SOILS FERTILIZEES. 425 



" Soils containing little or no phosphates of high solubility give practically 

 the same amounts of phosphoric acifl to successive extractions. 



'* Soils which have a fixing power of SO per cent or less have a fixing power 

 of about half as much from fifth-normal nitric acid solution. Soils which have 

 a fixing power over SO may fix equally as high a percentage from fifth-normal 

 nitric acid. 



" When the significance of the phosphoric acid extracted from a soil by fifth-* 

 normal nitric acid is to be decided, the fixing power of the soil for phosphoric 

 acid and the acid consumed should also be known. 



" Sulphite of lime increases the amount of phosphoric acid extracted from 

 soils high in iron. 



" Calcareous soils contain phosphates which are protected by the carbonate 

 of lime from the roots of plants, but which are exposed by solution of the car- 

 bonate of lime in acid solvents. 



" The amount of lime and magnesia dissolved may be estimated from the 

 quantity of acid consumed. 



" The quantity of material dissolved in second or succeeding extractions with 

 acid is sometimes large. 



" It would appear that the lime and magnesia are present in highly soluble 

 forms (carbonates and silicates), moderately soluble silicates and silicates of 

 low solubility. 



" Citric acid dissolves less iron, lime, and magnesia than fifth-normal nitric 

 acid. 



" It would appear that the phosphoric acid dissolved by fifth-normal nitric 

 acid in excess of about ten parts per million comes from phosphate of lime. 



" Judging the amounts of phosphates of lime presented to the roots of plants . 

 in a given soil, one must allow for the decrease due to absorption, and the 

 increase due to solution of incrusting material, so far as possible. 



" The author extracts the soil with fifth-normal nitric acid without correcting 

 for neutralization. 



" It is impossible to maintain only one variable in pot experiments, though 

 one may predominate. Soils may appear deficient for phosphoric acid and yet 

 be highly productive without phosphatic fertilizing. 



'• Soils containing less than 20 parts per million of phosphoric acid extracted 

 by fifth-normal nitric acid are highly deficient in phosphoric acid in pot experi- 

 ments. 



" Soils from which 20 to 100 parts per million of phosphoric acid are extracted 

 by fifth-normal nitric acid are usually deficient for phosphoric acid in pot 

 experiments, and the extent of their deficiency is related to the quantity of 

 phosphoric acid present. 



" Although the pot experiments were carried out under diverse conditions, 

 the average corn crop is closely related to the quantity of active phosphoric 

 acid in the soil. 



" Soils containing from 100 to 200 parts per niillion of active phosphoric acid 

 are possibly deficient in phosphoric acid in pot experiments, the chances being 

 even that they are or are not deficient. 



"The average possible corn crop, based upon the quantity of phosphoric acid 

 extracted from the soil in pot experiments, increases regularly with the amount 

 of active phosphoric acid extracted by fifth-normal nitric acid. Soils containing 

 less than 10 parts per million of phosphoric acid had an average possibility 

 of 4.5 bu. corn per acre. If they contained 10 to 20 parts, the possibility is 

 12.3 bu. If they contained 30 to 100 parts, the average possibility is 19.7 to 

 26.3 bn. corn per acre. If they contained 110 to 420 parts per million, the 

 average possibility was 50 to GO bu. corn per acre. 



