After talking with many farmers throughout Ohio during this year’s Extension meetings, one common question keeps popping up: What about nitrogen application to soybean? Yes, soybean plants have high nitrogen requirements due to the high protein content of grain. On average, approximately 4 pounds of nitrogen is removed per bushel of grain. (Corn only removes approximately 1 pound of nitrogen per bushel of grain.) Soybean nitrogen requirements are met through both nitrogen-fixing bacteria (Bradyrhizobia) and residual/mineralized soil nitrogen.
With higher soybean yield potential, do we need to use nitrogen fertilizer on our soybeans? In most situations, applying nitrogen to soybean has no yield benefit. Nitrogen application may be beneficial in soils with low residual nitrogen and/or low soil organic matter.
In 2013, we examined various nitrogen sources (polymer-coated urea, urea, and sulfur-coated urea), nitrogen rates (ranging from 30-400 pounds of nitrogen per acre), nitrogen placement (2-inch by 2-inch band and surface-applied), and nitrogen application timing (at planting and R3) at 10 locations throughout Ohio. Across all 10 locations, there was no yield benefit to any nitrogen source, rate, placement, or application timing. Our soybean yield averaged 43 to 75 bushels per acre depending on location. This research will be repeated in 2014.
One way to maximize nitrogen uptake by soybean plants without applying nitrogen fertilizer is by adjusting soil pH. Nodulation of soybean roots is adversely affected when soil pH drops below 6.0. In 2013, we collected soil samples from 65 farms throughout Ohio. Overall, 29% of the samples we collected had a soil pH of less than 6.0. Low soil pH occurred primarily east of I-71 (north eastern and eastern Ohio) where soils do not have as much lime content. However, there were a few areas in northwestern and western Ohio that had soil pHs less than 6.0. We suggest taking a soil sample and adjusting soil pH to be greater than 6.0 to maximize nitrogen uptake by soybean.