Dry ground: What’s the risk when applying manure to your fields?

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By Aaron Wunderlin, Discovery Farms  

An example of a soil crack in August 2018 in Brown County.

Discovery Farms staff members often discuss the risk for nutrient loss when manure or fertilizer applications are made on wet soils because there is increased chance of runoff. But there are also risks associated with applying manure on excessively dry soils.  

In fields with heavier soils, cracks can form when it gets dry. This is the result of clay particles drying out and shrinking. Cracks serve as preferential flow paths directly to tile lines or ground water. 

In Discovery Farms’ tile flow data, there are clear examples of preferential flow to tile lines during dry conditions. In August 2009, for example, a 1.8-inch rain event occurred on dry soil where cracks had been visible for weeks. Early in the rain event, tile flow increased sharply. When the rain ended, flow decreased sharply before tapering off. This sharp peak and decline was the result of water moving through macro pores, including cracks in the soil.  

In comparison, in the same field two days later when soil moisture increased to 35 percent, another 1-inch rain event occurred. When water moved more slowly through the wetter soil, there was a gradual recession after flow peaked. Discovery Farms continues to collect tile drainage data to further understand links between soil moisture and tile water quality. 

Water moving through cracks receives little filtration. When excess nutrients are present, they have the potential to be picked up and moved with the water. This presents risk to both surface and ground water quality. Remember to keep these risks in mind when making manure applications. All manure in the presence of enough rain can contribute to the nutrient load to water bodies. If soil cracks are present in areas with tile drainage or karst landscape, water quality could be at risk.  

Inspect your fields for soil cracks if it has been dry. Knowing soil cracks are there can help farmers make decisions regarding where manure applications should be made. If cracks are present and you have no other place to go, pre-tillage can break up the soil enough to prevent manure or runoff from entering these preferential flow paths.  

Managing manure on dry ground is just as important as managing it when the soil is wet. Knowing the conditions of your fields gives you one more piece of information to use when managing your manure.