SUMMARY 159 



crops, and then only if the inoculating material contains 

 living bacteria. The average fanner will probably never 

 need to inoculate his legume crop. Failure to get a crop is 

 usually due to some other cause more or less easily remedied. 



132. Summary. -For agricultural purposes, the soil may 

 be considered as that portion of the earth's crust which 

 can raise crops, and is composed of fine and coarse particles 

 or rock in all stages of decomposition, of organic matter 

 derived from decayed or decaying plants and animals, of 

 water, of bacteria, fungi, and other forms of life, and of 

 gases. A perfect soil is one which maintains a reserve supply 

 of food material and yet at the same time supplies sufficient 

 soluble material for the growing crop. It also is in such 

 physical condition that it can supply sufficient, water to 

 the crop and that it can permit of proper root movement. 

 Plant food is derived from rock particles and organic matter, 

 the former supplying the so-called mineral elements and the 

 latter supplying nitrogen. 



The top layer of soil is usually darker than the lower 

 layers and is called surface soil, in which crops can grow 

 best. Organic matter is defined as that part of the soil 

 which at one time or another was a part of living organisms. 

 As soon as an organism dies in the soil, it begins to decay 

 by the agency of bacteria which are minute, one-celled plants 

 that obtain soluble nutrient material for their growth by 

 decomposing and dissolving organic material largely. They 

 derive their energy by the oxidation of organic matter, or 

 in some cases by the oxidation of inorganic matter. The 

 enzymes which they secrete for the purposes of decomposing 

 food or energy material are capable of acting independently 

 after the death of the bacteria. There are two kinds of 

 bacteria in general: Aerobic, or those which require free 

 oxygen for growth, and anaerobic, which do not require free 

 oxygen for their growth. These two classes of bacteria 

 exist in the soil and regulate to a certain extent the manner 

 of decomposition of organic matter. Under aerobic condi- 

 tions organic matter tends to decompose completely to 

 carbon dioxide and water, together with residual mineral 

 salts and nitrates. Under anaerobic conditions there are 



