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United States Department of Agriculture

Agricultural Research Service

Author's Advisories for Nitrogen Decision Aid
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Nitrogen Decision Aid


We recommend that soils be sampled by soil mapping unit. Soil maps can be obtained from the local county NRCS office and they are free to landowners and farmers. Within a soil mapping unit, samples may be obtained by grid sampling. We find that most of the soil mapping has been done quite accurately. More detailed maps of a farm or field may be made by NRCS personnel and you should inquire about the possibility of obtaining this service if you feel that it might be helpful.

Most plants readily use nitrate-N. Soil microbes produce some nitrate-N throughout the year by digesting crop residues and soil organic matter. The amount produced varies due to temperature and moisture in the soil. Nitrate-N is also easily lost through leaching and denitrification. So, the amount of nitrate-N varies each year and it also varies between soil types. The N-decision aid is designed to estimate the nitrate-N in the soil as a result of varying spring (planting) conditions.

The N decision aid was developed and tested in Minnesota. Soils in this area freeze often to a depth of 60 cm (2 feet) or more. During the frozen period, soil mineral N concentrations remain unchanged. For that reason, the decision aid was designed for application shortly after the soil thaws in the spring. The decision aid 'runs' for a period of 90 days during which little plant growth occurs.

During this 90 day time period the crop producer will have an opportunity to make decisions about fertilization. For many areas of the world, soil freezing never occurs. In these areas the user must arbitrarily select a starting date and preferably one in which N can be applied with satisfactory results after the crop has been planted. Like soil tests and other measurements, computer decision aids may fail on occasion because of the limited amount of information that is processed.

We recommend that the decision aid be used in trial situations before widespread use. Certainly, the limited region and types of soils on which its development is based should not be regarded as representative of all soils. The critical level of soil nitrate-N varies and it is a value that the user must enter. While studies support use of a critical value of 20 ppm (micrograms per gram) of nitrate-N in the upper 2 feet of the soil profile, we also have observations where other values are better. The value best suited to local conditions must be determined through trial and error.

Soil clay content is an important property for the decision aid and it is also one of the more costly pieces of information that the producer supplies. If the textural class is known, a rough estimate of the clay content can be obtained by using the average value for that class. For example, the clay content for loam soils ranges from 7 % to 27 % and the average value will be 17 %. The estimate of clay content can be refined by considering that of adjacent mapping units. For example, if the adjacent units are clay loams that have clay contents ranging from 27 % to 40 %, it is likely that the loam soil has a clay content nearer 27 %. Estimates, of course, should be avoided as often as possible.

The decision aid ignores topography and we realize that this is a weakness. This is particularly true of depressions that receive runoff water from adjacent areas. In these cases, leaching is often greater than the decision aid predicts. On steeply sloping soils, we expect less leaching than the decision aid predicts. In these instances, the apparent critical value will be different from that for soils with moderate slopes.

The decision aid begins with an assumption that the soil available water content equals 90% of the available water holding capacity. Errors introduced with this assumption are small but they are still errors. The decision aid permits the user to enter the correct value if known. The assumption about initial moisture content was made because we know that most producers have little knowledge of the soil clay content, organic matter content or bulk density. People tend to forego determinations of these characteristics unless some use can be made of the information and, up to this point, obtaining this information has been of limited economical use. Much of the global climate is drier than that found here and a lesser initial water content may be more common; again, the user is able to enter more accurate values.

Soil is not always sampled by 6-inch depth increments or sampled to a depth of 2 feet. So, when the sampling depth is less than two feet, we make additional assumptions about the amounts and distributions of nitrate in the surface two feet of the soil. Again, these assumptions introduce small errors.

Leaching of nutrients in soil is a complex process. The decision aid simplifies this process. In doing so, the distribution of nitrate-N predicted departs slightly from that which is usually measured. The decision aid is designed to estimate the total nitrate-N in the upper two feet of the soil and to this end it has performed well. The disparity in nitrate-N distribution increases as the amount of rainfall infiltration during the 90-day period increases.

Soil bulk density is very important to the decision aid but it is rarely measured. This portion of the decision aid appears to have more uncertainty than many other features. To date, we seem to have accommodated soil bulk density adequately. Certainly, this feature needs improvement, but more research must be done on this property first.

The amount of fertilizer needed is given in terms of elemental N. The user must determine the amount of N in the fertilizer and adjust application rates accordingly.

The US is gradually converting to standard international units for measurement. In anticipation of this conversion, temperature is entered in degrees centigrade and rainfall is entered in millimeters.
All Fahrenheit measurements need to be converted by:

º C = 0.55*(º F - 32) .

All inch measurements of rainfall are converted by:
Millimeters of rainfall = 25.4*inches of rainfall.

.................... WARNINGS.............WARNINGS.............WARNINGS.............WARNINGS

I. Crops that follow a year of alfalfa usually don't respond to N fertilizer additions. Therefore, this decision aid should not be used when field has been cropped to alfalfa last year.

II. The decision aid may not process N added as manure accurately because of the variety of forms in which manure is added to the cropping system, the time of the year in which manure is added to the fields, or the rates at which N in manure is mineralized. Certainly, the pre-plant nitrate test may also require greater study when manure is a source of part of the added N.

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Last Modified: 12/1/2011
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