AVAILABLE PHOSPHATE AND POTASH 103 



The soil sample must be fresh, and immediately 

 after sampling is rapidly dried by artificial heat in order 

 to prevent the production of further quantities of 

 nitrates, which are otherwise readily formed during 

 manipulation. About 500 grams of the soil are pressed 

 into a large Buchner funnel connected with the pump ; 

 50 c.c. of hot distilled water are poured over the sample, 

 allowed to stand for a few minutes, and then sucked 

 through. This operation is repeated with successive 

 small quantities of water until all the nitrate has been 

 washed out. This should be done with about 200 c.c. 

 of water. 



The liquid extract is filtered, the nitrates reduced to 

 ammonia by means of the copper-zinc couple (48 (</)), 

 and the ammonia estimated, after distillation either by 

 direct titration or with Nessler's solution (212), accord- 

 ing to the amount. 



"Available" Phosphoric Acid and Potash. 



As an attempt to differentiate between the "total " plant foods 

 and that proportion of them which can be readily utilised, the 

 " available " phosphorus and potassium are estimated by extracting 

 the soil with dilute citric acid. The idea underlying this 

 procedure is an attempt to reproduce in the laboratory the natural 

 solvent agencies operative in the soil which govern the absorption 

 of the more readily available plant nutrients. 



87. Citric Acid Extraction. 



Two hundred grams of the " air-dried fine earth " are 

 placed in a Winchester quart bottle, together with 20 

 grams of crystallised citric acid and 2 litres of water. 

 Old acid bottles should be used for this purpose. The 

 mixture is shaken thoroughly from time to time during 

 a period of seven days. If a mechanical shaker is used, 

 twenty-four hours will be sufficient. 



The soil is now filtered off, and two portions of the 



