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considerable resistance to the plough. Sandy soils which 

 actually weigh heavier than other soils are called light soils 

 because they, offer least resistance to the plough. A stiff 

 clay soil which is said to be very heavy becomes lighter 

 i.e. less resistant to the plough, after there is a shower of 

 rain, though the rain actually adds to the weight of the soil. 

 The specific gravity of soils not as they actually occur but 

 after they are absolutely dry, varies from 2*5 to 2'8. The 

 specific gravity of soils very rich in organic matter is some- 

 times less than 2. The specific gravity of quartz is 2-65. 



83. Porosity. The fineness of division of the particles 

 of soil has great influence on vegetation. Food of plants must 

 pass into solution before it can be assimilated. The rapidity 

 with which dissolving action can take place is in direct ratio 

 to the surface. The finer the particles the greater the surface 

 and more the space the growing roots have for their develop- 

 ment and spread. But when the particles are too fine, the 

 soil becomes too compact for roots to penetrate, and it cracks 

 in drying which also interferes with the spread of roots. Up 

 to a certain limit therefore fineness of division of the parti- 

 cles of soil is desirable. The condition known as loamy 

 is the best in respect of porosity. 



84. Retention of water. This capacity of soils depends 

 mainly on the fineness of division of their particles. Humus 

 or vegetable organic matter in the soil has the greatest capa- 

 city for retaining moisture, and clay has greater capacity in 

 this respect than sand. Angular fragments have greater 

 capacity than round fragments for retaining moisture. 100 

 parts of sand take up about 25 parts of water by weight and 49 

 parts by volume ; clay, 40 parts by weight and 68 by volume ; 

 fine calcareous soil, 85 by weight and 80 by volume ; humus 190 

 by weight and 93 by volume. Ordinary Agricultural soil 

 takes up about 50% by weight of water, It will thus be seen 

 that an inch of irrigation or rainfall at a time is quite enough 

 to thoroughly soak it to a depth of i foot, and provision in the 



