18 THE CHEMISTRY OF THE FARM. 



active at summer temperatures ; it ceases apparently near 

 the freezing point. 



The fragments of rock present in soil, as stones, gravel, 

 and sand, are as a rule of little vahie to a plant, the 

 elements of plant food which they contain being in too in- 

 soluble a condition to be attacked by the roots. These 

 fragments of rock may however be slowly decomposed by 

 the mechanical action of frost, and by the chemical action 

 of water, and their contents thus gradually made available 

 to the plant. The solvent power of the water in a soil is 

 greatly increased by the carbonic acid, and perhaps also 

 by the humic acid it holds in solution. Water containing 

 carbonate of calcium in solution is especially capable of 

 attacking silicates. 



If water is allowed to drain through a soil it carries with 

 it a part of the readily soluble matter which a soil con- 

 tains. The substances chiefly removed by the water will 

 be the nitrates, chlorides, and sulphates of calcium and 

 sodium. When heavy rain falls these substances are 

 washed into the subsoil, and partly escape by the nearest 

 outfall into the springs, brooks, and rivers. The loss of 

 nitrates from highly manured land during a wet season is 

 very considerable. When dry weather sets in evaporation 

 takes place at the surface of the soil, the water of the sub- 

 soil is slowly brought again to the surface by capillary 

 attraction, and the salts it contains are concentrated once 

 more in the upper soil, forming in some rare instances a 

 white crust of salt upon the surface. Capillary attraction 

 has little influence in the case of Sandy soils. 



Of these readily soluble salts the nitrates are of the 

 greatest importance as plant food. The quantity of 

 nitrates in a surface soil will vary greatly, depending on 



