The Science of the Soil 



135 



weighed as magnesium phosphate. From the magnesium pyro-phosphate 

 obtained the weight of phosphoric acid can be calculated. One part of 

 magnesium pyro-phosphate = '64 parts of anhydrous phosphoric acid (P 2 O 5 ). 



Total phosphates are estimated by treating 5 gm. of dried soil with 

 25 cub. cm. of concentrated hydrochloric acid, and evaporated to dryness 

 over a water bath. The residue is moistened with concentrated sulphuric 

 acid, then treated with a mixture of 10 cub. cm. of hydrochloric acid and 

 10 cub. cm. of water, warmed, filtered while hot, the filter paper being well 

 washed, and the filtrate treated with an excess of ammonia, boiled, allowed 

 to cool, and filtered. It is then redissolved in nitric acid and dried with 

 40 cub. cm. of ammonium molybdate exactly as indicated in the paragraph 

 above for soluble phosphates. 



Potash. To estimate the potash a further 500 cub. cm. of the citric 

 acid solution are evaporated to dryness and incinerated in a basin over a 

 Bunsen burner to eliminate the organic matter. The residue is then treated 

 with boiling distilled water and filtered. An excess of platinum chloride 

 is then added to the filtrate, and the whole is slowly boiled until nearly 

 dry. The precipitate is filtered off and washed with alcohol until no yellow 

 coloration is to be seen in the filtrate. The least possible quantity of water 

 is then added, together with an excess of magnesium powder. The reaction 

 with this is completed by boiling. After cooling, the excess of magnesium 

 is dissolved in hydrochloric acid, the whole being filtered and the pre- 

 cipitated platinum, which remains upon the filter, is washed free from acid, 

 dried, and weighed. From this platinum is calculated the quantity of 

 potash present. One part of platinum = - 48 parts of potash (K 2 O). 



Iron. Available iron is estimated in a further 500 cub. cm. of citric 

 acid solution, this being evaporated to dryness over the water bath, and 

 incinerated over a Bunsen burner to drive off the whole of the organic 

 matter. The residue is dissolved in hydrochloric acid, then evaporated 

 in a conical glass flask. The iron is reduced to the ferrous condition by 

 the addition of metallic zinc (adding only sufficient to get it into complete 

 solution), and titrated with a deci-normal permanganate of potash solution. 

 Each cubic centimetre of permanganate of potash required until permanent 

 coloration is produced indicates the presence of '008 gm. of Fe 2 3 or ferric 

 oxide. 



Calcium Carbonate. This is estimated by 5 gm. of the dried soil 

 in a CO 2 apparatus, one portion of which contains a supply of hydrochloric 

 acid and a tube to enable it to be passed into the portion containing 

 the soil, and a further portion contains a tube filled with concentrated 

 sulphuric acid through which the escaping gases pass, in order to retain 

 in the apparatus any moisture that may tend to be carried away. In 

 this way the calcium carbonate is decomposed, the carbon dioxide being 

 evolved. The apparatus is then stood for a quarter of an hour on the water 

 oven to ensure the exclusion of the whole of the gas. The weight of 

 the whole apparatus being taken, with the soil in it, before commencing 

 the operation, it is now weighed again and the difference represents the 



