Chemical Composition Of Soil – Acidity, Salts and Organic Matter

What is the chemical composition of soil? Information about the acidity and salts, organic matter, composition of soil.


The great bulk of mineral soil horizons—not counting the organic matter—is made up of silicon, aluminum, iron, and oxygen, with lesser amounts of calcium, magnesium, sodium, potassium, and hydrogen. Many other elements are also present including other plant nutrient elements such as phosphorus, nitrogen, sulfur, manganese, boron, zinc, copper, molybdenum, and vanadium. The soil also contains fluorine, cobalt, and iodine that animals require in their food even though plants do not. The organic matter consists chiefly of carbon, hydrogen, and oxygen that plants take from the air and water, as well as small amounts taken from the soil.


Soil Characteristics - Soil Horizons, Profiles and Texture

Elements occur in the soil in many combinations that vary widely in their solubility and mobility, so that a total chemical analysis does not in itself indicate much about the dynamic, changing, and functioning part of a soil or about the amounts of the nutrients that are available to plant roots. The compounds in a soil depend not only upon those in the original rocks but also upon the great sorting actions of various kinds of plants, additions from the dust and gases in the air and from seepage water, and removals by leaching. Thus in different climates and under different plants, the composition of soils from identical rocks varies widely. And, of course, the natural balance may be easily and drastically upset. If the vegetation is harvested organic matter and chemicals may have to be added—rarely, if ever, in the same ratio—to compensate for those removed.

Acidity and Salts.

Soils also vary in their total amount and degree of acidity or alkalinity. The acids produced in soils are, for the most part, relatively insoluble and weak. The soil solution— the film water around the grains and in the fine pores—is quite dilute and contains ions of many salts. The degree of its acidity depends upon the relative amounts of hydrogen (H+) and hydroxyl (OH-) radicals in the solution. At exact neutrality, their concentrations are identical, but as one increases the other goes down, since the product of their concentrations is always the same. A mathematical notation called pH is used to express the degree or intensity of acidity on a scale of 1 to 14. A soil solution with a pH of 7 is neutral, and those with a lower or higher pH are increasingly acidic or alkaline, respectively.


As soils become depleted of calcium and other bases under leaching, they become acid, just under the surface, even though the parent material may contain lime. Normally the old soils developed under forests in humid regions are acid, and with strong leaching under a heather vegetation—as in western Ireland or Scotland and under pines or spruces in parts of Canada and the northern United States—they become extremely acid. In making good arable soils from naturally acid ones, farmers and gardeners add lime in accordance with the needs of the crops and the amount of acidity in the soil. At the same pH, the richer the soil is in clay and the higher the percentage of humus, the more lime it takes to produce a change in pH.

Soil Characteristics - Soil Horizons, Profiles and Texture

In subhumid regions where leaching is light and the grasses are heavy feeders on calcium, the solum is nearly neutral, and a layer of accumulated calcium carbonate (lime) forms just below the solum. In very dry regions the soil rarely becomes acid.

In low places salts often accumulate. Many crop plants are injured with more than about 0.2% salt, and few will tolerate more than 0.65%, although some wild plants will stand as much as 10%. if the salts contain sodium, the clays absorb it. Then, with leaching of the excess sodium salts, these clays become highly alkaline, very sticky when wet and very hard when dry. At this pH, the organic matter dissolves and coats all the grains to give the soil a black color, hence the old name for them—”black alkali soils.” By providing drainage, excess salts may be leached from the soil. By treatment with gypsum (calcium sulfate), the sodium in the clay can be replaced with calcium and the soil reclaimed for agriculture.


Organic Matter.

All soils contain at least some organic matter. The humus portion is essential to the formation of granular structure in clayey and loamy soils. In well-drained natural mineral soils, the highest amounts of humus are found in the subhumid climate where the soils have developed under tall grasses. The famous black soils of the Ukraine and of the American Great Plains—the chernozem soils—are very well supplied with organic matter. Under drier conditions there is less plant growth and less organic matter, while under more moist conditions the forces of destruction keep the amounts low. However, organic matter accumulates in wet soils, often in such thick deposits that the peat itself becomes the parent material for soils, comparable to rocks that give rise to mineral soil.


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