A comparison of the agronomic, economic and environmental effects of compost, poultry litter, and inorganic fertilizers on three organic farms demonstrated that soil chemical and physical properties were improved with agronomic rates of compost compared with low rates of compost, poultry litter or fertilizer. Accurately estimating nitrogen availability, not the source of nitrogen, was most critical to preventing groundwater contamination by nitrate-N and increasing yields.
The goals described above will be achieved by assessing the effects of compost, manure, and fertilizer on:
1. soil biological, chemical and physical properties indicative of soil quality;
2. nutrient leaching and runoff;
3. Average yield levels, net economic returns, and the variability of both yield and net economic returns over multiple production periods.
Use of compost rather than chemical fertilizers may be a more sustainable practice in vegetable production. Although the environmental benefits of compost use have been inferred from a limited number of short-term studies, long term research designed to determine whether the use of compost is more environmentally sound than commercial fertilizer or manure has not been conducted. Furthermore, many vegetable farmers are hesitant to substitute compost for fertilizers or manures because the relative long term economic returns of compost use have not been quantified.
Organic and biological farmers have long used animal manures and composts to enhance soil biological activity and to improve soil physical and chemical properties. Optimum rates of these amendments have not been determined because combined agronomic, economic, and environmental assessments have been lacking. Conventional farmers rarely use compost and apply manure largely for its nitrogen (N) value. The use of commercial fertilizers and excessive (when based on crop N needs) manure applications can impair soil and water quality. Farmers and the environment would benefit from research-based compost and manure application comparisons. Conventional farmers who understand the agronomic and environmental benefits of using compost as a soil amendment are likely to adopt organic/biological farming practices only if appropriate application recommendations can be demonstrated.
Agricultural Practices and Soil Quality
Soils are living systems that are vital for producing food and fiber, and for maintaining the health of ecosystems. Soil quality, or “the capability of a soil to produce safe and nutritious crops in a sustained manner over the long term, and to enhance human health, without impairing the natural resource base or harming the environment” (Parr et al., 1992), is integral to a sustainable agriculture. Soil quality can be affected positively or negatively by management. Erosion, compaction, acidification, loss of organic matter, and toxic chemicals may degrade soil quality. Adding organic matter and carefully managing tillage and other cultural activities can improve soil quality.
The quality of a soil is determined by a combination of chemical, physical, and biological attributes (Arshad and Coen, 1992; Parr et al., 1992; Stork and Eggleton, 1992; Visser and Parkinson, 1992). The most common soil quality attributes are indicators of soil tilth (the combination of properties that include aggregation and porosity, which increase aeration and plant available water-holding capacity and reduce erosion and runoff), nutrient supplying capacity (e.g., cation exchange capacity, available nutrients, organic matter, pH), and biological activity (microbial biomass and respiration, and the number and community structure of other soil organisms). Soils vary in quality because these attributes differ among soils.
Conventional agriculture has been designed to optimize soil fertility by adding inorganic fertilizer on the basis of soil testing to raise the concentration of readily available nutrients. Increasing soil nutrient-supplying capacity may better be accomplished by improving the soil’s biological activity. The availability, not the total amount, of essential nutrients in the soil limits crop production (King, 1990). In the long term, the use of inorganic fertilizer may reduce soil organic matter and biological activity (Fauci and Dick, 1994). Farmer use of inexpensive synthetic fertilizers permits short-term productivity improvement in nutrient-deficient soils but can impair soil and water quality (National Research Council, 1993) and neglects practices, such as crop rotation and recycling crop residues and manure, which enhance long-term soil quality.
Much of the recent scientific interest in organic agriculture has emerged from studies that have demonstrated benefits to soil physical, chemical, and biological properties and crop yields with increased levels of organic matter (Cole et al., 1987). Organic matter improves soil tilth, increases the soil’s abilities to hold and make nutrients available to plants through mineralization, cation exchange, and chelation, and permits resistance to the natural tendency of soil to become acid. Build-up of organic matter through additions of crop and animal residues has been shown to increase the population and species diversity of microorganisms, their associated enzymatic activity and respiration rates, and populations of invertebrates such as earthworms (Berry and Karlen, 1993; Kirchner et al., 1993; Weil et al., 1993).
Fertilization with manures has proven beneficial for crop yields because animal wastes contain relatively high concentrations of plant-available N and P. In addition, manure contains large amounts of organic matter and other essential plant nutrients that can be recycled by crops. Indiscriminate use of manure, like commercial fertilizers, has resulted in water quality degradation (National Research Council, 1993).
Agronomic, Environmental and Economic Benefits of Compost
Composting is an attractive practice for manure management because compost is more convenient to handle and store than uncomposted manure (Glenn, 1992); thus, composting reduces the potential for water quality impairment by preventing manure from being spread onto frozen or flooded ground or where soil contains high P concentrations. Composting manure and other agricultural wastes for use as a soil amendment is becoming more accepted among conventional farmers (Christian, et al., 1998), and compost use could be increased if farmers and educators understood optimum economic, agronomic and environmental application rates.
Compost application improves the chemical, physical and biological properties of the soil, which may increase crop yields or decrease yield variability (DeLuca and DeLuca, 1997; Evanylo, et al., 1998; Maynard, 1993b; Shiralipour, et al., 1992). Maynard (1993b) used compost to increase the yield of tomatoes by increasing nutrient availability to the plant. Virginia and Maryland farmers working with Evanylo, et al. (1998) were able to produce comparable melon and tomato yields by substituting yard waste and cotton trash composts for inorganic fertilizer. The composts increased soil organic matter, CEC, pH, and water-holding capacity and reduced bulk density during the two-year field study.
Biological properties of soil are also enhanced by the addition of compost. The increased microbial population of compost-amended soil has been shown to suppress certain soil-borne fungal pathogens, thus reducing the amount of chemical fungicides that must be used by farmers (Hoitink and Grebus, 1994).
Concerns about declining water quality from nitrate leaching into groundwater (Daliparthy et al., 1995; Goodrich et al., 1991; Maynard, 1989) and phosphorus transport into ground and surface water (Eghball et al., 1996; Mozaffari and Sims, 1996; Sharpley et al., 1994) due to nutrient overloading from land application of uncomposted manure and inorganic fertilizer require that these practices be reassessed. Composting converts the N in manure into more stable forms, which reduces the potential for nitrate leaching. Maynard (1989, 1993a) determined that nitrate leaching could be reduced by substituting compost for commercial inorganic fertilizer in vegetable studies ranging from 1-3 years. Brinton (1985) reported lower potential nitrate leaching from soils amended with composted than uncomposted manure for maize production.
Few field data have been reported for the impacts of compost application on P transport. Li et al. (1997) demonstrated in a column leaching study that various composts applied at rates to provide crop N needs increased the concentrations of soil leachate as high as 246 ppm NO3-N and 7 ppm PO4-P. These amounts constituted 3.3-15.8% of the total N and 0.2-2.8% of the total P in the composts studied. Thus, more research is required to determine whether compost applied at beneficial rates for crop production is environmentally safer than manure or inorganic fertilizer.
The economics of manure applications for crop fertility have been evaluated in such research projects as documented by Berends (1993), Govindasamy and Cochran (1995), Govindasamy et al. (1994), Herawati (1994), Jokela (1992), Pierce (1989), and Pierce et al. (1992). Most studies have involved transport and application of poultry litter to cropland at some distance from poultry production as a method to achieve fertility enhancement and water quality improvement in the poultry region.
There have been scattered reports such as those claiming reduced costs from compost-induced uniform maturity in cotton (Goldstein, 1994) and from decreased pesticide expenditures for vegetables (ibid.). Yields produced in the study conducted by Evanylo, et al. (1998) were not increased at all sites with compost despite the improvement in soil properties, but long term effects on crop yields were not determined. Compost rates were applied to supply crop N requirements at some locations and as a biological soil stimulant at other locations, which may have short-changed nutrient needs for optimum yields.
Economic benefits of compost applications for crop fertility have been reported in a few other publications (Ashley, 1993; Buchanan and Gliessman, 1991; Danforth et al., 1993; Farrell, 1996; Francesco and Lionello, 1992; Logsdon, 1995). Scientifically-based evidence regarding the positive residual and intangible (non-NPK) effects is lacking. Such documentation may stimulate greater use of compost by increasing producer confidence in compost benefits.
The value of compost in improving soil properties and supplying plant nutrients has been documented, but transitional and conventional farmers are hesitant to substitute compost for inorganic fertilizers or manures because the agronomic and economic benefits have not been well quantified. The value of using compost may be further enhanced if environmental advantages over commercial fertilizer and manure are demonstrated. The goal of this project is to compare the effects of compost, manure, and inorganic fertilizer on agronomic, environmental, and economic variables for sustainable vegetable production.
Two commercial farms and one non-governmental organization farm in the mid-Atlantic region of the United States have expressed the desire for research that compares the relative combined economic and environmental (agronomic and soil and water quality) impacts of manure, compost, and commercial fertilizer. Producers and researchers will learn how to credit compost for their specific systems, and producers, researchers, and extension personnel will explain how to determine optimum compost application rates for unique vegetable farming systems. The results of this program will demonstrate the economic and environmental value of compost for sustainable soil management. In the long term, enhanced soil quality will improve the economic viability of farms and minimize environmental degradation.
Treatment Design and Site Description
Field studies were conducted at three locations in Virginia: at the Virginia Tech Northern Piedmont Agricultural Research and Education Center (NPAREC, Orange) on a Fauquier silty clay loam (Fine, mixed, mesic, Ultic Hapludalfs), at the Bracketts Farm in Louisa County on a Jackland loam (Fine, smectitic, mesic, Aquic Hapludalfs), and at the Cascades Farm in Rockbridge County on a Weaver silt loam (Fine-loamy, mixed, mesic Fluvaquentic Eutrochrepts).
The productivity classes of the three soils were variable (Simpson et al., 1993), ranging from the high-yielding Weaver (mean corn grain yield = 10.0 Mg/ha or 160 bu/acre), to the moderately high-yielding Fauquier (mean corn grain yield = 8.55 Mg/ha or 130 bu/acre), to the low-yielding Jackland (mean corn grain yield = 4.28 Mg/ha or 65 bu/acre). The Weaver soil was formed in alluvial parent materials and is found on a gently sloping landscape of a stream terrace west of the Blue Ridge. The Weaver is a deep, medium-textured, well-drained soil with high water supplying capacity. The Fauquier soil is located on dissected uplands in the Piedmont region that was formed from residuum ranging from weathered mafic rocks to Triassic sediments. The Fauquier is a deep, well-drained soil with medium-textured surfaces and reddish-brown clayey subsurfaces, which is a moderate water supplier. The Jackland soil is a Piedmont soil that has formed from a variety of residual materials, including Triassic sediments, residuum from basic rocks, and other clayey sediments. The Jackland is a moderately deep soil with clayey-textured subsurface horizons and large components of shrink-swell clays. The soil is a moderate water supplier, but is somewhat poorly drained.
Eight treatments were established as follows: 1) CTL – Control (no amendments used), 2) LC – low compost rate (20% of the agronomic N need met with compost), 3) LCF – low compost rate with supplemental fertilizer (20% of the agronomic N need met with compost and supplemental fertilizer used to meet the needs of the balance of the N,P, and K needs), 4) AC – agronomic compost rate applied annually (N needs are met fully with compost), 5) BC – agronomic compost rate applied biennially (agronomic N rate of compost was applied in the first and third year of the study), 6) BCF – agronomic compost N rate applied biennially and supplemental fertilizer used to meet the balance of the N, P, and K needs, 7) PL – poultry litter (agronomic N needs were met with PL), 8) F – fertilizer (agronomic needs met with fertilizer only). Each treatment was replicated 4x in a randomized complete block design with individual plots measuring 3.6 m wide by 7.5 m long at NPAREC. At Bracketts and Cascades Farm, treatments were replicated 3x in a randomized complete block design and individual plots measured 3.05 m wide by 6.10 m long.
The compost (“Panorama Paydirt”) used at NPAREC and Bracketts farm was produced from a mixture of yard waste and poultry litter (Panorama Farms, Earlysville, Virginia), and the poultry litter was a granulated commercial product (Glen Hill Farm, Harrisonburg, Virginia). The compost used at Cascades farm was made on-farm from vegetable culls and other farm-generated vegetation. The sources of the fertilizer nutrients were ammonium nitrate (N), triple superphosphate (P), and muriate of potash (K).
Composition of the poultry litter and compost were employed to calculate agronomic N loading rates. Plant available N in compost and poultry litter were estimated by adding 100% of the NO3-N + NH4-N (incorporated immediately) and the fraction of organic-N estimated to be mineralizable during the first season. Mineralization coefficients used were 0.10 for compost and 0.6 for poultry litter. Fertilizer N, P, and K were applied at rates recommended by Virginia Cooperative Extension (2001) for results of the soil tests collected and analyzed each fall preceding the fertilization application.
Winter rye (Secale cereale L. Var. Wheeler) was planted as a cover crop each fall and incorporated into the soil each spring by disking. Soil amendments were hand-applied and incorporated within 24 hours in conjunction with the seedbed preparation by roto-tilling. Pumpkin (Cucurbita pepo Var. Magic Lantern), sweet corn (Zea mays L. Var. Silver Queen at NPAREC and Cascades and Zea mays L. Var. Silver Choice at Bracketts), and bell pepper (Capsicum annuum Var. Aristotle) were grown in 2000, 2001 and 2002, respectively. Target N rates were 84 kg/ha for pumpkin, 168 kg/ha for sweet corn, and 139 kg/ha for bell pepper.
Weed control was achieved by cultivation using a tractor-mounted rototiller and a barley straw mulch applied to a depth of 10 cm. We spot applied Glyphosate at Cascades and Gramoxone and Glyphosate at NPAREC in 2001 and Glyphosate at NPAREC and Cascades in 2002 to control emergency outbreaks of barley seedlings that unexpectedly emerged from the mulch.
We applied Armicarb at all sites throughout the 2000 growing season to suppress fungal diseases in pumpkins. To decrease pests in corn we applied mineral oil to corn tips and released beneficial parasitic wasps (T. ostriniae) in 2001. Supplemental water was provided by trickle irrigation at NPAREC and Bracketts farm.
Soil, Plant, and Water Analytical Procedures
Soil samples (10 cores/plot) were collected, air-dried, and sieved in spring 2000 and each fall following harvest for extraction and analysis of soil test (ST) phosphorus (weak Bray P), ammonium acetate extractable potassium (K), calcium (Ca), and magnesium (Mg), pH (1:1 soil:water, glass electrode), and cation exchange capacity (CEC, by sum of exchangeable cations) (Page, et al., 1982). Organic carbon (C) and N were analyzed using a VarioMax CNS macro elemental analyzer. For the Cascades site only, organic C was calculated from total C values as outlined in the Methods of Soil Analysis (Loeppert and Suarez, 1996). Soil was analyzed each fall for infiltration, bulk density, porosity, and soil moisture content as indicators of soil quality (Sarrantonio, 1996). The first and second year of the study CO2 respiration was measured with Draegar tubes (Sarrantonio, 1996). Detailed microbial analysis for active bacterial biomass, total bacterial biomass, active fungal biomass, total fungal biomass, protozoa (including flagellates, amoebae, and ciliates), and total nematodes was conducted after the second cropping season (Soilfoodweb, Inc., Corvallis, OR).
Soil cores (10/plot) were collected from the corn plots to a depth of 30 cm when the corn plants were 25 to 30 cm tall for analysis of nitrate N as an indicator of available soil inorganic N (Evanylo and Alley, 1997). Total nitrogen of corn earleaf samples collected from the plants at the early silking stage was determined by combusting the samples on a VarioMax CNS macro elemental analyzer.
At harvest, marketable vegetables were picked, counted, and weighed in the field for fresh weight yields. Pumpkins were harvested in one day, corn was harvested in two days, and peppers were harvested once a week for five weeks. Rye was sampled within 1×0.5 m quadrats from each plot at NPAREC in 2001, dried at 21ºC and weighed for biomass production, ground in a Wiley mill to pass a 2 mm sieve, and analyzed for total N and C on the VarioMax CNS macro elemental analyzer.
Zero-tension lysimeters, fabricated from plastic drainage pipe measuring 25 cm wide and 75 cm long, were installed into the soil to a depth of 90 cm in half the treatments (treatments: CTL, AC, PL, and F) following removal of soil with a tractor-driven auger. Each lysimeter, which was capped and sealed at the bottom, contained 5 cm of acid-washed sand and the carefully replaced soil augered from each hole. Tygon tubing ran from the sand reservoir to the soil surface, which was 15 cm above the upper rim of the drainage pipe. The lysimeters were installed in each replication of four treatments: CTL, AC, PL, and F. Water was withdrawn monthly with an electric pump. The soil solution was analyzed for electrical conductivity, pH, and NO3-N. Nitrate-N was analyzed using flow injection analysis and read on a Lachet instruments colorimetry detector at 550 nm and 660 nm wavelengths, respectfully.
Following the final pepper harvest, we conducted rainfall simulation on the four treatments which had lysimeters installed. A rainfall simulator was constructed using the plans recommended by the National Research Project for Simulated Rainfall (Miller, 1987). The simulator was run on each plot, collecting 2 water samples for each plot at 15 and 30 minutes after saturation. Water samples were filtered in the field and both samples, filtered and unfiltered were analyzed for ortho-phosphorus (PO4), nitrate (NO3-), ammonia (NH3), total P (TP), and total Kjeldahl N (TKN). The unfiltered water samples were analyzed for TSS (total suspended solids) and TOP (total organic phosphorus.) Soil samples were collected before and after simulation at 2” and 6” depths and analyzed for phosphorus using Mehlich 1, Melich 3, Bray and Kurtz P-1, and water soluble (using ICP-AES and Murphy-Riley) (Pierzynski, 2000).
All statistical analyses were conducted with the statistical package SAS (1996) using general linear models at 95% confidence. Additional least significant difference (LSD) separations and contrast tests were conducted on some data to obtain more detailed differences between treatments.
The compost used at NPAREC and Bracketts Farm contained 1.46 to 2.45% total N, of which less than 0.7% was in immediately plant available (inorganic) forms and 99% was in organic forms. The compost used at Cascades Farm contained 1.53 to 1.87% total N, with 5.8 to 8.4% of the N in plant available forms. The poultry litter contained more total N (4.6-4.8%) and available N (17-27% of the total N) than either of the composts. The organic C concentrations of the Cascades compost were 22.4% and the Panorama compost was 34.5%. The poultry litter contained 48% organic C. The C:N ratio of the poultry litter was lower than those of the composts (poultry litter = 10.2, Panorama compost = 19.5, and Cascades compost = 13.0), which indicates more available N in the litter than in the compost.
All organic materials contained high enough concentrations of P and K to satisfy crop nutrient requirements when they were applied at agronomic N rates. A potential concern with applying the composts and litter at agronomic N rates is P over enrichment of soil, which could be a source of surface water contamination. For example, the LC and LCF = 128 lbs P2O5/acre supplied by LC and LCF, the 638 lbs P2O5/acre supplied by AC, the 843 lbs P2O5/acre supplied by BC and BCF, and the 202 lbs P2O5/acre supplied by the PL far exceeded the 100 lbs P2O5/acre that was recommended by the Virginia Tech Soil Testing Laboratory for the fertilizer plots at NPAREC.
The availability of nutrients varied among treatment and site due to the differential amounts of nutrients applied and background differences at each site. Background levels of P, K, and Mg were high at Bracketts, while Cascades had higher CEC, organic carbon, calcium and pH than the other sites.
Soil test (ST) nutrients
At NPAREC, significant treatment differences for all nutrients were most notable after the third growing season. Treatments with annual or biennial applications of agronomic N rates of compost (treatments AC, BC, and BCF) had higher levels of ST-P, K, and Mg than the control, the low compost rate treatments, and the fertilizer treatment. Cation exchange capacity increased in the AC, BC, and BCF treatments with time due to the addition of organic matter. The greatest differences among treatments occurred in the third year, which demonstrated the cumulative effects of organic matter addition. Single degree of freedom contrasts were highly significant (P < 0.0002) for the comparisons of AC and BC with the control for ST-P, K, Mg, and CEC in the last year. The fertilizer treatment differed from the control only by elevated P levels. Annual agronomic N rates of compost provided significantly (P < 0.02) more ST-P, K, Ca, and Mg and higher CEC than the annual agronomic N rates of poultry litter.
At Bracketts, season, treatment and block almost always had a significant effect on ST-P, K, Ca, and Mg and CEC. Bracketts is characterized by high native soil K, Mg, and P. The initial K base saturation of 22.5% was two to four times higher than at the other two sites. This initially high level of exchangeable K decreased dramatically after the first cropping season in all treatments. Over the three years studied, Mg and CEC also decreased, while exchangeable Ca increased.
Treatment differences also occurred at Bracketts. The BCF treatment had higher concentrations of ST-P, K, and Mg and CEC than the control, fertilizer, low compost treatments, and the biennial compost treatment without supplemental fertilizer. These effects were not observed in the AC because of a strong block effect. Treatment randomization within blocks placed the BCF treatment at the end of each row where higher amounts of nutrients existed.
Calcium dominated the CEC (89% base saturation) during the first year at Cascades farm but decreased in all treatments after the initial application of amendments. The control treatment maintained a relatively high calcium concentration compared to the other treatments, whose Ca concentrations were diluted by the addition of other cations in the soil amendments. The concentrations of ST-P, K, and Mg increased with time, especially in the AC, BC, and BCF treatments. These treatments had higher concentrations of ST-P, K, and Mg relative to CTL, LC, and F. No treatment effects on CEC were measured.
Cropping reduced the high native soil concentrations of K at Bracketts and Ca at Cascades. The highest rates of compost raised the concentrations of soil test nutrients and CEC at all sites. Soil CEC can be increased by the addition of organic matter (Stevenson, 1994), which contains at least eight times the CEC of some clay materials (Sposito, 1989).
Although compost can be an effective buffering agent, soil pH was not affected by compost applications after three years at either NPAREC or Cascades. At Bracketts, the high compost and biennial compost with fertilizer treatments raised soil pH compared to the control, low compost, and fertilizer treatments due to a reduction in exchangeable acidity.
The increase in all nutrient concentrations was not necessarily a positive effect. At NPAREC, the agronomic N rates of compost increased the soil concentration of ST-P well above that necessary for optimum crop growth and above the concentrations resulting from the other treatments. The increase in P was a result of applying more P than required for crop uptake at agronomic N rates. The continuous use of large volumes of compost can increase soil P to concentrations above which can be bound by the soil. Such high concentrations can result in surface P transport from soil to water.
Carbon and nitrogen
NPAREC treatments AC, BC, and BCF had elevated levels of total C and N relative to the CTL, LCF, and PL. Contrast tests between AC and CTL, F, and PL were significant in the last year. The LC and LCF treatments did not differ from the control and fertilizer treatments, which demonstrated that low rates of organic matter are rapidly mineralized.
High applications of compost increased soil organic C and total N at Bracketts. The AC, BC, and BCF treatments had higher C and N concentrations than the CTL and F as assessed by contrast testing.
The average concentration of organic C at Cascades was 2.7%, which was more than double the soil C concentration at NPAREC and Bracketts. Organic carbon values were not different among treatments, possibly due to the high native C concentration and the analytical problems encountered when attempting to separate organic C from inorganic C for this high calcium carbonate-bearing soil. Nitrogen and organic C concentrations were highest in the AC treatment due to the high annual applications of C and N in the compost. The increase in soil C with compost addition is often dependant on soil type (Shiralipour et al. 1996).
High application rates of compost can increase soil organic C and total N. Additions of compost at rates considerably lower than that required to supply crop N needs and poultry litter at agronomic N rates are unlikely to affect soil organic C, total N, and other essential plant macronutrients in the short term. Low compost rate treatments were indistinguishable from the control and fertilizer treatments.
The measurement of carbon dioxide evolution in 2001 and 2002 may not have been sensitive enough to assess possible treatment effects on microbial biomass; therefore, we did not report mean CO2 concentrations. There were no differences among treatments in any of the microbial populations analyzed in the fall of 2002. This was unexpected because the AC soils contained 33% more organic C than the CTL soils. We expected to measure differences between these treatments because many microbial species are decomposers. Fauci and Dick (1994) demonstrated a relationship between organic C concentration and microorganism populations; however, our analytical tests may not have been suitably sensitive.
Soil physical properties
Soil bulk density was lower in AC, BC, and BCF than in CTL and F at the end of the second and third growing seasons. Even low compost rates lowered soil bulk density as demonstrated by contrast testing (i.e., BC and LC < C, LCF and BCF < F). A decrease in bulk density has been directly correlated to organic C additions (Khaleel et al. 1981). Similar patterns were observed for porosity.
Soil water content measurements were not significant until the final year, at which time the AC, BC, and BCF treatments held more water than the other treatments. The low compost treatments and PL treatment did not add enough organic material to benefit the soil water holding capacity.
All soil physical properties were improved with high compost rates within two to three years, but positive effects of low compost rates were slower in accruing or did not occur.
No treatment effects on pumpkin (Cucurbita pepo Var. Magic lantern) yield occurred at any of the three sites. The presence of residual nutrients (including plant available N) in the soil at concentrations that did not limit crop yield likely prevented yield responses to the treatments. Rainfall was plentiful, and soil moisture was unlikely to be limiting at any of the sites; therefore, no benefits from increased soil organic matter (e.g., increased water holding capacity) with the high compost treatments were realized. Mean yields at each site were 86.5 Mg/ha at Bracketts, 83.6 Mg/ha at Cascades, and 51.7 Mg/ha at NPAREC.
At NPAREC, the LCF, AC, BCF, PL, and F treatments produced higher yields than the CTL, LC, and BC plots. The F treatment also produced higher yields than the AC treatment. The effects of treatments on the pre-sidedress nitrogen test (PSNT) and the earleaf corn N concentration provided evidence that the lower yield in the AC treatment was due to limited plant available N. These results indicated that we overestimated the expected compost N mineralization rate.
The Bracketts’ yield was not affected by treatment (mean = 7.5 Mg/ha). Lack of treatment response was likely due to factors that limited overall yield on this low productivity soil. These factors included excess early soil moisture that reduced germination, seedling vigor, and N availability; poor farm management (i.e., irrigation timing and weed control); and severe mid- to late-season soil moisture deficit plus excessive heat.
Yields at Cascades were typically high (mean = 12.4 Mg/ha) for this highly productive soil, but no differences existed among treatments. High residual fertility and plant available water may have outweighed treatment effects. The high water table and soil organic matter concentrations prevented water and nutrient limitations.
At NPAREC, pepper yields in the BCF (3.1 Mg/ha) and PL (3.3 Mg/ha) treatments were higher than the other treatments, except AC, whose yield was indistinguishable from the CTL, LCF, BC, BCF, PL, and F treatments. The BCF and PL treatments apparently provided the least yield-limiting combinations of essential nutrients and organic matter, demonstrating the value of chemical and physical soil components on soil productivity.
The Bracketts’ crop yielded poorly, again due to yield-limiting environmental conditions (late spring frost, mid-season drought) and poor farm management. The jackloam soil is an inherently low productivity soil and even three annual applications of compost at agronomic N rates were not sufficient to improve productivity.
Cascades, as in the previous two years, attained high yields (mean = 4.8 Mg/ha) with no significant differences between treatments. All treatments were able to meet the crop’s needs of nutrients and water in the inherently productive Weaver silt loam. As previously described, Cascades had the highest concentration of soil C of all three sites (2.7%) and a high water table that rarely severely limited plant water needs. This naturally productive soil did not benefit from any of the treatments.
Highest rye biomass production was attained in the treatments (AC and PL) that had received high rates of organically-complexed N prior to the previous crop (pumpkin). The yield of the AC treatment was 3x the yield of the CTL. The F treatment also yielded significantly higher than the CTL, indicating there were considerable amounts of plant available nutrients in the soil almost 10 months after the previous application. The lack of a strong relationship between total biomass and total plant N (r2 = 0.28) was an indication that factors other than plant available N contributed to the rye yield response. The rye growth may also have benefited from enhanced soil physical properties resulting from the high compost rates.
No differences existed among treatments in total mass of nitrate leached annually. Treatment differences were found only for two of the 31 months sampled. Significantly higher concentrations of NO3- N occurred in the F (August 2001) and AC (January 2003) treatments. The potential for leaching losses of NO3-N is more closely related to correctly estimating the amount of soluble N that becomes available from the amendment than the actual type of amendment.
Increases in NO3- N in soil water occurred during two periods and was related to plant growth stage and season. The first occurrence of elevated NO3- N concentration was measured after the initial growing season, between October 2000 and July 2001, at which point the NO3-N concentrations presumably decreased due to the use of nitrate by the corn plants and evapotranspiration rates which exceeded rainfall plus irrigation rates. A second period of NO3- N rise in the soil leachate began in June 2002 following a bell pepper crop whose vigor was reduced due to a frost that occurred shortly after transplanting. The activity of mineralizing and nitrifying soil microbes decreases dramatically during the winter, but there is an apparent risk of nitrate leaching loss if the crop proceeding or during this time period does not utilize all the nitrogen.
Electrical conductivity (EC) varied with time, block and treatment and did not correlate well with either total nitrate or nitrate concentration. The leachate collected from the AC and PL treatments had higher mean EC’s than the CTL and F treatments, which may have been caused by continuous mineralization of the organic soil amendments. No effect of treatment on pH was observed.
Runoff and infiltration:
Simulated rainfall was applied to each treatment x replication for 30 minutes after the first signs of runoff. For the duration of the applied “rainfall,” infiltration volume of the simulated rainfall was not significantly affected by treatment, but there was a trend toward increasing infiltration in the order CTL<F<PL<AC. Runoff was lower in the AC treatment than in the CTL, with the PL and F treatments intermediate between the AC and CTL. The higher amounts of organic matter added to soil in the compost improved soil aggregation, infiltration, and water holding capacity, which prevented as much applied water from leaving the soil via surface runoff.
The surface loss of potentially water-impairing constituents followed the same trends a
did runoff. The total load of total suspended solids (TSS), comprised of mineral and organic solids, and various forms of N and P leaving the plots were lower in the compost-amended than in the control treatment. The values of these constituents in the runoff of the PL and F treatments were intermediate between the AC and CTL. Compost-amended soils contributed the least amount of TSS, N, and P to surface runoff despite having the highest concentrations of N and P in the surface soil because of the better “tilth” of these soils as measured by improvements in physical properties (viz., bulk density, porosity).
Educational & Outreach Activities
Bulluck, L.R. III, M. Brosius, G.K. Evanylo, and J.B. Ristaino. 2002. Organic and synthetic fertilizer amendments influence soil microbial, physical and chemical properties on organic and conventional Farms. Applied Soil Ecology 19:147-160.
Evanylo, G.K. and C.A. Sherony. 2002. Agronomic and environmental effects of compost, manure, and fertilizer use. p. 730-740. In 2002 International Symposium: Composting and Compost Utilization. May 6-8. Columbus, OH.
Evanylo, G.K., C.A. Sherony, and G.L. Mullins. 2002. Water quality effects of compost, manure, and fertilizer use for vegetables. Composting in the Southeast Conference, Palm Harbor, FL. October 6-9. CD-ROM.
Sherony, C.A., G.K. Evanylo, and J.W. Pease. 2002. Yield differences and economic implications of compost, poultry litter, and fertilizer amended soils. Composting in the Southeast Conference, Palm Harbor, FL. October 6-9. CD-ROM.
Evanylo, G.K. 2000. Effects of organic amendments on soil properties. Northern Piedmont Annual Field Day. Orange, VA. August 22.
Evanylo, G.K. 2001. Compost effects on crop growth, soil, and water quality. BioCycle Southeast Conference 2001. Atlanta, GA. Aug. 27-29.
Evanylo, G.K. 2002. Horticultural uses of compost. Virginia Annual Viticulture Conference. Charlottesville, VA. February 16.
Sherony, C.A. 2002. The Benefits of Compost. Extension Master Gardener Intern Class. Blacksburg, VA. January 29.
Sherony, C.A. 2002. Agronomic and Environmental Effects of Compost, Manure, and Fertilizer Use. NACAA (National Association of County Agricultural Agents) National Annual Conference. Savannah, GA. July 30.
Evanylo, G.K. 2003. Introduction to composting and compost use in horticulture. 2003. Mid-Atlantic Horticulture Short Course. Virginia Beach, VA. January 22.
Evanylo, G.K. 2003. Advanced composting and compost use in horticulture. 2003. Mid-Atlantic Horticulture Short Course. Virginia Beach, VA. January 22.
Evanylo, G.K. 2003. Composting and compost use in viticulture. Wineries 2003 Conference. Lancaster, PA. March 17-18.
Sherony, C.A. and G.K. Evanylo. 2003. Compost quality standards and soil quality research in Virginia. On-farm composting school. Dayton, VA. March 26.
Sherony, C.A. and G.K. Evanylo. 2003. Soil and Soil Amendments. Extension Master Gardener Intern Class. Blacksburg, VA. April 1.
Sherony, C.A., G.K. Evanylo, and D. Starner. 2001. Agronomic and environmental effects of soil amendments for vegetable crop production. In 2001 Agronomy abstracts. ASA, Madison, WI.
Mauceri, M., C.A. Sherony, and G.K. Evanylo. 2001. Effects of soil amendments on plant nutrition. In 2001 Agronomy abstracts. ASA, Madison, WI.
Sherony, C.A. and G.K. Evanylo. 2002. Composting and compost use. Virginia Tech Farm and Family Showcase, Kentland Farm, Whitethorne, VA. Sep. 5-7.
Sherony, C.A. and G.K. Evanylo. 2002. Effects of compost on soil quality. National Sustainable Agriculture Conference, NCSU, Raleigh, NC. October 23-24.
High annual application rates of compost improve soil physical (i.e., bulk density, porosity, infiltration, and water holding capacity) and chemical (CEC, organic C, N, plant available nutrients) properties, but crop production may benefit the most where native soil productivity is not extremely high.
Amendments that provide combinations of high organic matter and nutrient availability (viz., high compost rates, compost+fertilizer, and poultry litter) produce the highest crop yields. Low annual compost rates provide little advantage over the unamended soils, and low annual compost rates plus supplemental fertilizer provided little advantage over the soils receiving only fertilizer.
Biological properties commonly employed to assess the beneficial effects of organic soil amendments (e.g., soil respiration, soil microbial diversity) require additional evaluation to determine their value as a soil quality indicators.
Major discernible benefits of high compost application rates are the supply of plant available N and the improvement in soil physical and chemical properties. More accurate and precise estimates of mineralizable N is necessary to ensure adequate plant available N. A disadvantage of applying agronomic N rates of compost is the potentially deleterious concentrations of P that may accumulate in the soil.
Soil NO3-N leaching is due to excessive concentrations of inorganic and potentially mineralizable N remaining in the soil after plant uptake. The source of the N is less important than accurately estimating plant available N released during and after the growing season. Fertilizer, poultry litter, and compost can all provide adequate amounts of N to satisfy plant needs without endangering water quality. Increasing soil concentration of organic matter through additions of compost reduces runoff and increases infiltration, which reduces the total load of N, P, and other constituents leaving the soil in surface water flow.
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Estimation of Treatment Costs and Returns
The objective of the economic analysis is to determine whether or not there are significant differences in net returns by treatment. Estimation of treatment net returns considers the quantities and prices of inputs as well as price valuation of crop yield. Quantities and prices of most inputs were recorded during the experiment, as well as hours of labor invested for each management practice. If the data was not available, relevant secondary data was used for estimation purposes. Application rates and prices paid for fertilizer, compost, and poultry litter inputs, plus their associated application costs, constituted the cost differences between treatments. Capital inputs were minimal for this experiment, and most practices were accomplished by hand. In any case, capital inputs did not differ across treatments, so comparisons of gross margins (gross return minus variable costs provides an acceptable comparison of treatment economic effects.
Enterprise budgets by plot were constructed for the crops grown at NPAREC. Prices used for pumpkins, sweet corn, and bell peppers were 20.9 cents per Kg, 42.4 cents per Kg, and 66.5 cents per Kg, respectively (USDA 2003). Gross returns per plot were calculated as price times yield, and gross margins as gross returns minus enterprise variable costs per plot. Comparisons across treatments of gross margins rather than more completely as net economic return is justified because all inputs and practices other than those noted below are invariable across plots.
Mean treatment gross margins are negative or zero except for the pumpkin control treatment. Except for AC and BC, gross margins decline from 2000-2002. As noted above, it appears that the plots had relatively high residual fertility levels, and crop yields did not respond well to nutrient applications. There are large differences among the mean gross margins across treatments for a particular crop, but statistical analysis would serve only to answer the question whether some treatments lose significantly less money than others. Only seven plots in the experiment produced enough yield to cover the variable cost of production. Costs per plot are high because: 1) straw mulch is expensive to purchase and apply; 2) compost nutrients are relatively expensive, 3) the scale of operation makes inefficient use of capital resources, and 3) labor investment in these small plots is considerably higher than would be expected in a commercial operation.
Vegetable prices: http://usda.mannlib.cornell.edu/reports/nassr/fruit/pvg-bban/ Accessed April 11, 2003
At least 470 farmers and educators learned of the benefits of compost use through field days, workshops, short course, practitioner conferences, and extension-type meetings. An additional 220 extension agents and other educators attended professional conferences where we shared our data.
The extension of adoption by farmers is unknown, but we have been receiving increasing numbers of requests from extension agents and farmers for further information on appropriate sources and rates of compost to use as a soil amendment in agriculture.
Areas needing additional study
- Establishment of methods to precisely calculate plant available N in compost.
Assessment of the environmental ramifications of soil P buildup when compost is applied at agronomic N rates. This includes developing a better understanding of water movement and P transport in compost-amended soils.
Further evaluation of the value of soil biological tests commonly employed as indicators of soil quality.
Long term (>5 years) benefits of the application of annual low rates of compost.
Assessment of the benefits of plant growth regulators produced in compost on plant response to environmental stress (e.g., drought, salinity).
Continuing research on on-farm compost production and compost use economics.