342 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Used in small quantities, these fertilizers tend to deplete the soil 

 of nitrogen, unless provision is made to restore it otherwise. The 

 use of such fertilizers in large quantity is not to be recommended, 

 since a great deal more phosphoric acid is supplied than can 

 possibly be utilized by the crop. If more fertilizer is used, it 

 should be richer in nitrogen. 



Effect of Fertilizer on Succeeding Crops. The effect of fer- 

 tilizer carried over to a subsequent year is shown in the experi- 

 ments at Rothamsted. Plot 17 and 18 receive "mineral manure" 

 consisting of superphosphate and sulphates of potash, soda, and 

 magnesia one year, and 86 pounds nitrogen in sulphate of am- 

 monia the second year. These applications have alternated for 

 51 years. The average results are as follows: 



Bushels of wheat 

 per acre 



Plot 5 mineral manure " 14.9 



(A) Plots 17-18 when mineral manure etc., is applied 15.3 



(B) Plots 17-18 when sulphate of ammonia is applied 30.4 



Plot 7 complete fertilizer 32.9 



Thus plots 17-18 produce little more crops the year after the 

 application of sulphate of ammonia (A) than plot 5 which re- 

 ceives no ammonia. Plots 17-18 produce nearly as much the 

 year after the application of the phosphoric acid and potash, as 

 they do in plot 7 which receives them every year. That is to say, 

 the phosphoric acid and potash remain in the soil in an available 

 form, so as to be useful to the next crop, while the nitrogen was 

 leached out or otherwise rendered of little value to the plant. 



Fixation. It is commonly supposed that phosphoric acid and 

 potash of fertilizers are fixed and rendered less soluble almost as 

 soon as they are placed in the soil. This is not, however, the 

 case. Some of the lumps of fertilizer are '% inch in diameter, 

 and do not dissolve at once. When they do dissolve, 

 diffusion is not a rapid process, and the soil particles 

 nearest the fertilizer are brought in contact with the more con- 

 centrated solution, and fix larger proportions of it, than those 

 farther away. Hence the fertilizer becomes the center of a zone 

 of concentrated plant food, more dilute towards the outside. This 



