382 Scientific Proceedings, Royal Dublin Society. 



sample of calcium phosphate gave in cold saturated solution pH 7'0, which 

 on boiling for an hour and a half reached pH 57. From these examples it 

 may be seen that, in making up nutritive solutions for plant-growth to which 

 various nitrates, sulphates, and phosphates are added, a very indefinite 

 reaction may result according to the purity of the salts used. Boiling to 

 sterilize introduces further complications. M'Call and Haag (1920) have 

 shown that nutrient solutions containing acid potassium phosphate have 

 a lower hydrogen ion concentration than similar solutions with the acid 

 phosphates of calcium or magnesium. 



(c) Action of gypsum on alkali lands. — This type of study is, however, not 

 merely of academic or laboratory importance, but has applications in the 

 reclaiming af alkaline lands. In cases of extreme alkalinity sodium carbonate 

 is present, often in considerable quantity. It is well known that gypsum is 

 beneficial in counteracting the alkalinity. Now, even very minute amounts 

 of sodium carbonate suffice to bring a solution to over pH 10, but by adding 

 gypsuin the carbonate is precipitated as calcium carbonate till the ecpiilibrium 

 is obtained for the system in contact with the carbon dioxide of the air. 

 This results in the lowering of the pH value, as may be readily demonstrated. 

 For this purpose a solution of sodium carbonate was made up, and was well 

 over pH 10 as shown with thymol phthalein, which is the most useful 

 indicator for this range, though unfortunately its blue colour is not stable. 

 On adding a little solid calcium sulphate and warming for two to three 

 minutes in a water bath the value fell to pH 8'4. A portion of the same 

 mixture in the cold was at pH.9'4, but had fallen to 8'4 by the next day. 

 Further standing exposed to the air resulted in both heated and unheated 

 mixtures attaining to pH 7'8. Thus this very simple test enables one to 

 study the progress of the action of gypsum or flushing, and to judge the 

 quantities necessary in any given case. A soil which gives any blue with 

 thymol phthalein is probably too alkaline for most crops, as this reagent 

 gives no colour at pH 9'2, but a light blue at 9-4. Thymol blue (thymol 

 siilphone phthalein) gives a blue colour down to about pH 9, and is yellow 

 at pH 8. Cresol red may conveniently be used for the lower range of 

 alkalinity. Soil is certainly fit for crops in India at pH 9-0, or possibly in a 

 somewhat more alkaline state. It is of interest to note that in studying the 

 relation of the calcium content of Kansas soils to the hydrogen ion concentra- 

 tion, Swanson, Latshaw, and Tague (1921) found values up to pH 8'6 for a 

 loamy sand, and pH 8'40-9'03 for Oswego silt loam. These determinations 

 were made by electrometrie titration, and the value pH 9 '03 is a high value 

 for arable land. The soil originating from limestone in a quarry in Middle 

 Devonian strata near Plymouth, was at pH 9'0 as a maximum. Wherry (1916) 



