Nicole Tautges and Ian C. Burke, Washington State University, Pullman, WA
Soil fertility and weed control pose the biggest challenges during the three-year transition period. Without access to manure, growing green manure crops during the transition period is a common strategy to build soil fertility, but can decrease returns and increase risk. To address these challenges, an organic transition study was designed to examine the effect of inter-cropping pea green manure with small grains crops, enabling growers to invest in their soil health and fertility without having to sacrifice a year of crop production and revenue. During the first two years of the transition period, spring wheat and triticale was monocropped and planted in alternating rows with spring pea, which was terminated with a cultivator between grain rows. Residual effects of monocropping and intercropping were observed in winter wheat, planted in the third year. In years 1 and 2, intercropped spring wheat yielded 1180 to 2820 kg ha-1, which was 50 to 65% of monocropped spring wheat. Intercropped spring triticale yielded 1440 to 2790 kg ha-1, which was 68 to 80% of monocropped triticale. Relative weed biomass was similar between monocropped and intercropped small grains, but all intercropped treatments contained lower weed biomass than spring pea green manure alone, where weeds comprised 60 to 75% of the pea stand. Soil inorganic nitrogen levels after two years of intercropped triticale were greater than all monocrops at 100 kg ha-1, including spring pea green manure (70 kg ha-1). Correspondingly, year 3 winter wheat yields were greatest following intercropped triticale (4600 kg ha-1) and lowest following spring pea green manure (2590 kg ha-1). Winter wheat following intercropped wheat yielded 45% greater than following monocropped wheat. Intercropping small grains with pea green manure could increase grower returns while building soil nitrogen levels to a greater extent than pea green manure alone.