140 ANNUAL. REPORT OP THE Oft. Doc. 



dissolved bone, dissolved bone black and dissolved rock is dependent 

 upon the same reactions. The advantage occurring from this treat- 

 ment is the bringing of the phosphoric acid into a readily available 

 condition so that it can be immediately used by the plant when ap- 

 plied to the soil. So far as its use by the plant is concerned^ the re- 

 verted form is as available as that which is soluble in water, the 

 only advantage of the latter being the even and uniform distribution 

 in the soil and the fineness of subdivision. When we come to con- 

 sider the proper and most economical use of the phosphates, there are 

 many conditions which must be taken into consideration. It is a 

 well established fact that plants absorb material from the soil in 

 solution. The natural supposition would, therefore, be that the phos- 

 phoric acid should be applied in a readily soluble form, and there 

 are a great many cases where this is correct. A young plant 

 just starting its root system might utilize to good advantage phos- 

 phoric acid in a soluble form. It would be enabled to get a strong 

 healthy start and in the end work up the natural plant food material 

 in the soil to better advantage. Not only this, the object to be at- 

 tained must be considered. The trucker aims at quick returns and 

 must be guided by different considerations than one engaged in some 

 other line of farming. 



For the present, however, we will confine ourselves to a considera- 

 tion of the use of insoluble phosphoric acid, citing some of the con- 

 clusions arrived at by those who have conducted experiments in this 

 particular field. In considering the advisability of using insoluble 

 phosphates, the various conditions affecting its availability should 

 be taken into consideration. The character and composition of the 

 soil, the amount of organic matter in the soil, the kind of 

 plants to be grown, the source and general character of 

 the phosphate itself are factors bearing upon this point. The 

 amount of organic matter or humus in the soil will materially 

 affect the degree to which these insoluble phosphates can be utilized. 

 In the rotting of vegetable matter or manure in the soil, carbon 

 dioxide is produced in large quantities. This forms a weak acid with 

 water and is capable of considerable dissolving action. Not only 

 this but in the process of decay certain organic acids are formed, 

 which have a direct dissolving effect upon insoluble phosphates in 

 exactly the same manner, though not to the same extent, as the sul- 

 phuric acid which is used in connection with the manufacture of the 

 super-phosphates from rock phosphate. It is true that certain plants 

 have greater power for working up insoluble plant food material than 

 other plants. Different classes of plants vary widely in their ability 

 to dissolve mineral plant food. In general, it* may be said, that the 

 cereals and grass possess limited power in this direction, the legu- 

 minous plants to a considerably incrpaspd degree and root erops. 



