144 MIXED MANURES AND CALCIUM COMPOUNDS 



and water soluble, citric soluble and total phosphates, 

 are determined by the methods previously described. 

 The total phosphates are estimated after destroying 

 organic matter by ignition and rendering the silica 

 insoluble in the usual way. 



(c) Potash. — Ten grams of the sample are gently 

 ignited to destroy the organic matter, heated for ten 

 minutes with lo c.c. of concentrated hydrochloric acid, 

 and then boiled with 300 c.c. of water. The liquid is 

 filtered, boiled, and a slight excess of baryta water 

 added. The contents of the vessel are cooled and 

 diluted to 500 c.c. in a measuring flask, and then filtered. 

 Two hundred and fifty c.c. of the filtrate are made alkaline 

 with ammonia and treated with excess of ammonium 

 carbonate solution, and then, while boiling, with a little 

 powdered ammonium oxalate. The liquid is again 

 cooled, made up to 500 c.c, and filtered. Of the filtrate, 

 100 c.c. are evaporated in a platinum dish and the 

 residue heated gently over a low flame until all the 

 ammonium salts are expelled. The residue is then 

 treated with hot water, filtered if necessary, and the 

 potash determined by precipitation with platinic chloride 

 in the usual way. 



Lime and Limestone. 



Quicklime, slaked lime, and ground limestone (chalk) are all 

 used as manures, although not in themselves plant foods. They 

 act indirectly, either by preventing acidity, bettering the soil 

 texture, or by liberating the soil reserves of plant nutrients. 



137. Limestone. 



The calcium carbonate present is determined by the 

 method already described (36 {U), p. 57). Weight of 

 CO2 X 2-2727 = weight of CaCOg. 



A small quantity is dissolved in hydrochloric acid and 



