CHAPTER I. 



SOILS. 



BY S. S. DOUGALL, F.I.C., AGRICULTURAL CHEMIST TO THE 

 BUREAU OF AGRICULTURE. 



Earth, or soil, acts as the holder or conveyor of nourishment 

 necessary to the growth of plants, and also gives support to their 

 roots in enabling them to maintain a position best adapted to their 

 growth. 



Soils are the results of the disintegration, weathering, or 

 denudation of rocks. The forces that are concerned in the 

 formation of soils are both of a physical and a chemical nature. 

 The alternate action of heat and cold on rocks produces expansion 

 and contraction, creating small cracks and fissures which, by the 

 further actio i of water, are enlarged, and small fragments of the 

 rock are consequently disengaged. This action may go on until large 

 pieces of the rock are disengaged, which, in their downward course, 

 grind away part of the surface of he rock, which goes to make up 

 the soil. The water not only acts mechanically, but also chemically. 

 AU rocks contain something in their mineral composition that is 

 liable to chemical decomposition by the action of oxygen and 

 carbonic acid. The rain falling to the earth takes up oxygen and 

 carbonic acid from the atmosphere. The oxygen acting on the 

 sulphides oxidizes them into sulphates, which are dissolved in the 

 water and carried away, leaving the rock a porous mass which 

 eventually is broken up into fragments. 



Carbonic acid exerts a greater action on the rocks than oxygen, 

 even the hardest granite crumbling under its agency. It converts 

 the alkalies potash and soda- and alkaline earths lime and 

 magnesia into carbonates and bi-carbonates, which, being soluble 

 in water, are carried away in solution with the separated silica, 

 leaving, in the case of a felspathic rock, a deposit of kaolin. The 

 rain falling on these decomposed rocks carries away small particles 

 which are deposited on the lower levels, or in the nearest stream 

 or river, to be again deposited at some distance, or finally carried 

 out to sea. 



Soils are also formed /// loco by the weathering or disintegra- 

 tion of the surface of rocks which underlie the soil. Being com- 

 posed of particles of the rock from which they are derived they are 

 precisely of the same nature, or very little modified. When the 

 rock under the soil is a shale or slate, a clay soil will be found 

 on the surface ; sandy when overlying a sandstone rock ; calcarerous 

 when overlying limestone ; and overlying granite a soil composed 

 of clay and sand, rich in potash, and modified in its other mineral 

 constituents, according to the nature of the granite it is derived 



