THE SOIL 51 



will be deficient in air and growth will be retarded. This 

 need of oxygen can easily be tested by trying to germinate 

 seeds in boiled water which has cooled and has a layer of oil 

 over it to keep the oxygen out and in other water that has 

 been vigorously stirred to oxygenate it. Roots will seldom 

 enter soil, because of slow growth, where oxygen is limited 

 by lack of air spaces due to excess water or to the fact that 

 the soil particles are packed. 



The organic matter of any soil can be increased by fre- 

 quent applications of manure or other vegetable matter. It 

 has such a profound influence on the soil that it should be 

 given a great deal of consideration. Plant growth can be 

 improved more by building up the organic matter of the soil 

 through yearly applications than by any other method with 

 the exception of the use of water in cases of drought. Organic 

 matter not only gives a darker color to the soil which will 

 cause it to absorb more heat and make it warmer in the 

 spring, but it increases the water-holding capacity, since it 

 may hold several times its weight in water. Organic matter 

 decays slowly, forming acids which favor the dissolving of 

 rock particles, and the decayed matter furnishes the food 

 material for the growing plants. Recently it has been 

 ascribed greater importance as food for the organisms that 

 cause its decomposition. For example the bacteria that live 

 in the soil and fix nitrogen must have large quantities of 

 organic matter from which to get their food in order to 

 reproduce, grow, and fix the maximum amount of nitrogen. 

 Organic matter may some day be considered of most impor- 

 tance because of the soil organisms it supports. 



The living organisms of the soil, most of which are bene- 

 ficial, are so numerous that only a special student studying 

 small amounts of soil with the thoroughness of the bacteri- 

 ologist can appreciate their importance. Many of these 

 fungi and bacteria are important in bringing about the decay 

 of the organic matter of the soil, others as described in 



