PRINCIPLES OF MANURING. 53 



applies to the phosphoric acid that was at first rendered sohil)le in 

 water, but hy chemical change has become insoluble in that licjuid. 

 Its chemical and molecular conditions give it a value greater than 

 tliough it never had been soluble. Its value as comj^ared with that 

 which remains soluble is not determined ; some experiments seem- 

 ing to show that it is of equal value. In the market, however, it 

 takes a lower valuation. Sui)erphosphates made from ground phos- 

 phatic rock ( usually phosphorite ) , are more likely to contain 

 reverted phosphoric acid than those made from bone, owing to the 

 influence of the iron and alumina compounds in the rock. Moisture 

 also has a tendency to cause reversion. 



The commercial values of the different forms of phosporic acid, 

 soluble, reverted and insoluble, diminish in the order mentioned. 

 The insoluble form is valued in price according to the form and 

 condition of the substance containing it. "When in bone, its value 

 varies with the fineness of the bone. It is worth the least in ground 

 rock. It is well known that superphosphate has a much more 

 immediate effect on crops than bone or phosphorite that have not 

 been treated with oil of vitriol. This results from the solubility of 

 the phosphoric acid in the superphosphate, — at least, that is the 

 explanation offered. But how is it that solubility is able to effect 

 the value of this ingredient? Is it simply because that when solu- 

 ble, it remains in the soil and can thus be readily taken up by plants ? 

 Not at all. Phosphoric acid when applied to the land as a super- 

 phosphate, remains soluble but a verj' short time. The lime, ron and 

 alumina compounds in the soil soon precipitate it over the particles 

 of earth, but in a very finely divided condition, so that the soil, 

 water and roots are enabled to make a solution sufficient for the use 

 of growing plants. The state of solubilit}' seems desirable, chiefly, 

 as a means of obtaining an extensive, even and intimate mixture of 

 the phosphate with the soil. 



From the facts just stated, it would seem that the finer bone is 

 ground, the more nearly will its effect compare with that of super- 

 phosphate, so far as the phosphoric acid is concerned. The ditfer- 

 ence in the value of bone, according to its degree of fineness, is 

 recognized by the Connecticut Experiment Station. 



Nevertheless, bone meal is not a manure that generally causes a 

 large increase of growth the fii'st year of its application. At the 

 same time its phosphoric acid is the most valuable of any that can 

 be obtained in the insoluble condition. It is not advisable to appl}- 



