AGRONOMY 21 



is always undergoing chemical and bacteriological 

 changes — in fact, there is a perpetual change going 

 on — a soil being Nature's laboratory. 



Frost, rain, snow, water, oxygen, carbon dioxide, 

 bacteria, and other fungi are agents in the great 

 changes going on below the surface of the land. 

 Other agents, namely, those that are living, " work 

 upon the fabric of the soil." Trees grow, and the 

 fruits and other parts are removed, whUst their 

 roots remain in the soil — the dead portions of 

 which becoming detached help to increase the 

 humus or organic matter in the soil. Soil, there- 

 fore, consists of two portions — mineral and organic 

 matter; the former is derived from the decay of 

 rocks, and the latter from the decay of vegetable 

 and animal substances. 



Nearly all soils consist of sand, clay, humus, 

 and a varying amount of limestone. 



Soils may be classified as follows : — 



(1) Sandy soU . ['« "'^^ ^f" '^"H 10% of clay. 



\ more than J 



(2) Sandy loam . ,, 10 to 40% „ 



(3) Loamy soil . 



(4) Heavy clay . 



(5) Marly soil 



(6) Calcareous soU 



(7) Peaty soil . 



40 to 70% 

 85 to 95% 

 5 to 20% of calcium carbonate. 

 20% or more ,, „ 



20 to 70% of organic matter. 



A perfect soil for general cultivation would 



