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 after 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.