Research
The Nitrogen Index - Impact on Corn Production in Ontario
by Keith Reid, OMAF

We followed four principles to develop the N index:

1. When maximum economic rates (‘agronomic rates’) of nitrogen are applied, most of the added nitrogen is taken up by the crop and is not available to leach at the end of the season. Nitrate leaching from in-season nitrogen application is only a concern when agronomic rates are exceeded.

2. Nitrate can move only when water is moving down through the profile. In Ontario, the average rate of evapotranspiration is greater during the growing season than the amount of precipitation, so there is little or no opportunity for nitrate leaching out of the rooting zone during the growing season. Only nitrate in the soil at the end of the growing season is a concern. This nitrate may be from spring N applications above agronomic rates, or from fall-applied nitrogen in manure or other materials.

3. Soil type will have a large impact on the rate of downward movement of nitrate, and on the potential for losses by denitrification. Coarse sandy or gravelly soils transmit water very quickly, so there is a high risk of nitrate movement as well as a high probability of aerobic conditions, so there will be little denitrification. The opposite is true in fine-textured soils.

4. Soil Hydrologic Group is a reasonable indicator of both leaching risk and denitrification potential. In an ideal world, each field would be assessed individually for hydraulic conductivity and for denitrification potential, but these measures are neither quick nor easy to perform. The soil hydrologic group, based on the soil layer with the least potential for water movement, is an easily accessed estimate of soil conditions.

Calculation of the N index is triggered by application of available nitrogen to a crop at rates more than 15 pounds per acre over the recommended rate (OMAF general recommendations or recommendations from soil nitrate tests). For manure application on legume crops, the trigger becomes 30 pounds per acre over crop removal.

If a nitrogen index calculation is triggered by high N application rates, the Crop Removal Balance must be calculated, which is the amount of available nitrogen applied (from all sources) minus the amount of nitrogen removed in the harvested portion of the crop. This balance is used to determine ‘Value A’ from Table 1.

If a nitrogen index calculation is triggered, and manure or other nitrogen-containing materials are applied in the fall, the portion of the applied N that could be converted to nitrate before winter is determined, and this amount is used to determine ‘Value B’. The higher the nitrogen availability from a particular manure, the more nitrogen that could be available for loss. Nitrogen availability from manure depends on the ammonium content of the manure, the rate of mineralization of the organic portion, and how early in the fall the manure is applied. The ammonium fraction is immediately available, as opposed to organic N that needs to be mineralized first, so liquid manure generally has a higher N availability than solid manure. The mineralization of organic nitrogen is normally faster for manure from pigs or chickens than from ruminant livestock.

The impact of the N index on corn growers using only commercial fertilizer will be minimal, unless the grower is using higher rates of N than are recommended. Even in this situation, there is significant flexibility if the crop is being grown on medium to heavy textured soils. There will be much greater effect on livestock farmers using N rates above OMAF recommendations, particularly those using liquid manure on sandy soils. In these situations, the most common solution will be to shift from fall manure application to spring
application.

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