THE SOURCES OF PLANT FOOD. 21 



this means also capillary attraction is diminished, and the 

 land consequently suffers less from drought ; the water- 

 holding power of the surface soil is also increased. A more 

 important result of tillage is that the soil is thoroughly 

 exposed to the influence of the air. Soils containing 

 humus or clay will absorb ammonia from the atmosphere, 

 and thus increase their store of nitrogen. The organic 

 remains of former crops and manuring are also oxidised, 

 the nitrogen being converted into nitric acid. The rocky 

 fragments which a soil contains, as fragments of silicates 

 or limestone, will at the same time be more or less dis- 

 integrated by the combined action of water and air, as- 

 sisted by the carbonic and humic acids arising from the 

 oxidation of vegetable matter, and a portion of the in- 

 soluble plant food be thus brought into a state suited for 

 assimilation by the roots of crops. In winter time the 

 disintegration of the various ingredients of the soil is 

 greatly assisted by frost. Water in freezing expands, and 

 thus rends asunder the substance frozen. Of the various 

 results brought about by tillage, the increased production 

 of nitrates must be ranked among the most important. 



By drainage the various chemical actions we have just 

 mentioned are carried down to a greater or less extent 

 into the subsoil, for as the water level is lowered the air 

 enters from above to fill the cavities in the soil. By 

 drainage also the depth to which roots will penetrate is 

 increased, for roots will not grow in the absence of oxygen, 

 and rot as soon as they reach a permanent water level. 

 In a water-logged soil deoxidation is active, the nitrates 

 present are destroyed, a part of the nitrogen being evolved 

 as gas ; the soil may thus suffer a considerable loss of 

 plant food. 



