28 THE CHEMISTRY OF THE FARM. 



8. Tillage and Draining. The operations of tillage 

 and draining serve in many important ways to increase 

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

 crop ; some of these have been already noticed. 



By tillage, and the action of frost, the surface soil is 

 pulverised, and brought into an open porous condition, 

 favourable both for the distribution and action of roots. 

 As the absorbent power of roots resides entirely in the 

 root-hairs, a finely divided condition of the soil is essential 

 for the rapid development of a plant. Capillary attraction 

 is diminished by tillage, and the land consequently suffers 

 less from drought; the amount of drainage is at the same 

 time increased. By the destruction of weeds the whole 

 of the plant food is left available for the crop. Another 

 important result of tillage is that the soil is thoroughly 

 exposed to the influence of the air. Soils containing 

 humus or clay will absorb some ammonia from the atmo- 

 sphere, and thus increase their store of nitrogen. The 

 oxidation of the nitrogenous organic matter of the soil, 

 and the production of nitric acid, have been already 

 noticed, as also the disintegration and solution of the 

 particles of rock contained in soil. Of the various results 

 brought about by tillage, the increased production of 

 nitrates must be ranked among the most important. 



By means of pipe-drainage the various chemical actions 

 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 

 draining the depth to which roots will penetrate is also 

 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 pre- 



