82-84] NITROGENOUS MANURES 33 



washed away into the drains. This, however, does not 

 occur, as several substances in the soil cause it to ' revert,' 

 or turn into reverted phosphate, which, like slag phosphate, 

 is insoluble in water. The principal substances which 

 bring about this reversion are lime, oxide of iron, and 

 alumina. This reversion is shown in the next experiment. 



82. Action of Lime on Superphosphate. — Mix a 

 little superphosphate in a mortar with twice its weight of 

 freshly slaked lime. Grind them well together and intro- 

 duce a small quantity into a test-tube. Add distilled water 

 and shake well, then allow to settle. When the liquid has 

 become n2arly clear, filter and test the clear liquid with 

 ammonium molybdate. No phosphoric acid will be found. 

 The lime has reverted the superphosphate and rendered it 

 insoluble. 



* 83. Action of Ammonium Citrate on Reverted 

 Phosphate. — Take a little more of the mixture of lime 

 and superphosphate made in the last experiment in a test- 

 tube and treat it with ammonium citrate solution. Filter, 

 and test with ammonium molybdate. The reverted phos- 

 phate will be found to resemble slag phosphate in that it 

 dissolves in ammonium citrate. 



The vegetable acids in soil act very much in the same 

 way as ammonium citrate, so that the reverted phosphate, 

 which has been formed and thoroughly mixed with the soil 

 by the action of rain, lime, &c, on superphosphate, is slowly 

 redissolved and given up to the plants as food. 



Nitrogenous Manures 



84. The nitrogen in manures occurs in one or more of 

 three forms known chemically as organic nitrogen, ammo- 

 niacal nitrogen, and nitric nitrogen. In whatever form it 

 may occur, the action of the organisms in the soil is to turn 

 it into the last of the three, as this is the only form in which 



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