SOIL CHANGES DUE TO IRRIGATION 67 



judgment of experienced tillers of the soil, the soil is in 

 the best condition for plant-growth. In the case of the 

 clay soil, as more water was applied, the force of cohesion 

 continued steadily to diminish, with no definite point at 

 which a temporary hardening occurred. At a definite 

 degree of wetness, however, the clay soil is in the best 

 condition for working and for plant-growth. This is in 

 full harmony with the known properties of clay. 



The point of optimum water content is, approxi- 

 mately, identical with the field water capacity discussed 

 in Chapter II. It seems clear that, when the soil contains 

 a medium amount of water, that is, a quantity lying 

 between the maximum water capacity and the point of 

 lento-capillarity, it can be most easily worked, and is in 

 best condition for plants. It is interesting to note how this 

 intermediate point continually appears in the study of 

 the relation of soils and plants to varying water content. 



45. Volume changes of soils. It follows that, if such 

 differences in the force with which the soil crumbs are 

 held together are induced by the application of varying 

 quantities of water, the soil particles themselves must 

 actually move and rearrange themselves, as water is 

 added to or removed from the soil. Such movements of 

 the soil particles would naturally cause, also, correspond- 

 ing changes in the volume of the soil. This is an estab- 

 lished fact, well known to every practical farmer. If 

 wet clay is allowed to dry it shrinks, with the formation 

 of large cracks in the ground. When water is again added, 

 the clay swells and the cracks largely disappear. In a 

 large measure, this is true of all agricultural soils. As they 

 receive water, they swell; as they dry, they contract. 



The changes in the soil volume, due to the addition 

 of water, are very great. In clay and humus soils they 



