On-Farm Colonization of Tomatoes by AM Fungi

• 2008 annual report

Historically, before the advent of modern conventional agriculture with a reliance on off-farm inputs such as chemical fertilizers, productive farming systems depended on a closed system of recycling nutrients within the farm, building soil organic matter and enhancing conditions to maximize the agricultural ecology. The symbiotic associations of a majority of crops with arbuscular mycorrhizal (AM) fungi are one example of working within the natural agricultural ecosystem to maximize production. AM fungi have been demonstrated to enhance crops’ ability to mobilize and take up soil nutrients, particularly phosphorus, aid in water uptake and reduce disease pressures (Smith and Read, 1997). AM fungi have also been shown to produce a glycoprotein, glomalin, which is believed to play a key role in the stabilization of soil aggregates (Wright & Upadyaya, 1996, 1998). High soil phosphorus, from chemical fertilizer or from long term use of soil amendments such as compost and manures, has been shown to inhibit the colonization of crop roots by AM fungi (Mosse, 1973; Menge et al. 1978). Lack of mycorrhizal development would thereby deprive the crop of the benefits that mycorrhizal relations provide. Excessive soil phosphorus is also a pollution threat to fresh water bodies where erosion of high phosphorus soils are known to stimulate algae bloom. It is logical then, that avoiding a buildup of excessive soil phosphorus and maximizing access to existing but often immobile sources on the farm through crop/mycorrhizal relations, would serve as a benefit to low input sustainable farming practices. AM fungal inocula are commercially available to farmers. The cost of purchased inoculum must be borne by the farmer. Methods have been explored for producing inoculum on-farm (Douds et al., 2005) to reduce this expense and could make this technology and associated economic and environmental benefits more available to farmers. The purpose of this study is to explore on-farm production of AM fungi, and their use as an inoculum in field grown tomatoes on a small organic Maine farm. An organic farm was chosen for this study as they often face challenges to adequate soil phosphorus fertility due to the high cost of organic sources and the desire to avoid the practice of acquiring off-farm inputs. Tomatoes were chosen as a crop as they are known to have positive association with mycorrhizae (Mohandas, 1987) and because they are an important crop in small diversified vegetable and berry farms throughout New England.