SOILS 107 



noted above that nitrogen, phosphorus, potassium, and prob- 

 ably calcium and magnesium, were the elements likely to 

 become deficient. This being true, we must look to sources, 

 from which these elements may be economically obtained, 

 and maintain if possible and even increase the fertility and 

 productive capacity of the soils. Some sources from which 

 these elements may be obtained are crop residues, barnyard 

 manures, legumes, commercial fertilizers, and mineral sup- 

 plies. 



Crop residue. All the material which makes up the roots, 

 stubble, leaves, and other residue left after removing the 

 crop, contains plant-food elements derived from the soil and 

 air. To remove this residue or to burn it would be an evident 

 loss of soil fertility. Should these residues be returned, their 

 decomposition would not only add the elements to the soil 

 in large measure, but the organic material which they would 

 supply would give greater water-holding capacity to the soil, 

 raise its temperature, foster the growth of bacteria, and pro- 

 mote better physical condition. 



Legumes. Every school boy knows that upon the roots 

 of the legumes such as red clover, sweet clover, alfalfa, soy 

 beans, cow-peas, vetch, etc., are growing nodules which are 

 colonies of bacteria living upon the roots of these plants, 

 drawing the free nitrogen from the air and making it over 

 into an available form for the plant. This nitrogen is 

 assimilated into the leaf, stem, root, and seed of the plant. 

 About as much nitrogen is contained in the part of clover 

 above ground as is taken from the air. It will be seen, then, 

 that if these legume plants be plowed under and incorporated 



