(Editor’s note: A more naturally formatted version of this publication can be obtained at http://www.griffin.uga.edu/sare/documents/soilwater.doc)
This review summarizes knowledge gained from the Southern Sustainable Agriculture Research and Education (SSARE) program’s Research and Education projects (R&E) and Graduate Student (GS) projects about soil and water quality in sustainable food and farming systems.
The main objective of the review is to collect and organize the results of the SSARE soil and water projects in preparation for SSARE to create a “sustainability toolbox” for the southern region. A further objective is to identify topics that have been fairly well researched and topics that need more work. Suggestions for future SSARE-funded soil and water quality work are made.
Completed projects and ongoing projects with significant available information focusing on soil and water are included in the review. Projects are arranged chronologically within different topics such as Nutrient Cycling, Soil Conservation, etc. The review is presented in two parts following this abbreviated summary. Part I consists of section introductions followed by short abstracts of the main points of each project; Part II includes longer, more detailed summaries and literature citations.
Many projects examined more than one aspect of soil or water quality. Such interactions are mostly separated but cross-referenced in this review. Also, many projects investigated other topics, such as crop management, in conjunction with soil and water management. Non-soil and -water topics are not explored in this review. Such separation is not ideal; SSARE projects almost necessarily consider multiple factors. However, boundaries are necessary to limit this review to a manageable size. Future reviews will cover other topics.
Many if not most soil and water SSARE projects led to further work. This review includes only papers and information directly related to SSARE funding, partly to make the review manageable and partly to reflect SSARE’s specific accomplishments more accurately.
For the most part, project findings were not in disagreement with one another even in projects from quite different ecological situations. However, even when general project results were similar, important results concerning management details differed from place to place. One conclusion that can be drawn from the SSARE soil and water projects is that site specific and material specific management is necessary and that very few “one size fits all” answers exist. Even when those answers do exist – e.g., adding organic matter is beneficial to soil – implementation and consequences will vary widely from place to place and farm to farm.
Nutrient Cycling Biology, Cover Crops, and Compost
Soil enzymes, active organic matter, microbial diversity, macroscopic organisms, nitrogen (N) cycling, composting methods and feedstocks, compost application rates, compost contributions to soil fertility, cover crop management, legume contributions to soil fertility, managing mulches or living mulches for soil improvement.
Frequent organic matter addition stimulates soil biological functions and generally improves physical and chemical soil quality characteristics in agricultural soils, but cash crop yields may not increase over the time periods investigated. Applying and managing organic matter is difficult due to the unpredictability of nutrient release, and often due to the large amounts required. Appropriately managed cover crops supply adequate nitrogen (N) to most following crops. Site specific management of added organic matter or cover crops is necessary.
• High rates of organic matter addition were generally needed (tens of Mg ha-1 for agricultural by-products, composts, or manures).
• Agricultural by-products were more effective in increasing yield when used on damaged (eroded, leveled) soil than on intact soil.
• The C:N (carbon:nitrogen) ratio of agricultural by-products was key to their short term value as nutrient sources. Rice hulls, a high C:N ratio material, required N fertilizer as a co-amendment to prevent crop yield losses due to N immobilization.
• Cotton gin trash could be used composted or uncomposted, and was a valuable soil amendment that stimulated biological activity and suppressed some plant pathogens.
• Each agricultural by-product had specific management requirements for use as a soil amendment.
• Active organic matter addition resulted in heightened soil enzyme activity and changed the microbial community structure. Organic matter increased microbial diversity in some situations.
• Several years (more than 3) were generally required for measurable soil quality improvement, but one study showed changes in soil characteristics in 2 yr.
• Nutrient release, especially N, from organic matter was much less predictable than from fertilizers. The pre-sidedress nitrate test was not always adequate to predict N-mineralization from organic sources.
• Appropriate leguminous cover crops adapted to specific environmental conditions could provide adequate N for almost any following crop, even in no-till or agroforestry situations. However, if adequate N was available from fertilizer, extra N from leguminous sources did not increase yields.
• Cover crops increased cotton yield enough to pay for the cover crop, in addition to soil quality benefits.
• Inadequate N provision to harvestable crops and N losses due to leaching were common. Achieving synchrony between nutrient release from decomposing cover crops and nutrient requirement of harvestable crops was difficult due in part to inability to predict N-mineralization rates.
• Giving N-credits for legumes and using a combination of leguminous and non-leguminous cover crops to retain N in the system helped to reduce N leaching and to avoid over-supply of N to harvestable crops. Over-supply of N did not increase yields.
• Some cover crops, such as southernpea, could be harvested for immediate profit as well as providing nutrient cycling services.
• Rye was a successful N scavenger in almost any situation, but could disadvantage following crops due to allelopathy or N immobilization.
• A system of spring vegetable crops followed by fall cover crops was more effective at preventing N leaching than spring cover crops followed by fall vegetable crops.
• Other growth limiting factors such as moisture influenced the effectiveness of nutrient cycling from cover crops. Cash crops stressed by lack of moisture did not take up N effectively, leading to loss of N from the system.
• Organic mulches used with vegetable or herb crops increased water use efficiency and could replace C decomposed over the growing season.
1. Research investigating specific management recommendations for the use of different by-products as soil amendments will remain useful for SSARE’s goals of encouraging linkages among agricultural sectors and between agricultural and urban sectors.
2. Research on nutrient cycling in cover crops will remain useful if the research focuses on new situations such as no-till in organic systems, or creates more accurate and farmer-friendly models of nutrient release from cover crops.
3. Research to find ways that different cover crops can contribute to immediate farm profitability while providing benefits to soil quality will be a highly significant area of investigation.
4. To elucidate the relationships involved in microbial community structure and function will require detailed basic biological research which may or may not have immediate practical implications. However, answers gained from this kind of work may be broadly applicable.
Synthetic fertilizers, rates of fertilizer application, alternative fertilizers not including manure, compost, or cover crops, and soil fertility measurements related to fertilizer use. Nitrogen was the primary nutrient of concern; phosphorus (P) and potassium (K) were used to optimize N efficiency.
• Conservation tillage and no-tillage in semi-arid regions resulted in higher cotton and wheat crop yields at all fertilizer levels, indicating multiple benefits of retaining topsoil and increasing water holding capacity.
• Irrigation rate and rotation were more influential in increasing cotton and wheat yield in semi-arid areas than fertilizer rate.
• In some cases, even the best fertilizer management did not make unadapted crops and soils fit one another. Success in managing acid coastal soils for alfalfa production could not be achieved at a reasonable expenditure of time and money. Gypsum applied at up to 15 Mg ha-1 did not reduce aluminum (Al) to levels that alfalfa could tolerate in these soils.
• Complementary crops made fertilizer use more efficient. In an alleycropping situation, pecans scavenged 48 kg ha-1 unused fertilizer N from deeper within the soil profile than cotton could reach, redepositing the N on the surface through litterfall.
• Fertilizer produced by digestion of restaurant waste in an anaerobic digester was acceptable for field production but not for producing transplants; it burned sensitive transplants and did not provide enough nutrients for stronger transplants. The digester handled 2.27 Mg (2.5 tons) of food waste over about 15 months.
• Vegetable transplant production required more soluble minerals than field vegetable production, but care was needed to avoid burning.
• Expensive organic fertilizers (e.g., Fertrell) were effective in transplant production. Few inexpensive organically certified materials reliably provided soluble nutrients for transplant production.
1. Research investigating and optimizing more alternative fertilizers and micronutrient sources, especially creating and enhancing local nutrient cycling.
2. Research on novel crop combinations to improve profitability and increase fertilizer use efficiency.
3. Research on processing or formulating inexpensive organic materials to be suitable for organic transplant production or other delicate plant production, especially creating and enhancing local nutrient cycling.
Manure and Pollution
Manure usage to enhance soil fertility while minimizing potential pollution.
Appropriately managed manure is an acceptable nutrient source for almost any situation. Appropriately managed poultry litter is clearly acceptable in almost any form. Appropriate management includes manure and soil testing, consideration of crop requirements, nutrient scavenging crop rotations, timing of manure application close to peak crop demand, and soil erosion prevention. Site specific management is essential; allowable manure application rates found in one study do not readily transfer to other environmental situations.
• Grain crops could assimilate significantly more manure nutrients than vegetable crops, possibly due to the much greater biomass potential in grain crops.
• Legumes, especially crimson clover, in a rotation helped to limit P accumulation and N pollution potential from poultry manure usage, especially when used as fall cover crops following spring vegetable crops.
• Applying poultry manure in excess of recommended rates based on soil tests did not improve crop yields. Excess manure resulted in soil nitrate levels of 20 to 25 ppm NO3-N (exceeding water quality standards) at 80 cm soil depth.
• On no-tilled soil, comparable crop yields could be achieved with ammonium nitrate (168 kg N ha-1) and liquid dairy manure (336 kg N ha-1). The difference in N required was due to slower availability of manure N.
• Soil conservation and water quality protection measures such as filterstrips used in conjunction with manure application reduced surface runoff water nitrate N by up to 99% and surface runoff P up to 85%.
• Fertilizing sod with manure and selling the sod outside the local area of manure concentration cost-effectively exported more than 114,840 kg P away from a sensitive watershed.
1. Research that creatively integrates animal and non-animal agricultural enterprises or develops manure processing for export to provide avenues for positively handling animal waste.
2. Work that addresses structural changes to profitably limit the number of animals concentrated in local areas so that nutrient accumulation is reduced at the source.
3. Ongoing environment- and material-specific research, including work to create or improve methods for on-farm manure nutrient testing and manure nutrient availability modeling to encourage site specific management.
Manure from any source as a soil fertility amendment, application methods, application rates, and soil fertility measurements. Projects do not investigate potential pollution from manure.
These projects dealt primarily with high-value vegetable crops. Site- and crop-specific information and management is required when using manure for vegetables.
• Manure was applied to vegetable crops at less than 1 Mg ha 1, compared to an average of about 10 Mg ha-1 for grain crops.
• Vegetable crops seemed more sensitive to differing manure characteristics than grain crops. Different vegetable species and even cultivars responded differently to manure, requiring intensive site-specific management.
• Vegetable crops could be planted from 7 to 14 days after pastured poultry were on a plot, but optimum planting time varied by crop. (Planting may not be able to occur this soon if organic certification is a concern, depending upon the length of time to harvest.)
• Fractionating poultry litter into fine, medium, and coarse fractions allowed the re-use of the coarse fraction as bedding and use of the fine fraction as pelleted fertilizer with high N content. Storing fractionated or non-fractionated litter at low water content (less than 0. 5 kg water kg litter-1) reduced gaseous N loss during storage.
1. Research that creatively integrates animal and non-animal agricultural enterprises or develops manure processing for export, specifically directed toward high value crops.
2. Work that addresses structural changes to profitably limit the number of animals concentrated in local areas so that nutrient accumulation is reduced at the source.
3. Ongoing environment- and material-specific research, including work to create or improve methods for on-farm manure nutrient testing and manure nutrient availability modeling to encourage site specific management, directed toward high value crops.
Ground or surface water pollution, including nutrient and pathogen pollution, from any soil fertility source or animal or soil management practice.
There is no one answer to pollution prevention. Every watershed is unique and different nutrients behave differently in every watershed. Site specific recommendations were more effective than global recommendations in preventing pollution.
• Rye was the most effective of several cover crops in accumulating N from over-fertilized areas. However, N-uptake by rye occurred mainly in early spring, meaning that N had opportunity to leach over the winter months.
• Geotextile fabric installed under a loafing area on a dairy directed manure deposition into holding lagoons rather than allowing it to seep through soil to groundwater.
• A 1-acre constructed wetland effectively treated waste from a 500-hog facility. Wetland plants accumulated enough nutrients so that wetland discharge could be spread over 14% of the land area that untreated hog facility waste would require.
• Pasture runoff from an intensively managed rotational grazing dairy had only 1% of the pollution that runoff from a confinement dairy did.
• Fencing to exclude cattle from streambanks reduced streambank erosion and improved water quality. Recovery of already-eroded streambanks was slow in the streams under investigation because these bedrock-type streams had low sediment deposition.
• The location of a pollution source relative to surface water bodies influenced the source’s impact on water quality. Global assessment was not adequate to identify critical areas; field by field assessment was more effective.
Addressing remaining challenges:
1. Research in individual watersheds appears necessary, but is a long-term process. SSARE funding in this area may need to be directed toward the most sensitive watersheds and sites within watersheds.
2. Research on effective pollution control measures for specific problems, including continuing research on novel pollution control technologies.
3. Research investigating structural change to minimize pollution sources (e.g., high animal concentrations).
Soil and Water Conservation and Soil Physical Properties
Alternative tillage systems, erosion, terrace-building or other soil-moving practices to reduce erosion, strip-tillage, mulches, improvement of soil physical properties, water infiltration, and reduced irrigation use.
Keeping soil covered and limiting disturbance from tillage reduces erosion. Reducing tillage may increase crop yields. Site- and crop-specific management is necessary.
• Over 4 yr, no-tilled soil developed greater water holding capacity and water infiltration rates than tilled soil, even though bulk density of no-tilled soil was higher. Possibly due to the greater water-holding capacity, reduced tillage resulted in corn, soybean, cotton, and wheat yields as high or higher than with conventional tillage.
• Relay planting of soybeans into ripening wheat or planting soybeans into killed wheat stubble reduced erosion by 25 to 40% compared to monocrop conventional tillage. Soybeans planted into standing wheat yielded more than soybeans planted later into killed wheat, possibly due to higher moisture availability.
• In semi-arid areas, minimum and conservation tillage produced higher cotton yields than conventional tillage while using 25 to 35% less water and decreasing erosion. Efficient water delivery systems increased yields at every water level compared to conventional spray irrigation.
• No-tillage of a bean crop into banana plantations on extremely sloping tropical land created heavy erosion due to loss of plant residue cover on the soil. Situations such as this require permanent cover or other erosion-reduction strategies such as terracing.
• Hedges of stiff, thick-stemmed grasses as narrow as 1 to 2 rows reduced sediment leaving a field by an average of 75%. Within 10 yr, hedges created “terraces” that reduced erosion by an additional 30 to 50%. Hedges may be used in situations where no-till is not feasible.
Addressing remaining challenges:
1. Investigation and perfection of no-tillage or reduced tillage in organic cropping systems to reduce the dependence of these systems on cultivation for weed control to protect soil quality and reduce labor.
2. Site-specific investigation of alternatives or complementary practices to no-tillage or reduced tillage in situations where tillage is absolutely necessary or where reduced tillage will not completely solve erosion problems.
3. Environment-specific investigation of profitable but non-eroding intercropping alternatives for perennial or long-season annuals.
Projects investigating several aspects of soil characteristics including physical, chemical, and biological functions, not limiting their investigations to specific soil characteristics or functions; long-term or large scale systems projects.
Two of the three large scale systems projects found that over a number of years, differences in biological, physical, and chemical soil characteristics appeared among widely divergent systems. Organic matter content and microbial activity were generally improved in systems with decreased tillage and higher vegetation diversity or more permanent vegetation. Decrease in tillage resulted in higher bulk density but not in lower water infiltration rates. Heavy amendment with organic matter increased beneficial microbial species while tending to decrease pathogens, although specific exceptions to this trend occurred.
The systems projects were highly effective in creating platforms for smaller, short term projects that investigate specific questions. The main barrier to implementing long-term soil quality projects with a broad scope of investigation is separation among disciplines, with its implications for competition for limited funding.
• Returning CRP land to forage or crop production required suppression of old sod and addition of fertilizers. However, weather conditions, available water, and crop rotation were more influential in yield production than fertilizer management in the arid plains. No-tillage of wheat in former CRP land produced yields higher than conventional tillage.
• No-tillage of wheat in former CRP land did not result in increased erosion compared to soil remaining in CRP. Disk tillage resulted in erosion 3 to 4 times higher than no-tillage or non-disturbance.
• Tillage of former CRP land increased potentially mineralizable C by exposing formerly protected C and N pools to decomposition. No-tillage of wheat in former CRP land reduced potential volatilization of stored soil C by minimizing the movement of carbonate to the surface.
• A system of integrated crop and livestock production in the arid plains reduced water use by 23%, fertilizer use by 40%, and reduced erosion by more than 30% while maintaining cotton yields (per hectare of cotton in the system) and increasing profitability. Choosing water-efficient perennial forages influenced the overall efficiency of the system.
• Historic organic management increased soil P, CEC, plant available water, and selected crop yields compared to historic conventional management, regardless of current management. Current organic matter additions caused bulk density to decrease within 2 years, significantly earlier generally expected for change in soil physical characteristics.
• Perennial grassland and an organic system had higher soil microbial functional diversity than a conventional system. No-tillage and grassland systems had greater CO2 evolution than the conventional system. Organically managed soil supplied adequate available N for crop production over the season, and had the lowest bulk density and highest water infilration of all systems compared.
• A corn production system including crop rotation with legumes reduced system N and P requirements by 50% compared to conventional corn production. Corn following a mixture of hairy and bigflower vetch yielded similarly to corn fertilized with 140 kg N ha-1.
• Cover crops were effectively removed prior to corn planting by grazing, provided that animals were not allowed to compact wet soil. Leaving cover crop residues on the surface, killed by chemical desiccation, provided the best protection from compaction.
Other results from sub-projects:
• Vermicompost did not provide adequate N sources for sweet corn, resulting in marketable yields of 20 to 31% less than corn following crimson clover.
• Sorghum-sudan grass was an effective cover crop for summer weed suppression but did not allow enough time to establish fall cabbage transplants.
• Stream water quality improved when rotationally grazing cattle were supplied with non-stream water sources. Fencing was not necessary to deter cattle from the stream if alternative water was provided.
• The return of animal waste to the soil surface during grazing helped to sequester C and N, and increased soil microbial activity.
• Red clover-wheat-corn-soybean was the most favorable rotation to replace continuous corn in terms of soil quality characteristics and corn yield.
• N leaching after plowdown of alfalfa was high in the first year after plowdown. Transitions from perennial plantings to annual crops may pose short-term water quality concerns, especially when N release occurs out of synchrony with annual crop N uptake.
• Row crops were successfully be no-tilled into existing sod such as pastures in need of renovation.
• Lime-stabilized biosolids applied to pastures increased soil organic matter and supplied residual N without affecting groundwater.
Addressing remaining challenges:
1. Evaluation of the contribution of the large scale, long term systems projects to the SSARE mission and encouragement of broader use of these platforms for component work. The large scale, long term systems projects offer platforms for ongoing work if their support is stable, but they are expensive to maintain.
2. Supporting research or dialogue to definitively identify key soil quality variables (and other variables) for measurement when multiple variables cannot be measured due to limitations in research resources.
Review of SSARE-funded Soil and Water Quality Projects
Introduction and Methods
Researchers and farmers in sustainable or alternative agriculture have benefited directly and indirectly from Southern SARE research projects. This review summarizes knowledge gained from 77 SSARE Research and Education projects (R&E) and 4 Graduate Student (GS) projects about soil and water quality in sustainable food and farming systems. The main objective of the review is to collect and organize the results of the SSARE soil and water projects in preparation for SSARE to create a “sustainability toolbox” for the southern region. A further objective is to identify topics that have been fairly well researched and topics that need more work. Suggestions for future SSARE-funded soil and water quality work are made.
The review is presented in two parts: Part I consists of short abstracts of the main points of each project; Part II includes longer, more detailed summaries and literature citations. The review is arranged by categories such as nutrient cycling and compost, soil conservation, pollution, etc. Projects are described in chronological order within the categories. Categories correspond somewhat but not completely with basic soil science topics such as soil biology, soil chemistry, and soil physics. This is because SSARE’s focus has historically been more applied, with research problems identified according to farmers’ practical needs.
A challenge in performing this review was that many projects examined more than one aspect of soil or water quality. For example, cover crops were often used in soil conservation projects, but a researcher may also have measured the effect of cover crops on soil enzyme activity. Such interactions are for the most part separated in this review, but are cross-referenced so that a reader can locate different information generated by the same project. Also, many projects investigated other topics in conjunction with soils, such as the effect of cover crops on insect pests as well as soil quality. Non-soil topics are not explored in this analysis (but will be described in future SSARE reviews). This separation is not ideal, since SSARE projects almost necessarily look at multiple factors, but is necessary to limit this review to a manageable size.
Soil quality is not a completely operationally defined term. This review is based on Doran and Parkins’ (1994) definition: “The capacity of soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal health.” This definition fits the SSARE projects, all of which were or are concerned with protecting and enhancing soil biological functions, managing soil appropriately for water quality protection and soil conservation, and producing high-yielding crops and nutritious forages.
Potential measurements for describing soil quality are legion. Because research resources are always limited, choosing the best measurements for answering a particular research question is critical. Farmers and researchers, though, have different ways of considering soil quality, with farmers tending more toward qualitative descriptors (tilth, smell) and integrative measurements (organic matter content) that directly contribute to yield outcomes, while researchers prefer quantitative and mechanistic measures (bulk density, labile C versus humic C) (Romig, 1995). So far, the SSARE projects have not concentrated on basic soil quality research that leads to purely mechanistic explanations, but have focused more on quantitative measurements that correspond roughly to farmers’ qualitative concerns and the concern for acceptable crop yields.
In the SSARE projects, research on water quality always dealt in some way with farm-origin pollution of surface or groundwater, and was often intimately related to soil management issues. The main concerns were sediment and nutrient movement into surface water due to erosion, fecal pollution of surface water due to runoff of waste, and pollution of groundwater due to nutrient leaching. No projects dealt directly with pesticide movement into water, possibly because limiting and optimizing pesticide use was at least a secondary goal of many projects.
An unstated assumption seems to underlie most if not all the SSARE projects: that alternatives investigated are the best available management practices. No project used unsound practices as controls, even frequently used but unsound practices. For example, tillage to cover up rills or fill in gullies is common, but certainly does not solve the problem. No project even mentioned the possibility of using such practices. SSARE projects were committed to finding the best ways to create farm profits while protecting the natural resources that make those profits possible.
For the most part, project findings were not in disagreement with one another even in projects from quite different ecological situations. However, important results concerning management details differed from situation to situation. One often-repeated statement was that site specific management is necessary and that very few “one size fits all” answers exist in sustainable agriculture. Even when broadly applicable answers do exist – such as, adding organic matter is beneficial to soil quality – implementation and consequences will vary widely from place to place and farm to farm.
Only completed projects or ongoing projects with significant available information are included in this review. The review was performed by first considering the final report of each project, then seeking further papers and publications. In most cases, most information generated by each project was contained in the final reports and in publications listed in the final reports, but in some cases followup papers provided extra information. The easiest way to locate followup papers was simply to contact project directors. If project directors did not respond to repeated attempts to contact them or did not know of further papers, database searches were performed using project participant names and keywords. Often, such searches turned up related but non-SSARE-funded work, meaning that that information was not used in this review. The final reports for the first few projects funded (1988 to about 1990) were minimal at best and for one or two of those projects more information reliably linked to SSARE could not be found. For the Graduate Student projects, the final reports or papers mentioned in the final reports were the only information used, as these were shorter term projects whose results should have been contained primarily in the final reports. Several GS projects were sub-projects within larger R&E projects and are covered with those projects.
Many if not most SSARE projects led to further work in the same area. The decision was made to include only papers and information directly related to SSARE funding, again to limit the scope of the review to a manageable size, but also to reflect SSARE’s specific accomplishments more accurately. So, short-term projects that grew into new projects were not followed beyond the term of SSARE funding. Long-term projects (e.g., Virginia, Texas, North Carolina) that received initial SSARE funding were followed to the extent that it was possible to identify new work as being part of the same project.
Nutrient Cycling Biology, Cover Crops, and Compost
This section describes projects investigating soil enzymes, active organic matter, microbial diversity, macroscopic organisms, nitrogen (N) cycling, composting methods and feedstocks, compost application rates, compost contributions to soil fertility, cover crop management, legume contributions to soil fertility, and managing mulches or living mulches for soil improvement. This is the largest group in the SSARE soil quality projects, with more than 20 projects (including continuations) having significant information available in these areas.
The main result from these projects is unsurprising: frequent organic matter addition stimulates soil biological functions and generally improves physical and chemical soil quality characteristics in agricultural soils. The value of returning manures and crop residues to soil has been known for thousands of years. The devil is in the details of choosing which manures and agricultural by-products to use, at what rate, when, and how. These choices must be made on almost an individual basis for each farm. And cash crop yield responses to organic matter are less certain, as the SSARE projects found.
When fertilizers became widely available in the mid-20th century, fertilizers made managing organic matter less necessary at least in the short term, because the readily plant available nutrients in fertilizers raised crop yields enormously even as soil quality declined. A renewed focus on organic matter management has arisen for various reasons including the need to deal with accumulating organic wastes, the interest in food produced without chemical fertilizers, and the rising cost of energy. The SSARE projects have focused upon quantifying the benefits of organic matter to soil, and some have focused upon effective organic matter management techniques.
Managing organic matter additions to soil is difficult because of the relatively large amount of organic matter needed to make significant differences, along with the need to add organic matter frequently (at least annually). One estimate (Brady and Weil, 1999) suggests that 60 to 80% of a given batch of added organic matter is oxidized to CO2 within one year, an estimate supported by one SSARE project’s conclusion that last year’s organic matter did not confer residual benefits. Handling, processing, transporting, and incorporating such amounts of organic matter remains challenging and expensive. Furthermore, nutrient release from organic matter is much less predictable than from fertilizers. Decomposing organic matter may release nutrients either well before or well after cash crop peak nutrient requirements, meaning that crop yields may be low and valuable nutrients, especially N, may be lost from the system.
Six SSARE projects investigated the use of several local by-products for soil improvement, including cotton gin trash, rice hulls, composts, and yard waste. These materials were tested at rates between 4.5 and 60 Mg ha-1, about 4 and 55 tons acre-1. The C:N ratio of agricultural byproducts was critical to their success in encouraging crop yield; rice hulls, for example, have a very high C:N ratio and needed to be co-applied with N fertilizer to avoid N immobilization. Even incorporating crimson clover with rice hulls did not prevent N immobilization. Cotton gin trash seemed to be an especially important resource where available. Composted or uncomposted, this material improved soil biological activity and could suppress specific pathogens but encouraged the spread of other pathogens. Soils damaged by erosion or land-leveling responded to amendment with agricultural by-products especially well. One issue that individual SSARE projects could not readily address was the fact that each type of by-product had specific requirements for use.
Seventeen projects investigated cover crop or green manure management. These projects clearly demonstrated that appropriate leguminous cover crops adapted to specific environmental conditions can provide adequate N for almost any following crop. In the SSARE projects, cover crops often resulted in cash crop yields equal or greater to those achieved with fertilizers. One project found that even in no-till situations, cover crops provided adequate N to the following crop; tillage was not necessary to stimulate adequate N release in that environment.
However, achieving synchrony between release of nutrients from decomposing cover crops and the requirement for nutrients by growing harvestable crops is difficult. Inadequate N provision to the harvestable crop and/or leaching of N because the harvestable crop was not using N at the time it was available were common in the SSARE projects. No project provided a final resolution to this problem. The main recommendations were to give N credits for legumes in the system and to use a combination of leguminous and non-leguminous cover crops to retain N in the system. One project definitively found, for its environment, that fall cover crops were more beneficial for retaining N than spring cover crops followed by fall crops. Rye was a successful N scavenger in almost any situation, but could disadvantage following crops due to allelopathy or N immobilization.
Two projects found that the pre-sidedress nitrate test did not adequately predict N-mineralization from organic sources, and that predicting N mineralization accurately was necessary for ensuring adequate and non-polluting N supply to following crops. Two projects found that another challenge in using cover crops for fertility was that other growth limiting factors such as lack of moisture could reduce the effectiveness of nutrient cycling from cover crops.
Only a few projects so far have addressed the details of microbial community structure or specific enzymatic functioning. All projects that investigated these issues found that active organic matter addition resulted in heightened enzyme activity and could change the microbial community structure. In at least one project, microbial diversity was heightened by added organic matter, but the functional significance of this diversity was not clear. To elucidate the relationship between structure and function in microbial communities will require a great deal of detailed basic biological research. Such work may well not be of immediate practical use to farmers, although a nationwide survey of organic farmers by the Organic Farming Research Foundation found that soil biology was of fairly high interest to these farmers (OFRF, 1997).
Nutrient cycling, cover crops, and composts remains a complex topic with plenty of room for investigation. Research investigating specific management recommendations for the use of different by-products as soil amendments will remain useful for SSARE’s goals of encouraging linkages among agricultural sectors and between agricultural and urban sectors. Research on nutrient cycling in cover crops will remain useful if the research focuses on new situations such as no-till in organic systems, or creates more accurate and farmer-friendly models of nutrient release from cover crops. Research to find ways that different cover crops can contribute to immediate farm profitability while providing benefits to soil quality will be a highly significant area of investigation. To elucidate the relationships involved in microbial community structure and function will require detailed basic biological research, which may or may not have immediate practical implications. However, answers gained from this kind of work may be broadly applicable.
LS89-15. Enhancement of the stability of southern region agroecosystems through profitable transition to sustainable agriculture (Jones, 1991). Texas, Oklahoma, Arkansas.
This project facilitated various on-farm projects. One project investigated a system for growing cabbage in raised beds with rye or hairy vetch cover crops. Beds with cover crops maintained their height significantly better than beds with bare soil, indicating a reduction in erosion or soil compaction. The interaction of N fertilizer with cover crops was also tested. Bare soil beds produced the highest yields at every N rate, much higher than rye-covered beds. The lower yield of rye-covered beds was attributed to N immobilization by cover crops rather than loss of N.
LS90-20. Effective nitrogen for low-input forage and grain production in a thermicudic region (Stuedemann, J., personal communication, 2005). Georgia.
Crimson clover was evaluated as source of N for grain crops. Total N accumulated by crimson clover was 323 kg N ha-1, while total N uptake by grain sorghum planted into the clover was 454 kg N ha 1. The clover provided 71% of the N required by sorghum. Killing the clover early in the spring conserved soil water by reducing transpiration demand.
LS91-35. Improved nitrogen-use efficiency in cover crop based production system. (Wagger, 1994). North Carolina. (Cross reference: Pollution)
Legume-grass bicultures were evaluated for available N and N-scavenging ability. The C:N ratio of all bicultures was <30, indicating that net N mineralization would occur as residues decomposed. Inorganic N in the soil profile was lower in legume-grass bicultures than legume monocultures, suggesting that the grasses, especially rye, used leguminous N. When moisture availability was low, corn recovered only 7% of N from legume cover crops, meaning that a significant amount of N could be lost. Well-watered corn recovered 21 to 35% of leguminous N. The authors suggested that the most benefit is obtained when grass-legume cover crop mixtures are used and the following crop is not stressed by other growth-limiting factors. LS91-40 and LS94-40.1. Utilization of winter legume cover crops for pest and fertility management in cotton (Rothrock, 1995). Arkansas
This project investigated the interaction of cover crops and tillage in cotton production. Averaged over tillage systems, yield was greater when irrigated cotton followed clover, vetch, or native vegetation rather than wheat or rye. Under reduced tillage and with supplemental fertilization, cotton yield was similar following clover, vetch, or native vegetation; this suggested that leguminous N did not benefit cotton if fertilizer N was available. However, soil NO3-N accumulation under legume cover crops indicated the presence of excess N. Over-fertilization should be limited by giving appropriate legume N credits.
LS91-43 Cover crops for clean water: a national conference on the role of cover crops in improving water quality (Hargrove, 1991a). Multistate. (Cross reference: Pollution)
This conference proceedings provides information on the roles and management of cover crops and crop residues. Main biology topics include N cycling, weed and disease management, pest control, integrated production systems, and integrated crop-livestock systems. Specific information on various cover crop species and different environmental conditions is included. This resource could be helpful to farmers as well as researchers.
LS92-45. Organic nitrogen sources for sweet potatoes: Production potential and economic feasibility. (Collins, 1995). North Carolina.
This project investigated crimson clover as an N source for sweet potatoes and for rotational sweet potatoes and sweet corn. Crimson clover and 100 kg N ha-1 fertilizer produced equal sweet potato yields; yields did not increase as N increased further. Nitrogen use efficiency decreased as N increased to 200 kg N ha-1. Crimson clover also provided adequate N for corn, but corn yield suffered regardless of N source when corn followed sweet potatoes. Giving proper legume N credit when determining fertilizer requirements is essential to maximize sweet potato yields and N use efficiency.
LS92-49. Organic soil amendments of agricultural by-products for vegetable production systems in the Mississippi Delta region (Teague, 1995). Mississippi (Cross reference: Manure).
Various agricultural by-products were evaluated for use as soil amendments for horticultural crops. Unprocessed cotton gin trash resulted in severe weed problems. When cattle fed on piled trash over winter and the material sat undisturbed over summer, weeds were reduced. Gin trash produced no vegetable yield response at rates below 9.6 Mg ha-1 on undamaged soil. Composted gin trash applied at 60 Mg ha-1 to damaged soil improved cabbage yields beyond fertilizer alone. Straw and aquaculture waste compost at 4.5 Mg ha-1 increased collard yields but decreased southern pea yields. Up to 9 Mg ha-1 uncomposted rice hulls could be used with N-fertilizer as a co-amendment; legumes were not effective in reducing N immobilization by rice hulls. Agricultural wastes can be used as soil fertility amendments, especially when processed to reduce weed problems or high C:N ratios, and are helpful to remediate damaged soils.
AS93-07. Evaluation of recycled paper mulch as an alternative to black plastic mulch in vegetable horticulture (Schonbeck, 1995; Schonbeck and Evanylo, 1998). Virginia.
This project tested several mulches for tomato production. Hay, plastic, and paper mulches increased total tomato yields compared to unmulched soil. Hay mulch (100 mm deep) and leaf compost mulch (50 mm deep) provided enough organic N and C to replenish organic matter lost to decomposition during the growing season. Much of the N applied was not available in the first growing season owing to the high C:N ratio of these materials. Mulch treatments did not affect soil bulk density or water infiltration over one season, but earthworm populations were twice as high under hay mulch than black plastic, and intermediate under paper. Soil temperatures and moisture closest to optimum for tomatoes were produced by laying oiled paper just prior to planting to warm soil, and covering the paper with 10 cm hay about 7 weeks after planting to conserve moisture. This combination also provided effective weed control.
LS93-52. Dairy manure in low-input, conservation tillage animal feed production systems (Mullen, 1997). Tennessee. (Cross reference: Manure and Pollution).
This project investigated the potential to fertilize no-tilled corn silage with dairy manure to close farm nutrient cycles. Several categories of bacteria and fungi were more abundant from 0 to 5 cm soil depth than from 5 – 20 cm. Soil enzyme activity was highest from 0-5 cm, corresponding to higher organic C and N. Enzyme activities were higher in soils receiving manure than ammonium nitrate. After 3 yr of manure applications, manured soils and fertilized soils had nearly equal silage yields but manured soils had much lower pre-sidedress nitrate levels, indicating that the PSNT may not adequately predict N-mineralization.
LS93-53. Sustainable whole farm grain/silage production systems for the Southeast (Reeves, 1998). Alabama.
This project developed alternative grain and silage crops, especially lupin and tropical corn, for use in crop-animal systems. Wheat yields were highest following soybean, but could be matched following millet or corn fertilized with 134 to 202 kg N ha 1. When used as a green manure crop, lupin provided sufficient N for 46 Mg ha-1 silage corn yield, which was greater than silage yield following clover, winter fallow, or rye. Soybean and lupin reduced one another’s yield in rotation. Lupin was insensitive to acid soil pH but responded to P fertilization. No-till with in-row subsoiling encouraged lupin yield over conventional tillage.
LS93-55. Cover crop integration into conservation production systems (Dabney, 1994). Mississippi, Alabama, Tennessee, Arkansas.
This project evaluated several legume and legume-grass cover crops. Early flowering was negatively correlated with winter hardiness. Reseeding was more successful when cover crops were killed at least 36 days after full bloom. Biomass production varied from 5000 to 10,000 kg ha-1, with acid soils or low P reducing biomass. No-till cotton could be planted into cover crop residue 1 to 6 days following mowing and row cleaning. Cover crop mulches stabilized soil temperatures and increased early cotton growth. Rye suppressed weeds while hairy vetch encouraged grass weeds. Cover crops increased cotton yield enough to pay for planting the cover crop, in addition to benefits in soil quality.
LS95-69. Managing soil phosphorus accumulation from poultry litter application through vegetable/legume rotations (Earhart, 1998). Texas, Oklahoma. (Cross reference: Manure and Pollution).
This project investigated the use of poultry litter for soil fertility, while using cover crops to limit NO3- leaching or P accumulation. Over 4 seasons, the lowest residual P in the 0-15 cm soil depth occurred under a summer vegetable-fall cover crop rotation, followed by spring vegetable-fall fallow and spring cover crop-fall vegetable rotation. High biomass fall cover crops had the greatest potential to remove P from the system.
LS95-71. Developing municipal/farm linkages for on-farm composting and utilization of yard wastes: A regional resource issue (Evanylo, 1997). Virginia.
Cooperation between a city and several local farms led to the composting of 2600 cubic yards of yard waste. Approximately 6 parts leaves to 1 part locally available poultry litter were blended. A windrow turner produced a more desirable product, more quickly, than other methods of turning, but required a large flat area. Sweet corn yield was higher with commercial fertilizer due to higher plant available N, but soil in compost plots had higher total N, Ca, Mg, Mn, and B. Organic matter, total C, and CEC tended to be higher in plots receiving compost. Greenhouse studies showed that bedding plants could be grown in the composted yardwaste.
LS95-72. Agronomic and economic benefits of intercropping bean with banana (Li, 1998). Puerto Rico. (Cross reference: Soil and Water Conservation)
Banana is an important tropical cash crop with a growing period of 14 to 18 months. No-till greenshelled beans were intercropped with no-tilled banana to provide intermittent income. Beans did not affect banana yield and contributed only minimally to soil N or other soil nutrients during the bean growing season. Also, erosion was much greater with beans due to soil disruption even though beans were no-tilled. However, two successive intercroppings of bean increased farm income significantly.
LS96-75. Crop management systems for improving production of culinary herbs in the Virgin Islands (Palada, 2000). Virgin Islands.
Fresh herbs and vegetables are high value crops in the Virgin Islands, where tourism and gourmet foods are important. Combinations of organic and plastic mulches, organic fertilizers, green manures, and animal manures were used for sweet basil, chive, cilantro, parsley, and thyme on small farms. All green manures were acceptable for herb production. Sunnhemp and hyacinth bean tended to result in higher herb yields than cowpea, likely due to greater N supply. Herb response to animal manures depended upon residual fertility from previous management. Chicken manure at a rate of 900 kg dried manure ha-1 increased thyme yield in one farmer’s field. Hay was an acceptable mulch compared to other organic mulches or black plastic.
Microirrigation techniques and mulches were tested for tomato, cucumber, and bell pepper. Cucumber cultivars responded differently to different kinds of mulch and levels of irrigation; unfortunately the more flexible and productive cultivar was not locally preferred. In pepper, either organic or black plastic mulch increased irrigation efficiency and yield. In tomato, hay mulch with increasing irrigation increased fruit size but not total yield or number of fruits. In high rainfall seasons, the benefits of mulch and irrigation were less pronounced for all crops.
LS96-77. Sustainable cropping system for seedless watermelon and fall lettuce in rotation with green manures (Reddy, 1999). North Carolina, Virginia.
Various legume and legume-grass winter cover crop treatments prior to watermelon and lettuce crops were tested. Results were variable from site to site. Averaged across sites, watermelon yields were lowest following hairy vetch + rye. Watermelon following hairy vetch alone or austrian winter pea + rye produced yields slightly higher than melon with the recommended fertilizer. Fruit quality was not affected by cover crops. Lettuce could not be evaluated due to late melon harvests. Decomposing cover crops provided adequate N for watermelons but not in synchrony with melon requirements, resulting in NO3-N accumulation and leaching.
LS99-099. Economic and environmental effects of compost use for sustainable vegetable production. (Evanylo, 2002). Virginia.
This project sought to quantify longer term benefits that compost provides to soil quality and crop production. High and low compost rates with and without supplemental fertility were tested. Treatments did not affect soil microbiological properties. A high compost rate applied annually lowered soil bulk density and increased porosity, while a high compost rate applied biennially was sufficient to increase water infiltration. Rainfall runoff was lowest for the annual high compost rate, attributed to improvement in soil physical properties. The pre-sidedress nitrate test indicated that mineralization of NO3-N from compost was not as rapid as expected, while NO3-N leaching indicated that accurate estimation of plant-available N was more important than N-source in predicting pollution potential. Soil organic C, total N, and available P were highest in plots that received the annual high compost rate. Yield responses demonstrated that there was little benefit to low annual compost rates and that a one-time large compost application produced few residual benefits. This research showed that large amounts of compost are necessary to provide soil fertility and improve soil quality characteristics.
LS99-102. Demonstration of a sustainable integrated production system for native pecan and beef cattle producers and its effect on ecology in flood prone areas (McCraw, 2003). Oklahoma.
This project developed an integrated pecan-beef system using annual and perennial legumes as the main N source for the trees and for grazing. Pecan yields
The conclusion presented here represents the considered opinions of the primary author of this review. This conclusion has not been peer reviewed.
The SSARE soil and water projects have contributed an extensive array of practical knowledge for farmers in the Southern region. Projects seem to be generally one of two kinds: those that create new systems and those that nudge existing systems toward sustainability.
Some broadly applicable principles of sustainability have been identified, such as the principle that keeping soil covered with vegetation or crop residue and reducing disturbance by tillage will reduce soil erosion. This is a long-known and broadly applicable principle of sustainability that several SSARE projects showed is also beneficial for crop yields and profitability. But in the arid Texas High Plains, the current local agricultural environment will not accommodate this principle. Monoculture cotton is a deeply entrenched cash crop. In this system, water availability for cotton must take precedence over almost any other consideration – growing cover crops and keeping residue on the soil surface both reduce the available water for cotton. The soil is left bare, vulnerable to erosion and loss of organic matter. An extensive, long-term SSARE project is literally creating and perfecting a new system that can accommodate the principle of sustainability, to replace the monoculture cotton system.
Unfortunately very few projects can take on such enormous challenges, in part because rarely are the challenges so clear. Furthermore, most SSARE projects are limited by available resources. Instead of creating whole new systems, they encourage incremental steps toward sustainability by optimizing existing systems. Research that finds appropriate rates of manure application, tests agricultural by-products for soil amendments, puts together novel crop combinations, or evaluates the usefulness of plastic or paper mulch is of this type. Extremely site specific and material specific work is absolutely necessary to help farms and localities in various ecological situations to approach sustainability. And the work is unending. Researchers could try new materials and new crop combinations in new environments almost forever.
Areas needing additional study
The circle of farmers and non-researchers in involved in research and communication needs to grow. The SSARE farmer research projects that already exist serve a vital purpose, but they require a time-intensive relationship between farmers and researchers that only a very few people can engage in. A dialogue among researchers in various areas to come up with a protocol for dealing with anecdotal work might empower farmers and non-researchers to participate more easily in the expansion of knowledge, while not promulgating errors or myths.
SSARE’s challenge is to help engage more people in “tinkering” to nudge existing systems toward sustainability, while using the power of large systems projects to create new systems that are more inherently sustainable than old systems.
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