The Composition and Use of Fertilizers. 221 



stitutent presented in commercial fertilizers — in some cases it 

 may be entirely absent. The term " superphosphates " applies 

 truthfully to manv commercial fertilizers, but even these cannot 

 be correctly spoken of as simply " phosphates." This common 

 usage of the term " phosphate " for any form of fertilizer empha- 

 sizes the fact that there has been a tendency to overestimate the 

 value and importance of this constitutent, resulting in large appli- 

 cations of it without regard to the needs of soil or crop. 



2. Forms of Plant Food Essential to Fertilizers. 



In the absence of iron in the soil, plants turn yellow and cease 

 to grow; other elements, as magnesium, sulphur, etc., are essen- 

 tial to the complete development of a plant. But these elements 

 are used by plants in very small quantities, and, moreover, they 

 occur abundantly everywhere in soils, as already indicated. 

 Therefore, it is rarely necessary to supply these elements arti- 

 ficially to soils, and we do not need to consider them in connec- 

 tion with fertilizers. The elements of plant food which experi- 

 ence most often shows to be lacking in soils are these four: 



Nitrogen, Potassium (contained in potash compounds), Phos- 

 phorus (contained in phosphoric acid compounds or phosphates), 

 and in special cases, Calcium (contained in lime compounds). 



3. Stimulant or Indirect Fertilizers. 



A Stimulant or Indirect Fertilizer is one which does not in 

 itself furnish directly to the soil any needed plant food, but whose 

 chief value depends upon the power it possesses of changing un- 

 available into available forms of plant food. The stimulant or 

 indirect fertilizers which have been most commonly employed are 

 lime, gypsum and common salt. 



Gypsum or Land Plaster, known also as calcium sulphate or 

 sulphate of lime, has been much used in fertilizing crops. Its 

 value is due to its action as an indirect fertilizer. There has been 

 much difference of opinion as to the manner in which gypsum 

 acts. Probably it acts in at least three different ways, as follows: 



1. It has the power to form compounds with ammonia, in 

 which the ammonia is no longer in danger of loss by evaporation. 

 This power of fixing ammonia is probably of little value when 

 plaster is applied to the surface of the soil, but it may be of much 

 value when scattered over a heap of fermenting manure, and 



