Il8 AGRICULTURAL CHEMISTRY 



pounds of plants takes place in the cells of the 

 leaves, and is the result of chemical changes induced by 

 life processes. In order to promote cell activity, sun- 

 light and a suitable temperature are necessary. The 

 sun's rays take an important part in promoting chemical 

 changes in the leaves of plants. In addition to carbon 

 dioxid, water, heat, and sunlight, various mineral ele- 

 ments in the form of compounds of potassium, calcium, 

 phosphorus, nitrogen, iron, magnesia, sulfur, and pos- 

 sibly of a few others are required as plant food. Without 

 these essential elements and requisite conditions, the 

 growth of crops cannot take place. It often happens that 

 soils are unproductive because of the absence, in available 

 form, of some of the elements essential for plant life. 

 The production in the leaves of plants of the various or- 

 ganic compounds, as cellulose, starch, sugar, fat, albu- 

 min, etc., and a few of the complex chemical changes 

 which take place, have been studied. 



146. Decay of Organic Compounds. All organic com- 

 pounds, particularly those found in the tissues of plants 

 and used as food, are subject to chemical change com- 

 monly called decay. Such change is nearly always pro- 

 duced as the result either of the action of organized fer- 

 ments, or of the chemical products known as chemical 

 or soluble ferments. Fermentation changes and decay 

 take place whenever cell activity becomes feeble or 

 ceases ; then the material becomes food for micro-organ- 

 isms. Many chemical changes take place as the result of 

 fermentation ; some of these are necessary in plant and 

 animal nutrition. If the chemical changes, coordinate 



