472 Introduction to the Study of Science 



This condition of the soil, preventing the activities of the 

 nitrifying bacteria, is, on the contrary, favorable to the activities 

 of a different kind of bacteria called the denitrifying. The 

 bacteria of this group attack immediately and disintegrate 

 the nitrogen compounds already formed, setting the nitrogen 

 free in the air. That is, they deprive plants of the food mate- 

 rials provided by the other bacteria. 



These facts show that the productive soil is not dead, but 

 alive, very complex, interesting, and throbbing with energy 

 in every particle. To regulate and adapt it to the growth of 

 plants of economic value requires knowledge of its complex 

 nature and properties. This knowledge must not be limited 

 to the elementary facts that soil is composed of rock fragments 

 of different kinds and sizes and requires humus and certain 

 fertilizers for replenishing the substances which vegetation 

 removes. It must include also such facts as the soil's de- 

 pendence for fertility upon a sufficient amount of soil water, 

 soil air, and aeration, and the activities of beneficial species of 

 living organisms, both plant and animal. Such knowledge 

 will aid, moreover, in determining with what kind of bacteria 

 a soil should be inoculated for a given crop, such as alfalfa, as 

 is considered necessary in modern agriculture. 



226. Soil temperature. The study of soil temperature 

 involves many conditions, some of which have been considered 

 in other connections. In the study of heat (page 109) it is 

 shown that colors, kinds of surfaces, as rough or smooth, and 

 the position of objects relative to the sun or other heat sources 

 have much to do with the absorption and reflection of heat. 

 Some colors absorb heat rapidly, others slowly. Some retain 

 heat long or yield it up slowly, and others behave still differ- 

 ently. The kind of surface, rough or smooth, and the posi- 

 tion of the body, as directly or obliquely exposed to the heat 

 source, determine the amount of heat absorbed. A hill with 

 a north exposure is not as " warm " as one with a southern 

 exposure. In dealing with the temperature of the soil, these 



