Assessment of crop residue with an eye towards next spring

Following a successful harvest and uniform spreading of corn or soybean residue, the soil surface should have an even layer of residue and remaining root balls where the plant stalk was cut. Corn heads greater than 4 rows or grain tables wider than 15' present more of a challenge in getting uniform spreading across the harvest area as the residue must be jettisoned further to reach the margins of the harvest swath. A variety of choppers and spreaders are available and will affect the size and distribution of the residue coming out of the combine. Soybean yields in excess of 70 bushels/acre and corn yields greater than 180 bushels/acre should result in 90% soil cover if the residue is uniformly spread.

📸: BASF Contributing Writer/Featuring undisturbed corn residue with 95% soil cover (L) and soybean residue with 90% soil cover (R) following harvest.

How and when does decomposition occur?

Crop residues are broken down by soil microorganisms that are looking for food and energy. Like humans, heterotrophic soil bacteria use chemically reduced carbon sources for energy, oxidizing these sources and releasing carbon dioxide. As the bacteria feed their population grows dramatically. Bacterial feeding and residue decomposition occur at the highest rate in warm, moist soil with plenty of available nitrogen. As temperatures drop post-harvest, the activity of important soil bacteria slows to a standstill. Consider the post-harvest weather and soil moisture conditions in your area to get a sense of how much decomposition has occurred. In most locations, the bulk of the corn residue decomposition will occur in the Spring and into the summer, as the crop grows. The residue that is on the soil surface, or corn stalks that are standing upright, decompose much more slowly than residue with soil contact.

📸: BASF Contributing Writer/Featuring chisel-plowed corn (L) and soybean (R) residue, resulting in approximately 65% and 35% soil cover

📸: BASF Contributing Writer/Featuring three tillage approaches following soybean including chisel-plow (L), and disk-ripper vs. untouched (R).

Burial and physical reduction of residue

Tillage and other operations such as fertilizer injection and ultimately planting all reduce the amount of surface residue. The effects of each operation are multiplicative (big word for the day) meaning that if one operation reduces residue by 75% and another reduces residue by 45% then the net result is (0.75 x 0.45) =34%. For a deeper dive into residue reduction and an accompanying table matching agronomic operation with residue reduction see:

https://www.extension.purdue.edu/extmedia/ay/ay-280-w.pdf

Carbon-Nitrogen content of the residue is key

Soil microbes maintain a carbon-nitrogen ratio of about 8:1 in their bodies, so when decomposing high-carbon sources such as corn stalks (57:1 C:N) or wheat residue (80:1 C:N) they must find soil-available nitrogen to pair with the large amounts of carbon they are processing and building their population. Soil N may be limiting especially when corn residue is tilled into the soil. Soybean residue has a C:N ratio of about 15:1 and has sufficient N to fuel residue breakdown.

Conceptually, the idea of adding N to corn stalks in the Fall to speed decomposition seems reasonable but a study at Iowa State showed that there was no increase in decomposition rates on corn stalks treated with UAN fertilizer. Warm temperatures and soil moisture that favor biological activity are the key to faster residue decomposition rates. https://crops.extension.iastate.edu/cropnews/2014/04/myths-and-facts-about-residue-breakdown

📸: BASF Contributing Writer/Featuring application of UAN or ammonium sulfate to corn stalks. Application of UAN or ammonium sulfate to corn stalks has not been shown to accelerate Fall decomposition.

There are two considerations regarding residue remaining at next Spring's planting time. What sort of seedbed is needed and is residue an issue for pinning in the seed trench? The potential physical interference of residue can be reduced with vertical tillage post-harvest (sizing) or row cleaning equipment at planting. If corn is planted into corn or wheat residue what will competition for soil-available nitrogen do to early-season corn growth. Soybean residue is so fragile and has a favorable C:N ratio that will not cause nitrogen competition between soil bacteria and corn seedlings.

Crop residue that remains on the soil surface is a positive factor for erosion control, increased moisture infiltration, and in-season reduction of soil evaporation and weed growth. Managing residue to meet additional goals for soil warming and effective planting while avoiding potential nitrogen immobilization for early corn growth is also an important consideration.

Coming next: Soil reactions that alter soil pH

BASF provides the information in this article as a service to its customers; however, the views expressed by guest writers are their own and do not necessarily reflect the views of BASF.