Transition to Organic Vegetable Production by Large-Scale Conventional Farmers

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 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 (www.ciwmb.ca.gov/Publications/Organics/44202023.doc). 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.

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.