156 AERATION AND AIR-CONTENT. 



an extent that the accumulation of carbon dioxid is favored, and it is 

 suggested that this may be the toxin concerned. 



Russell (1912 : 111) grew six crops of rye in succession in sand to 

 which only nutrient salts were added to keep the food material con- 

 stant. A seventh crop was then grown at the same time as one on 

 perfectly fresh sand on which no crop had ever grown, though it was 

 also supplied with an equal amount of the same nutrients. Similar 

 experiments were made with buckwheat and spinach, and a parallel 

 series was carried out in soil cultures. There was no significant dif- 

 ference in the two crop yields, except in the case of buckwheat in sand, 

 an exceptional result that could not be confirmed. If the rye, buck- 

 wheat, or spinach excreted any toxin, the amount accumulated dur- 

 ing the growth of the six successive crops was insufficient to depress 

 the yield of the next crop appreciably. Thus, no lasting toxic effect 

 at least was produced by any of these crops, and the toxin hypothesis 

 fails to explain the advantages of rotation where there is always a 

 lengthy interval between crops. It is concluded that there is no 

 evidence of soluble toxins in normally aerated soils sufficiently sup- 

 plied with plant-food and with calcium carbonate, but toxins may 

 occur on sour soils badly aerated and lacking in calcium carbonate, or 

 in other exhausted soil. There is no evidence of any plant excre- 

 tions conferring toxic properties on the soil, but the Woburn results 

 show that a growing plant may affect its neighbor. 



Sherff (1912 : 428) observed that Sagittaria was able to invade 

 ponds of Nymphcea from the reed-swamp only when Nymphcea was 

 nearly or quite absent. The rhizomes of the latter are usually de- 

 cayed well toward the growing apex, and generally when the rhizomes 

 of Sagittaria penetrate the decayed parts, they also begin to decay. 

 Where the decayed Nymphcea rhizomes lay nearer the surface, 

 Sagittaria had grown underneath without harm. 



Hall, Brenchley and Underwood (1913, 1914) have studied the 

 growth of plants in soil solutions with especial reference to the theory 

 of Whitney and Cameron, and have obtained the following results: 



"We may now consider how far these results bear on the theory that crops 

 leave behind in the soil specific toxins which depress the growth of succeed- 

 ing crops of the same kind. In Series I, wheat and barley yielded almost 

 exactly the same weight of plant, whether they grow in solutions from the 

 wheat or the barley soils. As a rule the wheat plants were a little heavier 

 when grown in the solutions from the barley soils than when grown in solu- 

 tions from the corresponding wheat soils, but the barley plants were similarly 

 heavier in the solutions from the barley soils. The ratio of root to shoot is 

 very close in the two sets. Again, wheat and barley grown in the same 

 solution yield weights agreeing within the range of error of such experiments. 

 These facts alone would dismiss the hypothesis that the wheat soils contain 

 any soluble toxin injurious to wheat but not to barley, and vice versa, not- 

 withstanding the 60 years' repeated growth of these crops on the same soils. 

 In Series II the demonstration was pushed a stage further by including in 

 the comparison an artificial culture solution made from pure salts and con- 



