Soybean Yield and Nodulation Response to Crop History and Inoculation

Soybean [Glycine max (L.) Merr.] inoculation was imposed on a long-term continuous grain sorghum [Sorghum bicolor (L.) Moench] and soybean cropping systems study with and without manure application at Mead, NE. The objective was to determine the influence of 28-yr history of continuous grain sorghum and soybean production, inoculation, and manure on soybean yield and nodulation. Average 2-yr soybean grain yield was 0.5 Mg ha(-1) greater on plots with continuous grain sorghum crop history rather than soybean history, even aft er this history was broken by 2-yr crop rotation with grain sorghum in the 2 yr previous to the evaluation. Continuous production of soybean in the same field is undesirable, and besides short-term loss of yield from lack of crop rotation, also leads to long-term soybean yield reductions. Manure and inoculation application had no influence on soybean grain yield or number of nodules. Soybean in plots with 28-yr previous grain sorghum crop history had equal number of nodules to those in plots with soybean crop history and/or Bradyrhizobium japonicum inoculant. It is probable that this lack of response was due to B. japonicum movement among adjacent plots by wind erosion, water erosion, and/or soil transfer from contaminated tillage equipment used to till adjacent plots. Although the basis for long-term soybean crop history on yield is unclear, continuous soybean production should be avoided to prevent soybean yield depression.