MEANINGS OF TEXTURE AND STRUCTURE COMPARED 13 come to live in the soil by the millions because of the presence of organic matter which, because of the energy or food stored up in it, is the source of most of their sustenance and their energy. Some of these organisms are a detriment to soils, while others, because of fundamental changes they bring about, are of the greatest importance in crop production. (c) Air-Soil as a medium for plant growth must contain air. (d) Salts in greater or less amounts-in dry climates much. SOIL CLASSIFICATION Since the building materials of soils are largely mineral particles which remain more or less the same in size and amounts in any particular soil, we are afforded a basis whereby soils may be classified. A B C Crummy structure Compact structure Sandy structure FIG. 6. Diagrams illustrating soil structures. Soil Texture a Basis for Classification.-The most convenient and general scheme for soil classification is based on the amounts of sand, silt and clay soils contain or in other words, on the basis of "texture.' What Soil Texture Means.-Soil texture may be defined as a quality denoting comparative coarseness or fineness of soils as is determined by the relative abundance of sand, silt and clay contained in them. For example, a soil having course sand predominating is a course textured soil, whereas one having much clay is regarded as fine textured. Meanings of Texture and Structure Compared.-Texture should not be confused with soil "structure" which means the arrangement of the soil grains (Fig. 6), or which describes the relation of the soil particles to each other. It is important to remember that the soil components do not form a mere casual mixture in which every particle remains separate from every other particle. With the exception of sands, the soil components, because of the cementing nature of much of the organic matter, group themselves into compound particles and granules, thus developing a "crummy" or "granular" structure. When a "heavy" soil does not crumble, but is a hard, compact mass instead, it may be said to have a "compact" structure. A sand may be described as having a "loose" or "sandy" structure. Soils Classified According to Texture.-Eight principal soil classes based on texture are recognized. They are here given in an order ranging from the coarsest to the finest textured soils. Soil classes 1. Medium sand...... 2. Fine sand.. How distinguished Composed of 80 to 100 per cent sand (much medium and coarse sand). Composed mostly of fine sand. 3. Medium sandy loam. Having 50 to 80 per cent sand. 4. Fine sandy loam. 5. Loam... 6. Silt loam. 7. Clay loam. 8. Clay... . Having 50 to 80 per cent fine sand. Composed of 30 to 50 per cent sand and 50 to 70 per cent silt and clay. Containing 50 per cent or more of silt. Containing 20 to 50 per cent sand, 20 to 50 per cent silt, and 20 to 30 per cent clay. Having 30 per cent or more clay. A soil class is understood to mean all soil of the same texture. For example, soils consisting of about 50 per cent sand and about 50 per cent silt and clay belong to one and the same class regardless of where they may be found. By virtue of its texture this class of soil is named "loam." A loam is to be defined as a class of soil composed of about half sand and the remaining half silt and clay (more silt than clay). Again, a silt loam is to be defined as a class of soil, or a soil class, containing 50 per cent or more of silt. Of these classes of soils, silt loams are most widely distributed, loams take second place, and fine sandy loams third. In this classification no consideration is given organic matter. Soils containing much stone or gravel may be described as stony silt loam or gravelly loam, as the case may be. Gradations may also occur; such as, silty clay loam, loamy fine sand, etc. "Marsh" and "swamp" soils are to be regarded as class names though they are not included in the classification based on texture. Marsh is commonly interpreted to mean a wet level area covered with grasses. There are salt and fresh-water marshes. Swamp is usually understood to mean low, wet areas of fresh water formation covered with tree growth. Tamarack swamp, cedar swamp, and cypress swamp are familiar expressions. SOIL CLASSIFICATION 15 Mechanical Analysis and Mechanical Composition. In order to be sure to which class a soil belongs, the amounts of sand, silt and clay it contains must be determined by laboratory methods designated as "mechanical analyses." Such analyses give the "mechanical composition" of a soil; as, for example: * Per cents total 99.6—remaining 0.4 per cent consisted of stone and gravel. "Light" and "Heavy" Soils. A sandy soil, because of its high sand content, is comparatively easy to work-for this reason it is usually regarded as a "light" soil. A soil like clay, on the other hand, is termed "heavy" because it is much more difficult to till. By weight sand is the heaviest soil-clay is much lighter. Soil Classification Based on Mode of Formation.-It is convenient to study soils in relation to the manner in which they were formed or built up. This gives rise to quite a different classification, as follows: 1. Residual soils-remaining on rocks where formed. 2. Cumulose soils-deposits of partially decayed vegetation. 3. Alluvial soils-built up by alluvium deposited by flowing water. 4. Glacial soils-formed through glacial action. 5. Marine scils-formed by sediment carried into the sea. 6. Lacustrine soils-formed by sediment carried into lakes. 7. Loess (loes),-formed through the accumulation of dust carried by wind. 8. Colluvial soils-moved down steep slopes by gravity. The first two groups are sedentary" soils, since they have stayed where they were originally formed. The next five groups are "transported" soils-the material having been carried and laid down by water, glaciers and wind. The last named soils are so called because they moved down steep slopes, due to gravity. Residual soils are underlaid by the rocks from which they were formed-such as granite, limestone, sandstone, shale and others. Wherever such soils are found the graduation of rock into soil may easily be observed. These soils are widely distributed in the United States—including the Great Western Plains and the larger portion of the Southern States, excluding the river valleys and the Atlantic and Gulf Coastal Plains. If all the soils could have remained where they were formed then all the soils in the world derived from rocks would be residual. But this has been impossible because of the forces which have been and are still at work effecting changes on the earth. Cumulose soils are deposits of vegetable matter accumulated most commonly in what used to be shallow bays, lakes and ponds, and preserved because they were covered or saturated with water. It is common to see water plants such as flags, mosses, reeds and FIG. 7.-Diagram showing peat formation and a floating bog; cc, vegetable growth on surface of pond; dd, partially decayed organic matter accumulating on bottom; ee, climbing bog. (Shaler.) sedges, growing, for example, along the shores of a pond. These plants die, sink to the bottom and are wholly or partially preserved by stagnant water which prevents or inhibits their decay. Sometimes "floating bogs" are formed which become thicker and thicker, and as they thicken gradually sink to the bottom (Fig. 7). In either case there comes a time when the shallow body of water becomes filled with a soft, spongy mass-the final stage in the formation of "peat. Peat when dry is the lightest of soils, and may be black, brown or reddish in color. It is commonly described as "raw" when the plant remains can be easily recognized, and "well decomposed" when the plant remains have lost their identity. Peat is a material much used for fuel in countries of the Old World. Muck. When considerable sediment is mixed with peat the resulting soil is called "muck." It is more decomposed, firmer and heavier in weight than peat. Occurrence and Value of Peat and Muck Soils.—Peat and muck soils occupy local areas ranging from a few to thousands of acres in extent, and they may vary in depth from a few inches to THE SOURCES OF THE SEDIMENT 17 several feet. Some are formed in fresh and others in salt water (Fig. 8). They are not confined entirely to low lands, but may occur on hill tops and even on hill sides in depressions kept wet the year round. On many of them, in their natural state, wild grasses grow on others trees and shrubs. When artificially well drained most of them can be converted into valuable agricultural lands. Alluvial soils are found along streams, and are built of the alluvium carried and deposited by them during flood flow. When muddy streams overflow their banks, the flow over the flooded land is retarded, consequently the sand settles out, then the silt, and FIG. 8.-A bed of peat four feet deep underlaid by a 3-foot bed of marl, which in turn is underlaid by sand and then clay. finally the clay-if the flow is nearly or completely checked. Through this deposition, therefore, low level areas are gradually brought to higher levels. Many streams during the past ages have greatly subsided, leaving high and dry many level and productive expanses of these water-formed soils. The sources of the sediment carried by streams are mainly the uplands drained by them. Heavy rains and melting snows are responsible for the land erosion so commonly seen in hilly or "rolling" sections. The sand, silt and clay particles carried into streams may be gathered from many kinds of soils and which in turn form new kinds. Geologists have estimated that the United States is being planed down at an average rate of one inch in 760 years, |