146 PHOSPHATIC FERTILISERS [chap. 



of these weak acid solvents to arrange the various 

 phosphates in a scale of decreasing solubility, and argue 

 from that as to their availability to the plant, the order 

 of the table would not represent their relative value in 

 practice. 



In considering the action of the various phosphatic 

 manures in the field, the most important factor to be 

 taken into account is the soil, for the relative value of 

 the fertilisers will change entirely with different types 

 of soil. For example, the choice between super- 

 phosphate and basic slag or bone meal, as a phos- 

 phatic manure, must be determined, not by their 

 comparative solubility, but by the amount of calcium 

 carbonate and the wetness or dryness of the soil to 

 which the fertiliser is to be applied. A very large 

 number of experiments have been made to institute 

 a comparison between these fertilisers, but without 

 resulting in any very general information, just because 

 the question is really settled by those external soil 

 factors which are generally unrecorded. On certain 

 soils one or other of these manures will always give 

 the best results, on other soils their effects may be so 

 much alike that the choice between them can be 

 settled by price alone or by any consideration of 

 convenience that may enter. For example, in one 

 of the Rothamsted experiments one series of plots 

 receive superphosphate, another series basic slag, and 

 a third bone meal, in quantities supplying the same 

 phosphoric acid to each, the plots being otherwise 

 treated alike as regards nitrogen and potash. If we 

 abstract the results which refer to the )ields in the 

 year of the application of each manure and reduce 

 them to a common standard each year by taking the 

 yields of the unmanured plot as lOO, we obtain the 

 relat've figures in Table XXXIX. 



