MEASURING PLANT FOOD REQUIREMENTS 155 



ammonium nitrate and, finally, once or twice with cold water. Trans- 

 fer the filter to a beaker, and dissolve in standard potassium hydroxide 

 (1 cubic centimeter-0.2 milligram P), titrate the excess of potassium 

 hydroxide with standard nitric acid of the same concentration as the 

 KOH solution, using 0.5 cubic centimeter of phenolphthalein as in- 

 dicator. Subtract the number of cubic centimeters of acid used from 

 the cubic centimeters of KOH used, and multiply the remainder by 

 .0002 and the result will be the grams of phosphorus present in 200 cubic 

 centimeters of the soil solution. Determine the amount present in 

 500 cubic centimeters, and dividing by the weight of soil taken for 

 analysis (5), multiplying by 100 will give the per cent of phosphorus 

 present. 



(d) Nitrogen. Seven grams of soil are weighed into a large Kjel- 

 dahl flask, 0.7 gram of mercuric oxide is added, and to this is added 

 about 20 cubic centimeters of cone, sulphuric acid. The contents 

 of the flask are boiled and digested until colorless. Finely powdered 

 potassium permanganate is added while the contents of the flask are 

 hot, until a green solution furnishes assurance that the oxidation is 

 complete. After cooling, about 250 cubic centimeters of water are 

 cautiously added, then enough potassium sulfide solution to pre- 

 cipitate out the mercury, and some zinc filings to lessen the bumping 

 on boiling. Enough strong alkali is then added to neutralize the acid 

 and leave the solution strongly alkaline. The flask is immediately 

 connected to a still and the ammonia distilled off into a standard 

 solution of sulphuric acid. The excess of acid is titrated with standard 

 sodium hydroxide solution of the same concentration as the sulphuric 

 acid, using an alcoholic extract of cochineal as an indicator. Deter- 

 mine the number of cubic centimeters of acid neutralized by the 

 ammonia and multiply this by the nitrogen factor of the acid. In case 

 the acid is Fifth Normal this factor will be .0028. Divide the grams of 

 nitrogen by the weight of the sample and multiply by 100 and the 

 result will be percentage of nitrogen. 



(e) Total Potassium. This test is carried out as given on page 147, 

 Bulletin 105, Bureau of Chemistry, Department of Agriculture. One 

 gram of soil, very finely pulverized, 1 gram of ammonium chloride, 

 and 8 grams of calcium carbonate thoroughly ground in an agate 

 mortar are fused as directed in Fresenius' " Quantitative Analysis," 

 Vol. 2, page 1175, and by Hillebrand in Bulletin 305 of the United 

 States Geological Survey, where an illustration of the apparatus is 

 given. The fused mass is transferred to a porcelain dish, slaked with 

 hot water, finely ground with an agate pestle and transferred to a filter. 

 After washing with about 600 cubic centimeters of hot water, the fil- 

 trate and washings are run to dryness in a Jena beaker, taken up with 



