Applying MAP and DAP at Corn Sidedress

By Greg LaBarge

One fertilization strategy is to apply a two-year rotation phosphorus need ahead of the corn crop. The primary source of phosphorous fertilizer is the nitrogen-phosphorous (N-P) containing products of 11-52-0, Monoammonium Phosphate (MAP) and 18-46-0, Diammonium Phosphate (DAP). For example, the maintenance P need for corn yielding 180-bushel per acre and soybean at 60 bushels is 111 pounds of P2O5 per acre.

When applied in fall, the phosphorus from these products is solubilized and retained in the soil labile phosphorus pool. To supply this P need with MAP or DAP, we also apply 23 or 43 pounds of N with the application. This nitrogen is subject to environmental loss when fall-applied by leaching or denitrification. The net nitrogen result from fall MAP or DAP application is that little of the applied N is available to meet crop needs. By changing MAP and DAP application timing from fall to at sidedress, can we reduce the sidedress need from other N sources? Does this improve the economics of meeting nutrient needs in the rotation?

For this project, the total N rate was set at 180 pounds per acre, the Economically Optimal Nitrogen Rate (EONR) using a nitrogen to corn price ratio of 0.10. At planting, 40 pounds of N was applied in a 2 by 2 placement. The additional 140 pounds of nitrogen was applied to V4-V6 corn. The combination of nitrogen sources to meet the desired N rate was from MAP or DAP plus Urea, or 28% UAN depending on plot treatment to reach the total N need. The Urea product used was Environmentally Smart Nitrogen (ESN), a polymer-coated product to prevent against losses with surface applications. UAN 28% treatment was applied using a coulter injector. The dry fertilizer products were applied with a drop spreader.

Corn grain yields for 2020 and 2021 are shown in Table 1. No statistical differences were shown between the UAN 28% and DAP+Urea or MAP+Urea treatments in either year. The partial budget for the two-year fertilizer program may be advantageous for sidedress P even though nutrient sources have a higher per-unit N price. The calculation presented used fall 2021 fertilizer prices. The UAN 28% has an extra application trip with the fall P, plus the N from that fall application is lost.

Table 1. Corn yields and two-year fertilizer program cost using surface applied MAP and DAP at sidedress compared to coulter injected UAN 28%.

The point of this project was to start a conversation about different fertilizer application timing to increase fertilizer program efficiency. More equipment options are becoming available, making a dry fertilizer program possible. If you are in a liquid program, using 10-34-0 may be an option to consider.