DRAINAGE. 97 



This cooling of the soil, by the evaporation of its water, greatly 

 retards the growth of crops, and the fact that draining lessens 

 evaporation is one of the strongest arguments in favor of its adop- 

 tion. An idea may be formed of the amount of heat taken from 

 the soil in this way, from the fact that, in midsummer, twenty- 

 five hogsheads of water may be evaporated from a'single acre in 

 twelve hours. 



5. // greatly facilitates the chemical action by which the constitu- 

 ents of the soil are prepared for the use of plants^ and hy which its 

 mechanical texture is improved. Ordinary soils contain roots and 

 other organic matters, and the various minerals which aid, directly 

 or indirectly, in the nutrition of plants. Before the roots, etc., 

 which have been left in the soil by a previous crop, can become 

 useful to a new growth, they must undergo the process of decay, 

 which is a slow combustion, requiring the action of atmospheric 

 air. In a soil saturated with water, this decay cannot take place. 

 It proceeds most actively in thoroughly drained land, while in land 

 which is often too wet, it is greatly retarded. The mineral con- 

 stituents of plants can be taken up by roots only in solution of 

 water, which can dissolve them only from the surfaces of the 

 particles of the soil, and usually only after they have undergone a 

 chemical change from exposure to the air and moisture. The 

 more freely air is admitted into the soil, the more easily will the 

 coarser particles be disintegrated, thus exposing more surface, and 

 the more readily will the exposed portions be prepared for the 

 dissolving of their fertilizing ingredients. These chemical changes 

 also greatly improve the mechanical condition of the soil, tending 

 to make it more light and friable, and, both from its greater fineness 

 and from the increased amount of its decayed organic matter, to 

 enable it more readily to absorb fertilizing gases (2) from the air 

 and from rains, and to condense the watery vapor of the atmos- 

 phere in dry weather, (i.) 



6. It tends to prevent grass-lands from " running out." The 

 tillering of grasses — that process by which they constantly repro- 

 duce themselves by offshoots from the crowns of the plants — 

 goes on during the season of growth, as long as the roots can find 



