106 NATURE TEACHING 



possess to gather into masses, thus permitting a freer 

 circulation of water. Lime has a similar effect in 

 causing the particles to collect or flocculate, and is there- 

 fore often used as a dressing for stiff, clay lands, in 

 order to make them lighter, and more easy to till. 

 Kneading and trampling have the opposite effect ; 

 breaking up the little collections, groups or floccules of 

 clay, and thus closing the small openings. Hence it is 

 that brickmakers and potters, who require firm, compact 

 masses, thoroughly knead the clay they use before 

 working it into shape. The cultivator, on the other 

 hand, desires to bring his clay into a flocculent condition, 

 so as to permit the circulation of air and water; he 

 thus, at intervals, digs, forks, or ploughs the soil, 

 admitting the air and causing the clay to become 

 flocculent, while he is careful to prevent, as far as 

 possible, any trampling or walking over the soil which 

 he has tilled. 



Vegetable Matter in Soils. 



In digging down through the soil, it was seen that 

 the upper layers (surface-soil) were darker than the lower 

 (subsoil}. This is due to the presence of decaying leaves, 

 roots, and other vegetable matter derived from plants 

 previously growing on the spot, or brought there as 

 manure. Some soils are almost entirely made of decayed 

 vegetable matter for instance, in woods of beech, oak, 

 etc., where every year enormous quantities of dead 

 leaves are added at the approach of winter. In swampy 

 lands, covered with bog moss, enormous accumulations 

 of vegetable matter are formed, resulting frequently 



