In the previous blog we talked about different fertilizer strategies for P and K. In this blog I we will discuss the different approaches for N recommendations. Nitrogen fertilizer recommendations are a lot easier to interpret compared to other nutrient recommendations. Typically, to calculate a standard N fertilizer rate, you only need your yield goal and the N required per bushel or weight of crop to obtain your yield goal. The N required per bushel is often when differences come in when viewing recommendations from soil testing labs. You should ask what values they use and make sure they match with what you have seen on your own farm. For example, for corn I often use 0.8-1.0 lb N/bu to base recommendations off of instead of the traditional 1.2 to 1.3 lb N/bu. I have several on-farm data sets that support the reduced rate of N.
It is useful to know the organic matter (O.M.) content, soil sampling date, sampling depth (if more than 2 feet), the previous crop and the amount of straw residue remaining on the surface in continuously cropped fields. The following are some of the adjustments to a ‘generic’ N recommendation:
Adjustments to Standard Rate
1. The level of O.M. in your field should relate directly to how much N is released during the growing season. Twenty pounds of available N per acre is expected to be mineralized during the crop year for each 1.0 percent soil organic matter in the surface six inches for warm season crops (e.g. corn, grain sorghum), while 10 pounds nitrogen per acre is expected to be mineralized for each 1.0 percent soil organic matter for cool season crops (e.g. wheat). Most N fertilizer recommendations were created assuming a typical O.M. content of 2%. If the O.M. content is 1% or lower, 15 to 20 lb N/acre may need to be added to your standard rate because less N will likely be released from the O.M. present. Conversely, if the O.M. is 3% or higher, you can likely subtract 15 to 20 lb N/acre from your standard rate.
2. If the previous crop was a legume, N rates can be decreased because some N from these high N containing crops will likely be released during the growing season. Different legumes have different credits so do some research on determining the appropriate N credit from legumes.
3. If fertilizer N will be surface applied to no-till small grain stubble that remains on the surface from the previous year, some N will generally be tied up (‘immobilized’). If light residue add 10 lb N/ac and increase up to 40 lb N/ac as residue thickness increases.
4. Sampling time is important to capture the true amount of N that will be available at seeding because some O.M. decomposition occurs during the winter months, releasing plant-available N in a process called “mineralization”. The closer you can sample to planting the better in accurately determining soil nitrate levels. It is important to take the time to collect the 0-24” samples as often times you can have a tremendous amount of nitrate-N in the soil (50-150 lb N/ac).
Another factor that is important to consider is the efficiency of your applications, i.e. how many applications of N are made during the season. The more splits, the more efficient you are in utilizing the applied N. This can potentially decrease the lb of N/bushel that is required.
The only deviations from this traditional approach of N recommendations are sensor based approaches and methods to predict what N will be released during the growing season. These will be discussed in a later blog. Remember that the recommendations that come back are just recommendations and they need to be customized to your operation. I typically make several modifications when making fertilizer recommendations.