184 SOILS AND MANURES 



say phosphoric acid only half a crop could be pro- 

 duced, notwithstanding the presence of enough nitrogen 

 and potash for a full crop. The crop would not be 

 increased by adding more nitrogen or potash or all the 

 other constituents, but only by adding that which is 

 deficient phosphoric acid. 



This is doubtless true in the case of an artificial 

 soil in which all the constituents are present in an avail- 

 able state, but it is not strictly accurate in regard to 

 ordinary soils which contain plant foods in many differ- 

 ent degrees of solubility. It is found that when a soil 

 has become, by, exhaustion, deficient in both nitrogen 

 and phosphates, the addition of nitrogenous manures 

 alone will increase the yield and, further, that by con- 

 tinuing the applications of nitrogenous manures a larger 

 yield can be obtained for many years in succession. It 

 would appear as if the nitrogenous manures so stimu- 

 lated the growth of the plants as to enable them to 

 assimilate compounds .which were otherwise not avail- 

 able. The application of phosphatic manures alone, 

 in such a case, would also increase the crops. In neither 

 case, however, would a maximum crop be obtained. If 

 a soil be deficient in both nitrogen and phosphates a 

 maximum crop can only be obtained when phosphatic 

 and nitrogenous manures are applied together. The in- 

 crease produced by the joint action of the manures will 

 generally be much greater than that produced by the 

 two manures acting separately. This is prominently 

 brought out by some of the experiments at Eothamsted, 

 and may be illustrated by the table on p. 18,5 taken from 

 the records. 



Too Rich Soils. In general manures may be said to 

 be adapted to the requirements of the soil when they can 

 supply any ingredient of which there is a deficiency. 

 But an undue excess of any of the constituents may be 



