Seeking sustainability for organic cropping systems in the northern Great Plains : legume green manure management strategies

Abstract

Soil fertility in organic cropping systems of the northern Great Plains (NGP) is most often achieved through inclusion of leguminous green manures. The objectives in this study were to evaluate the efficacy of pea as green manure; and more specifically, measure the effects of pea green manure type, termination timing and method for soil water use and soil N contribution. A study consisting of winter pea, spring pea, mustard, buckwheat and fallow was conducted at Big Sandy Montana. In Bozeman, MT, termination methods of winter and spring pea were compared. Green manures were terminated at one of two timings, first bloom or first pod. Winter pea was the superior annual green manure in this study, optimizing the soil water conservation and N fertility goals of organic dryland growers in the NGP. Winter pea terminated at bloom also provided the greatest soil N at the end of the green fallow period relative to other green manures. However, pod-terminated winter pea may enhance soil N fertility due to greater N fixation over the long-term.

Although a N advantage of pod-termination was not illustrated in soil NO3-N data, greater shoot biomass N may eventually contribute greater soil N to the cropping system. Additionally, this study compared two experimental termination strategies, crimprolling and vinegar desiccation, with standard disk-tilling in attempt to reduce tillage in organic systems. The efficacy of termination strategy was dependent on pea growth stage. At first bloom, disk-tillage was the only method that effectively terminated pea growth. When terminating pea at pod, both disk-tillage and crimp-rolling were effective. Crimp-rolling at pod eliminated up to three tillage operations, while maintaining comparable soil water use and subsequent wheat yields to disk-tillage. Although the advantage of reducing tillage operations by crimp-rolling could not be directly measured in a 2-yr cropping sequence, longer-term tillage reduction may reveal multiple soil quality benefits for organic growers.