As fall field work is finishing up around the U.S. farm sector, one of the last tasks is the application of manure to get hog barn pits and lagoons ready for a long winter.
Daniel Andersen, Iowa State University assistant professor in Agricultural and Biosystems Engineering specializing in Manure Management, recently addressed questions of how manure moves through the soil profile in a recent edition of his blog “The Manure Scoop.”
Andersen, aka Dr. Manure, explains that there are some factors at play that determine how manure filters down — moisture content of the manure, soil type, soil structure, how wet the soil is, type of injector being used — and these can alter things a bit, “but in general for most liquid/slurry manure applications we tend to see roughly the same pattern.”
To illustrate how manure moves through soils, Andersen went to the kitchen and created a “Jell-O Poke Cake.” For those not in-the-know, a Jell-O Poke Cake is normally a white cake that after it is baked holes are poked in the cake surface with a toothpick, and while Jell-O is still in warm liquid form is poured over the cake. “The cake is a porous structure (similar to soil) and the Jell-O is a liquid-y manure slurry so it works pretty well to make the visual because you get a good color contrast.” See the picture to get an idea of the “soil” profile the cake illustrates.
“So what you see here is a red finger in a sea of white cake. The manure flowed down the injection trench and wicked sideways into the cake where it was held as the Jell-O cooled. Manure movement in soil looks surprisingly similar! In the following picture you can see an example of this movement in soil. In this case it was just a blue dyed water to make the movement more visible. What you see is pretty similar though; the liquid fills the injection slot and then wicks out to the side and downward. The sideways movement is caused by capillary pull from the soil while the downward movement is caused by gravity. The water (manure) keeps spreading out until the soil has enough capillary pull to hold onto what is remaining. When we put manure on, we want the soil to be able to hold all the manure in the root zone, where the nutrients in the manure (the nitrogen, phosphorus, potassium and others) will interact with the soil particles and hopefully stay there until our crop can use them,” Andersen writes.
Andersen goes on to explain that knowledge of various types of water in the soil profile is important to determine the amount of manure that can be applied. “I know what you are thinking, isn’t all the water the same? And it is, but there are different ways that soil can hold onto water — there is hygroscopic water, capillary water and gravitational water. Hygroscopic water is basically a water over the surface of the soil particles. It is held onto by the soil really tightly and in most cases plants can pry it off. Capillary water is held in the small pore space between soil particles. This is the water plants use to grow — it is water that the soil can hold onto tight enough to keep it from draining away, but not so tightly that plants can’t get it. … Gravitational water is water beyond what the soil is capable of holding onto and it will drain right through. The amount of water in the soil at these points can often be estimated based on soil texture.”
These properties, field capacity, permanent wilting point and water-holding capacity are generally thought to be related to the soils texture, but can vary considerably based on the amount of organic matter in the soil, the soil’s structure and the field’s recent tillage and management history.
Andersen further explains the water holding capacity of various soil textures, and other considerations producers and manure applicators need to thinking about to make sure the manure is kept in the field to be able to take full advantage of the fertility it has to offer.
Click here to read this blog, and other manure management blogs by Andersen.