Soil Fertility 57 



of the elements are used, and in others the terms ammonia, 

 phosphoric acid, and potash. 



Ammonia is a compound made up of nitrogen and hydrogen, 

 of which 82.3 per cent is nitrogen. To convert a number repre- 

 senting ammonia into terms of nitrogen, multiply by .823 ; 

 and to convert a number representing nitrogen into terms of 

 ammonia divide by .823 (or multiply by 1.215). (See para- 

 graph 41.) 



When the term phosphoric acid is used, it does not have 

 reference to the common chemical by that name, but to a 

 substance known as phosphoric anhydride, or phosphorus pen- 

 toxide, which contains 43.66 per cent of phosphorus. To con- 

 vert a number representing phosphoric acid into terms of phos- 

 phorus multiply by .4366 and to convert a number representing 

 phosphorus into terms of phosphoric acid divide by .4366 (or 

 multiply by 2.29). 



The term potash refers to potassium oxide, a compound 

 that contains 83 per cent potassium. To convert a number 

 representing potash into terms of potassium multiply by .83, 

 and to convert a number representing potassium into terms 

 of potash divide by .83 (or multiply by 1.024). 



Formerly the names of the compounds were most often used, 

 but the tendency of late is to use the names of the elements. 

 This latter method is much simpler. In some states the fer- 

 tilizer laws (paragraph 40) require that the phosphorus and 

 potassium content of fertilizers be expressed in the terms of 

 the elements ; in other states they must be expressed in terms 

 of the compounds. 



Maintaining the plant-food supply. In most virgin soils 

 the supply of plant-food is comparatively large and crops 

 can be grown for a number of years without returning any to 

 the soil. As crops continue to be removed, however, the 

 store of plant-food becomes less and less until finally the yields 

 decrease so much that, unless some rational farming practice 

 is adopted, the soil will wear out. Some virgin soils are 



