WHAT THE SOIL SUPPLIES 309 



and that the soil, in this respect, may be regarded as a chem- 

 ical laboratory where work is going on continuously. 



18. Physics of the soil. Not only is the chemistry of the 

 soil necessary to consider, but the physical properties are also 

 very important. This is a subject of great difficulty on 

 account of the complex mixture. The physical properties of 

 a mass of pure sand or of pure clay may be discovered with 

 comparative ease, but when many substances, with different 

 physical properties, are mixed together, the resulting physical 

 properties of the mixture as a whole form a far more difficult 

 problem. From this point of view, the soil may be regarded 

 also as a physical laboratory, which is but dimly understood. 



A few examples will illustrate what is meant by physical 

 properties. One of the most important physical properties 

 of soil is its relation to water. In fact, the capacity of soils 

 to receive water and to retain it in an available condition for 

 plants is the most obvious physical feature that cultivation of 

 the soil seeks to control. The clay constituent of a soil, 

 which is a very important one, may be taken as an illustration 

 of the relation of one kind of soil material to the water supply. 

 A certain amount of clay interferes with the free movement 

 of water, and therefore prevents it from draining away too 

 rapidly; it thus increases the retentive power of the soil. 

 But an excessive amount of clay interferes too much with the 

 movement of water and results in a water-soaked soil which 

 is very injurious to plants on account of the exclusion of air. 

 The converse is true in reference to sand. A certain amount 

 of sand makes the soil open to the movement of water, thus 

 increasing its receptive power ; but an excessive amount of 

 sand makes the soil too open, so that the water drains away 

 rapidly and the soil becomes dry. 



The lime constituents of the soil are very important both 



chemically and physically. Soils with insufficient lime are 



spoken of in general as " sour." Wherever decomposition 



of organic matter is going on, as is true of all good soils, there 



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