OEGANIC CONSTITUENTS OF SOILS. 11 



chemistry of agriculture had much to say but in regard to which 

 modern science was discreetly silent. With no established facts and 

 no methods of attack worked out progress was necessarily slow at 

 first but gained speed with each compound isolated or identified, 

 until to-day there have been isolated in these laboratories more than 

 25 1 definite compounds from soil organic matter, and the work is 

 progressing at a rapid pace. 



The search for this supposedly harmful constituent was rewarded 

 by the discovery, among others, of dihydroxystearic acid, a com- 

 pound which, on account of its frequent occurrence in soils, has been 

 rather thoroughly studied in regard to its effect on plant develop- 

 ment and growth. 



The isolated and purified dihydroxystearic acid was tested by 

 dissolving it in pure distilled water, and it was found to have de- 

 cidedly deleterious action on the wheat seedlings used in the tests. 

 The acid prepared in the laboratory behaved in the same manner. 



Its effect in the presence of nutrient salts was also extensively 

 studied in solutions containing calcium acid phosphate, sodium 

 nitrate, and potassium sulphate, alone and in combinations of two 

 and three of these salts, a total of 66 cultures being used in a single 

 test. The injurious effect of the dihydroxystearic acid was less where 

 all three of the nutrient elements were present than where only one 

 or two were present. The injurious effect was least in those cultures 

 of three nutrients where the nitrate was high. This indicates that 

 the action of the nitrate tends especially to overcome the harmful 

 effect of the compound, or else it enables the plant to resist or over- 

 come its effect. 



Dihydroxystearic acid has another quality which should be men- 

 tioned here as having a considerable bearing on its effect on crops, 

 even in such soils as contain much plant nutrient material in the 

 most readily available form. This is its influence on the absorptive 

 power of the roots of the plants growing in the soil, the soil solution, 

 or solutions of nutrient salts when dihydroxystearic acid is present 

 in them. The absorption of potassium and phosphate was greatly 

 interfered with, although both were present in soluble form in the 

 culture solutions; only the nitrate was consumed in any quantity. 

 This is in harmony with the fact stated above that when nitrates 

 were plentiful in the solutions the best growth was obtained and the 

 effect of the harmful compound was minimized or entirely overcome. 



The occurrence of dihydroxystearic acid was specifically studied. 

 For this purpose soil samples of good and poor fields were collected 

 and examined for this constituent. Soils from 18 different States, 

 extending from Maine to Oregon and southward to Texas, of widely 



i The total number is now 36; see Bulletins 88 and 89, Bureau of Soils. 



