May. i896. A NEW METHOD IN SOIL-ANALYSIS. 313 



although various attempts had been made to estimate it by extraction 

 with acetic acid. 



Passing over the long list of chemists, quoted by Dr. Dyer, who 

 have worked at this difficult subject, and some of whom have more 

 or less nearly anticipated him, we may point to the work of Stutzer 

 as giving the key to the present inquiry. It had long been admitted 

 that to determine the amount of available phosphates, in manures as 

 in soils, digestion with strong mineral acids was useless ; and the 

 practice of extracting from manures with ammonium citrate had come 

 largely into vogue, no doubt because the "precipitated" phosphates, 

 which are known from experience to be largely available as a plant-food, 

 had been found to be soluble in ammonium citrate. Against this 

 practice Stutzer, however, strongly protested, and proposed instead 

 the use of a i per cent, solution of citric acid as a solvent ; and he 

 showed that there was a considerable correspondence between the 

 solubility of various phosphatic manures in citric acid and their 

 availability for plant-food as tested by field-experiments. Unfortu- 

 nately, however, Stutzer's solution was open to the objection that it 

 was perfectly empirical; why should a i per cent, citric acid solution's 

 -dissolving power be an index to the amount of available plant-food ? 

 Here it is that Dr. Dyer steps in with a happy and original 

 suggestion. 



It had been known for long, in fact ever since Sachs' classical 

 experiment of etching a marble slab by exposing it to growing roots, 

 that the young roots excrete an acid liquid ; and it was a fair inference 

 that this acid excretion might be a means of bringing into solution 

 otherwise unavailable constituents of the soil which would not dissolve 

 in mere water. It therefore struck Dr. Dyer that, if one knew what 

 percentage of organic acid was contained in this root-sap, it would be 

 possible to place Stutzer's proposition on an altogether ;iew footing. No 

 data could be found in botanical works as to this acidity ; and Dr. Dyer 

 accordingly went to work to determine for himself the acidity of the root- 

 sap in various plants. The work was by no means simple, and involved 

 many difficulties of manipulation ; but eventually a large collection of 

 data was obtained. Since these data appear to be unique, and have 

 a very wide scientific interest, it will be well worth while to extract 

 the table entire. It will be unnecessary to trouble our readers with all 

 the statistics collected by Dr. Dyer in his tables, and we will therefore 

 be content with tabulating the acidity found in the roots in the moist 

 (natural) state, (i) in terms of hydrogen, (2) in terms of crystallised 

 citric acid. It will, of course, be understood that the acidity of the 

 roots is not due solely, or necessarily even at all, to citric acid, but no 

 doubt to a mixture of several acids ; but for the practical purpose in 

 hand it was convenient to calculate the determined acidity in terms of 

 crystallised citric acid ; the percentage in terms of hydrogen is, how- 

 ever, absolute, whatever be the acid. Thus, the formula of citric acid 

 is COOH, CH,, COH, COOH, CH„ COOH + H,0, molecular 



