Apply manure sooner or later?
A frequently asked question this time of year is: “What do you think about putting on manure in August or September?” The response is fairly simple: “What do you want to achieve with manure application?”
Liquid swine manure’s nitrogen form is primarily ammonium-N. Ammonium-N is positively charged and is attracted to negatively charged soil particles, preventing losses such as leaching. But this manure is similar to high ammonium-N content fertilizer, such as anhydrous ammonia that is subject to the same processes for nitrogen loss once it is converted to nitrate-N.
If your goal is to use liquid swine manure as a nitrogen fertilizer for corn the following year, then your primary objective would be to wait as long as possible in the fall to apply. As soil temperatures cool, microbial processes slow, and the conversion of ammonium-N to nitrate-N is also slowed down, thereby preventing losses of N as nitrate, primarily through leaching. Nitrate-N can also be lost through denitrification if soils become saturated. We saw this happen in 2014 when we had excessive rainfall and saturated soils in June.
Results of a two-year study in Minnesota showed when manure was applied in early September, corn yields averaged 10 fewer bushels per acre than with application in October or November. Similar studies showed that soil nitrate concentrations in June of the following year were much less in the plots where liquid swine manure had been applied in August and September versus the November application date.
Conversion to nitrate is dependent on soil temperature and moisture, whereas leaching is dependent on rainfall. The earlier the manure is applied, the more rapid the conversion takes place and the greater the time available to lose N due to leaching before the corn crop can access the N.
Incorporation and injection also slow gaseous losses of N by volatilization, as the manure has time to react with the soil. Losses of phosphorus and potassium should be negligible if manure is injected. Whereas if liquid swine manure is surface-applied, losses can occur if water flows across the surface or if erosion happens.
Solid and bedded manure
Solid and bedded manures have a higher concentration of N as organic N. This N must mineralize or breakdown into inorganic N forms that are available to the plant. The process of mineralization is increased in warm, moist soil conditions. We often see the impact of residual N as these manures continue to slowly break down.
Application of this manure in early fall would give more time for mineralization. But since these manure sources are surface-applied, the impact of N losses to the atmosphere and the time it’s exposed to potential runoff must be considered.
There are many reasons why manure is applied early in the fall. Sometimes manure storage is full, often we have longer periods of dry weather leading to good soil conditions for application, or more time is available in fall vs. spring when we concentrate on planting crops. If you have to apply manure, especially liquid swine manure, early in the fall, consider:
• applying only enough to alleviate storage concerns
• applying only a half rate so you can apply the remaining rate of N in the spring as commercial fertilizer or manure
• incorporating or injecting manure to maintain its value of P and K
For more information, read “Swine Manure Application Timing: Results of the Experiments in Southern Minnesota,” at http://www.extension.umn.edu/agriculture/manure-management-and-air-quality/manure-application/docs/manure-timing.pdf.
Rieck-Hinz is an Iowa State University Extension field agronomist.
This article published in the September, 2015 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2015.