Transition to Organic Vegetable Production by Large-Scale Conventional Farmers

Final 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:
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Project Information


Whole-farm analysis showed that flexible management techniques and careful planning helped large-scale California vegetable farmers convert successfully to organic production. Participatory research involved monitoring of 81 points on two ranches for three years. Biodiversity-based management included many crop species, cover crops, and insectary strips. No increase in disease, insect pests, or nutrient deficiency occurred during the transition. Soil quality improved due to higher microbial biomass and less potential for nitrate leaching. In specific experiments, yard-waste compost had similar effects as manure-based compost, and brassica cover crops caused no decline in mycorrhizal colonization or crop nutrient content. Organic transition was clearly viable in the midst of a conventional farming area.

Project Objectives:

Our objectives were 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.


Organic production has increased in the Salinas Valley of California, which is the largest producer of cool-season vegetables (e.g., lettuces, cole crops, celery) in the nation. Many large-scale conventional farmers are converting a portion of their land to organic production. Thus, there is a potential for organic production practices to be integrated into their conventional production as well. This project was designed to describe the changes and solve problems during the transition from highly intensive conventional vegetable production to organic production. Our grower-cooperator was Tanimura and Antle, Inc. (T&A), a large vegetable production company now committed to producing organic produce on hundreds of acres.

Participatory research between UC Davis researchers, farm advisors, and the grower-collaborator was designed to meet growers’ needs and to develop new scientific information on ecological changes during organic transition. Rather than use a typical agronomic experimental design, a three-year study was conducted to monitor changes during the transition to organic production on two conventional ranches (Storm Ranch and Daugherty Ranch in Salinas, California) by tracking many response variables and management inputs at 81 permanent points. This has provided a whole-farm analysis of the organic transition. In addition, other efforts were directed toward understanding soil fertility and soil quality. These included a 4-year trial on effects of different compost materials, and a survey of effects of brassica cover crops on crop nutrition and mycorrhizal colonization.

Organic management differed radically from prior conventional management. Conventional lettuce, broccoli, spinach, and celery production is done in 20-30 acre blocks with frequent inputs of inorganic fertilizers and pesticides. This was diversified into much smaller parcels of organic production of crops such as baby greens, leaf lettuce, radicchio, endive, escarole, fennel, cilantro, and parsley. New species mixes and cropping patterns were used, and soil fertility was managed with a combination of inputs including compost, chicken manure pellets, cover crops, and soluble fertilizers. A re-useable drip line for irrigation and delivery of soluble organic fertilizers not only conserved water and cut costs, but also kept the surrounding soil much drier, reducing incidences of weeds and diseases. A switch from legume cover crops to rye and mustard occurred because weeds became problematic with the legumes. Frequent hand hoeing kept weeds in check, and use of less susceptible crop varieties reduced impacts of aphids and leaf miners. The growers also shifted planting dates to avoid pest problems, knowing that leaf miners increase in late summer, and they grew more susceptible crops in early spring. Few pest outbreaks occurred, but at those times, organic pesticides were not very effective. The organic transition went smoothly due to careful planning by the grower, and the organic fields, set in the middle of a non-organic environment, did not become oases for large populations of nearby pests. Organic transition is clearly possible in the midst of a conventional growing environment.

As described below, a large volume of data have been collected and analyzed from the organic transition time course. This provides a strong, quantitative assessment of the successful transition to organic production and its concomitant improvement of factors contributing to environmental quality, e.g., higher soil carbon pools, low potential for nitrate leaching, no harmful pesticides, and lower water application compared to conventional production. In addition, yard-waste-based composts were shown to produce similar effects as manure-based composts, which would avoid some of the eventual problems, e.g., salinity, which comes with using manures. Brassica cover crops, which potentially can reduce pests and weeds, had little effect on mycorrhizal colonization or crop nutrition, based on a survey of these and other organic farms. Publications are now underway, and many presentations have been made to deliver the results of this project to a wide range of public and scientific audiences.


Click linked name(s) to expand/collapse or show everyone's info
  • Marita Cantwell
  • William Chaney
  • Karen Klonsky
  • Steve Koike
  • Liese Murphree
  • Richard Smith
  • Ron Yokota


Materials and methods:

Materials and methods are described in each section of the results and discussion.

Research results and discussion:
Organic transition time course

A large data set on biological responses and management inputs during the transition to organic production is now complete, and it describes changes 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. 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 experimental design of the monitoring study was designed using methods adapted from vegetation ecology, in which transects were repeatedly measured for three years. In June 2000, when certified organic practices were instated on the two T&A ranches, we set up permanent sampling points for frequent monitoring. On each of the 9 lots, we set up three transects. Each transect was a 2-bed strip across the entire field. There were three equidistant permanent sampling points (±5 m) along each transect. Thus, 81 sampling points existed; 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 have statistically tested 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 weeks 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. A final sample was taken in February 2004, for total soil carbon and nitrogen. The grower has provided us with information on management practices, irrigation, fertilizer, compost, and other inputs used for each of the transect locations for each sampled crop.

Since June 2000, 510 individual plots on 47 sampling days were sampled. A total of 25 different crop and cover crop species were assessed.

Seasonal cropping patterns changed markedly during transition. Many more species and smaller plantings were used compared to the 20-25 acre plantings of lettuce, broccoli, cauliflower, celery and spinach that are typical of conventional production. Initially, more crop species were planted, and plantings continued through the summer. 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. Nitrate was often close to zero, even though N concentrations in plant tissue were more than adequate. This can be best explained by the sampling location in the plant row, several inches away from the drip line where soluble nitrogen fertilizer was delivered. Plant roots probably depleted the nitrogen from this soil zone, accounting for the low nitrate values. Soil nitrate was also low at the lower depth (15-30 cm) suggesting that leaching of nitrate below the surface soil was not substantial. Potentially mineralizable N was variable, and hard to explain. Soil microbial biomass, however, increased at the Storm Ranch through time, as did total soil carbon and mycorrhizal colonization. These are all indicators of improving soil quality. At the Daugherty Ranch, which has higher soil clay content, no change occurred in these parameters, indicating that considerable variation between locations can occur even when the same management practices are used.

Pest damage and weeds did not increase during the transition period at either ranch. 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%), but root disease was higher at the Daugherty Ranch. Weed counts decreased with time, due to careful use of hand labor, but the assemblages of weed species were different between the two ranches.

Management inputs were obtained from the grower. A slight increase occurred in total N applied (chicken pellets, soluble fertilizer, and compost), as well as in irrigation over the three years, as reflected in higher water content at the time of sampling. High inputs of organic fungicides and pesticides were made during the second year, but were largely cut back after their efficacy was not shown to be useful.

Data analysis is still ongoing as we attempt various approaches for condensing the large volume of information. Multivariate analysis is now underway to examine relationships between biological response variables, soil parameters, and management inputs. Of particular interest is the finding that the two ranches differ in root disease, mycorrhizal colonization, soil microbial biomass, and soil carbon.

Compost quality trial

Compost was 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, chicken manure pellets, and soluble fertilizer applied through the drip system.

Two composts were 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 were applied at 7 yards/acre. The manure-based compost only contains 30% municipal yard waste, and other inputs are cow manure, clay, finished compost, and baled straw. The yard waste-based compost contains 75% municipal yard waste, along with additional manure and lime.
1. Yard waste-based compost (‘Commercial grade compost’) once per year (C1)
2. Yard waste-based compost (‘Commercial grade compost’) twice per year (C2)
3. Manure-based compost (‘High grade compost’) once per year (H1)
4. Manure-based compost (‘High grade compost’) twice per year (H2)

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 or brassica cover crops were used in subsequent years. The field was divided into two separate 10-acre experiments, each with four blocks per treatment. We sampled 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 was 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 were also measured. Damage from pathogens and insects was noted. There have been 17 sampling dates since the experiment began.

During most of the study, few differences were observed between any of the four compost treatments. These results are summarized in a report to the agency (California Integrated Waste Management Board) that provided matching funding ( Even in the most recent sampling from September 2004, there were no differences in lettuce biomass, nutrient content, soil microbial biomass, or inorganic nitrogen concentrations in soil. The only clear differences between treatments occurred when yard-waste compost increased radicchio dry weight in 2002 compared to the manure-based compost, but in this case, plant N content was reduced. This suggests that the yard waste-based compost may have affected plant growth in other ways besides N availability. At one other earlier sampling time, higher lettuce yields were observed after one year compared to the compost made from manure, especially in plots that had a small rather than large amount of cover crop biomass in the previous season. Throughout the study, there were no effects on soil or plants of one vs. two applications per year. Although the general lack of differences from compost materials are difficult to explain, these results point out that using the yard-waste compost is valuable, and costs substantially less than the manure-based compost. Use of the yard-waste composts over the long term may alleviate some of the problems caused by manures, such as salinity.

Brassica cover crops

During the summer of 2004, we conducted a survey of six different organic fields with vegetable crops to investigate the effects of using Brassica species as fall or winter cover crops. The impetus for this survey stems from recent research that indicates the secondary plant compounds found in Brassicas may reduce disease and weed pressures in subsequent production crops. Several recent studies have demonstrated a significant increase in the release of these compounds through the pulverization of plant tissue by flail mowing and immediate irrigation, a process coined as biofumigation. Some of the reported effects of biofumigation are reduced weed competition, nematodes, bacteria, post harvest pathogenic fungi, and, most notably, a reduction of some soil-borne pathogens.

Biofumigation holds great promise as an important management tool for conventional farmers who are currently under great pressure to find alternatives to chemical fumigation practices, and organic farmers who seek acceptable means of reducing soil pathogens to improve yields. There are, however, large gaps in our understanding of the mechanisms and impacts of biofumigation, particularly the potential negative impacts to beneficial soil organisms. Of particular concern are arbuscular mycorrhizal fungi (AMF), which are thought to play an important role in nutrient acquisition and uptake in low-input organic farming systems.

To investigate the possibility that brassica cover crops reduce AMF colonization to the detriment of subsequent vegetable crops we conducted a survey of five production fields planted after a brassica cover crop in the spring of 2004. We began the survey at a USDA organic cover crop trial on an experimental farm in the Salinas Valley in collaboration with Dr. Eric Brennan. The USDA organic trial is currently comparing eight cover crop treatments, which include standard application rates of a mustard, rye and a legume/rye cover crop mix, and treatments of the same cover crops at three times the application rate, along with a fallow and a fallow without compost. This experiment is set up in a randomized block design with four replications after having been cover cropped organically for three years. Romaine lettuce was the vegetable crop that was sampled in this study.

The rest of the survey was continued on organic vegetable farms in Monterey County and in San Benito and Yolo counties. We surveyed romaine, cantaloupe, onions, and both fresh market and canning tomatoes. The crops surveyed were selected because of their status as known mycorrhizal hosts, and their paired planting following either a brassica cover crop or non-brassica cover crop/fallow. Each crop was sampled within one week of harvest and analyzed for biomass, and both macronutrients and micronutrients. At the same time soil was sampled from 0-15 cm depth adjacent to the sampled plants and analyzed for total nitrogen, total carbon, and Olsen phosphorous while the roots were analyzed for AMF colonization.

The results of the survey indicate that there was little change in AMF colonization due to brassica cover crops planted at the average seeding rate. At the USDA station trial, the higher seeding rate resulted in a general trend showing that a rye cover crop gives the greatest colonization of romaine, followed by legumes, then mustard, and finally fallow. Although we found very little differences in vegetative and fruit biomass or nutrient uptake between crops grown following the various treatments, we did find a slight correlation between Zn uptake and colonization across all vegetable crops sampled in the entire survey (r2 = 0.12; P = 0.02). When each field was evaluated individually, we found a significant decrease in phosphorous uptake in the onions (P = 0.02) and an increase calcium (P = 0.01) uptake in romaine following mustard. In the USDA trial, romaine had a significant decline in AMF colonization following a higher application rate of mustard (3 times the standard rate). Yet the reduction in AMF colonization did not result as hypothesized in a detriment to the production crop, and in fact resulted in the highest mean wet weight of romaine in any treatment, and was statistically higher than the no cover crop treatments. It may be that the benefits (e.g. nutrient availability and pathogen and weed suppression) provided by higher amounts of a brassica cover crop preclude any loss in benefits from AMF colonization. Although the seeding rate for brassica cover crops used by our farmer cooperators did not result in a reduction of AMF colonization in their vegetable crops studied in this survey, there were no observed benefits from the brassica cover crops in terms of increased biomass or nutrient uptake in any of the on-farm trials, as compared to fallow or other cover crop species. Higher application rates of brassica cover crops or managing specifically for secondary compound release (pulverizing and immediate irrigation), however, may still have beneficial impacts on subsequent vegetable crops as well as cause a decline in AMF colonization.

Research conclusions:
  1. Organic transition is feasible by conventional vegetable farmers without large production risks, but it requires careful planning and implementation of alternative methods that require substantial capital and labor, such as drip irrigation and hand weeding.
    Environmental quality is enhanced by organic transition: soil carbon pools can increase, nitrate leaching potential can decline, and pesticides are no longer applied.
    The collaborative nature of this project that involved a major vegetable company, farm advisors, extension specialists, and faculty, had both direct and indirect impacts on the visibility and success of this organic transition project.
    The organic transition on these two ranches in the Salinas Valley was ‘uneventful’ in the words of our grower-collaborator, Ron Yokota of Tanimura and Antle, Inc. But this was a result of deliberate planning and monitoring during the transition period.
    Organic fields, set in the middle of a non-organic environment, did not become oases for large populations of nearby pests, and thus, organic transition was possible in the midst of a conventional growing environment.”
    The project has been introduced to public audiences and results will continue to be disseminated.
    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, field and lab skills, and participatory research with farmers. Over 20 students were involved.
Participation Summary

Research Outcomes

No research outcomes

Education and Outreach

Participation Summary:

Education and outreach methods and analyses:

The following presentations addressed factors involved in the transition to organic production in vegetable production and, in most cases, specifically described this SARE project:
Weeds During the Transition to Organic Vegetable Production. Nov. 12, 2002. Salinas. UC Cooperative Extension Weed Meeting. Speaker: Murphree

Roots, Microbes and N Cycling: Soil Ecology in Salinas Valley Agriculture. May 23, 2002. Palo Alto. Stanford University Seminar. Speaker: Jackson

Minimum Tillage and Organic Matter Management. Mar. 12, 2002. Davis. DANR Conservation Tillage Workgroup. Speaker: Jackson
Cover Crop Field Day. Feb. 21, 2002. Salinas. UCCE Cooperative Extension Monterey County. Speakers: Brennan and Smith

Cover Crop Field Plot Demonstration. Jan. 16, 2002. Salinas. Irrigation and Nutrient Management Conference 2002. Speakers: Brennan and Smith

Organic Matter Management, and Soil and Plant Health. Jan. 16, 2002. Salinas. Nutrient Management Conference 2002. Speaker: Jackson

Nutrients and Irrigation. Jan. 29, 2002. Hollister. Water Quality Short Course. Speaker: Smith

Minimum Tillage and Organic Matter Management. Mar. 12, 2002. Davis. DANR Conservation Tillage Workgroup. Speaker: Jackson

Soil Biology and Transition from Conventional to Organic. Jan., 2002. Ag Alert Newsletter. Staff Writer: Bob Johnson

Soil Aspects of the Transition to Organic. Dec. 5, 2001. Davis. DANR Vegetable Crops Continuing Conference. Speaker: Jackson

Transition to Organic-A Multidisciplinary Approach. Dec. 4, 2001. Salinas. Annual Entomology Seminar. Speakers: Smith and Chaney

Weeds in the Community. Nov. 16, 2001. Salinas. WOW Weed Symposium. Speaker: Jessyka Wengren (CSU Monterey Bay intern)

Salinas Valley Organic Strawberries and Vegetables: Research Results and Implications for Production. Nov. 1, 2001. Davis. DANR Organic Farming Workgroup. Speaker: Jackson

Effects of Organic Amendments and Tillage Practices on Soil Microbial Biomass, N Availability and Crop Yield in Intensive Agriculture. Aug. 9, 2001. Madison, WI. Ecological Society of America National Meeting. Speaker: Jackson

Ecological Principles: Components of a Sustainable Organic System (What we know and don’t know). July 22, 2001. Sacramento. American Society for Horticultural Science National Meeting. Speaker: Jackson

Overview of Main activities and Recent Reports. Jan. 30, 2003. Salinas. UC DANR Optimizing Soil Management for Cool-Season Vegetables Workgroup. Speaker: Jackson

Transition to Organic Production in Cool Season Vegetables in Salinas. Feb. 6, 2003. Modesto. ASA California Plant and Soil Conference. Speaker: Jackson

Cover Crops for Nutrient Management in Organic Vegetable Production. Mar. 6, 2003. Sebastopol. Community Alliance with Family Farmers Field Day. Speaker: Jackson

Changes in Plant and Microbial Nitrogen Cycling During Transition to Organic Production. Nov. 25, 2003. Davis. Department of Vegetable Crops Seminar, UC Davis. Speaker: Jackson

Bus Tour Leader and Cover Crop Presentation. Jan. 22, 2003. Salinas/Watsonville. Ecological Farming Conference. Speaker: Smith

Nutrient Management of Row Crops. Jan. 28, 2003. Hollister. Water Quality Short Course. Speaker: Smith

Vegetable Agriculture in the Salinas Valley. Feb. 25, 2003. Pacific Grove. NRCS Western Region Agronomy Consortium. Speaker: Smith

Soil Management for Organic Crops – presented in Spanish. Feb. 26, 2003. Chualar. Soil Fertility Class at ALBA. Feb. 26, 2003. Chualar. Speaker: Smith

Nutrient Management Practices to Prevent Runoff and Leaching. Mar. 4, 2003. Watsonville. Water Quality Short Course. Speaker: Smith

Fertilizers and Fertilization of Row Crops. Mar. 12, 2003. Salinas. Hartnell College Soils Class. Speaker: Smith

Nutrient Management Practices to Prevent Runoff and Leaching. Nov. 6, 2003. Salinas. Water Quality Short Course. Speaker: Smith

Transition to Organic Production. Dec. 5, 2003. UC Davis. DANR Cool-Season Workgroup. Dec. 5, 2003. UC Davis. Speaker: Jackson

Cover Crops and Soil Fertility. . Mar. 18, 2004. Bolinas. Marin Co. Cooperative Extension. Speaker: Jackson

Overview of Advantages and Disadvantages of Cover Crops. Mar. 18, 2004. Bolinas. Marin Co. Cooperative Extension. Speaker: Smith

Project Mention. USDA Western SARE ‘Simply Sustainable’. April 30, 2004.
Organic Transition at Tanimura and Antle. May 25, 2004. Salinas. DANR ‘Optimizing Soil Management for Cool-Season Vegetables’ Annual Meeting. Speaker: Jackson

Exploring Plant-Microbial Nitrogen Cycling in Organic Farming Systems. Jan 23, 2004. Flagstaff, AZ. Seminar at the Biology Dept., Northern Arizona University. Speaker: Jackson

Transition to Organic Production by Large-Scale Conventional Growers. Jul. 13-15, 2004. Berkeley. California Conference on Biological Control. Speaker: Jackson

From Conventional to Organic Agriculture: Analyzing the 3-Year Transitional Period of a Large-Scale Vegetable Grower in Salinas Valley, CA. ASA-CSSA-SSSA Annual Meeting. Seattle, WA. Nov. 3, 2004. Speaker: Smukler

Overview on Organic Transition in Cool-Season Vegetables. Dec. 3, 2004. UC Davis. DANR Cool-Season Workgroup. Speaker: Jackson

Results of a Three-Year Study on the Transition to Organic Production in Cool-Season Vegetables. Jan. 18, 2005. Salinas. UC Organic Vegetable Production Short Course. Speaker: Jackson

Large California Farm Transitions to Organic with Ease. USDA-SARE Highlights. Spring, 2005

Education and Outreach Outcomes

Recommendations for education and outreach:

Areas needing additional study

Pest management is an area identified by growers and extension personnel as a much needed topic for organic production. Another issue is the build-up of phosphorus in soils managed with high manure inputs. From a larger perspective, the potential for widespread adoption of organic practices is currently unknown, yet would justify further research at larger scales if expected to occur.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.