Decomposition Processes 101 



stood arc taken into consideration. The common loodstult analyses 

 are not of sufficient value for this purpose, since they give a rather 

 limited concept of the chemical composition of the plant materials 

 that find their way into the soil. 



A series of analyses of various plant materials, as obtained by 

 the proximate method, is gi\ en in Table 15. The chemical composi- 

 tion of the plant \ aries not only with its nature, but also with its age 

 and with conditions of growth and nutrition. Tables 16 and 17 

 indicate the effect of age upon the chemical composition of rye and 

 corn plants. At an early stage of growth, plants are rich in water- 

 soluble substances, including sugars and amino acids, in proteins, 

 and in mineral constituents; the older the plants become, the less is 

 the proportion of these constituents and the greater is the concen- 

 tration of cellulose and lignin and, to some extent, of the hemicellu- 

 loses and polyuronides. This change in composition has an im- 

 portant bearing upon the rapidity of decomposition of the plant 

 materials. In leguminous plants, such as alfalfa, the protein content 

 also decreases with the maturity of the plant, and the cellulose and 

 lignin contents increase. This affects both the digestibility of the 

 plant materials by animals and their decomposition in soil. 



Decomposition Processes 



When plant and animal residues are added to the soil or are placed 

 in composts under favorable conditions of moisture and aeration, 

 they are attacked by a great variety of microorganisms, including 

 bacteria, fungi, actinomycetes, protozoa, worms, and insect larvae. 

 As a result of the activities of these organisms, considerable portions 

 of some of the constituent chemical elements in the residues, notably 

 the carbon, nitrogen, phosphorus, and potassium, are rapidly liber- 

 ated in forms available for plant growth. The process is at first 

 rapid but gradually slows down; the rate of decomposition depends 

 upon the nature of the residues and upon the conditions under 

 which decomposition is taking place. If the residues are low in 

 nitrogen, as in straw, this element wffl not be liberated for some 

 time and is, therefore, not made available for plant growth. 



This is illustrated in Table 18, where the formation of nitrate in 

 the soff is used as a measure of liberation of the nitrogen in an avail- 

 able form. A definite quantity of root material obtained from several 



