30 BOTANY: PRINCIPLES AND PROBLEMS 



dioxide being relatively high. Plowing tends to increase greatly 

 the air content of a soil, since the structure of the whole mass is 

 loosened and the crumbs are more widely separated. A soil in 

 this condition is said to be in good tilth. Where water occurs 

 only in capillary form much air is present in the larger spaces, 

 and such a state of the soil is therefore clearly the most favor- 

 able for plant growth since then, and then only, is a plentiful 

 supply of water combined with a plentiful supply of oxygen. 



Organic Matter. — All rich soils contain a considerable amount 

 of material derived from the dead bodies of organisms, particu- 

 larly of plants. Roots which die and remain in the soil, and 

 leaves and other plant parts which fall on the soil surface, ^re the 

 sources from which this organic matter is mainly derived in nature. 

 In the practice of agriculture it is increased in amount by various 

 artificial means. After entering the soil it soon begins to undergo 

 decomposition, and for the most part is finally broken down into 

 simple end-products — carbon dioxide, water, and ammonia. As 

 this organic material decays it becomes characteristically dark in 

 color and undergoes a series of complex chemical changes. In 

 this condition it is known by the general name of humus. 



Humus is of importance to plants in many ways. It improves 

 the physical condition of the soil, for because of its coarse and 

 fragmentary character it tends to separate the particles and thus 

 to increase materially the air-content of the soil. Since humus 

 absorbs water readily, its presence also adds to a soil's water- 

 holding capacity. The decomposition of humus liberates certain 

 nutrient materials, notably an abundant supply of nitrogen com- 

 pounds, which ultimately become available to plants. Humus is 

 also the seat and food-supply of the soil bacteria, minute organ- 

 isms which are indispensable in plant nutrition. Any treatment 

 of the soil which will increase its humus content will therefore 

 tend to increase its productivity, and whatever decreases the 

 humus content will impoverish the soil. 



Dissolved Substances. — Soil water is by no means pure water 

 but carries dissolved within it a great variety of substances. 

 Anything which is to be taken in by the roots of plants must be in 

 solution, and it is consequently obvious that these dissolved 

 substances are the only portion of the soil, aside from water itself, 

 which is directly available as nutrient material for plants. 

 Their origin and chemical composition are therefore of much 

 importance botanically. 



