CLAY AND PEAT SOILS. 183 



if this is always practicable. We can merely con- 

 sider generally those manures which experience has 

 proved to produce the best mechanical as well as 

 chemical effect on a soil of a certain general char- 

 acter. 



Cla}/ earths. These have a great amount of fer- 

 tility stored in them ; but they are often so tenacious 

 as to prevent the entrance of the air, which is im- 

 portant to the production of valuable compounds. 

 This is partially remedied, as we have seen, by 

 thorough draining and pulverization ; but a still 

 further benefit can be realized by an application of 

 sand to the depth of from one-half an inch to an 

 inch mixed with the surface, thus not only produc- 

 ing a mechanical efi"ect by loosening the soil, but 

 inducing a chemical change by the introduction of 

 air. Ii^ such cases very coarse sand is to be pre- 

 ferred; that which is rich in lime is the best, as 

 by its fermentation it produces still greater porosity. 

 Horse-manure, and others which ferment rapidly, 

 lime, marl, etc., are best for such land. 



Peaty soils are also benefited by applications of 

 sand or fine gravel. On such lands many plants 

 fail to succeed because the carbon contained in the 

 humus absorbs the oxygen as it enters the earth to 

 form from it carbonic acid. By the increased po- 

 rosity produced by sand, oxygen is not only pre- 

 sented to the roots, but the decomposition of the 

 humus is hastened. Manures possessing vegetable 



