Aluminum is one of the ions of positive charge that react with basic solution to form insoluble oxides and hydroxides. Aluminum occurs as a replaceable cation in acid soils in far greater quantity than any of the other cations that show this property. So long as the soil remains neutral, these aluminum oxides and hydroxides remain insoluble and inert in the soil.
Because calcium ions replace both aluminum and hydrogen ions during the neutralization of an acid soil, the aluminum ions can be said to contribute to the total acidity of the soil. In most methods for determining total acidity of soils, the replaceable aluminum and hydrogen ions are lumped together.
Soils become acid through a process that is almost the direct reverse of the liming process shown in the diagram. The soil parent materials usually contain colloidal material, which is nearly saturated with basic cations like calcium and magnesium. But through the centuries during which soil development takes place, hydrogen ions carried by downward percolating waters gradually replace these calcium and magnesium ions. The calcium and the magnesium are carried away by the drainage waters.
The replacement of bases by hydrogen ions from the water moving through the soil profile is a slow process, but soil formation usually takes place over many centuries. The more water moving down through the soil, the faster the process. Therefore the soils of humid regions are generally more acid than the soils of subhumid regions, and acid soils are rarely found in and regions. Also, since sandy soils can hold lesser amounts of replaceable bases, these sandy soils usually become acid more rapidly than do fine-textured soils.
Many important chemical properties of the soil are dependent on the kind of replaceable cations held by the clay-organic colloidal fraction of the soil. Calcium, potassium, and magnesium held as replaceable ions constitute the major source of these important plant nutrients in most soils.
The physical properties of the soil are often affected also by the replaceable cations. In some of the alkali soils of the West, excesses of replaceable sodium ions bring about very undesirable physical properties and very slow movement of soil water. In these alkali soils, hydroxyl ions in the soil solution greatly exceed the hydrogen ions and pH values of 8 to 10 are common.
One of the most important experiments dealing with the effect of pH on plant growth was conducted by D. I. Arnon and his associates in California. They grew plants in water solutions in which the pH varied from very acid to very alkaline; the solutions contained a liberal supply of all the important plant nutrients at all pH values. The plants grew well except at extremely acid or extremely alkaline pH values values that are only very rarely encountered in field soils. Throughout the range of pH values common in soils that is, from about 4 to 9 plant growth was not greatly affected by the pH of the solution.
This experiment of Dr. Arnon and his associates might well cause one to question why soil scientists so frequently measure the pH of the soil in order to diagnose troubles in crop production. One might also question the value of tables of the pH values at which various plants grow best.
The answer to these questions lies in the fact that the effects of pH on plants growing in soil are indirect, while Dr. Arnon's solution culture experiments were designed to measure the direct effects of pH. The solution cultures used contained neither deficits nor excesses of the essential plant nutrients.
In soil, however, the solubility and availability to plants of many important nutrients is closely related to the pH of the soil. It is this indirect effect of pH on the availability of plant nutrients that justifies the frequent use of pH measurements for diagnosis of soil problems, and makes tables of so-called pH preferences of plants useful under many conditions.
Changes in the acidity of soils may change the availability to plants of different nutrients in different ways. As the pH of an acid soil is increased by the addition of lime, ions such as aluminum, iron, manganese, copper, and zinc become less soluble. In acid soils these ions may be found in dissolved form in quantities sufficient to become toxic to plants. As the soil is neutralized, these ions form inert oxides and hydroxides, and the toxicity is corrected. As the pH of the soil is increased still further, the solubility of these ions becomes so low that deficiencies of those (iron, copper, manganese, zinc) needed by plants may occur.
Bacteria and other micro-organisms living in the soil convert nitrogen, sulfur, and phosphorus from organic compounds, in which these nutrients are unavailable to plants, to simpler inorganic forms that plants can take up. Neutralizing an acid soil usually makes the soil condition more favorable to the growth of bacteria and may thus indirectly speed up processes by which important nutrients become available to plants.
The bacteria that live in association with the roots of legumes are less effective in their important role in nitrogen fixation in acid soils than in neutral or alkaline soils.
In farm practice, compounds of calcium and magnesium are the basic materials used to treat acid soils.
Along with the decrease in acidity, the addition of these materials increases the supply of calcium and magnesium for use by plants growing on the soil. In some cases, a lack of available calcium or magnesium may be the most important defect of an acid soil.
The availability to plants of the phosphorus in soils is changed in a rather complex manner when the acidity of the soil changes. Phosphate availability in many soils is highest when the soil is neutral or slightly acid, and it declines as the soil becomes either strongly acid or alkaline.
Boron and molybdenum are other plant nutrients that show changes in availability with changes in the pH of the soil. Boron deficiencies frequently occur when too much lime is added to an acid soil. On the other hand,molybdenum is most often deficient in acid soils, and becomes more available as the soil is limed.
In any discussion of the relationships between pH and nutrient availability in soils it should be emphasized that these relationships differ in different soils. In organic soils (peats and mucks), the relationships between pH and nutrient availability are not the same as for mineral soils. Copper, for example, may be deficient in acid organic soils but is rarely so in acid mineral soils.
Many soils are naturally alkaline and contain an excess of lime. The availability of plant nutrients in these naturally alkaline soils quite often differs from the availability of nutrients that results when a naturally acid soil is treated with an excess of lime. For example, many naturally alkaline soils have an adequate supply of available boron, whereas overliming of a naturally acid soil usually brings about boron deficiency.
Since the relationships between pH and the availability of plant nutrients are complex, pH measurements of soils are not easy to interpret in the solution of problems of soil fertility.
While the measurement of pH may give some valuable clues concerning the reasons for poor plant growth, it is generally necessary to follow up these clues with additional tests before an accurate diagnosis of the trouble can be made.
Among the many recent publications about chemical reactions in soils the following may be cited: Chemistry of the Soil, by F. E. Bear (Reinhold Publishing Corp., New York, 1955); Clay Mineralogy, by R. E. Grim (McGraw-Hill Book Co., Inc., New York, 1953); Cat- ion on Exchange in Soils, by W. P. Kelley (Reinhold Publishing Corp., New York, 1948); Soil Conditions and Plant Growth, by E. J. Russell and E. W. Russell (Longmans, Green and Company, New York, 1950); "Formation Constants for CU (1l) Peat Complexes," by N. T. Coleman, A. C. McClung, and D. P. Moore (Science, February 24, 1956).