Last time we started the conversation on precision manure application and some of the challenge of variable rate manure application, this time we’ll dive into an example and see how the economics play out.
Precision agricultural methods and variable rate application can improve nutrient management and help reduce environmental impact. We talked last time about doing this with nitrogen and the challenges it presents, but as manures also provide phosphorus and potassium. These nutrients and their need for upcoming crops can be well predicted by soil sampling. This means grid soil sampling can be used to tailor phosphorus and potassium recommendations across the field and put down a sufficient, but not an excess amount anywhere. In some cases, this may allow us to cover more acres with manure and save the purchase of commercial fertilizer for those additional acres.
The process of soil sampling and developing the “to apply” maps are similar to what would be done for commercial phosphorus or potassium fertilizers. The first main difference from commercial fertilizer is you get a guaranteed analysis, whereas with manure you have to sample, sample, sample to get a handle on its composition. This could range from testing every load to taking a composite sample, the key being you have to trust the analysis to have less variability than the rate change over the field. In some cases, this may be easy, if the manure was well mixed as it is loaded and stockpiled it will be more uniform than if it was loaded from different areas of a pen. Getting a handle on this variability is critical to make variable rate application a success.
A study by ISU on variable rate manure application showed it wasn’t unusual for fields to span four to five soil test phosphorus classes within a field. By varying the manure application rate across the field they were able to positively impact yield similar to what was obtained with a fixed application rate, not apply manure to areas that were optimal or high in soil test P and in so doing, save manure to be utilized in other areas. Moreover, the variable application rate decreased soil test variability, reducing soil P in areas that were high (and as a result more susceptible to phosphorus loss) and increasing phosphorus content in areas that were low, and the phosphorus in the soil positively impacted future crops.The variable rate allowed for better manure management by conserving manure where it wasn’t needed and placing it where more value could be obtained while also reducing the risk of phosphorus delivery from the field to water resources.