Update:

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Water quality troubles in the Chesapeake Bay are related to both nitrogen and phosphorus, but much less is known about the impacts of cover crops on phosphorus losses than on nitrogen losses. The main pathway for phosphorus transport from croplands to bodies of water is via surface runoff during intense rainstorms or heavy snow melt. A secondary pathway in areas of poorly drained sandy soils is leaching of phosphorus to drainage ditches. There is little research on how cover crops impact phosphorus losses. Some studies that suggests that cover crops might increase soluble phosphorus at the soil surface where it would be susceptible to becoming dissolved in runoff water. Cover crop mechanisms that cycle P and make soil P more soluble and plant–available may also allow high productivity on Maryland farms with lower levels P fertilization. This could be part of a long-term strategy to make farming more sustainable both economically and environmentally.

Cover crops can affect the loss of phosphorus by several, somewhat contradictory, mechanisms. Cover crops might reduce the volume of runoff water from a storm; increase the amount of rain required to start runoff from fields; reduce the concentration of P-carrying sediment in runoff water; increase the concentration P dissolved in runoff water; reduce phosphorus in surface soil because of plant P uptake; increase P concentration by freezing injury that releases soluble phosphorus from cover crop tissues. The goal of the proposed research is to provide data on how a range of cover crop practices impact the loss of phosphorus by surface runoff.

The results of this year’s runoff research are perhaps best summarized by the data which show the cumulative amounts of runoff water, sediment loss and phosphate-P loss in runoff for all the samples analyzed to date. Between 18 October 2019 and 24 February 2020, an average of 12.8% to 30.2% of the rainfall was lost as runoff during eight runoff-generating events totaling 250 mm of precipitation. This is not counting several rain events that were too light to cause any runoff from any of the plots. Cumulative sediment losses from the first three events were very modest, ranging from 32 to 53 kg sediment per hectare. To put these values in perspective, since they were from only three storms over two months, we could multiply these losses by 6 times to estimate annual rates of sediment loss between 192 and 315 kg/ha. These figures can be compared to the 2 to 4,000 kg/ha annual loss that is considered “tolerable” (T-value) for similar soils by the USDA/NRCS. The amount of dissolved phosphate-P lost in the runoff from the first five events over 4.5 months ranged from 8 to 36 grams of P per hectare (0.1 to 0.5 ounces/acre). If we again assumed a similar rate of P loss through the year, the annual loss of dissolved phosphate-P would range from 21 to 107 g P/ha. While other forms of P (organic and sediment bound) in the runoff remain to be analyzed, these very low levels of dissolved phosphate-P loss in runoff from moderately high P fertility soils under no-till management with crop residue cover should be encouraging.