No. 7 Article 6/May 21, 2010

Determining How Much Nitrogen Is in the Soil

Trying to maximize returns on investment is at the forefront of every farmer's mind. One of farming's largest and most expensive investments is the application of nitrogen for corn. Every year producers have to determine how much nitrogen needs to be applied to maximize profitability and reduce the potential for water quality degradation that is associated with nitrogen use in farming operations. A corn nitrogen rate calculator was developed based on a robust database of recent corn trials conducted under many environments in the state and across many years. This is a very useful tool to help farmers decide what the economical optimum nitrogen rate would be when corn is following corn or when following soybean. This tool, however, cannot be used to predict how much nitrogen may be available in your soil when manure or other nitrogen-fixing legumes besides soybean have been grown in the preceding year. So it is important in some situations to first determine how much nitrogen is in the soil, and then to use that information and adjust the rates provided in the online calculator.

Pre-Sidedress Nitrate Test

One of the tests that farmers typically use to determine the amount of nitrogen present in soil is the pre-sidedress nitrate test (PSNT). By sampling later in the season (late May to early June), you can get a measure of the amount of nitrogen mineralized into plant-available forms from organic nitrogen plus the amount of carryover nitrogen still present.

Just like with other tools, this test can be useful, but it is important to know under which conditions that is true. The PSNT is often more accurate in high-yielding environments and in fields that have received manure or other organic fertilizers in the recent past or that have had legume crops with high nitrogen content, such as alfalfa. Usually a small starter rate (20 to 30 lb of nitrogen per acre) can be applied without compromising the value of the test. The PSNT is not very useful for fields with soybean or corn as the previous crop or where commercial inorganic fertilizers were applied, unless a substantial amount of carryover is suspected.

Given weather conditions last year, it is very unlikely that much nitrogen was carried over to this year. Something to keep in mind is that if late-spring temperatures are below normal, the test tends to overestimate nitrogen application needs because of lower soil-test values. This is the result of slow rates of mineralization in the soil. Daily minimum soil temperatures in the last 10 days have ranged from the upper 60s (Fahrenheit) in southern Illinois to the low 50s in northern Illinois and represent slightly cooler soil temperatures than normal.

How to collect and handle samples for the PSNT. The reliability of this procedure depends very much on ensuring that samples are collected and processed correctly. Some people suggest collecting samples two feet deep, but research has shown that samples from the first foot-depth were as useful at predicting nitrogen needs. Since two-foot sampling is impractical in most situations and does not improve the predictability of the PSNT, collecting samples only from the top 12 inches of the soil is recommended.

Collect samples when corn plants are 6 to 12 inches tall (V4 to V6 development stage). If the field had a history of broadcast applications, randomly collect 20 to 25 samples from an area no greater than 10 acres. If band applications of fertilizer or manure were used to fertilize the previous crops, collect at least 10 sets of three cores each from two corn rows with 30-inch spacing. Collect the first core 3 inches to the right of one corn row, the second core midway between the two rows, and the third core 3 inches to the left of the next corn row. Place all the cores in a bucket and take a subsample after the cores have been thoroughly mixed. If mixing the entire sample to produce a representative subsample is too difficult, it is better to use large sample bags and keep the entire sample. Collecting a sample less than the full 12 inches or not collecting all the cores will produce unreliable results.

If the samples cannot be delivered to the laboratory the same day, either freeze or air-dry them. If you air-dry samples, dry them as fast as possible by spreading the soil out on paper, crushing the cores, and blowing air with a fan. Since drying can be difficult without proper facilities, freezing is the best option for most people. Make sure to tell the laboratory that you want to measure nitrate-nitrogen. If you send the entire sample, request that the whole sample be dried and ground before a subsample is taken.

Interpreting results. If the PSNT lab results indicate levels above 25 parts per million, you can be certain that no additional nitrogen is needed (there will not be a response to additional nitrogen applied at sidedress). On the other hand, a full rate should be applied if nitrate-nitrogen levels are less than 10 parts per million. With test levels between 10 and 25 parts per million, adjust nitrogen rates proportionally. When calculating the rate, make sure to subtract any nitrogen that was already applied in the starter or in any other way.--Fabián G. Fernández

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