It is to be noted that when the pH of soils goes higher than 8.5, the availability of their phosphorus tends to increase again. This is because in this higher pH range the soil solution contains small amounts of sodium hydroxide which reacts with insoluble soil phosphates and forms small amounts of the highly soluble and available sodium phosphate. Since little or no leaching usually accompanies this high alkalinity, most if not all of the sodium phosphate thus formed is available for use by such crops as will grow at this high alkalinity.

It will be noted in the chart that conditions associated with acidity, especially of a pronounced degree, are unfavorable as regards the availability of sulfur, potassium, calcium, and magnesium. At strong acidity, the conditions for both the accumulation of sulfur in organic matter and its subsequent transformation to sulfate for plant use are not favorable. With increasing acidity, the amounts of the other three elements in exchangeable or readily available form usually decrease because of the attendant severe leaching; furthermore, the elements are held more tightly by the excess of insoluble acids against solution and plant feeding.

The availability of the other nutrient elements, iron, manganese, boron, copper, and zinc (often referred to as the minor nutrient elements), is influenced greatly by reaction. The influence on boron and manganese has been discussed. It should be noted that beyond pH 8.5, the influence of reaction is again in the direction of making boron more available; that is due to the presence of sodium hydroxide in the soil solution, which forms soluble salts of boron that are available for plant use.

The influence of reaction on the availability of iron is similar in nature to that of manganese, but due to the great abundance of iron in soils compared to manganese, deficiency of the former in available form does not occur nearly so often as of the latter. Below pH 6.5, small amounts of iron tend to exist in the ferrous state. In that form the iron is soluble in carbonic acid, and so is readily available. In fact, at extreme acidity and restricted aeration, toxic concentrations of ferrous iron sometimes exist. Also, at strong acidity, the ferric oxide, which is always present, dissolves in sufficient amounts to supply crop needs. As the pH rises above 6.5, soluble ferrous iron tends to become oxidized to ferric oxide, which is so insoluble under neutral and alkaline conditions that a deficiency for crop growth sometimes occurs under such conditions. As is the case with manganese, a good supply of active organic matter tends to overcome a lack of available iron at high pH because it not only carries and furnishes available iron, but also produces local reducing areas where iron is kept in the readily available ferrous state.

Crops need only minute amounts of copper and zinc. But, even so, deficiencies sometimes occur, particularly under calcareous and highly alkaline conditions, which greatly lower the solubility and availability of the two elements. Also, in strongly acid soils the amount present in readily available forms may be too low for crop needs because of depletion by leaching during the development of the acidity and strong retention by the soil acids of the other portions which exist as exchangeable bases.

A most important point to note is that pH 6.5 is a very favorable reaction as regards the availability of all of the elements listed and obtained by plants from the soil proper. That is why for general farming it is usually recommended that acid soils be limed to pH 6.5. Of course, for the control of potato scab and the culture of certain ornamental plants that need a lower pH, that condition may not be feasible or advisable. I also emphasize that the chart is a generalized diagram. Because adequate and precise data relating to certain aspects of the subject are still lacking, I had to make some assumptions in its preparation and so there are undoubtedly some inaccuracies in it. There will be cases that do not conform to the diagram because of the inaccuracies, or special and peculiar conditions that are involved, e. g., conditions that are associated with orchard crops. But in the main, especially in the reaction range of pH 5.5 to 8.5, I believe the diagram presents a fairly reliable picture. It should be added that where rather pure calcium lime is used, a deficiency of available magnesium may be created at pH 6.5 or any other pH value.