used to prepare a seedbed. Sweet corn was planted in May and June each year using the growers’ planting equipment, and standard fertilizer and weed management practices were used to grow the crops.
Data Collection. Cover crop biomass was estimated by randomly selecting 4-6 locations within each cover crop block by tossing a 0.25m2 aluminum quadrat. The quadrat was worked through the cover crop foliage to the soil surface and the foliage clipped within the quadrat. Individual cover crop species within the mixtures were separated into paper bags and returned to laboratory for drying and weighing. Samples were taken to the OSU Central Analytical Laboratory for analysis of percent carbon and nitrogen.
Corn yield was determined using the participating growers’ commercial harvesting equipment (Fig. 2). Corn was hauled to the Norpac processing facility where harvest weights and quality grades were determined. Harvested plot areas in the field were measured to calculate crop yields. Cover crop seed costs were obtained from the seed suppliers for the economic analysis.
Objective 2. To evaluate the nitrogen contribution of legume-based cover crops to organic vegetable production
Materials and Methods
Year 1. An experiment was initiated in October 2006, at the OSU Horticulture Farm near Corvallis, OR. The soil is a Chehalis silt loam. The field was planted in blackberries from 2001 until 2003 and was fallowed for two years before the cover crop trials were planted during the first week of October 2006. Although the land had not been certified “organic” by a certification organization, based on the time from prior chemical applications, the land would have been eligible for certification.
In preparation for planting, the field was disked and rolled with a cultipacker. Experimental treatments consisted of five cover crop treatments and a no cover crop control (Table 4). A randomized complete block design with four replications was used. Cover crop treatment plots were 4.6 m x 36.6 m. To ensure accurate and uniform seeding rates, cover crop plots were subdivided into 4.6 m x 6.1 m subplots, and strings were stretched to define the subplot boundaries. Cover crop seed was weighed separately for each subplot and the seed distributed by hand. The vetch in all treatments was inoculated with Rhizobium leguminosarum at approximately 4g/kg of seed. A few drops of water were added to the seed before mixing in the Rhizobium. Because of the small size of the phacelia seed, a “seed shaker” was made using a 0.5 L mason fruit jar with 0.2 cm diameter holes drilled in the lid. After seeding, strings were removed and a tine-harrow drag was pulled longitudinally down the length of the plots to cover the seed. After planting, irrigation was applied with overhead sprinklers several times to assure germination and establishment during a very dry October.
Cover crop biomass, carbon and nitrogen content. Two biomass samples were taken on May 11, 2007 in each cover crop treatment by mowing across each replication using a 0.76 m-wide powered sickle bar mower (Fig. 6). Cover crops were raked within a 4.6 m long section and weighed using a hanging scale. A sub-sample was then pulled from the pile and the legumes, weeds and grasses were separated and placed in paper bags. The samples were oven dried at 65oC for 72 hours and then weighed to determine percent dry matter. Sub-samples were analyzed for percent C and N by the OSU Central Analytical Laboratory using a LECO CNS-2000®.
Effects of cover crops on N availability
and yield of broccoli
A Tortella power spader and Lely Roterra ® were used to incorporate the above-ground cover crop biomass and prepare a seed bed in mid-May (Fig. 7). A split-plot randomized block design was then established. Each cover crop plot was split into four sub-plots with different N fertilizer rates (0, 100, 200, and 300 kg N/ha), which was randomized within each cover crop treatment plot. Feather meal, an organically approved source of N, was selected for this experiment because it has an N-P-K analysis of 12-0-0. Each cover crop plot was 4.6 m x 36.6 m and the N fertilizer sub-plots were 4.6 m x 9.2 m. Feather meal was weighed and hand applied in an approximate 15 cm-wide band over the row and incorporated to a depth of 5 to 8 cm using a tine harrow.
Transplant production. ‘Arcadia’ broccoli seeds were planted in the greenhouse on in 200-cell trays using an organically approved potting mix. Seedlings were transplanted on May 30 through June 1, 2007 using a mechanical trans-planter and irrigated immediately afterward. Broccoli rows were on 90 cm centers, with plants spaced at 46 cm apart within the rows.
Pest management. Tractor-mounted sweep cultivators and hand hoeing were used for weed control. Insect population abundance was sampled weekly by visually examining broccoli leaves. Pyganic® was applied to control cabbage aphid, Safer’s BioNeem (0.09% azadirachtin) was applied to control flea beetles and Entrust® (80% spinosad) was applied to control control cabbage loopers and imported cabbage worms.
Broccoli Yield. Fifteen broccoli plants were harvested from each treatment. The broccoli plants selected for harvesting had plants on both sides and preference was given to the interior two rows of each treatment. Heads larger than 10 cm in diameter were harvested upon maturity. Broccoli was harvested on 3 to 6 day intervals, from August 14 through August 30, 2007, for a total of five cuttings. The number, weight and diameter of the broccoli heads was recorded.
Following each broccoli cutting for yield data, surrounding mature broccoli was harvested, and the crop boxed and sold through several local natural and organic food stores, including a regional organic foods wholesaler. The average price received for the crop through all markets was $2.20/kg ($1. 00/lb.)
Soil Nitrate Sampling. Five soil cores 6” deep were taken from the 0 N treatments in all cover crop treatments on 4 dates throughout the growing season. Soil cores were taken outside of the broccoli root zone, in the center of the rows and submitted to CAL for nitrate analysis.
Data Analysis. PROC MIXED (SAS 2007) was used to analyze the split-plot design, testing hypotheses that cover crops and N-rate affect broccoli yield, and examining the interaction between the two variables. A least squares mean test was used to calculate matrices of p-values comparing individual cover crop and N-rate treatments. P values are the SAS-calculated probability values use to examine the “statistical significance” of comparing treatment means. P-values range from 1.0 (no significance) to a very low number. The lower the number, the greater the probability that the treatment means are truly different.
Year 2. The experiment was relocated in October 2007 to an adjacent plot at the Horticulture Farm. The field had been planted in grasses from 2003 through 2004, assorted vegetables in 2005, summer buckwheat in 2006 and fallowed in 2007 before the cover crop trials were planted. In preparation for planting, the field was disked and rolled with a cultipacker, and prilled lime was applied on September 24, 2007 at a rate of 1 ton/acre and incorporated with a tine-harrow drag. Experimental treatments consisted of the same five cover crop treatments and a no cover crop control, and the same N rates.
The same experimental design as 2007 was again used, however the shape and size of the plots were reduced to make more efficient use of land and water resources. Cover crop treatment plots are 4.1 m x 24.4 m. To ensure accurate and uniform seeding rates, cover crop plots were subdivided into 4.1 m x 6.1 m subplots, and the cover crop seed weighed and hand sown. Inadvertently, the vetch treatments were not inoculated with Rhizobium leguminosarum at as they were last year. After seeding, a tine-harrow drag was pulled longitudinally down the length of the plots to cover the seed. After planting, irrigation was not needed due to plentiful rains in October. Cover crop biomass and N-content was sampled in early May, and analyzed as in the previous year. Cover crops were flail mowed and incorporated as in Year 1, however a modified rotary strip cultivator was modified to till 15 cm (6 in) wide bands in the plant row. Feather meal was distributed within the N rate plots by hand, and the strip cultivator was used to incorporate the feather meal and prepare a seedbed. Broccoli transplants were produced in the greenhouse, as in the previous year and hand transplanted to the field on July 10. Between row spacing was reduced to 76 cm (30 in) and in-row spacing was reduced to 35 cm (14 in). Soil nitrate sampling was conducted as in Year 1. Weed and insect control procedures were also similar to Year 1. Broccoli was harvested over five dates in late August to early September to estimate yield.
Objective 1. To enhance farmers’ ability to select and manage cover crops in conservation tillage vegetable crop production systems
Corn yield response to cover crop treatments varied across the farms, however when averaged across the six fields, sweet corn yields were 1.0 tons higher in the oat-legume cover crop plots compared to fallow treatment, an 11% increase (p = .12). There were no yield differences among the cover crop treatments.
Averaged across the six fields, all cover crops produced similar biomass, approximately 5,000 lbs per acre. Total N content was highest in the phacelia-vetch (185 lbs N/acre), with N content in the oat-vetch mixture of 153 lbs N/acre double of the N content of the oats (75 lbs N/acre). Cover crop performance and N content varied considerably among the farms and years.
Carbon-to-nitrogen ratios have been used to indicate the relative percent nitrogen in cover crops, since the percent carbon of most plant tissues remains relatively constant. Percent nitrogen content of plant tissue varies considerably, not only between species, but also within various growth stages of a given species. For example, an immature cereal crop has a higher nitrogen content (and hence lower C: N ratio) than a mature cereal crop. The C: N ratio is commonly used as a relative indicator to predict how rapidly the plant tissue will degrade after it is killed, either in the soil or on the soil surface as a mulch. Plant tissue with high C: N ratios (>30) will be degraded more slowly than tissue with low C: N ratios such as <15. In these trials, the C: N ratios (and percent nitrogen) of the cover crops, as well as the increased quantity of cover crop nitrogen, may be a key factor in the increase of corn yields in the oat-vetch cover crop mixture. The oats grown in the mixture with vetch had a higher nitrogen content (1.9%) and lower C:N ratios (25) compared to the fallow plots with 1.5% nitrogen content and a C:N ratio of 36. Apparently, the legumes in the mixtures are releasing nitrogen in the soil, which is taken up by the oats, reducing the C: N ratio of the oat tissue. A combination of increased N fixation associated with the legumes, as well as more rapid mineralization of the higher N containing oats in the oat-vetch mixtures likely contributed to the increased plant available nitrogen during the growing season and the increased sweet corn yields. However, legumes are also known to improve a variety of soil quality parameters, including enzyme activity. The lack of increase in corn yields in the strip-till fields by oat cover crops alone compared to the no-cover crop plots, may be due to several factors, including immobilization of soil nitrogen by soil bacteria engaged in degrading high C: N ratio plant material. Oats may contain allelopathic compounds that could retard corn growth as well. Objective 2. To evaluate the nitrogen contribution of legume-based cover crops to organic vegetable production Cover Crop Biomass and Nitrogen Contribution. Year 1. The phacelia-vetch mixture produced the highest above ground, dry matter biomass (9,380 lbs/acre) and the highest amount of nitrogen (185 lbs/acre), followed by the oat-vetch mixture at 8,210 lbs of biomass per acre and 165 lbs N. In both of these mixtures, the vetch component produced higher biomass than the non-legume component, and with the higher percent N of the vetch (2.5% compared to 1% for oats and 0.8% for oats, the vetch contributed the greatest proportion of nitrogen to the mixture. In both of the mixtures, the non-legume component contained a higher percent N than the same crop grown as a sole crop, showing the increased uptake of N from the associated legume. The high C: N ratio of the oat (68:1) and the phacelia (66:1) were likely responsible for nitrogen immobilization and subsequent reduction of broccoli yield described above. Year 2. Cover crops established very poorly in the fall of 2007 as result of unusually onset of cold, rainy weather. Cold wet weather throughout the fall and winter produced rather weak cover crop stands in the spring of 2008. Legume nitrogen in the vetch and vetch mixtures was about a third of that produced in 2007. Broccoli Yield. Year 1. SAS analysis revealed a highly significant interaction between cover crops and N-rates (p = .02), therefore cover crop treatment effects were examined for each N-rate separately. Cover crops affected broccoli yields for both the zero and 100 kg N/ha rates (p = .01, and .02 respectively), whereas the cover crop effects were lost at the higher N rates of 200 and 300 kg N/ha (p = .32 and .75 respectively). Therefore, most of the following discussion of cover crop effects will focus on the zero and 100 kg/ha N rates, with p-values comparing individual treatment means are shown in the p-value table. At the zero N-rate, oat sole crops reduced broccoli yield by 67% compared to the no-cover crop fallow (p = .01), and the phacelia sole crop reduced broccoli yield by 33% (p = .12). At the 100 kg N rate, oats continued to reduce yield compared to the fallow treatment (p = .10), however there was no yield loss in the phacelia blocks (p = .77) These yield reductions were likely caused by nitrogen immobilization by soil microorganisms. Many other cover crop studies have shown crop yield loss following relatively high C: N ratios of the cover crop residue at time of incorporation. The addition of legumes in the oat-vetch (OV) and phacelia-vetch (PV) mixtures compensated for the yield-suppressive effects of the oats and phacelia, with broccoli yields exceeding the fallow treatment in the PV blocks at zero nitrogen (p= .06), and OV mixtures increasing yields in the 100 kg N rate treatment (p = .03). The OV mixtures produced nearly three times greater broccoli yields than the oats alone at both zero and 100 kg N/ha (p = .02, .01). The PV mixture increased broccoli yields over phacelia alone at the zero N rate (p=. 01), however increases were not significant at the 100 kg N rate (p = .18). Vetch as a sole crop failed to increase yields over the fallow in the 100 kg N rate (p = .73), and although average broccoli yields were increased over the fallow by one ton per acre in the zero N rate treatment, the effect was not likely different (p = .20). Although there as no overall cover crop treatment effect in the 200 kg N rate treatment, the PV treatment increased yields over the fallow, oat, and OV treatments (p = .08, .05, and .06). Year 2. SAS analysis revealed no interaction between cover crops and N rate, with all treatment yields increasing linearly with increasing fertilizer rate. Both the oat and phacelia cover crops (without legumes) reduced broccoli yields across all N rates. Although the total N accumulation in the oat-legume or phacelia-legume mixtures was considerably less than in Year 1, this was enough to overcome the apparent N immobilization occurring with the oats and phacelia. Soil Nitrate. Soil nitrate values through the season for both years followed the pattern of broccoli yield, with the highest soil nitrate found in the vetch and vetch mixtures and the lowest soil nitrate found in the oat and phacelia sole crops. Soil nitrate in the fallow plots was intermediate between the vetch mixtures and the sole crops. These data clearly reveal the nitrogen immobilization effects of high C:N ratio cover crops, and how the addition of legumes to the mixture can alleviate this effect.
See sections on Economics and Farmer Adoption.
Educational & Outreach Activities
Garrett, A. 2009. Improving nutrient management with cover crops in organic broccoli production. M.S. Thesis, Oregon State University. 87 pp.
Luna, J. M. 2007. Reduced tillage for vegetables in the Pacific Northwest. Proc. Empire State Fruit & Vegetable Expo. Syracuse, NY. 3pp.
Luna, J. M. and A. Garrett. 2008. Improving yield with cover crops in organic broccoli production. In Good Tilth. Vol. 19(4): 19.
Luna, J. M. 2009. Managing cover crop and conservation tillage systems to enhance vegetable crop yields, economic returns and environmental quality. Final Report to the USDA-SARE Program. Available on-line at: http://hort.oregonstate.edu./faculty-staff/luna
Outreach and Extended Education
Luna, J. M. 2009. Cover crops and nitrogen contribution in organic broccoli production systems. North Willamette Valley Horticultural Society. Canby OR. (75 attendees)
_____ 2008. Nitrogen contribution of cover crops. Cover Crop Field Day, St. Paul, OR. (34 attendees).
_____2008. Improving soil quality using cover crops. Oregon Association of Nurseries. Mt. Angel. (45 attendees).
_____2008. Cover cropping systems for nursery production. Bailey Nursery In-Service Training. Yamhill, OR. (8 attendees).
_____2008. Cover crop bicultures in organic broccoli production. Field day for Small Planet Foods field representatives. Corvallis, OR. (7 attendees)
______2007. Reduced tillage for vegetables in the Pacific Northwest. Proc. Empire State Fruit & Vegetable Expo. Syracuse, NY. (46 attendees).
_____ 2007. Developments and opportunities in sustainable agriculture. Growing Green Enterprise Workshop, Monmouth, OR. (26 attendees)
_____ 2006. Soil quality and pest management for organic gardening. Insights into Gardening Workshop, Corvallis, OR. (145 attendees).
_____ 2004. Strip tillage vegetable production : experience from conventional and organic systems research. OSU Soil Biology Workshop, Aurora, OR. (47 attendees).
_____2004. Conservation tillage in Oregon. Western Region Conservation Tillage Conference, Five Points, CA. (87 attendees)
____2004. Conservation tillage systems. OSU Soil Management in Field Nurseries Workshop. Aurora, OR. (60 attendees).
_____2004. Conservation tillage vegetable production. Far West Agribusiness Assn. Salem, OR. (40 attendees).
Objective 1. Cover crop establishment costs depend on seed costs, seeding rates, planting equipment, and labor costs. Seed costs per acre for the cover crop treatments were oats, $9.00; oat-legume, $17.40; and phacelia-legume, $23.40 (Table 3). Planting equipment and labor costs are based OSU Extension cost estimates for two disk passes to prepare a seedbed, followed seeding with a grain drill ($33.00/a). Estimated benefits from the cover crop treatments can be calculated by subtracting cover crop costs from the net return (graded yield x corn price).
The oat-legume treatment produced a net increase of about $50/acre in net profit compared to the fallow. Obviously, the high corn yield from this treatment produced the high economic returns.
Objective 2. The advantages of legume-based cover crop mixtures over sole crops are apparent in this study, both for increasing broccoli yield, but also for increasing soil organic matter. In year 1, broccoli yield was severely reduced by oat cover crops at the zero and 100 kg N rate, requiring up to 200 kg N/ha of supplemental fertilizer to overcome the yield loss. The phacelia-vetch mixture, however, increased broccoli yield compared to the fallow treatment by an average of 1.3 tons per acre (1.5, 0.8, and 1.5 tons broccoli/acre at zero, 100 and 200 kg N). Based on our actual market price paid for organic broccoli of $0.91/ lb ($1,820 /ton), the economic value of the crop yield increase for phacelia-vetch cover crops was $2,370 per acre.
The oat-vetch cover crop mixture also increased broccoli yield, but only at the 100 kg N rate. Broccoli yields were virtually identical at the other three N rates (see Fig. 9). At the 100 kg N rate, however, the oat-vetch treatment produced the highest broccoli yield, 5.8 tons/acre, which was 1.9 tons higher than the no cover crop fallow plots. Economically, this increase was worth $3,460/acre. Averaged across the three N rates, however, the average yield increase was only 0.6 tons, worth $1,090 per acre.
Both the oat-vetch and the phacelia-vetch mixtures at 100 lbs fertilizer N per acre produced similar yields to the fallow treatment at 200 kg N/ha, suggesting a 100 kg of “N fertilizer equivalency” or fertilizer replacement value. The cost of the organic fertilizer used in this experiment, feather meal, was $2.33 per pound of N ($.28 lb meal with a 12% N content). Consequently, at 100 lbs N contribution per acre, the fertilizer replacement value of the cover crop mixtures was estimated at $230 /acre.
Cover crop seeding costs were higher in the phacelia-vetch mixture ($42/acre) compared to $21/acre for the oat-vetch mixture, due to the higher cost of the phacelia seed ($7/lb). However, because the cover crop was broadcast seeded in this experiment, a higher seeding rate of phacelia (3.5 lb/acre) was used than rates (2 lbs/acre) used previously in other trials where the cover crop was planted with a grain drill.
No estimates were made of the economic contribution of the cover crops to short and long-term soil quality. However the cover crop mixtures contributed 4 to 5 tons of dry matter biomass per acre to the soil. Averaging 41% percent carbon, this represents a carbon contribution of 1.6 to 2 tons C per acre. Increasing soil organic matter is critically linked to short and long term soil productivity and sustainability.
The yield impacts reported in this study are relevant to crops typically planted in mid to late May or early June, across the maritime Pacific Northwest. Because yield impacts are likely related to total mineralizable N accumulated by the cover crops, killing the cover crops and planting vegetable crops earlier in the spring will not likely produce the same level of response. Cover crops killed earlier will likely have less legume nitrogen, since vetch is growing rapidly in the spring. At the same time, sole crops of cereals, such as oats in this study, will also not be as mature and will have a higher percent N in the tissue. This would likely produce less N immobilization and yield loss as was seen in this experiment.
Two farms involved in this on-farm research project have adopted integrated cover crop and strip-till systems extensively, with Hendricks Farms farming more than 400 acres of strip-till sweet corn, and Stahlbush Island Farm using cover crops and strip tillage on more than 2,500 acres of vegetable crop land. Three Mile Canyon Farm, near Boardman, OR, has purchased two strip-till machines and used them to strip till more than 4,000 acres of corn land. The farm manager reported that this system saved large quantities of fuel and helped reduce wind erosion.
Most recently, one Willamette Valley processed-vegetable grower has purchased a strip-till machine and built a new “second pass” strip till machine. He is using the system on his 500 acre farming operation, and in addition is doing custom strip-till work for four other farmers in his area.
This project clearly demonstrates the potential “win-win-win” possibilities for integrated production systems using legume-based cover crops and strip-tillage. Not only can this production system reduce soil erosion and improve soil quality, but also it can significantly increase net income by increasing crop yields and simultaneously reducing the need for nitrogen fertilizer. In addition, both strip tillage and legume cover crops dramatically reduce the need for nonrenewable energy inputs (diesel fuel and chemical fertilizer), thereby contributing to current national goals of reducing energy consumption.
Areas needing additional study
Additional research is needed on cover crop selection and mechanical kill practices for strip-till, organic vegetable production. In our work (not reported here) we experimented with mechanical methods of killing phacelia-vetch cover crops. Using a sprocket roller (cultipacker), we demonstrated good kill of this cover crop mixture, but unfortunately we experienced stand establishment failure attempting to grow organic sweet corn. Because of the fragile structure of the stem and of the roots of phacelia, this cover crop offers excellent promise as a substitute for cereals as cover crops. Cereals must be grown to near maturity for roll down mechanical methods of killing to work, and in the Pacific Northwest, this occurs too late in the season and the large quantity of biomass in the cereals is very difficult to incorporate. Even in minimum tillage situations, the residue remaining from cereal cover crops interferes with mechanical cultivation equipment. Phacelia residues shatter easily and make a much better seedbed than cereal cover crops. One potential drawback of phacelia is that it is not cold tolerant and is killed at temperatures below 17 deg. F. For the Pacific Northwest and California, however, with relatively mild winters, this cover crop offers excellent potential. Additional research is also needed in precision, in-row weed control technologies to facilitate strip till systems for organic production.