THE CHEMISTRY OF THE FA KM. 413 



hydrous silicates, and humus. Different crops have very different powers 

 of attacking these various forms of plant food. 



The operations of tillage and drainage serve in several ways to increase 

 the amount of plant food which is at the disposal of a crop. 



By tillage the surface soil is kept in an open porous condition, favour- 

 able for the distribution of roots. By this means also capillary atti 

 tion 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 disintegrated by the combined action of water and air, 

 assisted by the carbonic and humic acids arising from the oxidation of 

 vegetable matter, and a portion of the insoluble plant food be thus brought 

 into a state suited for assimilation by the roots of crops. In winter time 

 tin disintegration of the various ingredients of the soil is greatly assisted 

 by frost. Water in freezing expands, and thus rends asunder the sub- 

 :ice frozen. Of the various results brought about by tillage, the in- 

 creased production of nitrates must be ranked among the most im- 

 portant. 



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 



1 is lo\veivd the air enters from above to fill the cavities in the soil. 



By drai; o the depth to which roots will penetrate is increased, for 



ill not grow in the absence of oxygen, and rot as soon as thev 



nianent water level In a water-logged soil deo.\idatin i> 



acti present are destroyed, a part of the nitrogen i 



; the soil may thus sutler a considerable 1">^ <!' plant 



food 



ionally resorted to a> a means of increa^in^ the avail- 

 able pi, : and improving the texture of a heavy soil. The soil is 



burnt iii lirap^, which an; then spread over the land. If the soil con- 

 y to see that ' phates of the limestone inav 



by the complete disintegration \vhieh attend- the 



conversion into lime. The lime will also attack the of the soil at 



a hi and liberate a pai 1 of the potash from its in-oluble 



combinatioas. To pn>duce the bed n^ults it is essential that the bun 



