14 PRINCIPLES OF FARM PRACTICE 



water have become warm. Heavy clay is an example of a 

 cold soil; it is not the clay but the water held by the clay 

 that makes it slow to heat. Sandy soil, on the other hand, 

 which holds but little water, is regarded as a warm soil. 



Another use for water in the soil is that it meets the need 

 for moisture of certain microscopic plants, chiefly bacteria, 

 that are always present in fertile soils. These organisms need 

 water quite as much as other forms of life. 



How water is held by the soil. In order to understand 

 how the soil holds water, we must keep in mind the fact that 

 it is largely made up of particles of rock. These particles are 

 irregular in shape and size, so that spaces are left between 

 them. The arrangement may be suggested by thinking of a 

 number of rocks and bricks thrown loosely together. Here 

 spaces of various sizes and shapes occur among the rocks 

 and bricks just as they occur among soil particles, differing 

 only in size. When these spaces are filled with water, the 

 water is known as free water. When water appears at the 

 surface of the soil or at lower levels, as when a hole in the 

 ground becomes filled with water from adjoining soil spaces, 

 it furnishes an example of free water. 



When water clings to the surface of soil particles and is 

 held in the sharp angles between them, but does not occupy 

 the spaces, it is known as film or capillary water. If a pencil 

 is dipped in water, the part coming in contact with water 

 will become wet. Here the solid substance of the pencil 

 attracts and holds a thin layer of water on its surface. It is 

 in a similar way that solid particles of the soil hold films of 

 water on their surfaces. Capillary water is important be- 

 cause it is almost exclusively the form of water used by the 

 plant. When the spaces of the soil in which the root of a 

 plant is growing are filled with water, the air supply is cut 



