17 



Acidity values remained about the same when potassium sulfate was 

 applied, but increased after applications of acid phosphate, ammonium 

 sulfate or disodium phosphate, being proportional in each case to the 

 amount put on the soil. The increased total acidity following applications of 

 disodium phosphate (which is alkaline to phenolphthalein) was unexpected 

 and a more detailed study was made of the sap from these plants. Ether- 

 soluble acids were absent and none of the phosphate was extracted by 

 moisture-free ether. Phosphate was determined in i cc. portions of 

 Samples 12-15 and the total acidity of the solution calculated on the as- 

 sumption of the phosphorus being present (i) as orthophosphoric acid, 

 and (2) mono-alkali phosphate,* the values being given in Table XI. 

 Table XI. — Acidity op Sap by Titration and Calculation. 



Acidity. 



The values calculated as XH2PO4 agree more closely than those for 

 H3PO4, pointing to the presence of the phosphate as mono-alkali phos- 

 phates. Subtraction of the "check" value for Mg2P207 from each 

 of the other values to obtain the increase in phosphate intake due to 

 applications of disodium phosphate and comparison of the titratable 

 acidity calculated from these results with the excess of acidity of the solu- 

 tions over that of the "check" gives the following results: 



Table XII. — Acidity and Phosphorus Content Due to Overfeeding. 



Increase in PiOs. 



The ratio between the value of H determined by titration and by the 

 gravimetric method at 15° was determined to be 0.905, so that the ratios 

 obtained are in the same direction, although the lower values for the sap 

 indicate that some of the phosphate may have been present as the mono- 

 hydrogen phosphate. 



This method was applied to the problem of determining the salt in form of which 

 phosphorus enters the plants. In every case increasing applications of disodium phos- 

 phate gave higher acidity values. When brown rock phosphate was used (nasturtiums 

 grown in sand culture with Hopkin's nutrient solution omitting phosphorus after the 

 first application) a regular increase up to a maximum in size of plants followed by a 



* Two hydrogens of orthophosphoric acid and one of monosodium phosphate 

 when the solution is concentrated at 0° and phenolphthalein is the indicator.''* At 

 higher temperatures, hydrolysis of the salt increases the alkalinity of the solution. 



