Studies of Methods for Measuring Forms of Available Soil Nitrogen

In modern agriculture, crop growth is more often limited by N than by any other nutrient element. Despite this, there is as yet no method for testing soils for N available to a crop that is generally considered to be satisfactory. Several attempts have been made to develop soil tests for evaluating N fertilizer needs under specific field conditions. Commonly, the efficacy of these tests has been evaluated from data on crop yield responses to applied N fertilizer. Few if any attempts have been made to evaluate this efficacy from measurements of the contributions of indigenous soil N to the N nutrition of plants