310 ELEMENTARY STUDIES IN BOTANY 



is a liberation of acids, and if these accumulate, the soil 

 becomes literally sour, a condition which the presence of lime 

 corrects by neutralizing the acids. But lime is also of great 

 value in clay soils in a physical way, by mixing with the clay 

 and making the soil of better texture for the handling of 

 water. 



The humus (organic material) of the soil greatly increases 

 its water-holding capacity. In fact, a forest soil, in general the 

 best example of a humus soil, has the physical properties of a 

 sponge in receiving and retaining water. 



It becomes evident that in tfiis single matter of water rela- 

 tions the proper balance among the various soil constituents 

 is of great importance, and that this balance may be secured 

 in a variety of ways. It should not be forgotten that 

 the physical properties of the soil mixture in reference to 

 water represent only one phase of the physical properties of 

 soil. 



19. Life of the soil. The soil is not only a chemical lab- 

 oratory and a physical laboratory, but it is also a biological 

 laboratory. This means chiefly that it contains multitudes of 

 the exceedingly minute plants called bacteria. The bacteria 

 of the soil have come to be recognized as the most important 

 agents for putting the soil in proper condition for such plants 

 as we cultivate. Bacteria are active in all decompositions 

 of organic material, which is thus reduced to substances that 

 the plants can use. They are active also in many other use- 

 ful changes of soil material. Conspicuous among their 

 activities, however, is their relation to the nitrogen supply of 

 the soil. Certain bacteria, unlike other plants, can use the 

 free nitrogen of the air, and in this use the nitrogen enters 

 into compounds that cultivated plants can use. This is 

 spoken of as the " fixation " of nitrogen, which simply means 

 taking free nitrogen and putting it into compounds. It is 

 evident, since plants must always have a nitrogen supply 

 from the soil, that this work of the nitrogen-fixing bacteria 



