Impact of Degree of Fertilizer and Manure Incorporation and Timing of First Runoff Event on Phosphorus Losses to Surface Runoff

Traditional teaching on phosphorus (P) behavior in soil emphasized its retention, and therefore conservation programs to reduce P loss from agricultural soils focused on reducing soil erosion. More recent observations in the Great Lakes watershed suggest that losses of dissolved forms of P are increasing. Combined trends in tillage practices, along with timing and placement of fertilizer and manure, may drive this increase in loss of soluble P. This project aims to assess the effect of timing and placement on losses of dissolved and particulate forms of P. The project has three major objectives: i) controlled runoff studies assessing fertilizer and manure P placement/incorporation; ii) controlled runoff studies assessing commercial fertilizer placement and timing to first runoff event; and iii) a field study relating runoff and incorporation methods on P losses from fertilizer and manure.

For the first objective, three soils (of clay, silt loam, and sandy loam texture class) were placed in controlled environment runoff boxes (40 inches long, 8 inches wide and 4 inches deep). Treatments compared surface broadcast, incorporation, surface band, and subsurface band application methods. The three P sources were monoammonium phosphate fertilizer, solid cattle manure, and liquid dairy manure, each applied at a rate to match two years of crop removal from a typical corn-soybean rotation. Artificial rainfall, representing a storm of 1 in 10 years frequency, was applied 7 days after P application, and again 7 days after the first runoff event. Runoff water samples are now being analyzed for total P and dissolved reactive P. Analysis of 2016 measurements is continuing and a site has been identified for the field study to be conducted in 2017.

Preliminary results from the controlled environment runoff box studies indicate that the highest losses occurred when P sources were left on the surface. Incorporation reduced losses considerably, but losses were still around five times greater than the control. The greatest loss reduction occurred with subsurface band placement.

One undergraduate and one graduate student are involved in this project. The information being generated is highly applicable to quantifying the water quality benefit associated with timing and placement of P.