The Transition from Conventional to Low-Input or Organic Farming Systems: Soil Biology, Soil Chemistry, Soil Physics, Energy Utilization, Economics, and Risk

The Transition from Conventional to Low-Input or Organic Farming Systems: Soil Biology, Soil Chemistry, Soil Physics, Energy Utilization, Economics, and Risk

Summary

A 12-year comparison of organic, low-input, and conventional farming systems showed that yields were similar among all systems, with differences between systems less than those between years. The organic system with premium prices was the most profitable. Soil organic carbon was doubled in 10 years in the organic system. Runoff from cover-cropped systems was 1/3 that from conventional systems. The conventional farming systems were least efficient at storing excess N. Arthropods, pathogens, and nematodes had little influence on crop yields. Weeds resulted in small but detectable yield losses, and higher production costs in some years in the organic systems.

Objectives/Performance Targets

1. 1. Over a twelve-year period encompassing three, four-year rotation cycles, compare four farming systems with different levels of reliance on non-renewable resources with regard to:
   a. Crop growth, yield, and quality as influenced by different pest management, agronomic and rotational schemes of the four farming systems.
   b. Abundance and diversity of weed, pathogen, arthropod, and nematode populations and their impact on crop growth, yield, and quality.
   c. Changes in soil biology, physics, chemistry, and water relations and their impact on soil quality and productivity.
   d. Cost of production inputs, value of production, economic risk, energy budgets for agricultural production under the four farming systems.
   2. Compare and evaluate novel low-input and organic farming tactics, with emphasis on innovations that correct deficiencies, enhance profitability or decrease risk in each farming system.
   3. Distribute and facilitate adoption of information generated by this project to all interested parties as it becomes available.

Accomplishments/Milestones

Work Accomplished The Sustainable Agriculture Farming Systems (SAFS) project was established in 1988 by a multidisciplinary group of researchers, farmers, and farm advisers, to study the transition from conventional to low-input and organic systems. The SAFS project has compared four farming systems: organic, low-input, and conventional 4-yr rotations, and a conventional 2-yr rotation (Appendix A, Figure 1). Cash crops in the 4-yr rotations included processing tomatoes, safflower, dry beans, wheat, corn. The 2-yr rotation is tomatoes and wheat. Conventional management was based on current farming practices in the region. In the low-input and organic systems, use of synthetic fertilizers and pesticides was reduced or eliminated primarily through cover cropping, addition of organic amendments, mechanical cultivation and residue management, and modifications of irrigation and planting schedules. Among the most important recent findings from the SAFS project are the following:

· Yields of all crops in all systems continue to hover at or slightly above Yolo county averages. The organic system with premium prices is the most profitable of all the systems; however, without premium prices the organic system loses money.
· Soil organic carbon was doubled in 10 years in the organic system, increasing by 10 t/ha. These results were obtained despite additional tillage operations in the organic plots due to cover crop considerations.
· Due to changes in soil physical properties resulting from cover cropping and organic matter increases, infiltration rates are over 50 percent higher in the organic and low-input systems. In the winter of 1999-2000, less than 15 percent of rainfall in these systems was lost as runoff, compared to 43 percent in the conventional systems. Soil water storage is significantly greater in the organic and low-input systems.
· The differences in soil moisture content have differing impacts on yields and crop quality in different crops and different systems. For example, corn yield losses have resulted from water penetration problems in the conventional systems. However, tomato fruit quality was found to be inversely related to soil moisture content just prior to harvest, and was highest in the conventional systems and lower in the organic and low-input systems. Future research will focus on optimizing irrigation management in these systems.
· The conventional farming systems have been least efficient at storing excess N while the organic farming system has been the most efficient. Thus, N losses have been greatest from the conventional, intermediate from the low-input, and least from the organic farming system. However, nitrogen availability has been an occasional problem in the organic corn and tomato systems. A study examining possible solutions to this problem indicated that summer cover crops and fall irrigations promote bacteria-feeding nematode populations and N mineralization which leads to higher tomato yields.
· Arthropods, pathogens, and nematodes have had little influence on crop yields. Weeds have resulted in small but detectable yield loss, and higher production costs in some years, particularly in the organic system.

The first twelve years of the project were completed with the growing season of 2000. Data from 2000 are still in the process of being analyzed. Results of the final analyses will be published in newsletters, peer-reviewed publications, and in a final integrated report. Now the project is in transition to a study of the potential for conservation tillage in organic, low-input, and conventional rotations, and will be renamed the Irrigated Agriculture Conservation Tillage (IACT) Project. Some integrated results from the 12-year SAFS project are already available and are reported below by objective.

Objective 1. Over a twelve-year period encompassing three, four-year rotation cycles, compare four farming systems with different levels of reliance on non-renewable resources with regard to:
a. Crop growth, yield, and quality as influenced by different pest management, agronomic and rotational schemes of the four farming systems.

Yields. Comparable crop yields are obtained from the organic, low-input, and conventional farming systems (Appendix A, Table 1). Tomato and corn yields in the organic system are generally lower than those of the low-input and conventional systems, mainly due to N limitation; however, differences between years are generally much greater than are yield differences between systems (see Appendix A, Table 2).

Quality Fruit soluble solids for the organic (4.45 brix) and low-input (4.72 brix) tomatoes were found to be below the average fruit soluble solids for the state (5.31 brix), while tomatoes from the conventional system had higher soluble solids (5.85 brix) than the average value for the state. The soluble solid concentrations of the tomato crop in each system was inversely proportional to the amount of irrigation water applied to that farming system. The higher infiltration rate and corresponding amount of water that percolated through the soil profile in the organic and low-input systems appear to have a negative impact on fruit quality in the alternative systems (Colla, 2000). Future research will involve fine-tuning irrigation management to solve this problem.

b. Abundance and diversity of weed, pathogen, arthropod, and nematode populations and their impact on crop growth, yield, and quality.
Weeds. Weeds are most abundant and most problematic in the organic farming system and least abundant in the conventional systems where chemical intervention is used. The general trends are for the weed community to shift from annual to predominantly perennial species under conventional management and from broadleaved weeds to grasses under low-input and organic management (Poudel et al., 2000).
Microbial communities. The farming systems influence the composition of soil microbial communities. Microbial biomass was usually higher (up to 2x) in the organic and low-input systems than in the conventional systems; microbial activity, when measured, showed the same pattern. Microbial communities in low-input plots were intermediate in composition between the conventional and organic communities (Ferris and Scow, 1999).
Intensive, year-long sampling (1998) of microbial communities (by phospholipid fatty acid analysis–PLFA) in soils under different crops (tomatoes, corn, wheat/beans, safflower) and management systems (organic, low input, conventional 2 and 4 yr rotation) was performed at the Sustainable Agricultural Farming Systems Project (SAFS). Three years later (2001), a single sampling of the same plots planted uniformly with conventionally managed wheat was completed. Strong differences with crop and management observed in 1998 disappeared one year after all soils were planted with the same crop and managed in the same way. The strongest evidence of the previous treatments was the fact that microbial biomass was higher in organic and low input than in conventional soils. Correspondence analysis showed small differences in the 2001 samples between organic and low input treatments versus conventional and high input treatments, but the magnitude of this difference was small compared to differences between the 1998 and 2001 samples.

Pathogens. Symptoms of corky root disease in tomato were significantly more severe in plots with a two-year rotation than in plots with a four-year rotation. Soft root tips caused by Pythium or Phytophthora sp. and red root rot caused by Fusarium sp. were generally also more severe in the two-year rotation than in the four-year rotations. Apparently the short, intense rotation reverted to host crops too frequently to allow for natural regulation of root diseases. In the other systems, levels of plant-parasitic nematodes and fungal root pathogens were being regulated by the length and diversity of the four-year rotation (van Bruggen and Semenov, 1999).
Nematodes. Using nematode faunal analysis, we have found that by creating conducive conditions for microbial activity in the early fall (adjusting soil moisture and enhancing carbon levels) we could enhance the abundance of grazers on the microbial biomass the following spring. That, in turn, increased mineralization rates in the spring to meet nutritional requirements during the vegetative growth period of the summer crop (Shouse and Ferris, 1999; Ferris et al, 2000).

c. Changes in soil biology, physics, chemistry, and water relations and their impact on soil quality and productivity.

Soil Carbon and Organic Matter. After 8 years of differential management, levels of soil organic matter in the organic and the low-input farming systems were 20 and 10% greater, respectively, than in the conv-2 system. The changes in organic matter content are consistent with rates of organic inputs into each system. Carbon levels in the organic system were doubled, with 10 t/ha being sequestered over an eight-year period (Appendix A, Table 3) (Devevre and Horwath, 1999; Horwath et. al, 2000).

Soil Fertility. The challenge in determining nutrient availability in alternative cropping systems designed to accumulate SOM is 1) assessing the temporal availability of nutrients, 2) interaction of input N with organic N pools, and 3) relationship of SOM turnover dynamics to nutrient availability. The SAFS’s agronomic management systems were designed to assess changes in N availability during the transition from conventional to low-input/organic management. We specifically examined seasonal patterns and mechanisms of N uptake of each input in each system. The following hypotheses were examined:

1. The addition of fertilizer-N or manure-N will increase the N mineralization of a winter vetch cover crop to the succeeding summer crop.

2. The C and N from the winter vetch residue will become more stabilized as compared to C and N from the summer crop through the formation of stabilized microbial and degradation products during long-term decomposition.

A field experiment with 15N labeled fertilizer and 15N labeled vetch residue was conducted to determine the temporal pattern of N release from both sources in conventional and alternative cropping systems. The experiment was conducted within conventional (fertilizer), low-input (fertilizer and organic N), and organic (organic N only) cropping systems established 9 y previously. Availability of 15N from the labeled inputs (fertilizer and vetch) was determined based on uptake by maize (Zea mays L.).We showed distinct temporal patterns of N availability from these different sources of N, vetch, manure and fertilizer. Uptake of total N and 15N by maize in each cropping system was monitored at 10 d intervals from 50 to 90 d after seeding for determination of uptake rates. Uptake of 15N from fertilizer in the conventional system was greater than uptake of 15N from vetch in the low-input and organic systems. Uptake of 15N from vetch was delayed, but with a sustained availability later in the season. Uptake rates of 15N from fertilizer peaked at 4.3 kg N ha-1 d-1 between d 70-80 while those from vetch residue reached a maximum of 0.6 kg N ha-1 d-1 during the same time period. Grain and N yield did not differ between cropping systems at harvest despite different temporal and quantitative availability of N from organic and inorganic N inputs, supporting N synchronization for optimum yields can be achieved with decreased levels of inorganic fertilizer in conjunction with legume residues.
In support of our first Hypothesis 1, we observed that when the fertilizer (inorganic N or manure) and vetch were added together there is a strong interaction changing the availability of vetch N to the summer corn crop. Under the conditions of this study, two inputs complementary to a vetch cover crop (mineral fertilizer-N or aged manure) were found to increase net recovery of vetch-derived N in corn. When fertilizer-urea was added with the vetch residue the vetch supplied an additional 25 kg N to the corn compared to vetch alone. This increase was observed when supplying half the traditional rate of fertilizer N. Similarly, manure increased the uptake of vetch N to corn upto 15 kg N. These observations showed that the pathways influencing N availability were influenced by both C and form of fertilizer N input.
Work was simultaneously carried out to describe the N-supplying and C-mineralizing properties of these soils as a result of years of different management to address Hypothesis 2. Chemical fractionation of soils revealed the dynamic nature of SOM and the large potential effect each input can have on C cycling and nutrient release. We showed that vetch C was preferentially stabilized into soil organic fractions compared to corn C by taking advantage of the C4 signature of corn. While efforts in promoting sustainable agriculture are often focused towards accumulating as much SOM as possible, our results indicate that more is not necessarily better. Under low-input management, where N inputs are less than in conventional or organic systems, SOM content is not as high as in the conventional or organic systems, but exhibits a strong ability to provide plant-available N while conserving C.
A number of publications have resulted from this research including Horwath et al., 2002; Kramer et al., 2001a; Kramer et al., 2001b, Devevre and Horwath 2001, Poudel et al., 2001a, Poudel et al., 2001b; Clark et al., 1998; Clark et al., 1999; Hadas et al. 2001. Increased understanding of input interactions and SOM maintenance is required to develop optimum agricultural practices that most efficiently utilize available resources and efficiently store C. The storage of soil C and N and management to manipulate the timing of soil N release are critical research areas that will need to be addressed to design productive and environmentally sound cropping systems. Future research will examine the importance of the form of N in stabilizing soil C. By examining whether residue-N or fertilizer N is more important in stabilizing C in SOM , we will address the mechanisms controlling the sequestration of C and N in SOM. This understanding will become important to develop management practices designed to accumulate SOM and sustain N availability in future cropping systems.
Soil physics and water relations. Due to changes in soil structure, water infiltration rates are 50 percent greater in the low-input and organic systems. In the winter of 1999-2000, the proportion of winter rainfall lost as runoff was less than 15 percent in the organic and low-input systems, compared to 43 percent in the conventional systems (Appendix A, Figures 2 and 3). Volumetric water content is correspondingly higher in the organic and low-input systems (Joyce et al., 2001).

d. Cost of production inputs, value of production, economic risk, energy budgets for agricultural production under the four farming systems.
Economic viability. While the organic system with premium prices has performed better than the conv-4 or low-input systems, until 2000 the cumulative net return for the conv-2 system was always higher than that for any other system due to the greater frequency of high-value tomato crops in this rotation. This changed in 2000. (Appendix A, Figure 4). For the 2000 growing season, the organic system with premium prices had the highest net returns ($329 per acre), followed by conventional 2 ($289 per acre). Performance for the other systems was poor. Conventional 4 and low input net returns were only $43 and $42 per acre, respectively. The organic system with conventional prices lost $120 per acre. The tomato crops all performed better than in 1999 but there were net losses for all systems in corn and safflower. The conventional bean crop was destroyed resulting in a loss of $254 per acre. In contrast the organic bean crop and low input crop showed profits. The economic and environmental viability of the conventional system are threatened by the greater prevalence of pests and diseases, and the greater reliance on pesticides.
Pesticide inputs. Cumulative pesticide usage over the course of the project has been greatest in the conv-2 system, followed by the conv-4 system, the low-input system and finally the organic system (Appendix A, Table 4). Total usage is related to the philosophy and protocols of the farming systems, and the crop rotation. The major uses of pesticides are in the management of weeds in the conventional and low-input systems.

Objective 2. Compare and evaluate existing and/or novel low-input and organic farming tactics, with emphasis on innovations that correct deficiencies, enhance profitability or decrease risk in each farming system.

Niche-specific cover crops. 1994-95 studies had shown that summer cover crops, planted following the harvest of early field tomatos, would recycle free soil N and produce large amounts of biomass, if the cover crop could be established no later than Sept 1st. A large 1997 trial demonstrated the great potential of mixtures containing sorghum-sudan, the excellent compatibility of lablab with sorhum-sudan, and the potential to choose cowpea genotypes less sensitive to photoperiodic effects, to Lygus attack, and to root knot nematodes. Based on 1997 data, 10 cowpea genotypes were tested in a 1998 study which specifically examined the compatibility of cowpea genotypes with the sorghum-sudan and lablab components of the planned CC mixture. Cowpea candidates were further narrowed to the 3 genotypes planted in 1999 and 2000 tests, and for which data for component biomass and N are still being analyzed.
Fall management practices to enhance activity of bacterial- and fungal-feeding nematodes. We have conducted several studies on fall management practices to enhance activity of bacterial- and fungal-feeding nematodes in cover crop decomposition and soil fertility. Manipulations have included an irrigated late-summer cover crop, fall irrigation alone, and/or a winter cover crop. Nematode and microbial communities were measured, as well as soil nitrogen and performance of the following tomato crop. Generally, the ratio of bacterial:fungal-feeding nematodes was greater when the soil was irrigated in the fall. Dry soil in the fall selected for fungal-feeding nematodes and the so-called general opportunists, perhaps reflecting the prevalence of fungal-mediated decomposition under those conditions and the lower ability of enrichment opportunists to exploit the conditions. Fall irrigation only and fall irrigation plus a late summer cover crop provided significantly greater available N in the following spring and yield enhancement of the subsequent tomato crop. We conclude that “feeding and activating” the soil foodweb during the early fall when soil temperatures are conducive to biological activity, increases the bacterial-grazing community the following spring. A consequence for the farming system is measurably greater amounts of soil mineral N during the early growth of the subsequent tomato crop and increased tomato yields. (Shouse and Ferris, 1999; Ferris et al., 2000).
Reduced-tillage tomato production. A series of recently organized farmer/scientist focus sessions by UC-SAREP indicated strong growers’ interest in testing reduced/no-till practices in the field row-crops production system in Sacramento Valley (Mitchell et al., 2000). General concerns that farmers brought up in relation to reduced tillage were: (1) how to deal with fertilizer/herbicide inputs? (2) furrow irrigation which is very different from Midwest reduced-tillage practices, (3) crop rotation and cover crops are difficult to find to combat weeds, and (4) economics of reduced tillage. Farmers are considering evaluating several types of no-till equipment, including Buffalo planter, Sub-surface tiller transplanter, Ferguson strip till machine, and 5-ft cover crop seeder. Reduced tillage appears to be an opportunity to increase the sustainability of production systems in the Sacramento Valley. Current SAFS research in the companion area is evaluating the feasibility of reduced- and no-tillage methods of tomato production. SAFS researchers are now developing a reduced-tillage tomato system which uses nonchemical or reduced-chemical cover crop management, transplanting, and cultivation under high-residue conditions.

Objective 3. Distribute and facilitate adoption of information generated by this project to all interested parties as it becomes available.

In addition to over 85 peer-reviewed journal articles, 7 popular press articles (see Appendix B for complete list), and a home page (http://agronomy.ucdavis.edu/safs/home/htm), results from SAFS have been presented in more than 80 national and international conferences. SAFS has hosted more than 1500 visitors from over 30 countries at its eleven annual field days, six workshops, tours, and individual group visits. The SAFS plots serve as a living laboratory for field trips, and provide lab samples, for numerous UC Davis classes and Cooperative Extension courses in soils, agronomy and the pest sciences.

Work still to be accomplished
1. Complete analysis of 2000 data. Data on the changes in the full suite of soil physical, chemical, and biological properties over the third rotation period still await analysis.

2. Impact assessment survey. A survey of the impacts of the 12-year SAFS project will be carried out in the winter of 2001-2002.

3. Final newsletters and publications. Newsletters and publications reporting the results of the above data analyses as well as an integration of the full 12 years of the project will be published in 2002.

Impacts and Contributions/Outcomes

In a survey of USDA-funded research pertaining to organic agriculture, the Organic Farming Research Foundation lauded three SAFS-associated projects as the “state-of-the-art of university-based organic farming systems research” throughout the entire U.S. The SAFS project has positively impacted farming practices and agricultural communities in the Sacramento Valley, the state, the nation, and many countries around the world. These changes include: a greater interest in cover crops, legumes and crop rotations; increased organic acreage of field crops; increased monitoring by growers of water use/efficiency, pest thresholds and soil and crop nitrogen requirements; a growing recognition of the importance of soil ecology; and heightened interest in a more holistic view of soil quality.
The SAFS project has specifically demonstrated that it is profitable to grow tomatoes organically; that yields can be maintained in organic and low-input systems; that pesticide use can be reduced by half with little or no decrease in yields; and that cover-cropping can provide multiple important benefits, including sequestration of carbon, decreased nitrogen leaching, increased infiltration and decreased runoff, and improved soil quality. We are currently attempting to place dollar values on some of these benefits.

Collaborators: