Organic wheat-clover intercrop effects on soil nitrate levels and subsequent corn yields

Background/Question/Methods

Undersowing red clover (Trifolium pretense L.) into small grains was a common practice in grain producing systems to add nitrogen to the soil, provide forage and ground cover, but declined when mineral nitrogen fertilizers became widely available. For farmers that need to replace mineral fertilizer, legume-cereal intercropping presents an opportunity to add organic matter and nitrogen, but how much nitrogen is available to the next crop is less clear. Our research objectives were:  1) to determine biomass production of intercropped red and white clover; 2) to determine the amount of nitrogen produced by each clover; 3) to investigate the effects of clover-intercropping on a subsequent corn (Zea mays L.) yield. In March of 2011, 2012, and 2013, we underseeded red clover and white clover (Trifolium repens L.), respectively, into a winter wheat (Triticum aestivum L.) stand near Mead, Nebraska. Split-plot treatments were time of termination (fall or spring) and mowing. Control treatments included no fertilizer treatment, a post-harvest soybean (Glycine max L.) cover crop, and manure. We measured above-ground biomass production, above-ground legume nitrogen content, soil nitrate (NO₃^- -N) concentration, and corn yield.

Results/Conclusions

In 2011, red clover produced 3.04 t ha^-1 to 5.45 t ha^-1 of biomass, depending on time of termination and mowing. White clover produced 1.49 t ha^-1 to 3.03 t ha^-1. Soybeans produced 4.15 t ha^-1. At fall termination, soybean plant N content was 4.15%, red clover was 3.86% and white clover was 3.56%. In 2012, precipitation was 30% of average and clover biomass production was less than 50% of the biomass produced in 2011. Corn grown after clover yielded significantly less than corn grown after the control treatments. Based on biomass production and N content of the clovers, we expected soil nitrate levels to be highest under red clover, but soil nitrate tests showed no significant difference between clover and the control treatments. Decomposition rates of clover might be slower than those of soybeans, with more of the N available in the second and third year after clover incorporation. Red clover can produce large amounts of biomass suitable as forage or green manure. In water-limiting conditions, the benefits of using a red clover intercrop need to be weighed against potential soil water deficits red clover leaves to a subsequent crop.