The Composition and Use of Fertilizers. 203 



3. Another effect of nitrogen is to increase in a very marked 

 degree the relative proportion of nitrogen in the plant. This is 

 something which we cannot see with our eyes and which can be 

 ascertained only by chemical analysis. The importance of this 

 fact may not be readily apparent, but it becomes so, when we con- 

 sider that much of the value of some foods depends upon the 

 amount of nitrogen compounds, that is, proteids, contained in 

 them. For example, some samples of wheat may contain only eight 

 per cent, of proteids, while others may contain twice this amount. 

 This difference in composition makes a great difference in food 

 value. 



Phosphorus. 



(a) Source. — Phosphorus is always found in nature combined 

 with other elements. It occurs combined with oxygen and cal- 

 cium (or lime) and this compound is called calcium phosphate or 

 phosphate of lime. It also occurs in soils as phosphate of mag- 

 nesia, and phosphate of alumina and iron. Calcium phosphate or 

 phosphate of lime is found in some minerals and in the bones of 

 animals. 



(b) Description. — Phosphorus, when uncombined with other 

 elements, it is yellowish, waxy-looking, solid substance. It is soft 

 and can be cut as easily as ordinary beeswax. It is very poison- 

 ous. It takes fire very easily and, therefore, has to be kept under 

 water. When phosphorus burns, it simply unites with the oxygen 

 of the air, forming a compound which contains oxygen and phos- 

 phorus; this compound of oxygen and phosphorus is commonly 

 called phosphoric acid. 



(c) Importance of Phosphorus Compounds. — The phosphates, 

 like the nitrates, are found everywhere in the soil and are of great 

 value in their relations to plants. The phosphates found in the 

 bones are taken into the animal body in the food. All plants 

 used as food contain small quantities of phosphorus compounds 

 which they get from the soil. The phosphates taken into the 

 body are partly given off in the excrement and urine. 



(d) Relation to Fertilizers. — - The fact that phosphorus com- 

 pounds are absolutely necessary for the maturity of plants indi- 

 cates that phosphates are essential to complete fertilizers. Soils 

 become deficient in available phosphates quite rapidly, especially 

 in grain-growing regions. 



(e) Specific Functions in Plant Growth. — 1. Phosphorus 

 compounds aid in the general nutrition of the plant by furnish- 



