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How is the amount of crop available N affected by weather?

In some years, corn is clearly nitrogen deficient, and in others the same amount of fertilizer appears to be adequate. This happens because nitrogen availability is very dynamic: In warm weather nitrogen mineralizes faster from the organic matter in the soil to become available to the crop, while in colder weather mineralization is slower, and so less becomes available. In a dry spring nitrogen mineralizes, and remains in the root zone where the crop can take it up once it reaches its rapid growth phase in early summer. However, in a wet spring (see the brown dotted line in Figure 1), nitrogen can be leached out of the crop’s reach, or can be lost to the atmosphere through denitrification.

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Figure 1. The influence of early season weather on optimal N rates. Larger view.

In such a spring, more nitrogen is required at sidedress time than after a dry or normal spring. Losses can be even greater in a situations where there are high nitrogen levels in the soil early, for example when fertilizer is applied before planting (Figure 2) or in a manured scenario (Figure 3).

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Figure 2. Risking greater losses with pre-plant application. Larger view.

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Figure 3. Risking greater losses with pre-plant application. Larger view.

It is therefore not possible to predict how much nitrogen is needed in a given year before the early season weather has passed. Therefore, many growers currently apply “insurance fertilizer” in case they lose a lot of N during the spring, which is unnecessary in most years. The Adapt-N tool, allows the grower to assess more precisely how much is needed in their field at sidedress time, and therefore ADAPT to the year’s weather conditions. In most years, this will result in lower fertilizer rates and cost savings. In some other years, it predicts that more N is needed and prevents yield losses. Also, the trend towards more extreme weather events due to climate change makes such seasonal adjustments more compelling.

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