PHOSPHORIC ACID LIME MAGNESIA. 99 



and added to the weight of the main quantity of the precipi- 

 tate. From the sum of these weights the amount of potash in 

 the soil can be calculated, knowing that 94 of potash (K 2 O) 

 correspond to every 485 of the double chloride. 



This method w r orks very well and is a great saving in time 

 and labour over the old methods requiring the preliminary 

 removal of the iron, aluminum, lime, and magnesium. Unfor- 

 tunately it prevents the simultaneous determination of the 

 sodium. 



The so-called "total potash" obtained in this way is not 

 necessarily the whole quantity of potash which the soil con- 

 tains, for hydrochloric acid does not remove all the potash 

 from silicates. 



If the real total potash be required, it is necessary to 

 analyse the residue insoluble in hydrochloric acid, exactly as is 

 done with a mineral silicate. However, the potash in a soil 

 insoluble in hydrochloric acid is probably unavailable to plants 

 for many years to come, so that its determination is not often 

 of importance. 



Determination of "total" phosphoric acid. This can con- 

 veniently be done in the portion of the soil taken for determi- 

 nation of loss on ignition. The residue in the platinum dish 

 is treated with strong hydrochloric acid, digested for some 

 time, evaporated to dryness, and heated to render the silica 

 insoluble, moistened with strong hydrochloric acid, taken up 

 with water, filtered, and evaporated with strong nitric acid. 

 It is again diluted and mixed with excess of ammonium 

 molybdate solution and allowed to stand in a warm place for 

 18 or 24 hours. It is then filtered, washed by decantation 

 with dilute nitric acid and once with water, then dissolved in 

 dilute ammonia (filtered if necessary), mixed with "magnesia 

 mixture,"" and allowed to stand 12 hours. The ammonium 

 magnesium phosphate, NH 4 MgP0 4 , is then filtered off, washed 

 with ammonia, dried, ignited slowly and carefully, and 

 weighed as Mg. 2 P 2 7 . Phosphoric acid may also be deter- 

 mined in a portion of the hydrochloric extract of the soil. 



Determination of the lime and magnesia. Except in some 

 few cases, the amount of these constituents is so small that 



* A solution containing magnesium chloride, ammonium chloride, and ammonia. 



H 2 



