METHOD OF TESTING SOIL ACIDITY. 27 



phthalein are added, and into this is stirred drop by drop from a 

 graduated glass tube provided with a stopcock, known as a burette, 

 a measured amount of some alkaline solution of known strength, 

 commonly a one-twentieth normal solution, as it is known to chem- 

 ists, of sodium hydrate. When a sufficient amount of the sodium- 

 hydrate solution has been dropped into the beaker, the acidity of the 

 acid solution becomes neutralized and it turns pink. A reading is 

 made on the burette showing the exact amount of the sodium-hydrate 

 solution used in elfecting the neutralization. From this reading is 

 computed the degree of acidity expressed in fractions of a normal 

 acid solution. Now 100 c. c. of a normal acid solution would require 

 for its neutralization 100 c. c. of a normal solution of sodium hydrate, 

 or 2,000 c. c. of a one-twentieth or 0.05 normal solution. In a test of 

 one of the acid nutrient solutions used in the blueberry cultures, 

 18 c. c. of a 0.05 normal solution was required to neutralize the acidity 

 of 100 c. c. of the acid solution. Since 18 c. c. of a 0.05 normal 

 solution is the equivalent of one-twentieth that amount, or 0.9 c. c. of 

 a normal solution, the degree of acidity of this acid solution is 0.009 

 normal. It requires an equal amount of a 0.009 normal alkaline 

 solution to neutralize it. 



In applying this phenolphthalein test to soils the same scale is 

 used. A soil is regarded as having normal acidity when the acid ex- 

 isting in a gram of the soil if dissolved in 1 c. c, of water gives a nor- 

 mal acid solution. If a soil were described as having an acidity of 

 0.02 normal, it would mean that the extract of 100 grams of it in 100 

 c. c. of water would be a 0.02 normal acid solution ; that is, that 100 

 c. c. of the solution would contain 2 c. c. of a normal acid solution. 



The method of extraction followed for all the soil acidity tests 

 given in this paper is as follows : The soil is first air dried at an ordi- 

 narj'^ room temperature. Ten grams are then weighed out, shaken thor- 

 oughly with 200 c. c. of hot water, and allowed to stand over night. 

 In the morning 100 c. c. is filtered otf and boiled to drive away any 

 carbon dioxid present. The solution is then titrated with a 0.05 nor- 

 mal solution of sodium hydrate, using phenolphthalein as an indi- 

 cator. All the tests were made by Mr. J. F. Breazeale. of the Bureau 

 of Chemistry, to whom the writer is greatly indebted for many cour- 

 tesies and suggestions on the chemical side of the experiments. 



The expression " normal solution " used in this paper, it must be 

 understood, is the normal solution of chemists, not of surgeons. 

 Surgeons use the expression " normal salt solution " to describe a cer- 

 tain weak solution of common salt in water which has the same 

 osmotic pressure as the blood. A normal solution in chemistry is a 

 solution of certain fixed strength, or concentration, based on the 

 molecular Aveight of the substance under consideration. Normal solu- 



193 



