158 AERATION AND AIR-CONTENT. 



accessory food substances, as well as upon mineral nutrients, and 

 thinks that the very small amounts necessary are at first supplied 

 by the seed and later by the humus of the soil. Bacterized peat 

 results from the action of certain aerobic soil organisms at 26° C. 

 which decompose it and convert a large amount of the humic acid 

 present to soluble ammonium humate. 



Bergen (1915 : 491) observed that an exceptional rainfall in July 

 led to much greater growth in perennial mesophytes growing along- 

 side of a belt of deciduous trees. The stems of Aster novce-anglice, 

 Asclepias tuber osa, and Helianthus grosse-serratus were about twice 

 as tall and much more robust than during the ordinary season. A 

 suppressed plant of Chelone glabra grew luxuriantly and flowered 

 freely. The dwarfing in ordinary seasons was ascribed to the lower 

 water-content, due to the demands made by the trees. 



Amos (1918), in a study of the causes of clover sickness, finds scant * 

 evidence that it is due to the excretion of toxic substances by the pre- 

 ceding clover crop. 



Hart well, Pember and Merkle (1919) have conducted experiments 

 on the effect of one crop upon another, in which five different crop 

 plants were grown for 2 to 3 years in the same soil, and then fol- 

 lowed by a particular crop plant. When onions were grown after 

 each of the individual crops, the yield was least with buckwheat and 

 mangels, larger with rye and onions, and best with redtop. When 

 buckwheat was the succeeding crop its yield increased after crops in 

 about the following order: redtop, buckwheat, mangels, rye, and 

 onions. The divergent effect of crops on those that follow seems not to 

 be due, principally at least, to the amount of nutrients removed, since 

 the smallest yield may not occur after the crop removing the largest 

 amount of the nutrient most needed. Soil acidity was affected 

 differently by the various crops, and generally the best yield of 

 onions, which are sensitive to the conditions accompanying acidity, 

 followed the crops giving rise to the least acidity. This relation was 

 supported by the fact that the effects of the crops on the following one 

 were much less divergent when acidity was reduced by liming. 



CONCLUSIONS. 



The original assumption of Livingston, Schreiner, and their asso- 

 ciates, and of Bedford and Pickering as well, to the effect that plant 

 roots excrete substances toxic to the plants themselves and to other 

 plants, seems no longer to be accepted even by them. While 

 Livingston (1918 : 93) regards the general hypothesis that unpro- 

 ductiveness in agricultural soils is frequently due to soil toxins as well 

 estab fished and generally accepted, he states that: 



"The evidence that crop plants do actually excrete toxic substances into 

 the soil is not veiy strong in any of this work. Better than to assert that they 

 are so excreted is to state that there is evidence that the soil frequently 



