SOIL TEXTURE AND COMPOSITION 85 



These particles as they exist in a soil are not in close contact 

 but are surrounded in a dry soil by a film of air and in a moist 

 soil by a film of water. The space occupied by this air or water 

 in natural conditions often equals one-half the entire mass. Work 

 indicates that for maximum bacterial activity this should be com- 

 posed of 60 per cent of water and 40 per cent of air. 



We more fully appreciate the action of water, plants, and bac- 

 teria on these particles when we are told that the total surface of 

 all the grains of soil found in one cubic foot would be equal in 

 area to from two to three acres. A soil composed of the larger 

 particles such as sand is referred to as a light soil, although a 

 unit volume weighs more than does a soil containing much clay 

 which is called a heavy soil. A light soil is more easily cultivated 

 than is a heavy soil. It also loses its water more rapidly and 

 warms up earlier in the springs hence, it is referred to as a warm 

 or early soil. 



The texture of a soil depends upon the rock from which it has 

 been derived and also upon the methods of formation. The 

 weathering of sandstone results in a sandy soil. The weathering 

 of shale on the other hand yields clay soils. Soils which have 

 resulted from the grinding of native rock with little weathering 

 yield soils of coarse texture. Older weathered soils are of a finer 

 texture than are younger. The bacterial activities of a soil are 

 dependent on the texture of the soil, as will be shown later. 



Soil Structure. — Soil structure signifies the arrangements of 

 the particles which constitute the soil. The particles may be 

 separate so each can move freely or they may be arranged in small 

 groups as granules which act as a single particle. The former is 

 known as the separate-grain structure and the latter as the granu- 

 lar or crumbly structure. Good tilth in clay and loam implies 

 a granular structure, and poor tilth a separate-grain structure. 



The conditions affecting structure are (i) texture, (2) quan- 

 tity of organic matter present, (3) lime, (4) presence or absence 

 of alkali salts, (5) wetting and drying,_(6) freezing and thaw- 

 ing, (7) tillage, and (8) plants and animals. 



Nothing the farmer can do will modify the texture of the 



