Transition to Organic Vegetable Production by Large-Scale Conventional Farmers

2003 Annual Report for SW01-057

Project Type: Research and Education
Funds awarded in 2001: $123,399.00
Projected End Date: 12/31/2004
Matching Non-Federal Funds: $128,246.00
Region: Western
State: California
Principal Investigator:

Transition to Organic Vegetable Production by Large-Scale Conventional Farmers


Organic transition is clearly feasible in the Salinas Valley of California. A three-year monitoring of 81 points on two ranches has ended, and preliminary analysis shows that nutrient deficiencies, weeds, disease symptoms, and insect damage did not increase through time. A compost quality trial indicated higher radicchio biomass and lower N with yard waste- vs. manure-based compost; differences may be related to a complex set of soil changes after three years. The role of arbuscular mycorrhizae for nutrient uptake by vegetables following brassica cover crops will be pursued. Better understanding of effects of management practices will be emphasized in 2004.

Objectives/Performance Targets

Organic production is currently increasing in the Salinas Valley of California, which is the largest producer of cool-season vegetables (e.g., lettuces, cole crops, celery) in the nation. There is widespread interest among large-scale conventional farmers to convert some of their land to organic production. There is potential for organic production practices to be integrated into conventional production as well. Our project is designed to describe the changes and solve problems during the transition from highly intensive conventional vegetable production to organic production. Our farmer-cooperator is Tanimura and Antle, Inc. (T&A), a large vegetable production company that made the decision to allocate some of their acreage for organic production. Our objectives are to: 1) Monitor changes in crop species and yield, soil organic matter and soil microbiology, diseases, insects, and weeds during the three-year organic transition. 2) Design experiments to target specific management and pest problems as they arise. 3) Track changes in agronomic management, economic issues and decision-making. And, 4) disseminate findings via field days, public meetings, workshops, and publications.

A three-year monitoring study was proposed to examine changes during the transition to organic production on two ranches under conventional management by tracking many response variables on 81 permanent points. We have now completed the field sampling and most of the lab analyses for this project. The transition went smoothly and resulted in changes in crop species, seasonal cropping patterns, and management inputs, but did not result in major changes in pests or other problems (see below). We also proposed to troubleshoot specific problems with specific experiments. Soil fertility and organic matter management has been our focus for this aspect of the proposal. We set up an on-farm compost quality trial where we have been comparing yard waste-based vs. manure-based composts since 2000. We are now starting another project on the effects of brassica cover crops on soil microbial communities, with an emphasis on arbuscular mycorrhizae, as many growers have recently included these cover crops in their rotations.

The project so far has generated a large volume of time-course data on the three-year organic transition, and we are in the midst of the data analysis phase. During the coming year, we will be finalizing these analyses with various statistical approaches and preparing manuscripts for publication, and increasing the visibility of the project to the public.


Organic transition time course

Our researcher-grower collaboration has resulted in a large data set describing the transition to organic production at the whole farm scale. Organic transition is clearly possible and relatively non-problematic in the Salinas Valley, even though it demanded many changes in management compared to conventional production (see below). To our knowledge, this is the first time that researchers have monitored entire ranches to examine how production, pest, soil quality, and management inputs change during transition to alternative cropping systems.

The three-year monitoring study was designed in the following way. In June, 2000, when certified organic practices were instated on the T&A ranches, we set up permanent sampling points for frequent monitoring during the 3-year transition. On each of the 9 lots, we set up three transects. Each transect is a 2-bed strip across the entire field. There are 3 equidistant permanent sampling points (±5 m) along each transect. Thus, 81 sampling points exist; 56 on the Storm Ranch and 27 on the Daugherty Ranch, which are approx. 3 km apart. A set of measurements was taken in June, 2000. We will statistically test the assumption that the 9 lots initially have similar means and variance for these characteristics. Since then, these permanent points were re-sampled approx. 5 times for soil (e.g., potentially mineralizable N, soil microbial biomass (MBC), nitrate, and ammonium) and plant characteristics (e.g., biomass, nutrient content, and mycorrhizal colonization), diseases and insect pests, and weeds. Every 7-10 days from late spring through late fall, and at least every three days during the rest of the year, transects were sampled when they were at 0-7 days before harvest by the grower. They were sampled again in February, 2003, before vegetable crops were planted for the 2003 season. The grower has provided us with all information on management practices, irrigation, fertilizer, compost, and other inputs used for each of the transect locations for each sampled crop.

Since June, 2000, 171 transects have been sampled (516 individual plots) on 48 sampling days. (Over 80 of these transects were done during the SARE grant period.) A total of 25 different crop species have been sampled. Here we summarize preliminary trends, but more comprehensive analysis using various statistical approaches will be conducted during the coming year.

Seasonal cropping patterns changed markedly during transition. Initially, more crop species were planted, and plantings continued through the summer (Figure 1). As the transition progressed, there were fewer crop species, and fewer crops during the summer, due to the growers’ concern for leaf miner damage in late summer crops. Nutrients, i.e, N, P, and K, in lettuce, as an example, were in excess of sufficient supply in all three years. The two ranches showed generally similar trends. Soil analyses showed surprisingly low concentrations of nitrate and ammonium throughout the three-year transition (Table 1). Nitrate was low at the lower depth (15-30 cm) suggesting that leaching of nitrate out of the surface soil was not substantial. Potentially mineralizable N was variable, and microbial biomass analyses are not complete, so further data analysis is needed to determine how soil microbial processes have changed throughout the transition.

Pest damage and weeds did not increase during the transition period (Figure 2). Insect damage was mainly limited to minor damage due to leaf miner stings and leaf lesions from chewing insects. Leaf miner damage was greatest during late summer and fall of 2002, and caused the grower to reduce plantings during this season in the following year. Only a small percentage of samples experienced root or leaf disease symptoms (<20%). Weed counts decreased with time, due to careful use of hand labor. Hoeing costs are forthcoming and will serve as another measure of weediness. Arbuscular mycorrhizal colonization increased slightly with time.

Management inputs are still being obtained from the grower. Data obtained so far indicate a slight increase in total N applied (chicken pellets, soluble fertilizer, and compost), and irrigation over the three years (Figure 3), as reflected in higher water content at the time of sampling (Figure 2). There has been no consistent increase in any products applied for pest control (Figure 3).

These summaries are based on much more detailed data that are still being analyzed. We will also re-sample the fields for total soil carbon and nitrogen. We are on track with our goal to focus on multivariate and other approaches during winter of 2004.

Compost quality trial

Compost is applied to all fields on both ranches at least once per year, with the goal of supplying a longer-lasting nutrient supply than is available from short-lived cover crops and soluble fertilizer applied through the drip system. Nutrient availability is not only a major concern of conventional growers who are making the transition to organic production, but also for conventional growers who are limiting their use of inorganic fertilizers to prevent N loss.

Two composts have been compared for their effect on soils, yield, plant nutrients, and pest problems on a 20-acre field on one of the ranches described above, the Storm Ranch. Each treatment plot is 0.6 acres. The composts are applied at 7 yards/acre. The manure-based compost (‘high-grade compost’) only contains 30% municipal yard waste, and other inputs are manure, clay, finished compost, and baled straw. The yard waste-based compost (‘commercial-grade compost’) contains 75% municipal yard waste, along with additional manure and lime. Use of the commercial-grade compost applications is 25% less expensive than the high-grade compost. A cover crop mixture of legumes and non-legumes was used in every treatment in 2000-01, and Merced rye was used in 2001-02. The field is divided into two separate 10-acre experiments, each with four blocks per treatment. We sample soil at two depths (0-15 cm and 15-30 cm) at two locations per plot for inorganic N, potential N mineralization, MBC, total C and N, and EC. Sampling is repeated at harvest of all subsequent crops. Aboveground biomass, plant N, P, and K content, and density, identity, and biomass of weeds in 1m2 quadrats are also measured. Damage from pathogens and insects is noted. There have been 16 sampling dates since the experiment began.

Our recent results indicate that the compost based largely on yard waste inputs increased radicchio dry weight in 2002 compared to the manure-based compost (Figure 4), but that plant N content was reduced. This suggests that the yard waste-based compost may be affecting plant growth in other ways besides N availability, and supports our early findings in the study, when higher lettuce yields were observed after one year, compared to the compost made from manure and a lower percentage of yard waste, especially in plots that had a small rather than large amount of cover crop biomass in the previous season. We are exploring possibilities by taking samples for bulk density, moisture holding capacity, and organic matter constituents, using pyrolysis techniques in collaboration with Dr. Francisco Calderon of the USDA-ARS in Beltsville, MD. There were no effects on soil or plants of one vs. two applications per year.

The first differences in soil properties were also observed in 2002 (Figure 5): potentially mineralizable N tended to be higher when compost was applied at two times per year, with little difference between compost types. This was not accompanied by changes in soil microbial biomass C. The general lack of differences so far in soil N supplying power and soil microbial biomass between the four compost treatments has been surprising, and warrants further investigation so that growers can have more information about the value of this expensive input.

Brassica cover crops

Lettuce growers in the Salinas Valley have greatly increased their use of brassica cover crops. Several recent studies have demonstrated methods to significantly increase toxins released as brassica cover crop tissue is decomposed in soil. This process, known as biofumigation, has been reported to reduce weed competition, nematodes, bacteria, post harvest pathogenic fungi, and, most notably, some soil-borne pathogens. Biofumigation holds great promise as an important management tool as conventional farmers are currently under great pressure to find alternatives to chemical fumigation practices, while organic farmers search for acceptable means of reducing soil pathogens to improve yields. Although there is a growing body of literature indicating an increasing understanding of the mechanisms that cause biofumigation and the means of improving its potency to suppress soil-borne pathogens, there is a striking gap in our understanding as to the effects of this practice particularly in the rhizosphere. Some studies have shown that the effect of biofumigation on soil organisms may depend on the particular brassica species used. Future research may entail identifying which brassica species reduces soil-borne pathogens while maintaining rhizosphere functions, particularly the interaction between vesicular arbuscular mycorrhizal VAM fungi and lettuce. A series of greenhouse and field trials coupled with on-farm surveys will help elucidate the effects of biofumigation on microbial populations and ultimately the effect on yield particularly in low-input and organic systems that may be more dependant on VAM fungi interactions.


The following presentations addressed factors involved in the transition to organic production in vegetable production and specifically described this SARE project:

Presentations by L. Jackson
1/30/03, UC DANR Optimizing Soil Management for Cool-Season Vegetables Workgroup, Salinas, Overview of main activities and recent reports, 12 attendance
2/6/03, ASA California Plant and Soil Conference, Modesto, Transition to organic production in cool season vegetables in Salinas, 50 attendance
3/6/03, Community Alliance with Family Farmers Field Day, Sebastopol, Cover crops for nutrient management in organic vegetable production, 25 attendance
11/25/03, Department of Vegetable Crops Seminar, UC Davis, Davis, Changes in plant and microbial nitrogen cycling during transition to organic production, 20 attendance

Presentations by R. Smith:
1/22/03, Ecological Farming Conference, Salinas, Bus tour leader and cover crop presentation, 45 attendance
1/28/03, Water Quality Short Course, Hollister, Nutrient management of row crops, 16 attendance
2/25/03, NRCS Western Region Agronomy Consortium, Pacific Grove, Vegetable agriculture in the Salinas Valley, 25 attendance
2/26/03, Soil Fertility Class at ALBA, Chualar, Soil management for organic crops – presented in Spanish, 25 attendance
3/4/03, Water Quality Short Course, Watsonville, Nutrient management practices to prevent runoff and leaching, 24 attendance
3/12/03, Hartnell College Soils Class, Salinas, Fertilizers and fertilization of row crops, 19 attendance
11/6/03, Water Quality Short Course, Salinas, Nutrient management practices to prevent runoff and leaching, 43 attendance

Impacts and Contributions/Outcomes

  1. The preliminary analysis of results from this research and education project is demonstrating to conventional vegetable farmers that transition to organic production is feasible without large production risks. Moreover, growers also are interested in using some of the organic transition practices in their conventional production, e.g., compost and cover crops. This message is largely made possible by our collaboration with a large, well-respected company in the area, and the description of changes over entire ranches, which encompasses a large range of management scenarios.

    The collaborative nature of our project, which involves a major vegetable company, farm advisors, extension specialists, and faculty, has both direct and indirect impacts on the visibility and success of this organic transition project.

    Organic transition is a long-term process, and because our results are not yet complete, we have not yet published intermediate findings until the full data set is available.

    So far, the organic transition on these two ranches in the Salinas Valley has been ‘uneventful’ in the words of our grower-collaborator, Ron Yokota of Tanimura and Antle, Inc. Since no major problems have arisen, next year’s work will be oriented toward describing ways that organic production can be optimized, for both production and environmental quality.

    The project has been introduced to public audiences and results have been disseminated, as described above.

    The internship program for undergraduates from California State University Monterey Bay (CSUMB) with our project, under the guidance of Dr. Murphree, has been effective in training science students about sampling design and field and lab skills. So far, nearly 20 students have been involved.


Marita Cantwell
Extension Specialist
University of California at Davis
Dept. of Vegetable Crops
One Shields Ave.
Davis, CA 95616
Liese Murphree
California State University Monterey Bay
100 Campus Center
Seaside, CA
William Chaney
Farm Advisor
UC Cooperative Extension, Salinas, CA
1432 Abbott St.
Salinas, CA 93901
Richard Smith
Farm Advisor
UC Cooperative Extension, Salinas, CA
1432 Abbott St.
Salinas, CA 93901
Karen Klonsky
Extension Specialist
University of California at Davis
Dept. of Agricultural and Resource Economics
One Shields Ave.
Davis, CA 95616
Ron Yokota

Ranch Manager
Tanimura and Antle, inc.
P.O. Box 4070
Salinas, CA 93912-4070
Steve Koike
Farm Advisor
UC Cooperative Extension, Salinas, CA
1432 Abbott St.
Salinas, CA 93901