Unlike commercial fertilizers that can be mixed to achieve a desired nutrient content, manure comes with fixed nutrient ratios. These ratios often don’t align perfectly with the needs of field, and this can lead to over or under application of certain nutrients, including nitrogen and phosphorus.
In a University of Minnesota Extension Crop News article, extension educator Chryseis Modderman posed the question, “Should you apply manure based on nitrogen needs or phosphorus needs?” The answer, she wrote, depends on the soil and the manure test results.
If a field is already high in phosphorus, a farmer may decide to apply manure on a phosphorus-based rate to avoid further build up. If phosphorus levels are low, a nitrogen-based rate would work well as long as it won’t elevate phosphorus levels too high.
Even though phosphorus is less mobile than nitrogen, Modderman explained there is still a risk of loss through runoff and erosion. To avoid phosphorus build up, she shared two strategies. One is to apply the nutrient at a phosphorus-based rate. An alternative method is to apply manure at the nitrogen-based rate, but then not apply any more manure to that field until the excess phosphorus has been utilized.