Mike Rankin
Crops and Soils Agent - Fond du Lac County
University of Wisconsin - Extension
There are many different sources of nitrogen (N) in a crop production system. These include purchased N, legumes, organic matter, and manure. There are also several mechanisms in which N can be lost such as denitrification when soils are saturated and leaching when crops are grown on sandy soils or when over applications of N are made. All of this makes N management as much of an "art" as it is a "science".
Lots and lots of liquid manure
The trend on many dairy farms is to increase cow numbers and install manure storage systems for liquid or semi-solid manure. Applications of manure to crop fields are typically made once or twice per year. When manure is applied to fields in this type of system, it is often incorporated into the soil immediately. This captures all of the available N for a future crop. Over a period of several years, these manure applications have a significant impact on the N status of the soil, not only for the year following application but also in subsequent years.
192 bushels per acre without applied N
In a 1998 research trial, a corn field was selected on the John Pickart farm, Malone, to test the accuracy of the pre-sidedress N test (PSNT). This was a field scheduled to receive a sidedress application of N (no preplant N was applied). It was a second year corn field that had received 12,000 gallons per acre of liquid manure in fall, 1997. The 1996 crop was soybean. The PSNT taken one week before the sidedress application yielded results of 35 ppm nitrate-N, well above the critical level of 21 ppm where no additional N would be needed. Armed with this information, a randomized and replicated field study was done with treatments of 0, 50, and 100 lbs. N per acre applied sidedress as anhydrous ammonia. Final yield results are presented in Figure 1.
As the PSNT predicted, there was no yield response to additional N. Obviously, a previous manure application and winter/spring weather friendly to N carryover had supplied more than enough N to meet the needs of the crop.
Using the pre-sidedress nitrogen testThe PSNT offers several advantages to its cousin the pre-plant nitrogen test. First, samples only need to be pulled from a one foot depth versus a two foot depth. Second, because it is done at a much later date than the pre-plant test, any N lost from denitrification because of heavy early spring rains or N mineralized from a previous fall manure application is accounted for.
The primary disadvantage to the PSNT is that a producer must commit to a sidedress-N program on fields where benefits to using the test are probable. This means taking extra time during the summer to make N applications and counting on cooperation from the weatherman to get the job done.
The procedure for taking PSNT samples is relatively straight forward. One to two weeks prior to the planned N application date, one-foot soil samples are taken and a composite sample representing each 20 acre area is submitted to the soil testing lab. Based on the amount of nitrate-N in the top one foot of soil profile, an N application recommendation is made. The critical level is 21 ppm. Over this level, no additional N is needed.
Should more N be applied as sidedress?
Agronomically, there is no advantage to applying N sidedress vs. pre-plant unless conditions arise where pre-plant N is lost because of denitrification in early spring. However, on dairy farms where liquid manure is applied at high rates over a period of years, the need to accurately account for long-term N contributions can only be done on a field by field basis using the PSNT. This doesn’t mean that every corn field must be subject to this management strategy , but only those fields where there is some doubt as to N need. These would primarily encompass 2nd or 3rd year corn fields. The PSNT clearly helps to take some of the guess work out of past application rates, variations in manure analysis, and soil N reactions driven by environment. The producer benefits economically, and we all benefit environmentally.
For
more information contact Mike Rankin