2 SOILS: PBOPEBTIBS AND MANAaJEJMJSNT 



air in the struggle for life. Primitive plants have been 

 followed by more highly organized ones as the incursions 

 have gone on, and always to the profit of the soil, until 

 the soil has accumulated a great store of organic matter 

 and a teeming population of microscopic life. 



This debris of rock and plant residue that has accumu- 

 lated through the centuries of struggle is the arable soil 

 from which man obtains his bread. The study of this 

 soil is a history of strife and struggle, and as the light of 

 investigation is turned on it, new contestants, new opera- 

 tions, new results, and new principles are brought to view 

 and the story must be retold. 



1. Composition of the soil. — Broadly speaking, the 

 soil is composed of two general classes of materials, rock 

 and organic matter. The former usually makes up the 

 bulk of the soil, while the latter occurs under normal 

 conditions in relatively small amounts. In spite of this 

 low proportion, however, its presence is of vital impor- 

 tance to productivity. The soil has also three general 

 phases — the physical, the chemical, and the biological. 

 In the physical phase, the size and shape of particle, the 

 movement of air and water, and other physical proper- 

 ties are dealt with ; in the chemical phase, the composi- 

 tion of the particle, of the organic matter, and of the soil 

 solution is of dominant importance ; in the biological phase, 

 the soil is seen to be not an inert material, but teeming 

 with life — minute forms of life, to be sure, but of great 

 importance m the manufacture of food for plants. Under 

 these three general phases, then, the changes going on ia 

 a son may be studied, a.d they are found to be directed 

 primarily toward the production and maintenance of con- 

 ditions favorable for plant growth. The soil is not a 

 simple medium to study, but is extremely complicated 



