THE SOIL. 49 



capillarity, rolling the soil increases its water capacity. 

 Plants absorb their water from a certain volume of soil 

 and hence the water capacity of soils is better expressed 

 relative to the volume of soil. 



" Shrinkage" is the name given to the diminution in 

 volume consequent on the loss of water from the soil. 

 From the table* it will be observed that the finer the soil 

 particles the greater is the shrinkage; whereas in sandy soils 

 the volume scarcely changes when the soil is dry. In clay 

 soils it is the decrease in volume of the colloid bodies on 

 drying which is the chief reason of the great shrinkage of 

 these soils. Natural!}', on watering, such soils expand, 

 but, for some time at least, the watering will not bring 

 the soil back to its original volume; as a certain amount of 

 permanent contraction will take place owing to the closer 

 packing of the soil particles. It is this firm packing which 

 gives the soil its power to support the heaviest buildings. 

 The shrinkage takes place both vertically and horizontally. 

 When the surface is rapidly dried, it shrinks and the lower 

 soil layers being still in a state of expansion, the outer 

 layers crack. This cracking causes great damage by 

 breaking the plant roots and by restricting their growth 

 to, the lump of soil on which they are growing. Further, 

 the surface of the fissures present a large area from which 

 the soil water is quickly evaporated. On the other hand 

 the cracks are more or less indispensable for the ventilation 

 of the soil which, when the soil is firmly crusted, would 

 be reduced to a minimum. To prevent the rapid drying 



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