Submitted to: Compost Science and Utilization
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/19/2007
Publication Date: 10/1/2008
Citation: Douds, D.D., Nagahashi, G., Reider, C., Hepperly, P. 2008. Choosing a mixture ratio for the on-farm production of AM fungus inoculum in mixtures of compost and vermiculite. Compost Science and Utilization. 16(1):52-60. Interpretive Summary: Arbuscular mycorrhizal [AM] fungi are beneficial soil fungi that colonize the roots of most crops, forming a symbiosis. The benefits received by the plants include increased nutrient uptake from the soil and increased resistance to drought and diseases. In return, sugars, the product of photosynthesis, are sent to the roots and released to the AM fungi. Utilization of these fungi should be an important tool for organic and other farmers seeking to minimize or eliminate chemical fertilizers and pesticides. Earlier we developed a method to grow AM fungi with bahiagrass plants in media consisting of compost and vermiculite (a light weight, inert ingredient in many potting mixes). The resulting media could be used as inoculum by farmers. Earlier experimentation showed that the proportion of the two ingredients was very important. Compost is a very nutritious media, and from the plant’s perspective, why should it sacrifice some of its sugar to the fungus when grown in a high compost media where nutrients are abundant and no help is needed finding them? In situations of too much compost in the mixture, the plant restricts colonization of its roots, which in turn, limits production of the fungus for agricultural use. We conducted experiments over two years using three composts, each diluted to four levels with vermiculite, to develop equations to predict the optimal mixture of compost and vermiculite for production of three AM fungi. Three simple equations were produced which allow a farmer to predict the optimal compost and vermiculite mixture once the compost has been analyzed for routine parameters such as nitrogen, phosphorus, and potassium.
Technical Abstract: Arbuscular mycorrhizal [AM] fungi are potentially important tools in sustainable agriculture due to their roles in crop nutrient uptake, disease resistance, and water relations and in stabilizing soil aggregates. Inocula of these fungi can be effectively produced on-farm in mixtures of compost and vermiculite with a suitable plant host, such as bahiagrass (Paspalum notatum Flugge). Success of this method, however, depends upon utilizing the optimal compost and vermiculite mixture ratio. Experiments were conducted over two years utilizing a complete factorial design with three composts, for mixture ratios, and three AM fungi with the objective of producing regression equations to predict optimal mixture ratios using routine variables of compost nutrient analyses. Growth of colonized P. notatum in yard clippings and dairy manure + leaf composts; which were high in N, low in P, with moderate K levels; produced more spores of AM fungi at mixture ratios of 1:2 to 1:4 [v/v compost: vermiculite] relative to higher dilutions. Dilution ratios of 1:19 and 1:49 were best for controlled microbial compost, which was high in P, low in N, and moderately high in K. Simple equations were developed which predict the optimal fraction of compost in the mixture for each of the three AM fungi studied (Glomus intraradices, Glomus mosseae, and Gigaspora rosea). Percent N, P, and K and N:P ratio were the significant independent variables. These equations allow a farmer to choose a mixture ratio for the on-farm propagation of AM fungi knowing only the nutrient analysis of the compost to be used.