14 THE SOIL AND THE PLANT 



chemical changes constantly going on in the soil, principally as 

 agents of destruction of the complex organic molecules synthe- 

 sized by plants and transformed by animals. Coincident with 

 this destruction is the construction or formation of new com- 

 pounds. Some of the end products of such transformations are 

 inorganic or mineral materials. There is no breaking down that 

 is not accompanied by a proportionally equivalent building up. 

 The activities of microorganisms upon the mineral constituents 

 of soils are numerous and varied. These may be of the nature 

 of oxidation, reduction, hydrolysis, and carbonation. 



Were these microbes eliminated from the soil, the organic 

 matter constantly added would accumulate until all the combined 

 nitrogen, phosphorus, and some of the potassium would be in 

 that form. These elements which are so important for the 

 growth of cultivated and uncultivated plants would not be avail- 

 able to a new crop, simply because plants cannot utilize in any 

 amounts materials in complex organic forms, but must have them 

 supplied as simple mineralized substances. To understand these 

 relationships we must know something of the nutrition of the 

 plants and of their composition. 



The Nature of Plant Nutrients. — All plants require for 

 their nutrition the elements, carbon, hydrogen, oxygen, nitrogen, 

 phosphorus, potassium, calcium, magnesium, iron, sulfur, and 

 possibly some other elements in small amounts (see Fig. 6). 

 Very few of these elements, however, become factors hmiting 

 plant growth as a result of their presence in insufficient amounts 

 in soils. This is due to the fact that either many of the elements 

 are required by plants only in very small amounts or because 

 some of these elements exist in soils in considerable abun- 

 dance. Under continuous culture of soils in humid regions it is 

 frequently necessary to replenish the store of three elements in 

 particular — nitrogen, phosphorus, and potassium, and occasionally 

 sulfur. These are required by plants in greater amounts than 

 any of the other mineral elements obtained from the soil, and, 

 therefore, become depleted most rapidly. To overcome this 

 deficiency a great variety of substances are utilized. Nitrogen 

 is added in such inorganic forms as compounds of ammonia, 

 nitrate, cyanamid, as well as in organic substances such as urea, 

 packing-house refuse, and guano. This last is the natural com- 

 post of excrements and carcasses of such animals as bats and 



