SUBSOIL PLOUGHING. 287 



is made to extend, varies greatly in different countries 

 and in different soils. The ordinary period in Eng- 

 land is four or five years, but it is sometimes extended 

 to nine, or even more. In the ordinary four or five 

 course system, wheat is only grown once in four or 

 five years, manure being given only once, at the 

 commencement of the course, when a crop of turnips 

 being grown, is fed off by sheep ; the land thus re- 

 ceiving a rich dressing of sheep's dung (812). 



757. With respect to the natural sources of saline 

 matters in the soil, it must be observed that, al- 

 though their nature and quantity are in a great mea- 

 sure influenced by the composition of the subsoil, and 

 stony substratum ; yet it frequently happens that, 

 from want of mixture, the soil is deficient in some of 

 those very substances which the subsoil is rich in ; 

 and in consequence the soil becomes considerably im- 

 proved by mixture, or by spreading over the surface 

 some of the decomposing stones dug from below. 



758. We are apt to think that a soil must of 

 necessity contain portions of those substances which 

 enter into the composition of the substratum ; but 

 this is by no means always the case. Thus the sur- 

 face-soil of a chalk district is not unfrequently nearly 

 wholly destitute of calcareous or chalky matters ; 

 and soils in such situations are, therefore, often im- 

 proved by spreading lime over the surface. 



759. It is consequently of high importance to 

 know the nature and composition of the subsoil, as 

 well as the surface soil ; for by a judicious use of the 



