2002-2004 Specialty Crops Research Program
University of California

Principal Investigators: Louise Jackson and Kate Scow

Project title: Nutrient Management and Soil Microbial Ecology for Organically-Grown Fresh-Market Tomatoes

Vegetables, which are some of California’s most important specialty crops (Kuminoff et al. 2000), are typically supplied with high levels of inorganic nutrients to maximize yield and quality. Organic production, one of the fastest growing sectors of vegetable production in California (Klonsky et al. 2002), relies solely on organic sources of nutrients. When farmers rely on nutrients in soil organic matter, they are dependent on soil microbial processes that control the turnover and availability of forms of nitrogen (N) and phosphorus (P) that can be taken up by plants. The ecology of soil microbial communities and their activity directly affects the ecophysiology of crop nutrient uptake, growth, and biomass allocation, and thus yield and quality. These community-level processes also affect the systems-level flows of nutrients, and their loss from the farm via leaching, runoff, or gaseous efflux.

The premise of this research is that optimal soil management for organically-grown crops promotes soil microbial communities that enhance N and P availability to plants during periods of peak crop demand, and minimizes nutrient losses to the environment. Our central hypothesis is that arbuscular mycorrhizae (AM), symbiotic fungi associated with the roots of almost all plants, are beneficial for organic production because they increase crop uptake of N and P from soil, and may stimulate nutrient release from soil organic matter and the occurrence of other microbes that further increase the nutrient supply to plants (Fig. 1). Thus, management practices that boost N and P to high concentrations should be avoided because they decrease mycorrhizal symbiosis (Smith and Read 1997) and increase losses of nitrate (NO3--N) and P to the environment.

Our specific objectives are:
1) Survey fresh-market tomatoes on many organic farms to obtain an inventory on soil properties, mycorrhizal colonization and management practices.
2) Compare fresh-market tomato yield and quality in response to N and P availability, with and without mycorrhizae, on soils from organic farms.
3) Investigate the effects of manure on soil microbes, implications for plant N and P uptake, and the potential for N and P loss to the environment.

We have resources and preliminary information already available for the project including: an exploratory survey of organic farms; genotypes of fresh-market tomato either susceptible (wild type) or not susceptible to mycorrhizal colonization (Barker et al. 1998), and experience with quantification of microbial activity and diversity using molecular techniques.