2(] .KG I 'P TJ. I. JV A G1U C UL TV RE. 



4. The soil water. Infiltration, irrigation, and rain 



water, with solutions of salts. 



5. The soil air. 



A small part of the mineral matter of the soil is derived 

 from the remains and ashes of animals and plants, but 

 most of it consists of decomposed rock particles of different 

 sizes, shapes, hardness, and composition, depending upon 

 the nature of the rock from which the soil was derived. 

 In sandy soil the particles are comparatively large, while 

 in a clay soil they are mostly very small. This results 

 from sandy soil being largely composed of very hard 

 material (quartz) which is difficult to break up, whereas 

 clay soil is formed from, and in part composed of, soft 

 mineral particles (mica, felspar, etc.) w T hich are easily 

 weathered and broken up into fine material. Pure clay 

 weathered felspar and pure sand are equally devoid 

 of plant food. In ordinary soils it is only by the constant 

 weathering of felspar, mica, etc., that a steady supply of 

 available plant food is obtained. Generally, clay soils 

 contain more un weathered felspar, etc., than sandy soils, 

 and hence the former are usually richer in the mineral 

 plant foods. "Loam" is the name given to soils in which 

 the mineral particles are in size, smaller than those in 

 sandy but larger than the fine particles of clay soils. The 

 physical properties of a soil, its drainage, irrigation and 

 cultivation, depend largely on the size of the particles of 

 which it is composed. 



The organic matter in the soil is generally known 

 under the name humus. The humus consists chiefly of 



