The agrobiologic test for normality in fertilizer experiments and variety comparisons. II. Fertilizers

The foregoing paper is the ninth of a series of contributions by the writer to this JOURNAL (beginning in 1943) concerning uses of the standard yield diagram. On the basis of extended experience with this diagram the following conclusions have been reached: 1. The Mitscherlich-Baule yield equation y=A (1-10(-0.301x) accurately represents the course of crop increase under the action of increasing concentrations of a growth factor only when the soil is agrobiologically uniform and free from elements unfavorable to plant growth. 2. The very fact that the Mitscherlich-Baule equation holds with exactness only in normal environments makes it an invaluable tool for the use of agronomists, soil scientists, and plant physiologists who are called on to assess the quantitative relations between plants and their nutrients under the heterogenous conditions of practical plant culture. The standard yield diagram is a very convenient means of applying the Mitscherlich-Baule equation to such conditions. 3. When so applied, the diagram functions as a standard of reference by which it may be ascertained whether the course of crop increase in a field test or a pot test with plant nutrients has been normal or has been interfered with by nonnormal influences; at the same time a measure of the effect of the interfering influence may be obtained. 4. In well-replicated tests there are two principal causes that distort the normal course of crop increase under the action of increased concentrations of a plant nutrient, viz., fertilizer unbalance due to an excess of the more concentrated nutrient (far-end depression), and initial inhibition of the normal effect of one nutrient (near-end depression). 5. Application of the diagram to the data of a field experiment with a plant nutrient establishes the A value or maximum yield that may possibly be obtained by extended use of that nutrient on that field in its apparent present condition. Comparison of the attained level of production with the calculated perultimate level or AQ value of the crop plant will indicate the range within which it may be possible to raise further the level of production by removal of adverse conditions or by increasing the concentrations of other nutrients. Chief among the adverse nutritional conditions are the sources of the two types of yield-depression. 6. In the preceding paper (3) it was shown how the law of homologous yield curves establishes criteria for distinguishing between normality and abnormality in a combined variety-fertilizer test and provides a basis for a more accurate rating of the yielding abilities of varieties of crop plants. In this final paper of the series, it is shown that the rule of halved increases establishes an additional criterion for the normality of a two-nutrient test, and how application of this rule assists in evaluating the influence of far-end depression.