In May 2000, the California Integrated Waste Management Board (CIWMB) (now known as the California Department of Resources Recycling and Recovery or CalRecycle) initiated a contract with the University of California Davis for a two-year demonstration and research project on optimizing the use of compost in intensive vegetable production systems in the Salinas Valley. The purpose of this project was to determine the effects of using compost in the intensive vegetable production systems of the Salinas Valley, and examine factors that could influence the use of composts made with municipal organic materials.

The approach was to study responses of soil carbon and nitrogen availability in response to addition of compost and cover crops to vegetable production fields, and examine the effects on plant yield, nutrient content, and pests. An economic analysis of using cover crops and compost was also conducted. The project focused on fields that were in organic production, or in transition to organic production, because these are situations that typically utilize high inputs of organic matter inputs.

Project Update

The focus of the project was on the combined use of compost and cover crops for two reasons: (1) The two sources of organic matter provide both readily available (plant residues) and more degradation-resistant (compost) carbon compounds, which together were hypothesized to increase and sustain the soil microbial biomass and the capacity for retaining carbon in the soil; and (2) many organic and organic-transitional vegetable growers use both inputs in their crop rotations, and this research was aimed at providing information on these practices for this growing sector of the California agricultural industry.

Three main studies were supported by the project and are summarized as follows:

  • A compost quality trial on a Salinas clay loam soil showed that compost derived largely from municipal yard waste increased lettuce yields after one year, compared to a 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. There were no effects on assays for soil carbon and nitrogen availability, including soil microbial biomass, potentially mineralizable nitrogen, ammonium and nitrate. There were no effects of one vs. two applications of compost per year. The field was in transition to organic production and received cover crops and organic fertilizers in addition to compost. Soil microbial biomass and potentially mineralizable N increased across all treatments throughout the 1.5 year period.
  • A second trial on a Metz silt soil comparing winter bare, cover crop, cover crop+compost, and compost treatments showed higher lettuce yields in the treatments with compost, but with no consistent effect of these amendments on the soil assays for soil carbon and nitrogen availability. Since this trial was flooded temporarily, the experiment is being repeated on an Antioch loam soil. Preliminary results show that lettuce yields were similar in all three compost or cover crop treatments, and these were higher than the winter bare treatment.
  • Economic analysis of cover crop+compost use in relation to all other management costs for vegetable production on a Salinas silt loam soil showed that these amendments increased yield with acceptable net economic returns during a two-year period. The additional cost of cover crop+compost treatments paid off in terms of net returns for a broccoli crop, due to a high yield response but less so for lettuce crops. Thus, use of cover crop+compost inputs was economically viable, and when compared to non-amended soils. As determined in a previous study, these inputs also improved some aspects of soil quality, including increased soil microbial biomass and decreased leaching of nitrate.

The management implications of this project are that composts made of municipal yard waste are recommended for vegetable production to increase yield, especially in circumstances where a cover crop is not possible, or only a short-term cover crop can be accommodated due to scheduling. The role of compost in increasing yield was not clear and cannot be simply attributed to changes in soil carbon and nitrogen availability.

The final report, Maximizing Benefit and Utilization of Compost in Vegetable Production (#442-02-023) presents data from field trials studying the effects of using compost in the intensive vegetable production systems of the Salinas Valley and examines factors that could influence the use of composts made with municipal organic materials.