98 THE SOIL. 



by their constant humidity, for the cultivation of cereal 

 plants and the superior kinds of forage grasses, have 

 been reclaimed by drainage, and made lit to produce 

 food for man and beast. When the farmer, by means 

 of drainage, keeps within bounds the amount of water 

 in his fields, he controls its injurious influence at all 

 seasons ; and by the speedier removal of the water, 

 which soaks the earth and destroys its porosity, a path 

 is opened for the air to reach the deeper layers of the 

 ground, and to exercise upon these the sanre beneficial 

 influence as upon the surface soil. 



In winter, the earth at a depth of 3 to 4 feet is 

 warmer than the external atmosphere ; hence the air 

 coming up from the drain-pipes may contribute to keep 

 the temperature of the arable surface higher than it 

 would be without this current of air. The air in the 

 drains is generally richer in carbonic acid than is the 

 case with atmospheric air. 



The effect which drainage produces upon the fer- 

 tility of land may in itself be deemed a proof thai, 

 plants cannot derive their food from the water moving 

 about in the soil. This view is strongly supported by 

 the analysis of well, drain, and spring water. (See Ap- 

 pendix D.) 



The drainage-waters contain all the substances which 

 the rain-water, percolating the surface soil, is capable 

 of dissolving : they contain various salts in trifling pro- 

 portions, and among these mere traces of potash ; am- 

 monia and phosphoric acid are generally absent. In 

 analyses specially made for this purpose, Thomas Way 

 found that in four (drainage) waters no appreciable 

 quantity of potash could be detected in 10 pounds of 

 water ; three other waters were found to contain from 

 2 to 5 pounds of potash in 7,000,000 pounds of water. 

 In three waters no appreciable quantities of phosphoric 

 acid could be discovered : four other waters were 

 found to contain 6 to 12 pounds of phosphoric acid, 

 and 0'6 to 1'8 pounds of ammonia in 7,000,000 pounds 

 of water. In a similar series of analyses, Krocker 

 found that in six drainage-waters no appreciable traces 



