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Organic crop production systems are increasingly being adopted by producers in the northern Great Plains. This study evaluated the expected net returns and risk of organic crop rotations, compared to conventional rotations. Field plot data of organic and conventional crop rotations were used to determine the net returns of the systems, using four different levels of premiums for organic produce. The risk of returns was evaluated using estimated cumulative density functions and stochastic dominance. The most profitable organic rotation required high price premiums to dominate the most profitable conventional rotation. However, the most profitable organic rotation dominated some conventional rotations that are commonly used in the northern Great Plains. The organic rotations had slightly higher risk, but the relative risk of rotations had little impact on the optimal rotation.

S I' M M A R YThe relationship was examined between the density of vesicular-arbuscular (VA) mycorrhizat inoculum and the growth of Phleum pratense L. and Agropyron trachycaulum (Link) Malte. The most probable number method (MPN) was used with combinations of host, VA mycorrhizal inoculum and a series of dilutions of field soil. Tbere was a positive correlation between the proportion of field soil and the amount of mycorrhizal colonization. The numbers of tillers and leaves, shoot and root dry weights, and root:shoot ratios were negatively affected by increasing proportions of field soil inoculum. The addition of fertilizer alleviated the effects of increasing proportions of field soil on plant growth and reduced the amount of colonisation by mycorrhizal fungi. The relationship between increasing proportions of field soil and plant growth was not species-specific. We conclude that there was an interaction between the inoculum density of VA mycorrhizal fungi and soil nutrient availability which affected plant growth.