MEASURING PLANT FOOD REQUIREMENTS 157 



taken up and washed about fifteen times with 80 per cent alcohol, 

 three times with ammonium chloride solution, and again fifteen times 

 with alcohol. The precipitate is then washed through the filter with 

 hot water into a platinum dish, evaporated on the steam bath to dry- 

 ness and heated in an air oven at 110 C. for an hour, cooled in a 

 desiccator, and weighed as K 2 PtCl 6 . Duplicate samples should 

 not differ more than 1.5 milligrams in the final weight. The weight 

 of K 2 O can be determined by multiplying the weight of the K 2 PtCl 6 

 by the factor .1941. 



A correction must be made for the amount of potassium in the 

 reagents, which is found by making a blank determination, using no 

 soil. 



(Ammonium chloride solution is made by dissolving 200 grams 

 NH 4 C1 in 1000 cubic centimeters water and saturating with K 2 PtClc.) 

 (/) Calcium. Calcium may be determined as described by Hop- 

 kins in his Soil Fertility and Permanent Agriculture, page 632. Five 

 grams of soil (or less if high in calcium) are decomposed by heating 

 10 grams of sodium peroxide in an iron crucible. This is then taken 

 up with water and hydrochloric acid and made up to 500 cubic centi- 

 meters, as in the phosphorus determination. After being allowed to 

 settle over night, 200 cubic centimeters of the supernatant solution 

 are heated to boiling and precipitated from the hot solution with 

 ammonia. The precipitate is filtered out on a 15-centimeter filter 

 and washed with hot water until but a slight test for chlorides is 

 given with silver nitrate. The filtrate is again evaporated to dryness 

 and heated (to dehydrate any remaining silica), taken up with water 

 and hydrochloric acid, brought to a boil, and ammonia added to pre- 

 cipitate any remaining aluminum. The precipitate is filtered out on 

 a small filter and washed with hot water. It should not be washed 

 more than necessary to remove the chlorides, as the wash water car- 

 ries aluminum through into the filtrate. On heating this filtrate and 

 allowing it to stand overnight, more aluminum may be found to pre- 

 cipitate out. All of the aluminum must be removed by repeated pre- 

 cipitations. The solution is then made slightly alkaline with ammonia, 

 brought to a boil, and to it is added slowly, while it is being stirred, 

 enough concentrated ammonium oxalate solution to precipitate the 

 calcium and to change the magnesium to the oxalate. After boiling 

 until the precipitate has a granular appearance, it is allowed to stand 

 three hours or longer, decanted into a filter, and washed twice by decan- 

 tation. The precipitate in the beaker is then dissolved with a few 

 drops of hydrochloric acid, a little water added, and the calcium 

 reprecipitated, boiling hot, by adding ammonium hydroxide to slight 

 alkalinity. A little ammonium oxalate is added, the solution allowed 



