78 FERTILITY AND FERTILIZER HINTS 



with sulphuric acid) before being applied as fertilizer. The 

 production of rock phosphates in the United States has almost 

 entirely discouraged the importation of the mineralized or phos- 

 phatic guanos. 



Form of the Phosphates. — The phosphoric acid in bone phos- 

 phates and rock phosphates is in the form of tricalcium phos- 

 phate. Bone phosphates are always as phosphate of lime while 

 rock phosphates contain more or less impurities as iron, alumina 

 and silica. It is customary to apply the name, "bone phosphate 

 of lime," to the phosphate present in rock phosphates, although 

 tricalcium phosphate is the correct name. The phosphoric acid 

 in basic slag is not in the same form as in the other phosphates. 

 It was formerly accepted that the phosphoric acid in basic slag 

 existed as tetra-calcium phosphate, but HalF claims that the 

 phosphoric acid is in the form of double phosphate and silicate 

 of calcium Ca,,(CaO) (PO,).,CaSiO,. 



Availability of the Phosphates. — All of the phosphates are 

 slowly available as plant food and practically insoluble in water. 

 The phosphoric acid in phosphates is not entirely used the first 

 year so that maximum crop returns cannot be expected im- 

 mediately, but the continued use of phosphates give good results. 

 For quick growing crops the phosphates are not always desirable. 

 The phosphates from bones are perhaps more readily decomposed 

 than the rock phosphates. There is more or less organic matter 

 in bones which decays quite rapidly and attacks tHe phosphoric 

 acid with which it is closely associated. In the rock phosphates 

 there is no organic decay and the impurities as iron and alumina 

 retard to a certain extent the fermentation and decomposition 

 of the phosphoric acid present. Basic slag phosphate as shown 

 by the statistics in this chapter, is used extensively in Europe. 

 European experiments show that this material is of higher avail- 

 ability than the insoluble bone and rock phosphates. 



The nature of the soil has a great deal to do with the avail- 

 ability of phosphates. Soils in good tilth will disintegrate the 

 phosphates more readily than those in poor physical condition. 

 The sandy and gravel soils are liable to give poorer results than 



' Fertilizers and Manures. 



