Nutrient Management Research Database

Studying the management strategies suited to large-scale organic production, particularly during the mandated 3-year transition period from conventional management, is a unique research challenge. Organic production traditionally relies on small, diverse plantings and complex management responses to cope with soil fertility and pest pressures, so research should represent decision-making options of an organic grower at the farm scale. This study analyzes crop, soil, pest and management changes during the organic transition period on two ranches (40 and 47 ha) in the Salinas Valley, California in cooperation with a large conventional vegetable producer, Tanimura and Antle, Inc. Permanent transects were established across the two ranches at the onset of adoption of organic practices, and soil and plants were sampled at harvest of almost all crops, while all management operations were recorded by the co-operator. The 10 ha blocks were divided into many small plantings, and 17 different cash crop and cover crop species were planted during the transition period. Management inputs consisted of a range of organic fertilizers and amendments, sprinkler and drip irrigation, cultivation and hand-hoeing, and several types of organic pesticides. Results from the 3-year period followed these general trends: increase in soil biological indicators (microbial biomass and arbuscular mycorrhizae), low soil nitrate pools, adequate crop nutrients, minor disease and weed problems, and sporadic mild insect damage. Multivariate statistical analyses indicated that some crops and cultivars consistently produced higher yields than others, relative to the maximum yield for a given crop. Multi-factor contingency tables showed clear differences in insect and disease damage between crop taxa. Although Tanimura and Antle, Inc. used some of the principles of organic farming (e.g., crop diversity, crop rotation, and organic matter (OM) management), they also relied on substitution-based management, such as fertigation with soluble nutrients, initially heavy applications of organic pesticides, and use of inputs derived from off-farm sources. Their initial production of a large number of crop taxa in small plantings at staggered intervals proved to be an effective strategy for avoiding risks from low yields or crop failure and allowed them to move towards a smaller number of select, successful crops towards the end of the transition. This study demonstrates the feasibility of large-scale producers to transition to organic practices in a manner that was conducive to both production goals and environmental quality, i.e., increased soil C pools, low soil nitrate, and absence of synthetic pesticides.

• Research began in June 2000 on two large ranches in the Salinas Valley.
• Sampling ended in March 2003, giving an accurate depiction of the 3 year transition period from conventional to organic agriculture.
• In most cases, crops were direct seeded into tilled and shaped beds.
• Drip tape was buried for irrigation at 3cm depth. C
• over crops were normally planted once per year, and consisted of mainly Merced rye, a legume mix of 50% bell beans, 40% cowpeas, and 10% cayuse oats, or a legume mix/Merced rye mix. Compost was applied at least one time per year, and the C:N ration averaged 18.
• Pelleted chicken manure fertilizer (2.5-2-2.5) was applied prior to plantings, supplying 28kg N/ha, then a soluble fertilizer (6.0-0.4-0.2) was applied multiple times through drip, with N application ranging for crop.
• Organically certified pesticides were used throughout the transition.
• Soil monitoring samples were taken and analyzed for gravimetric soil moisture content, nitrate and ammonium, and potentially mineralizable nitrogen.
• A soil properties dataset was also assessed for total soil N and C, electrical conductivity, particle size distribution, Olsen P and soluble K.
• Plant samples were analyzed for total N, P and K.
• Yield was also assessed.

• Crop performance increased over the study period.
• Average nutrients were above the critical value of most crops for which critical deficiency values were available, although broccoli shoots were below the critical value for P.
• Insect damage increased between the first and second years and then declined in the third year for summer and fall observations.
• There was a trend towards increased soil biological activity and microbial biomass carbon during the transition period.
• Soil nitrate concentrations at the soil surface generally decreased over the study period.
• There were no differences in mean soil C and N values at either ranch between the beginning and end of the study.

other crops included were endive, escarole, fennel, frisee, parsley, radicchio