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Fertilizer Nitrogen: Its Role in Determining Crop Yield Levels¹

The frequency and magnitude of crop response to N was found to be generally greater than those to P and K in representative cropping areas of the humid region of the United States. P and K are rather immobile nutrients and crop recovery is lower than for N. Consequently, P and K tend to accumulate in most soils as a result of fertilizer applications. In contrast, N acts as a relatively mobile nutrient and crop requirement and removal tend to be high. Since N is also lost from soils by leaching and volatilization, little accumulation occurs in humid areas and applications need to be made largely on the basis of current crop needs.

Results of this study have important implications for soil testing, the need for high P water solubility, and the present number of fertilizer grades and ratios.

² Agronomists.

Received for publication March 18, 1966.