Impacts of Cover Crops on Phosphorus and Nitrogen Loss with Surface Runoff

Aug 2019


Surface runoff accounts for most of the phosphorus (P) delivered to streams from Iowa fields. However, important fractions of runoff P are dissolved reactive P (orthophosphate P) and particulate (sediment-bound) P. Dissolved P is readily available to aquatic organisms and a large proportion of particulate P becomes available over time, depending on properties of the receiving water body. Recent surveys of Iowa streams and in the Lake Erie watershed suggest the amount of dissolved P loss from fields and its impact on water quality is greater than often assumed. Iowa research has shown higher orthophosphate P loss with fertilizer than with manure, and some conservation practices that reduce erosion and particulate P loss may not reduce -- or may even increase -- dissolved P loss. Assessment of dissolved P loss has become especially important because it is the most active runoff P fraction impairing water quality.


This research continues evaluations already underway for one more year. This is needed because weather at the research site (mainly rainfall) did not provide consistent runoff during the last four years and no reliable conclusions can yet be drawn.

  1. To continue assessing for one more year (2019 crop year) the impacts of a winter cereal rye cover crop on soil, P, and nitrogen (N) loss with surface runoff in a field testing very high in P, managed with a corn-soybean rotation and no-till or chisel-plow/disk tillage.
  2. To complete data management, statistical analyses and final reporting of soil-test measurements, crop yields, soil loss and several N and P fractions for five crop years (2015 through 2019).


The project has been developed at a 42-acre field at the Hermann Iowa State University farm located south of Ames where 12 areas ranging from 1 to 3 acres accommodate four systems replicated three times. Evaluations include, as in previous years: (1) no-till without cover crop, (2) no-till with a cereal rye cover crop each year, (3) chisel-plow/disk tillage without a cover crop, and (4) chisel-plow/disk tillage with a cereal rye cover crop each year.