Tuesday, November 4, 2014

Fall vs. Spring - the whats and whys we need to think about in terms of manure application timing

Storing our manure reduces or eliminates the need to collect and spread the manure on a daily basis. The primary reason to store manure is to provide the capability for the farm to land apply the manure at a time that is compatible with both the climatic and cropping characteristics of the land receiving the manure. For example, in the Midwestern United States, we generally try to avoid applying manure in the winter as the snow cover and frozen soils make it difficult to get the manure where we want it and to do a good job. Similarly, during the summer when our crops are actively growing it can be difficult to find good places to land apply our manure. This leaves us with two big windows for manure application, in the spring after the soils start to warm up and before we plant crops for the up-coming year and in the fall after our crops have been harvested but before the soils freeze.

So, this brings up the question is spring manure application better than fall application? Well, applying in the spring leaves less time for decomposition of organic material in the manure and conversion of manure nitrogen into nitrate before the crop is up and actively growing. This can be a good thing as it can reduce the loss of nitrate, but often times spring is a busy time with planting and other fieldwork and any delay caused by waiting for manure application might reduce our yield potential. Additionally, in the spring we are often dealing with wet soils that might make soil compaction a concern. If we apply in the fall, it gives microbes in the soil time to decompose the manure, which can make the manure nutrients available to the crops as soon as they are planted. On the other hand, it gives us more time for nitrogen loss before the crop is up and growing.

A recent summary of nitrogen application time was performed as part of the Iowa Nutrient Reduction Strategy.  The summary was not specific for manure, but was geared at how timing of nitrogen application influenced losses through leaching and crop production. They estimated that switching from fall fertilizer application to a pre-plant nitrogen application in the spring would reduce nitrogen losses by 6% on average. These studies also indicated that switching from fall application to spring application would increase corn yield by 4% on average, (For completeness they found that switching from spring nitrogen application to sidedress application or a split pre-plant/sidedress application would reduce nitrate leaching by 4-7 %.)

In summary, I don’t think there is a perfect answer for when we should apply our manure, both fall and spring application have different opportunities and challenges. The potential benefits of reduced nitrogen losses from less fall application need to be balanced with practical concerns such as time and equipment availability in the spring and the soil conditions we will be applying our manure too. However, before moving on let’s dive into a bit of the science on this and see what we can find.


Although it is actually a cycle, it is often convenient to think of it as a flow of nitrogen, where the organic nitrogen will first be converted to ammonium nitrogen, and then eventually nitrate nitrogen. To be usable by plants the nitrogen has to be either in the ammonium or nitrate form, so conversion of organic nitrogen to ammonium makes it more plant available. Once it is in the ammonium form there are three things that can happen, plants and microbes can use the ammonium immediately, it can be converted to ammonia and lost to the air via volatilization, or it can be converted to nitrate which is susceptible to denitrification and leaching. Many of these processes (mineralization of organic nitrogen, nitrification, denitrification) are performed by microbes in the soil. The rate these microbes perform these processes at is related to the soil temperature, warmer temperatures faster reaction, colder temperatures, slower reactions. To conserve as much of our nitrogen as possible, we don’t want to let it get to nitrate nitrogen. This is because nitrate is the form most easily lost from the soil as it is extremely water soluble, making it easy to leach into groundwater or tile drains. Ammonium on the other hand will often stay in the soil as long as it isn’t exposed to the atmosphere where it can be lost by volatilization.

So what this means to use if we are using manure as a fertilizer source is if we have ammonium rich manures we want to wait until our soils are cooling before we apply it. Otherwise, it will be converted to nitrate yet this fall and be susceptible to losses all winter and spring before our crops have the opportunity to use it. If we are using manures that are mostly organic nitrogen we still want to be aware of this but probably can apply a little sooner as are manure nitrogen has to go through several conversions before it can be leached. As a general recommendation, ammonium rich manures should be applied after the soil temperatures are 50-degrees and cooling. If manure applied to soils when soil temperatures are above 50 degrees F, the inorganic nitrogen converts rapidly to nitrate-nitrogen, which is a very mobile form of nitrogen and increases the risk of nitrogen leaching into the ground waters.

  This desire to delay manure application until the soil is cooler needs to be balanced again the availability of equipment and the risk of a quick change in weather conditions (frozen soils) preventing manure application. So like most choices in life, there isn’t one easy answer that will work for everyone, it’s all about balancing your risk of not completing your manure application with your desire to maximize its fertilizer value.