EFFECT OF SUPERPHOSPHATE ON WHEAT YIELD, N.S.W. 7 



At the outset of the investigation, before commencing work with the 

 soils, several of the recommended methods for the estimation of phosphoric 

 acid were tested. Of the official methods, "The Provisional Official 

 Method" (Wiley, "Principles and Practice of Agricultural Analysis," 

 Vol. I, page 514) was chosen as likely to give consistently accurate results 

 with a minimum expenditure of time and labour. The following is an 

 outline of the method: 



To the solution containing the phosphate, 20 c.c. of nitric acid and 25 c.c. 

 of 50 per cent, ammonium nitrate solution were added, and the liquid heated 

 to 50 degrees C. A solution of ammonium molybdate was also brought 

 to this temperature, and sufficient to ensure the complete precipitation of 

 the phosphate as ammonium phospho-molybdate added slowly and with 

 constant stirring, the vessel then being stood in a warm place until preci- 

 pitation was completed. The yellow precipitate of ammonium phospho- 

 molybdate was filtered off and washed free from acid with a 1 per cent, solution 

 of sodium nitrate. With the filter paper the precipitate was transferred to 

 the beaker used for precipitation, and the precipitate dissolved in excess 

 of standard sodium hydrate solution containing 323-7 c.c. of normal sodium 

 hydrate per litre; hence 1 c.c. of the solution was equivalent to -001 m. 

 of PoO,. Special precautions were taken to prevent the carbonation of tfc^ 

 solution through contact with carbon dioxide, by the use of soda-liM 

 tubes. Phenolphthalein was added as indicator, and the excess of alkali 

 over that used in dissolving the .yellow precipitate neutralised with a 

 standard solution of hydrochloric acid equivalent to the sodium hydrate 

 solution. The amount of sodium hydrate used in dissolving the precipitate 

 was thus determined, and from this the equivalent amount of P,O 5 

 calculated. 



Having decided upon the method to be adopted, and having proved it 

 by determining the P 2 O 5 present in solutions containing known quantities 

 of pure salts, the work of determining the amount of P 2 5 in the soils 

 submitted was commenced. 



For the estimation of the total P 2 O 5 , 10 gr. of the fine soil was used- 

 Ting was boiled for ten hours in a conical flask fitted with air-condenser, 

 with 100 c.c. of hydrochloric acid of constant boiling strength. The flask 

 was thoroughly shaken each hour, and at the end of the ten hours the 

 extract filtered and the residue washed till free from chlorides. This 

 extract was then evaporated to dryness with nitric acid, and after heating 

 for some time, moistened with hydrochloric acid, taken up with hot water 

 and filtered. This operation was repeated and then the P 2 O 6 precipitated 

 and estimated by the method described. 



Two hundred grams of the air-dried soil was taken for the determination 

 of the available P 2 5 . This was placed in a Winchester flask with 20 grams 

 of citric acid and 2,000 c.c. of distilled water, and the whole agitated for 

 twenty-four hours. After allowing the soil to settle, the supernatant liquid 



