COMMERCIAL FERTILIZERS 211 



286. Deficiency of Two Elements. If the prelim- 

 inary trials indicate a deficiency of two elements as 

 nitrogen and phosphoric acid, in verifying these 

 results, both elements are used together, in the same 

 way as described for deficiency of nitrogen, with addi- 

 tional plots for the separate application of nitrogen 

 and phosphoric acid. 



287. Importance of Field Trials. While it is a 

 difficult matter to determine the actual needs of a 

 soil, it will be found that both time and money are 

 saved by a systematic study of the question. Suppose 

 fertilizers are used in a "hit or miss" way year after 

 year on a soil, deficient only in phosphoric acid. It 

 might take 8 years to indicate what the soil really 

 as deficient in if a different fertilizer is used each 

 year, and during all this period, either the soil 

 fails to receive its proper fertilizer, or expensive and 

 unnecessary plant food is provided. Field tests to be 

 of value must be continued for a number of years and 

 the results verified. 



288. Will it Pay to Use Commercial Fertilizers? 



This question can be answered only by trial. If a 

 soil is in need of available plant food, the additional 

 yield of crops should pay for the fertilizer and 

 the expense of using it. Some fertilizers have an 

 influence on two or three succeeding crops, and only 

 partial returns are received the first year. When 

 large crops must be produced on small areas, as in 

 truck farming, commercial fertilizers are generally 

 necessary. In the production of large areas of staple 



