II. R. Hutchinson and K. MacLbnnan 85 



tube is then closed with a plug of cotton wool from the middle of which 

 a strip of potassium iodide starch paper is suspended. After a short 

 period the degree of soil acidity can be gauged by the intensity of the 

 blue colouration produced. Another method, suggested by Baumann 

 and Gully (21). consists in the addition of a solution containing 2-0 per 

 cent, potassium iodide and 0-1 per cent, potassium iodate to 1-3 grm. 

 of the soil in a test tube ; after being shaken and then allowed to stand 

 for 15 minutes the solution is filtered and an equal quantity of dilute 

 starch solution added. The intensity of the colouration provides an 

 index of soil acidity. 



In a test proposed by Albert (22) the soil is suspended in water, a 

 few grains of lithium phosphate added and then allowed to stand until 

 no further colour change occurs; the tint ranges from light brown to 

 brownish-black and depends on the formation of soluble lithium humates. 



(6) Quantitative Methods. 



One of the earliest methods, and one which is still largely used on 

 the Continent, is that introduced by Tacke (23) based on the capacity 

 of the soil to hberate carbon dioxide from finely divided calcium car- 

 bonate. The soil is placed in a flask through which a slow stream of 

 pure hydrogen is passed to remove residual carbon dioxide from the 

 soil and the apparatus is then opened, by means of a T-piece, to a 

 Pettenkofer absorption tube containing N/5 or N/10 sodium hydroxide 

 solution; an excess amount of a suspension of finely divided calcium 

 carbonate is then allowed to flow from a separating funnel to the soil 

 bottle. The passage of hydrogen is then continued for 21 hours and 

 the amount of carbon dioxide evolved is estimated by titration of the 

 soda against phenolphthalein after the addition of barium chloride 

 solution. The method has given satisfactory results on the whole and 

 has the advantage over various other methods in that the change upon 

 which it depends is the one occurring naturally in the field; its dis- 

 advantages are that the determination is somewhat wearisome and 

 that, during the period of the determination, there proceeds a slow but 

 appreciable degradation of organic matter with the production of 

 carbon dioxide. To obviate this source of error, Siichting (24) has 

 suggested the employment of a definite amount of calcium carbonate 

 and the estimation of the residual carbonate after all change has ceased. 



Wheeler, Hartwell and Sargent (25) attempted to render the Tacke 

 method more rapid by boihng the mixture, but found that degradation 



