We will look at nitrogen. Gomes and Loynachan (1984) at Iowa State University studied the effect of growing degree days on the amount of anhydrous ammonia applied to the soil. They found that about 80% of this form of ammonia was immediately recoverable after application. A mystery appeared to exist as to the use or disappearance of the other 20% of the amount applied. Nevertheless, it is apparent that immediately after application of anhydrous ammonia to the soil, 72-95% can be recovered.

Growing degree days begin to accumulate from the moment of application of anhydrous to the field. The days are correlated with the rate of conversion into another form of nitrogen. This is known as nitrification. Nitrification will proceed according to the degree day accumulation. It is assumed that the moisture is correct (if it wasn't you couldn't have applied the anhydrous). By the time 1000 growing degree days have accumulated, 90% of the applied nitrogen has been converted to the nitrate form.

Now, why is this important? For example, in the spring of 1997, anhydrous that had been applied in November had received something less than 200 GDD when heavy rain fell by mid-April. Farmers were concerned about whether or not the anhydrous that had been applied had been lost because of the heavy rains. Since there were fewer than 200 GDD, only about 10% of the applied anhydrous had been converted to a form that could be lost by denitrification or by leaching. So the most that the farmers could have lost by that time in mid-April due to the heavy rains would have been 10%.

Suppose that heavy rain came after complete nitrification; then 90% of the applied nitrogen would be susceptible to being lost. That does not mean it would be lost. It just means it would be susceptible to loss.

Taylor and Killorn (1994) created a simple computer program to calculate the fate of nitrogen. The fate of nitrogen program estimates the loss that could have been and even what it likely is for various temperatures or moisture conditions. The important thing to realize is that growing degree days can be used as a model of the nitrification in the soil.

The upper line in Figure 1.13 represents the nitrification process under the influence of a nitrogen stabilizer, so-called by the companies that sell nitropyrene or some other chemical that will inhibit the biological activity in the soil that brings about the nitrification. Much less nitrification takes place if a stabilizer or a nitrification inhibitor is present in the soil. So in some cases, that could be a significant factor, as indicated by the lower line (Figure 1.13). We will discuss nitrogen-GDD connections in lesson 4.

Fig. 1.13 Influence of heat unit/growing degree day on nitrification rate, with and without nitrogen stabilizers