Cover Crop Diversity: How Important Is It for Soil Quality and the Subsequent Crop?.

Increasing cover crop diversity is thought to be beneficial to soil quality and possibly to the subsequent crop, yet almost no studies have been published on this topic, especially in semiarid regions. In Montana, we initiated a study in 2012 at four locations to assess the effects of four plant functional groups (N fixers, tap rooted, fibrous rooted, and brassicas) with two species per functional group. Treatments consisted of a “Full” mixture (with all four functional groups), four mixtures that have all but one functional group, the four individual functional groups, a sole pea cover crop (“Pea”), and a chemical fallow control. After two cycles, there were some soil quality differences between fallow and cover crop treatments, but very few differences in soil quality between the Full and Pea cover crops. A better predictor of microbial biomass and enzymes than diversity, was total aboveground biomass. Grain yield and grain protein in the subsequent year (after one or two cover crop cycles) was the same following Full and Pea treatment in all but one site-year. While diversity has potential to reduce risk and use resources more efficiently, diversity has not importantly benefited soil quality or agronomics of the subsequent crop to date. This is not necessarily surprising in a dryland semi-arid system, based on relatively low biomass production, and we expect that if functional groups do have differential effects on soil quality, we will need more rotations to see those changes.  Alternatively, choosing cover crop species that maximize biomass, without using excessive soil water, may be the best goal in semiarid environments. We hope to have two more site-years of data to present at the annual conference.