CHAPTER 2 



PHYSICAL, CHEMICAL AND BIOLOGICAL PROPERTIES 



Texture of Soil. Texture pertains to the size of the mineral particles 

 that make up the body of the soil. In the laboratory, texture is deter- 

 mined by a mechanical analysis. This is described in Chapter 1. The 

 clay portion of a soil will range anywhere from a fraction of one per cent 

 to as high as fifty per cent of the body of the soil. The particles of clay 

 are so small that they can be seen only by the use of a high-power micro- 

 scope. When clay is thoroughly mixed with water the particles will 

 remain in suspension for several days. It is this clay that is chiefly re- 

 sponsible for the turbid condition of the streams of water flowing from 

 the land after heavy rains. Clay, when thoroughly wet and rubbed 

 between the thumb and finger, has a smooth, greasy feel. 



The silt may also range from a very small percentage to sixty per 

 cent or more of the body of the soil. It forms the group of particles next 

 larger than clay. It produces practically no perceptibly gritty feel when 

 wet and rubbed between the thumb and finger. Silt particles will remain 

 in suspension in water for only a short time, seldom more than one-half 

 hour. 



The various grades of sand consist of particles very much larger than 

 those of either clay or silt, and can be seen with the naked eye. The per- 

 centage of sand in soils like that of clay and silt varies between wide 

 ranges. Sandy soils may contain seventy-five per cent to ninety per cent 

 of the different grades of sand. All of the sandy soils give a distinctly 

 gritty feel when the wet soil is rubbed between the thumb and finger. 



Water-Holding Capacity of Soils. The texture of the soil is very 

 important and determines in a large degree the water-holding capacity 

 of the soil, the rapidity of movement of water and air in the soil, the 

 penetration of plant roots, ease of cultivation and, above all, the crop 

 adaptation of the soil. Texture is determined by the relative amounts of 

 the particles that fall into the several groups mentioned. The textural 

 effect is modified by the structure of the soil (discussed later) and its 

 content of organic matter. 



The larger the proportion of fine particles, such as clay and silt, the 

 greater is the surface area of these particles in a unit volume of soil. In 

 a well-drained soil all gravitational water passes away and only capillary 

 water is retained. This capillary water consists of very thin films of water 

 adhering to the surface of the soil particles and surrounding them in such 

 a way as to make a continuous film of water in the soil. Through this 

 continuity of the film, water moves by capillarity from a point where the 



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