File:Truog 1947 pH and nutrient availability.jpg

English: A diagram showing the availability of different plant nutrients at different levels of soil pH. Adaptations of this diagram are still widely used today.

From: Truog, Emil. 1947. The Liming of Soils. In: USDA Yearbook of Agriculture 1943-1947. pp. 569-570. https://naldc.nal.usda.gov/naldc/download.xhtml?id=IND43893966&content=PDF

Caption in the source text: "This chart illustrates the general trend of the relation of reaction (pH) and accompanying conditions to the availability of plant-nutrient elements."

First part of the section of the paper that refers to the diagram:
"Liming and the Availability of Plant Nutrients

The influence of reaction and accompanying conditions on the availability of the plant nutrients obtained from the soil proper is shown diagrammatically on page 570, which is simpler in form but more complete in several respects than that published by N. A. Pettinger. Reaction is expressed in terms of the pH scale. The reader is reminded that on this scale, a pH value of 7.0 (the middle vertical line in the diagram) represents the neutral point, while values to the left and progressively less than 7 express increasing acidity, and values to the right and progressively greater than 7 express increasing alkalinity. Also, a change in pH of one unit expresses a tenfold change in reaction; thus, pH 5 expresses acidity which is 10 times as intense as that at pH 6; likewise, at pH 9 the alkalinity is 10 times as intense as at pH 8. This change in intensity of acidity and alkalinity is shown in the diagram by the change in width of the heavily cross-hatched area between the curved lines.

In the drawing the influence of reaction and accompanying conditions on the availability of each nutrient element is expressed by the width of the band (the wider the band the more favorable the influence) carrying the name of the respective element. Thus, for the maintenance of a satisfactory supply of available nitrogen, a reaction or pH range of 6.0 to 8.0 is the most favorable. This does not mean that if the reaction of a soil falls in this range that a satisfactory supply of available nitrogen is assured. All it means is that as far as reaction is concerned, the conditions are favorable for a satisfactory supply of available nitrogen. Also, the narrowed band for nitrogen at pH 5 does not necessarily mean that a deficiency of this element will prevail at that pH; it means that as far as reaction is concerned, the conditions are not favorable for an abundant supply of available nitrogen; other factors than reaction and the usual accompanying conditions may even promote the presence of an abundant supply; moreover, certain crops having a low requirement may be fully satisfied with a low supply. What I have said about nitrogen holds also for the other nutrient elements.

At both strong acidity and strong alkalinity, the conditions as regards the fixation of nitrogen by legumes and the transformation of organic nitrogen to forms available to plants become increasingly less favorable."