With the high cost of fertilizers, particularly Nitrogen (N), growers will want to credit all sources of N on the farm. These include previous legume crops, recent manure applications and an often unmentioned source--the supply of nitrogen from the soil organic matter. A well managed, healthy soil with frequent applications of manure has the ability to supply significant quantities of N to a growing corn crop.
In fact, the largest "pool" of N in the soil is contained in soil organic matter (OM). This N is released from soil OM in the ammonium form through the natural microbial consumption of OM called mineralization. The process begins as soils warm in April and increases with rising soil temperatures, continuing through the growing season.
You can credit 35 lbs./acre of available N per % soil OM. Mineralization of soil OM coupled with frequent applications of manures and cover crops to build soil quality can easily supply 100 to 150 lbs/a. or more of available nitrogen. Soil management practices that help to conserve and build soil organic matter, such as no-till and conservation tillage, can take the largest credit for this source of N. How do you know how much N to credit? You need to create a balance sheet of all the available sources of N on your farm including:
- A reliable estimate of the amount of manure applied to each field
- A manure analysis (estimates are available in the Penn State Agronomy Guide)
- An estimate of the percent (%) of manure N available to the crop based on your handling practices (These are available in your Agronomy Guide (p. 38, table: 1.2-14, 2007 edition)).
- Amount of N from starter fertilizer
- Amount of N from any previous legume
If the sum of available N is within 50-75 lbs. of that needed by the crop (1 lb. N/bu of corn) and you have built up the soil with good management and consistent manure applications, you may be in a position to "wait and see" if this N is actually needed. The PSNT (a soil nitrogen test) and Leaf Chlorophyll Meter both can be used when corn is approximately 8-12" to determine if you need any additional N. These tests are specifically designed for fields with high fertility (manure, other organics or after alfalfa). The only limiting factor is that they are most reliable if you do not apply broadcast N before planting--and limit starter N to 15-20 lbs. Later, you can test to determine if additional N is needed. It would then be applied as a side dress treatment.
The efficiency of nitrogen fertilizers follows this order: sidedress, at planting, and preplant broadcast. Dribbling the balance of your N as UAN on the surface when corn is 8-12" is the most efficient and should be considered when practical. In many cases, a little starter N followed by the bulk of your N at sidedress time can be a very efficient program. Penn State data has shown that similar amounts of N applied as a sidedress vs. broadcast consistently results in more N being available to and utilized by the corn crop. Another benefit of waiting to apply N is that you can determine if it is actually needed. This is very helpful in no-till situations where manures and residues are left on the top. These tools will show if the N from recent manure applications and the mineralization of cover crops and past manure use will allow you to reduce or eliminate that expensive N. Many county Extension and Conservation District offices now have chlorophyll meters or will assist you in taking a soil nitrate (PSNT) test. In some cases, you can sign out a meter and receive training in its use. Meters are available on a first come basis, so call several weeks ahead to get your name on the list.