Tillage, Fertilizing and Rotation 105 



The percentage of available plant-food elements in a 

 complete fertilizer is usually marked on the container. 

 State laws require this of manufacturers and state in- 

 spection is usually very rigid. A 2-8-10 formula means 

 2 per cent nitrogen, 8 per cent phosphorus and 10 per 

 cent potaS'h. The nitrogen is often listed as ammonia, 

 which in interpreting formulas is considered to be about 

 ^%7 as valuable as nitrogen. Fertilizers should always 

 be purchased on the basis of the available plant-food 

 which they contain and not according to the price or 

 brand of the fertilizer. The fertilizer lowest in price 

 is often the most expensive medium through which plant- 

 food may be purchased. In general, the actual plant- 

 food contained in higher-grade fertilizers is least ex- 

 pensive.^ 



Home-mixing of fertilizers. 



Thorough familiarity vrith fertilizing constituents, 

 better facilities for handling and the ability to pur- 

 chase in large quantities, makes legitimate the commer- 

 cial manufacture of complete ready-mixed fertilizers. 

 In spite of this, many sweet potato growers can save 

 money by mixing their manures at home and if intelli- 

 gent care is exercised as good results can be secured with 

 the home-mixed goods as with that purchased ready- 

 mixed. In many cases there may be an actual improve- 

 ment in results as well as saving in cost, since the 

 " carriers " are known and mixing can be done to suit 

 specific conditions. 



The approximate composition of the fertilizers de- 

 sired must first be decided on and the form in which 

 each constituent is to be applied must be determined 



1 Bull. 392, N. Y. Exp. Sta., 1914. 



