Wheat Yields, Nitrogen Uptake, and Soil Moisture Following Winter Legume Cover Crop vs. Fallow

Long-term use of fallow in dryland wheat (Triticum aestivum L.) systems can increase erosion and decrease soil fertility. One possible solution is to grow a green manure crop during the fallow period. To evaluate the short-term effects of substituting a winter legume green manure for fallow in a 2-yr wheat rotation, a series of experiments were conducted in the Sacramento Valley of California from 1994 to 1996. Soil water content, inorganic soil N, and yield of fertilized wheat after fallow were compared with those of unfertilized wheat following a woollypod vetch-field pea [Vicia villosa Roth subsp. varia (Host) Corb.; syn. V. dasycarpa Ten.-Pisum sativum] green manure crop. Supplemental experiments (1995-1996) compared unfertilized wheat following (i) fallow with incorporation of legume biomass imported from another plot, (ii) growth and incorporation of green manure plus supplemental legume biomass from another plot, or (iii) growth of a legume crop with removal of aboveground biomass. Soil water content after a green manure crop was 6.6 cm less than after fallow in 1994 (to the 90-cm depth), but only 1.5 cm less in 1995 (to 150 cm). During the wet winter of 1995-1996, fertilized wheat plots after fallow had higher inorganic soil-N levels than unfertilized plots following a green manure crop, but wheat yields were similar. Where a green manure crop was grown and incorporated, there was no wheat yield response to additional legume biomass. Similarly, without a green manure crop, there was no yield difference between fertilized plots receiving 112 vs. 28 kg N ha -1 . Results could be different in drier years or in soils with lower N fertility, but the data indicate that possible long-term yield benefits of green manuring are not necessarily preceded by lower short-term yields.