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.

In the case of phosphorus, you will note that in the pH range of 6.5 to 7.5 conditions are most favorable for high availability. Below pH 6.5 the influence as regards availability rapidly become less favorable. That is an important reason why acid soils should be limed to pH 6.5. In fact, if lime produced no other benefit than its favorable influence on phosphate availability, it would usually pay to use it. At pH 6.5, lime is sufficiently abundant and available to keep a considerable portion of the phosphorus in the form of calcium phosphate, which is soluble in carbonic acid and hence is readily available to crops. This holds for both the phosphorus naturally present in soils and that applied as manures and fertilizers.

When lime is present or added in amounts so as to raise the pH beyond 7.5, the influence on phosphate availability gradually becomes less favorable, although this is usually not serious until the pH goes beyond 8 and there is present 2 to 3 percent and more of free calcium carbonate. The reason for this is explained by these reactions :

CaCO3+H,CO3=Ca(HCO3)2 Ca3 (PO4) 2 + 2H2CO3=Ca2H2 (PO4)2 + Ca (HCO3) 2  

The action of carbonic acid on calcium carbonate is expressed by the first reaction, and on calcium phosphate by the second reaction. The solubility of all the products is low, but adequate for plant nutrition.

You will note that calcium bicarbonate is formed in both reactions. Now, if a great abundance of calcium carbonate is present in a soil, the action of carbonic acid on it will keep the soil solution saturated with calcium bicarbonate, and this will greatly retard or even stop the second reaction, because it also involves the formation in solution of calcium bicarbonate. In other words, when the. soil solution is once saturated with a certain product, such as calcium bicarbonate, all reactions involving the formation of this product are brought to a standstill until some of the product is removed by leaching or plant feeding. The explanation coincides with the principle of chemistry usually referred to as the law of mass action. Our observations indicate that usually a soil must contain several parts per hundred of calcium carbonate that is, enough to permeate all the minute areas of a soil in order to retard seriously the availability of phosphate. Of course, since 1 percent of calcium carbonate in a soil represents 10 tons of lime to the acre, a condition of this kind is seldom, if ever, produced by liming.