630 ' ANNUAL REPORT OF THE Off. Doc. 



either disintegration or decomposition. In the first case the rock 

 mass is simply broken into finer portions, the minerals of which 

 it is composed remaining intact, while in the case of decomposition, 

 the mineral form is changed. Decomposition proceeds more readily 

 as disintegration advances, on account of there being more surface 

 exposed. 



The agents which cause weathering are (1) the atmosphere, (2) 

 water, and (3) plants and animals. In the atmosphere are certain 

 gases, which act chemically upon rock substances and cause them 

 to become decomposed. The expansion caused by heat and cold, 

 often causes a mechanical rending of rock particles. Wind, espe- 

 cially when ladened with dust and fine sand particles, acts as an 

 abrasive agent which is responsible for the wearing away of much 

 rock surface. 



Water contains many substances which act chemically upon rock 

 substances. Most rocks are not soluble in pure water, but let 

 carbon-dioxide or some such substance be present, and its action 

 is very marked. Water, again, has a great erosive power, and by 

 this action aids materially in weathering. Water, as ice, has a great 

 crushing power, aside from its transporting quality. The mechani- 

 cal action of tree roots is familiar to all. Earth worms and allied 

 animals in the soil serve to disintegrate the soil particles. Bacteria, 

 also found in the soil, serve a similar purpose. 



The above forces, sometimes one and sometimes another, serve 

 to rend and decompose even the hardest rocks. Some varieties are 

 much more easily weathered than others, but when the time element 

 is considered, we see that the resulting material is, indeed, of 

 great abundance. To this material, which is spread as a blanket 

 over the earth, we give the name, soil. 



A soil composed wholly of rock fragments, however, is not all 

 that is necessary from an agricultural standpoint. Some organic 

 matter must be present. This is furnished by decaying portions of 

 plant and animal matter. The organic matter helps to make avail- 

 able portions of the soil constituents needed as plant food. The 

 amount of organic matter in any soil depends upon many factors 

 among which may be mentioned its method of formation, climatic 

 conditions, previous treatment, etc. 



PROPERTIES OF THE .SOIL. 



Let us now consider a few of the properties of the soil and show 

 their relation to crop production. One substance that must be 

 present to make a soil available for cropping, is water. Without 

 moisture, no matter how rich the soil may be, we can have no pro- 

 duction. A plant derives its food by the action of water, which 

 carries it in solution to all parts of the plant body. The small root 

 hairs penetrating the intra-spaces of the soil must find water. Water 

 in the soil is held in three forms, hydroscopic, capillary and gravi 

 tational. Hydroscopic water is held as a thin film about the soil 

 particles, capillary water is that contained in the small tube-like 

 openings made by the intra-soil spaces. Gravitational water is that 

 which will drain away from the soil, due to the force of gravity. 

 It is from the capillary water that the root hairs secure the moisture. 

 By the laws of capillary attraction, the smaller the tubes the higher 



