pH, Soil Acidity, and Plant Growth

W. H. Allaway.

When crop plants do not grow well, one of the first questions the soil scientist usually asks is, "What is the pH of the soil?" or, "Is the soil acid, neutral, or alkaline?"

The reason for these questions lies in the fact that the pH, or degree of acidity of the soil, often is a symptom of some disorder in the chemical condition of the soil as it relates to plant nutrition.

A measurement of soil acidity or alkalinity is like a doctor's measurement of a patient's temperature. It reveals that something may be wrong but it does not tell the exact nature of the trouble.

The acidity or alkalinity of every water solution or mixture of soil and water is determined by its content of hydrogen ions and hydroxyl ions. Water molecules break up, or in chemical language, ionize, into two parts hydrogen ions and hydroxyl ions. When there are more hydrogen ions than hydroxyl ions, the solution is said to be acid. If there are more hydroxyl ions than hydrogen ions, the solution is alkaline (or basic). Solutions with equal numbers of hydrogen and hydroxyl ions are called neutral.

Only a very small percentage of the water molecules present are broken up into hydrogen and hydroxyl ions at any one time. If one attempts to express the concentration of these ions in conventional chemical ways, some cumbersome decimal fractions result. In order to avoid these cumbersome numbers, the Danish biochemist S. P. L. Sorenson devised a system called pH for expressing the acidity or alkalinity of solutions.

The pH scale goes from 0 to 14. At pH 7, the midpoint of the scale, there are equal numbers of hydrogen and hydroxyl ions, and the solution is neutral.

pH values below 7 indicate an acid solution, where there are more hydrogen ions than hydroxyl ions, with the acidity (or hydrogen ion concentration) increasing as the pH values get smaller.

pH values above 7 denote alkaline solutions, with the concentration of hydroxyl ions increasing as the pH values get larger.

The pH scale is based on logarithms of the concentration of the hydrogen and hydroxyl ions. This means that a solution of pH 5 has 10 times the hydrogen ion concentration of a solution of pH 6. A solution of pH 4 has 10 times more hydrogen ions than one of pH 5 and 10 times 10, or 100 times, the hydrogen ion concentration of a solution of pH 6.

A measurement of the pH of a solution of a strong, or highly ionized, acid measures essentially the total strength of the acid. But a pH determination of a solution of weak, or slightly ionized, acid measures only a part of the total strength of the acid, because pH is a measure of hydrogen ions only and does not measure all the acid molecules that can potentially ionize to form hydrogen ions.

In a soil, hydrogen ions exist in a number of different chemical combinations and states of adsorption on the surfaces of solid particles. The number of hydrogen ions in the soil solution at any one time is small in relation to the number held in a less active form in various nonionized molecules and on the surfaces of the solid particles.

When the soil is limed in order to bring it to neutrality, enough lime must be added to react not only with the so-called free hydrogen ions of the soil solution but also with those held in the less active forms. This is so because as the neutralization of the soil progresses, ionization of the less active forms of hydrogen likewise progresses and new free hydrogen ions are formed as long as the supply of less active forms holds out.

Thus it is possible to think of the total acidity of a soil as being composed of two parts.

One part, often called the active acidity, is made up of the hydrogen ions in the soil solution. These are the hydrogen ions measured when the pH of the soil is determined.

The second, and much larger, part of the total soil acidity is often called potential acidity. The potential acidity is due to hydrogen ions held in various chemical combinations and adsorbed on the surfaces of solid particles. These hydrogen ions are in chemical equilibrium with the free hydrogen ions of the active part of the soil acidity, and as the free hydrogen ions of the soil solution are neutralized or removed from the soil solution in other ways, hydrogen ions from the less active (or potential acidity) source enter the solution.

Most of the hydrogen ions in the potential acidity forms are held on surfaces of solid particles of clay or soil organic matter. These clay and organic particles are very small, and consequently have a large surface area per unit weight. They make up what is called the colloidal fraction of the soil. Since most of the potential acidity of soils is due to hydrogen ions held on the clay and organic particles, it follows that fine-textured soils, which are high in clay and organic matter, can have a higher total acidity than sandy soils of low clay and organic content.

There are different kinds of clay in different soils, and these different kinds of clay can hold different amounts of hydrogen ions in the potential acidity form. Generally speaking, the clay found in soils of cool-temperate and arid regions can hold a greater quantity of potential-acidity hydrogen ions than can the kind of clay found in soils of warm-temperate and tropical regions. General relationships between kind of soil, pH, and the amounts of lime required are shown in the table.

The hydrogen ions of the potential acidity form are held to the colloidal material because of their electrical charges. The colloidal material is predominantly negatively charged. The hydrogen ions are positively charged, and the attraction between the unlike charges accounts for most of the binding of hydrogen ions to the colloidal surfaces. When hydrogen ions change from the potential to the active acidity forms, their places on the clay-organic colloidal material are taken by other ions carrying positive charges. Ions of calcium, magnesium, potassium, and sodium are positively charged and may take the place of hydrogen on the colloidal complex.

Ions with positive charges are called cations. The process whereby positively charged ions of one kind are replaced on the surfaces of the clay-organic colloidal material is called cation exchange. The total amount of all kinds of cations held by the clay-organic complex at any one time is called the cation-exchange capacity. The ions held to the clay-organic surfaces are called exchangeable or replaceable cations.

The basic process involved when lime is added to an acid soil is the replacement of hydrogen ions held by the clay-organic colloidal material with calcium ions from the lime.

The diagram shows this process. The small numbers by the symbols for the different cations indicate the number of positive charges carried by that cation. Where no number appears, one positive charge is present. The numbers of different kinds of ions involved in an exchange reaction depend on the number of charges carried by each ion. Thus one calcium ion with its two positive charges will replace two singly charged hydrogen ions. Three doubly charged calcium ions are required to replace two triple charged (or, in chemical terms, trivalent) aluminum ions.

Aluminum ions have a unique role in soil acidity. In the diagram it can be seen that the replaceable aluminum ions of the acid soil have been replaced by calcium in the neutral soil. The aluminum ions thus replaced react with the soil water to form insoluble hydroxides and oxides.