ELLSHA MITCHELL SCIENTlr.'C SOCIETY. 127 



We see from the above experiments that consequent upon the 

 addition of acid phosphate to the soil a precipitation occurs by 

 which some of the phosphoric acid soluble in water becomes no 

 longer soluble in that liquid, but is readily dissolved by ammonium 

 citrate at 65° C. This precipitation may continue until later a 

 basic phosphate of iron or alumina, insoluble in ammonium 

 citrate, forms. 



The resuits brought out by these investigations are: 



1st. The sesquioxides of iron and aluminium present in the 

 soil, more than any other ordinary ingredients, precipitate to a 

 marked degree the soluble phosphoric acid of the superphosphate 

 applied to them. 



2d. The precipitation varies in direct proportion to the content 

 of ferric oxide and alumina. 



3d. That this precipitation commences immediately when moist- 

 ure is present, and contiuues to increase gradually till all the 

 soluble is so precipitated. 



4th. In soils containing much of the oxides of iron and 

 aluminium basic phosphates of iron and alumina, iusoluble in 

 ammonium citrate at 65° C, commence to form at once, and 

 increase very slowly. 



5th. Organic matter by decomposition furnishes organic acids 

 which prevent the precipitation of soluble phosphoric acid. 



6th. The organic acids so formed either dissolve the tricalcic 

 phosphates of the superphosphates, or by preventing the precipi- 

 tation of soluble phosphoric acid allow it to act on the uudecom- 

 posed phosphate, rendering it available to plants. 



7th. The greater the proportion of soil to soluble phosphoric- 

 acid the greater the precipitation, but this is not proportional to 

 the quantity of the soil. 



8th. That a limit is soon reached beyond which an additional 

 quantity of soil has very little or no effect on the superphosphate. 



9th. This limit is caused by the impossibility of a larger 

 number of soil particles coming in contact with the particles of 

 superphosphate. 



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