562 APPENDIX C. 



each tube, unless of uniform diameter, and is ascertained by taking a 

 soil whose phosphoric-acid percentage has been determined gravimet- 

 rically and giving it the following quick treatment ; which must, of 

 course, be closely followed in each soil to be examined : 



Two grams of the fine earth is ignited in a platinum dish to destroy 

 the organic matter, transferred to a test- tube containing 5 cc. of nitric 

 acid and made to boil for only a couple of seconds, thus preventing the 

 solution of silicates to any material extent. It is not allowed to stand, 

 but a little water is immediately added and it is quickly thrown on a 

 small filter and washed with a little water. The phosphoric acid is then 

 precipitated with molybdic acid at the proper temperature ; allowing it 

 to settle, the liquid is drawn off and the precipitate transferred to the 

 measuring- tube. It settles into the small part in a short time if the 

 latter is not too narrow, and is then measured with a millimeter scale. 

 This represents the percentage as found in the soil by the gravimetric 

 method, and serves as a guide for other examinations, whose agreement 

 with gravimetric determinations is generally quite close, and quite suf- 

 ficient for practical purposes. The rapidity with which the solution is 

 made and separated from the soil is a matter of special importance for 

 comparative results, or determination of percentages ; for if the acid 

 solution be allowed to stand for some time before filtration from the 

 soil, silica passes into solution also, and the volume ot the molybdate 

 precipitate is increased by it ; thus vitiating the results and adding to 

 the time required for the method. By this short method the practically 

 important phosphoric acid in the soil may be approximately deter- 

 mined within half an hour. 



SHORT MI I HOD FOR ALKALI SALTS. 



The old method of obtaining solutions of the salts by leaching the 

 soil on a filter until all of the alkali had been washed out has been 

 replaced by the following short one. 50 or 100 grams of the well- 

 mixed soil is placed in a bottle containing 200 cc. of water, shaken up 

 occasionally during 12 hours and allowed to settle. The solution may 

 then be passed through a common filter (or preferably a pressure filter) 

 and an aliquot part (usually 50 cc.) of the filtrate evaporated to dry- 

 ness in a platinum basin and ignited at a temperature just below redness 

 to destroy any organic matter that may be present. The basin and 

 contents are weighed and the soluble salts are dissolved in a very little 

 water and separated by filtration through a small filter into a 50 cc. 

 cylinder and the alkali carbonates and chlorids determined by titration, 

 being calculated as sodium compounds. 



