TOXIC ACTION OF ONE CROP ON ANOTHER 311 



that bacterial assistance is not necessary for this change, for it 

 has been found that considerable change occurs in soil solutions 

 equally in the presence or absence of bacteria. 



The great rapidity of the change in nature of the substances 

 in soil when they have been extracted from it to form a soil 

 solution, may not hold good while they are still present in the 

 comparatively dry soil, for it has been found that they adhere 

 to that soil with considerable tenacity, and cannot be washed 

 from it in the same easy way in which neutral salts can. Never- 

 theless, the changes would appear to be fairly rapid even in dry 

 soils : direct experiments, based on chemical reactions, have 

 established this, and so has the examination of the effect on 

 germinating seeds of soils from immediately adjoining plots of 

 tilled and grassed land ; for fourteen such pairs of soils when 

 examined, both in July and October, within ,a day or two after 

 the samples had been collected, showed that germination was 

 more rapid (to the extent of 4 per cent.), and that the percentage 

 of seeds germinating was greater (to the extent of i'5) in the 

 soil from the grass, than in that from the tilled land ; so that 

 any substance toxic towards germination which may have 

 originally been present in the former, had become altered in 

 its nature during the interval elapsing before the seeds could 

 be germinated in it (XIII, 131). 



Much work has been done of recent years by the Bureau of 

 Soils of the United States x on the substances present in fertile 

 and infertile soils, assuming, which does not always seem justifi- 

 able, that the substances extracted from soils with the aid of 

 various chemical reagents are those which are actually present 

 in the soils originally. Amongst the deleterious substances thus 

 obtained, dihydroxystearic acid is prominent. But it is certain 

 that this is not the substance which the Woburn experiments 

 now in progress indicate as being the toxin produced by plant- 

 growth : for this toxin, or the substance to which it imme- 

 diately gives rise, is a reducing substance, that is, it absorbs 

 oxygen, and converts iodine into hydriodic acid, whereas di- 

 hydroxystearic acid does not do so. The Woburn results have 

 shown that this reducing substance is present in all soils, and 

 that the proportion of it is generally increased, often very 



1 Schreiner and Reed, " Certain Organic Constituents of Soil in Relation 

 to Soil Fertility," U. 5. Dept. of Agric. Bur. of Soils, Bull., No. 47, 1907. 



Schreiner and Shorey, '.' Chemical Nature of Soil Organic Matter," 

 ibid., Bull., No. 74, 1910. 



Schreiner and Lathrop, " Dihydroxystearic Acid in Good and Poor 

 Soils," Journ. Amer. Chem. Soc., 1911, xxxiii., 1412-17. 



