SOILS 15 



appearance) and various minerals containing compounds 

 of iron and other metals with alumina and silica. 

 Under the combined effect of the intense heat of day 

 and comparative cooling of night the rock splits and 

 allows water to soak in from above. This water con- 

 tains carbonic acid (obtained partly from the air while it 

 fell through it as rain, and, still more, from the roots 

 of plants) and also various organic acids produced 

 by the decay of vegetable matter. These acids act 

 on the minerals in the rock, and break them up into 

 simpler compounds. The soluble compounds are car- 

 ried away below, while the silica, which is insoluble, 

 remains as separate particles of sand, and the insolu- 

 ble compounds of alumina with silica and certain 

 metals remain as clay, an entirely distinct substance. 



Soil, therefore, is produced from the underlying rock 

 by a long process in which the sun's heat, rain water, 

 the carbonic acid of the atmosphere and that given 

 out by the roots, and lastly organic acids produced by 

 the decay of plant remains, all play a part. The action 

 of these agents is continually to wash away the surface 

 soil, and to renew it from below, and there is added 

 the decaying remains of animals and plants decaying 

 naturally or partially digested and dropped as dung 

 and these constitute humus, a very important consti- 

 tuent of all good soils. 



The various kinds of soil differ from each other 

 mostly in the different proportion of these three consti- 

 tuents sand, clay and organic matter (humus). 



In the dry beds and banks of rivers which flow 

 only during the rains, the soil is almost pure sand, 

 because though ordinary rich soil is washed down off 



