265-267] Soil Analysis 159 



moisture. From this estimation calculate the quantity of dry 

 soil in the 200 grams, 



265. Available Phosphoric Acid.— Evaporate 500 c.c. of 

 the clear liquid on a water bath until it measures about 100 c.c. 

 Allow it to cool thoroughly and add 40 c.c. clear ammonium 

 molybdate dissolved in nitric acid (paragraph 208). Allow to 

 stand for forty-eight hours with occasional stirring. Decant the 

 liquid through a filter and wash several times, first with dilute 

 nitric acid (1-4), then with pure water in very small doses, and 

 finally transfer to the filter and wash free from excess of acid. 

 The ammonium phospho-molybdate is dissolved in dilute am- 

 monia, allowing the solution to run into a weighed platinum 

 dish. When the filter has been washed two or three times, the 

 dish is placed on the water bath and the ammoniacal liquor 

 evaporated to dryness. It is finally dried in a steam oven and 

 weighed. The residue contains 3*5 per cent, of its weight of 

 phosphoric acid (P 2 5 ). 



266. Available Potash. — Five hundred c.c. of the clear 

 liquid is evaporated on the water bath with a little strong hydro- 

 chloric acid ; when quite dry it is gently heated over an Argand 

 until all the organic matter is burned off and the residue is nearly 

 white. This residue is dissolved in HC1, and evaporated slowly 

 with 5 c.c. platinum chloride solution (10 per cent). If the 

 evaporation be conducted slowly, the platinum potassium 

 chloride settles out well, in spite of the various salts present. 

 When nearly dry, decant through a filter paper, wash twice with 

 small quantities of platinum chloride solution, and finally with 

 strong alcohol. When the alcohol comes through clear, dry the 

 filter at ioo°, then wash through with hot water into a tared dish. 

 Evaporate to dryness, dry in a water oven and weigh. Cal- 

 culate the percentage of potash as described in paragraph 36. 



267. Calculation. — The 200 grams originally dissolved in 

 the citric acid contained a certain amount of moisture, which 



