DRAINING — IRRIGATION. 317 



texture of the soil is improved, or substances are 

 added to it in which it was previously deficient. 



858. It is almost unnecessary to say anything re- 

 specting the importance of draining, and the change 

 which it produces in too retentive soils, and in soils 

 which from their position are naturally wet and 

 swampy. The facts which have previously been 

 stated show that too much moisture in a soil is hurt- 

 ful, preventing the free circulation of air throughout 

 it, and in several ways interfering with the healthy 

 growth of plants ; this is remedied by draining. 



859. By draining, and subsoil ploughing, the ac- 

 cess of air is facilitated, and the decomposition of 

 silicates and similar stony compounds in the earth is 

 assisted. In mixing, or by adding mineral manures, 

 we increase the quantity of certain inorganic compo- 

 nents of the soil. 



860. Amongst the modes of fertilizing the soil and 

 augmenting the growth of crops, the subject of irri- 

 gation must not be omitted. The effects produced 

 by flooding grass-land are well known; they depend 

 on several distinct causes. 



861. Very dry parched land is therefore much im- 

 proved by irrigation, when from the heat of the climate 

 the greater part of the natural moisture of the soil is 

 evaporated; under such circumstances, the artificial 

 application of water is necessary for the successful 

 cultivation of plants, and accordingly it is a constant 

 practice in tropical countries, in the cultivation of 

 rice and similar crops. 



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