CHAPTER XV. 



SOURCES AND COMPOSITION OF FERTILIZERS. 



When knowledge that certain elements are essential to plant 

 life was first secured, the action of various elements were tested 

 in practice upon soils in order to see which of these are not pres- 

 ent in sufficient quantity. For example, at Rothamsted, plots are 

 still fertilized with sulphate of magnesia. In the process of time, 

 it was found that phosphoric acid, potash, and nitrogen were the 

 substances needed for plant food, and a fertilizer is now generally 

 defined as a substance which contains phosphoric acid, potash, or 

 nitrogen, or a mixture of them, and is used as an application to 

 the soil to promote the growth of plants. A further requirement 

 is that the potash, phosphoric acid, or nitrogen be in such forms 

 as to be readily taken up by plants. The value of the fertilizer 

 is based upon the amount of these three substances it contains. 



Fertilizers have other effects on the soil in addition to their 

 supply of plant food. They may affect its acidity or alkalinity, 

 its physical structure, etc. Substances which contain little or 

 no phosphoric acid, potash, or nitrogen, and are used upon the 

 soil for other reasons, are termed amendments. Lime, for 

 example, is an amendment. 



Nitrogenous Fertilizers. Nitrogenous fertilizers are divided 

 into two groups, inorganic and organic. The two inorganic 

 materials, nitrate of soda and ammonium sulphate, may be directly 

 assimilated by plants, though the ammonium sulphate usually 

 undergoes some nitrification and is converted partly into nitrates. 

 Organic substances, such as dried blood, cottonseed meal, tank- 

 age, etc., must first undergo changes in the soil, by which the 

 nitrogen is converted into ammonia or into nitrates, or into 

 organic compounds which can be assimilated by plants. The dif- 

 ferent nitrogenous fertilizers have different agricultural values, 

 depending on the readiness with which they can be assimilated. 



Inorganic Nitrogenous Materials. Nitrate of soda is found in 

 the rainless districts of South America mixed with dirt and com- 

 mon salt, as deposits termed caliche. It contains on an average 



