COMMERCIAL FERTILIZERS AND THEIR USE 251 



elements are used together, in the same way as de- 

 scribed for deficiency of nitrogen, with additional plots 

 for the separate application of nitrogen and phosphoric 

 acid. 



301. Importance of Field Trials. While it is a diffi- 

 cult 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 eight 

 years to indicate what the soil really lacks 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 num- 

 ber of years and the results verified. 



302. 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 

 should pay for the fertilizer and the expense of using 

 it. Some fertilizers have an influence on two or three 

 successive 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. They have not yet been ex- 

 tensively used in the western prairie states in the pro- 

 duction of large tracts of staple crops, as wheat and corn. 



