CHAPTER IV

HOW ROCKS AND CLIMATE AFFECT SOILS

AMONG the factors which determine soil types was mentioned "kind of material" which refers largely to the kind of rock from which the mineral particles came. Any traveler interested in soils can easily observe the marked effects that rocks and climate have upon soils in different sections of the country. In this chapter we shall consider some of these effects both from a physical and chemical point of view in order that we may understand certain facts and soil conditions that may come within our observation, experience, or reading.

ROCKS IN THEIR RELATION TO SOILS

Kinds of Rocks and Their Changes.-There are many kinds of rocks, all of which may be divided into three main groups, according to the manner in which they were formed.

Igneous rocks were the first to form on the earth. They are also formed by the solidification of molten material from within the earth. Lava rocks formed through volcanic eruptions are, therefore, igneous rocks. Other igneous rocks are granite, basalt, and syenite.

Sedimentary rocks are bedded rocks formed from sediments such as sand, shells, mud, etc., deposited in sheets through the action of water and wind-mostly through water action. Through pressure and cementation these sediments gradually change into rocks; such as sandstone, limestone and shale. Most sedimentary rocks have been formed under sea water. All materials forming these rocks came originally from igneous rocks. Sedimentary and igneous rocks may change into metamorphic rocks.

Metamorphic rocks are so called because they are rocks which, through long periods of time, have changed their structure as a result of great pressure, heat, and water solutions. Slate, marble, quartzite, and schist are examples. A limestone may change into a marble, which, under proper conditions may further change its structure and become a schist. Thus a sedimentary rock may change into a metamorphic rock, and, in turn, may undergo a further change sufficient to be designated as another kind of metamorphic rock; namely, a schist.

A Rock is an Aggregate of Mineral Particles.-On examining rocks closely we find them composed of mineral particles massed or cemented together.2 In some rocks, as sandstone, the particles are mostly of the same kind. In others they are of different kinds. In a granite, for example, the dissimilar particles may be easily distinguished by differences in hardness, color and crystal form. They are called "rock-forming minerals."

Rock-forming Minerals of Many Kinds. We shall not attempt to study all the many kinds of minerals of which rocks are composed, but only the more common and important ones as are given in the following table:

The Common and Important Rock Minerals

A study of the chemical composition of these rock minerals will make clear to us the source of the mineral elements necessary for plant growth.

No nitrogen is found in any of these minerals.

Quartz particles are the grains of which sandstones are mostly formed.

Why Some Soils Are Deficient in Some of the Important Mineral Elements.-A sandy soil composed of quartz sand, or derived from pure sandstone would naturally contain only a very small amount of the important mineral elements. Why?

Soils composed of mineral particles derived from rocks containing such minerals as feldspars, hornblende, micas, apatite, etc., are usually well supplied with the mineral plant-food elements. The fact that peat soils do not contain any appreciable amount

2 Lava or glassy rocks are exceptions.

of mineral particles containing the necessary mineral elements explains why they are generally deficient in potassium and phosphorus.

Muck soils usually contain more potassium and phosphorus than peats, because these soils contain considerable mineral matter (Chapter II).

MORE ABOUT WEATHERING

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Products of Rocks Weathering. We have learned that the framework of mineral soils consists of sand, silt and clay-the common products of rock weathering.

When a granite, for example, is transformed by weathering into soil, many chemical changes occur during the transformation. A residual soil from granite is not merely a powdered form of that rock, represented by a combination of sand, silt and clay. Many of the soil particles, it is true, are the same kind of mineral particles as are found in the granite; others, however, are quite different from any that ever occurred in the parent rock. The formation of the new kinds, or secondary minerals, is a result of the chemical changes, or decay. In the decay, or chemical changes, many of the complex rock minerals are split up, chemically, and their.elements recombine in different ways, or unite with other elements, water and gases.

In addition, therefore, to the common products of rock weathering-sand, silt and clay, there are formed other products, among which are true clay (kaolin), carbonate of lime, and salts.

True clay (kaolin) is a definite, fine material originating mainly through the decay of feldspars and micas.

Carbonate of Lime, or lime carbonate, is lime combined with carbon dioxide gas. (CaO + CO2). It is formed in rock decay when the calcium (Ca) in the rock enters into a new combination with and through the action of carbon dioxide and water.

The mineral calcite is a pure form of carbonate of lime. Limestone, shells, coral, marble, dried lime mortar, air-slaked lime and marl are other forms of material containing lime carbonate.

Salts. Among the many salts formed during the process of rock decay are:

Chlorides. Common table salt is an example.

Sulfates.-Glauber's salt is an example.

3 Soils made up of materials derived from rocks are commonly referred to as "mineral soils."

Carbonates. Such as washing soda.

Phosphates. As lime phosphate.

From Granite Into Soil.-A residual soil resulting from the weathering of a granite is usually a sandy loam or loam. The transformation of granite into soil is illustrated and summarized in the following diagram:

* Sand, silt and clay particles are mainly the products of the breaking up of rocks, while true clay, carbonate of lime and salts are products of rock decay.

Soils From Other Rocks.-Sandstone usually gives rise to sand or sandy soils; shale and slate to heavy clays, and limestone to silt loams and clay loams.

THE EFFECT OF CLIMATE

Soils in Humid Climates Are Leached.-In humid climates the heavy rainfall causes much soil to be washed away and carried into the ocean. Sand, silt and clay are not the only materials carried into the sea-carbonate of lime and salts are also thus carried.

In a humid region much of the rainfall sinks into the ground, and as the water passes down through the soil it dissolves carbonate of lime and salts and carries them in solution until it comes to the surface in the form of springs; thence into rivers, and finally into the sea. All drainage waters, therefore, carry to the sea, carbonate of line and salts in solution and sediments.

4 When a region has an average annual rainfall of more than 30 inches it is regarded as having a humid climate. A region having an average annual rainfall of between 20 and 30 inches the climate is commonly referred to as sub-humid.

What Becomes of the Materials Carried Into the Sea.-The sand settles out near the shore, and in time becomes sandstone (Fig. 17).

The clay settles to the bottom farther out and finally turns to shale.

The salts remaining in solution become the "salt of the sea." Shells. The carbonate of lime is taken out of solution in the ocean water by myriads of tiny shell animals, whose shells sink to the ocean floor when they die. Coral animals, certain water plants and bacteria also cause carbonate of lime to be deposited. Usually, however, on the sea bottom beyond the accumulations

FIG. 17.-The ocean receives all the materials washed from the continents. The sand settles out of the river waters near shore, and the clay is carried farther out. The salts make the sea-water salty. The dissolved carbonate of lime is taken out of solution by tiny shell animals.

of sand and mud there are deposited shells and particles of carbonate of lime which in time turn into limestone. As a general rule, whenever limestone, sandstone or shale is found, there at one time existed a sea or lake.

The mud and sand, which are the impurities of limestone, constitute the soil-forming material when limestone weathers.

Alkali Soils. Soils in regions of little or no rainfall are not leached as they are in humid or sub-humid climates. The salts formed through weathering processes, therefore, accumulate in these soils. In places the soils are so salty that cultivated plants cannot grow in them (Fig. 18). Such soils are called "alkali soils." Salty crusts commonly form on their surfaces. When