PRACTICAL AGRICULTURE. 219 



have not put within the reach of plant-roots all that 

 they require ; that still food is to be brought to them 

 all the way from far above the surface Ox the field to 

 far below it, and that water and air are the carriers. 



418. There is hardly a more important principle in 

 agriculture than the one I have now endeavored to 

 illustrate — that of deeply ploughing and finely pul- 

 verizing the soil. A caution is, however, here neces- 

 sary. Suppose a field has hllherto been skimmed 

 over to a depth of only five inches. Just at the ter- 

 mination of these five inches is what may be denomi- 

 nated the plough-floor — that stratum of earth on. 

 which the plough has always run, about as hard as a 

 cart-path. Above this is a thin and exhausted soil. 

 All below is hard, impenetrable by the roots of 

 plants, and almost impervious to water. 



419. If now the plough be put down to twice the 

 depth before reached, and the whole ten inches invert- 

 ed, it is manifest that the surface will be made up of 

 soil that never saw the light before ; and that the ori- 

 ginal top-soil will be buried at too great a depth. It 

 would seem to be a safer course to lower the furrow 

 one inch a year till the requisite depth were reached. 

 In this case, the change would be less violent; the 

 upper and lower soils would be perfectly mixed, and 

 the whole would be thoroughly pulverized. 



420. Nothing is better established than the benefit 

 of mixing unlike soils ; as peaty with sandy or with 

 clayey soils ; or swamp muck with any soil essentially 



