THE SOURCES OF PLANT FOOD. 15 



sand, clay, carbonate of calcium, and humus ; as each of 

 these preponderate the soil is said to be sandy, clayey, 

 calcareous, or peaty. 



Sand is either composed of pure quartz (silica), or con- 

 sists of fragments of more complex minerals — mica, for 

 example. When the former is the case, the sand will 

 supply no plant food ; but in the latter case the gradual 

 decomposition of the mineral will slowly increase the ash 

 constituents available for the plant. 



Clay is a silicate of aluminium, produced by the decom- 

 position of felspar and other silicates ; if absolutely pure it 

 would furnish nothing to the plant ; it always, however, 

 contains some potash, and frequently a considerable quan- 

 tity. Clay has the important i^roperty of absorbing and 

 retaining phosphoric acid, ammonia, potash, lime, and 

 other substances necessary for plant nutrition. 



The calcareous matter of soils supplies lime to the 

 plant ; limestone also generally contains phosphoric acid. 

 Carbonate of calcium is beneficial to the soil in many 

 ways. It preserves the particles of clay in a separate 

 coagulated condition, thus making heavy soils friable and 

 pervious to water. It enables clay to exercise its absorbent 

 power on various salts, which would otherwise escape its 

 action. It also promotes the decomposition of vegetable 

 matter, and the formation of nitrates in the soil. The 

 presence of some salifiable base is essential for the per- 

 formance of the chemical operations belonging to a fertile 

 soil ; the salifiable bases usually present are either carbo- 

 nate of calcium, or the alkalis derived from the decom- 

 position of silicates. 



The humus, or decayed vegetable matter of soils, has its 

 origin in the dead roots, leaves, &c., of a previous vegeta- 



