Researchers evaluated loads of total phosphorus, dissolved phosphorus, particulate phosphorus and total solids against flow volumes to learn how phosphorus and sediment losses differed between plots. Shallow-disk injection of manure was found to be more effective than broadcasting manure in promoting dilution of dissolved phosphorus and to a lesser extent, total phosphorus. The broadcast manure plots experienced more runoff of particulate phosphorus than did the injection plots.
Importantly for no-till advocates, no difference was detected between application methods for total solids in the runoff — meaning manure injection, with its slight disturbance of the soil surface, did not cause sedimentation. No-till practitioners, who constitute slightly more than half of the dairy farmers in Pennsylvania, have been slow to adopt manure injection due to concerns about the practice causing sedimentation and muddying streams.
However, the precision and accuracy of the study, recently published in Agriculture, Ecosystems and Environment, was constrained by hydrologic variability, conceded Jack Watson, professor of soil science and soil physics, Penn State. His research group in the College of Agricultural Sciences conducted the study. Watson pointed out that the findings demonstrate that, even at a small scale, the effectiveness of a practice in accomplishing water quality benefits varies.
"This has been the case with previous phosphorus-mitigation field studies, as well," he said. "Even studies done with carefully constructed research plots like ours, which allow us to collect, measure, test and contrast runoff, are confounded by hydrologic variability."
But despite the variability, the findings showed that manure injection decreased the overall phosphorus losses, according to lead researcher Melissa Miller, a master's degree student in soil science when she conducted the study.
"When we looked at the total phosphorus losses from the plots, we were able to see a strong trend," she said. "It was revealed in both overland and subsurface flows following rain events."