Summer applications of manure to recently harvested wheat fields are a great option for managing manure volumes on a livestock facility. In an ideal situation, soil samples are collected following the wheat harvest and an agronomically appropriate rate of manure can be applied based on the future cropping plans for the field. However, we do not live in an ideal world. Whether the soil is too dry and hard to sample, or the manure pits are about to overflow, summer manure applications are often done without recent soils data to guide the rate. Here are some expectations and management tips for managing nitrogen, phosphorus, and potassium following a summer manure application.
Nitrogen in manure exists in two primary forms, ammonium and organically bound nitrogen. Depending on the type and storage method of the manure, the percentage of nitrogen in either form can vary greatly. The ammonium fraction is immediately plant available. While ammonium is relatively immobile in the soil, this form of nitrogen is easily converted to nitrate when soils are moist and warm which makes the nitrogen prone to leaching and denitrification. The organic fraction of nitrogen requires microbial decomposition to become plant available. A portion of the organic nitrogen will carry over to the next growing season. The relative amounts of organic and ammonium nitrogen forms are based on the type and handling of the manure.
Consider using a cover crop to help recover some of the plant available nitrogen to be released to a future crop when the cover crop residue decomposes. To help account for nitrogen that does carry over to the following growing season, use soil nitrate and ammonium testing so that an appropriate application of supplemental nitrogen fertilizer can be applied.
The general rule of thumb for managing phosphorus from manure, is that if you can keep the manure and soil on the field, you will keep the phosphorus as well. Cover crops do not take up an appreciable amount of phosphorus from a manure application, but they can help reduce runoff and erosion, keeping the phosphorus in the soil profile. Subsurface injection or light incorporation of the manure will also help minimize phosphorus losses. Collecting soil samples shortly after a manure application will not give you an accurate assessment of the availability of the phosphorus for the next crop. While there is no exact rule for how long to wait before soil sampling a manure field, most crop advisors would agree that you should wait 3 to 6 months to give enough time for the fresh source of phosphorus to come into equilibrium with the soil.
Potassium from a manure application is considered relatively immobile in the soil because it is a positively charge ion that can be held in the soil profile by the cation exchange capacity (CEC). Practices for minimizing loss of potassium and soil testing for it are the same as mentioned above in the phosphorus discussion. However, the soil’s ability to maintain potassium until the next growing season is going to be highly dependent on the type of soil the manure was applied to. Sandy, low CEC, and high organic matter soils have the lowest ability to hold potassium. If possible, select fields or sections of fields with more finely textured, higher CEC, soil to apply manure during the summer.
With the current price of conventional fertilizer, it is becoming increasingly necessary to not only utilize the nutrients in manure but to properly assess what is in the soil profile to avoid over applying any fertilizer.