Effect of Field History and Tillage on Crop Productivity and Soil Quality in Organic No-till Soybean Systems.

This experiment was conducted to determine the effect of tillage and field history on organic soybean crop productivity and soil quality. Crop rotations at this certified organic site at the Iowa State University Agronomy Farm (Boone, IA) were Rotation 1: corn–oat/alfalfa–soybean–oat/alfalfa; or Rotation 2: oat/alfalfa–corn–soybean–oat/alfalfa. Rye was planted at 80 to 145 kg/ha in late October 2013 to 2015, and each following Spring, half of the rye plots were disk mowed before planting, and, in the other plots, the rye cover was rolled/crimped. Soybeans were planted into the mulch in 76-cm rows at 405,080 seeds/ha. Rye mulch coverage in soybean row middles was greater in rolled no-till plots, averaging 44% coverage, compared to 27% in mowed rye plots. In 2014, soybean yields averaged 2,958 kg/ha, with no effect from tillage or rotation treatments. In 2015, soybean yields averaged 2,084 kg/ha in mowed plots, with no-till plots greater at 2,824 kg/ha. A drought in June 2016 prevented planting until July 6 when rains returned, so soybean plants were chopped for livestock feed on September 13. Soybean dry matter yield at harvest was equivalent between treatments, averaging 4.9 MT/ha. Soil samples, taken to a 15-cm depth for soil quality analysis after crop harvest in 2014 and 2015 revealed an impact of no-tillage on macroaggregate stability. Macroaggregates (> 250 µm in diameter) comprised a greater proportion of the total soil mass in the no-till soil compared with the mowed treatment. Mehlich-extractable P was higher in the mowed treatment compared to the no-till for both rotation sequences, but the effect was significant only in 2014. Macroaggregate stability is an integral indicator of soil dynamic change, where enhancement is also related to changes in soil structural stability, water infiltration and storage, and carbon storage.