246 SOIL CONDITIONS AND PLANT GROWTH 



If either the rye, buckwheat, or spinach excreted any toxin the 

 amount accumulating during the growth of six successive 

 crops was insufficient to cause any appreciable depression in 

 yield in the next crop ; the exceptional result given by buck- 

 wheat in sand could not be confirmed. 



These and similar experiments show that no lasting toxic 

 effect is produced by any of the crops studied, and they rule 

 out any toxin hypothesis as an explanation of the advantages 

 of rotations ^ where there is always a lengthy interval between 

 the crops. They do not, however, show that there is no tran- 

 sient effect, and they are thus quite consistent with some re- 

 markable observations by Pickering on the effect of grass on 

 apple-trees (225^). It was found at Woburn — and the observa- 

 tion has since been confirmed elsewhere — that the effect of 

 growing grass round apple-trees is to arrest all healthy growth 

 and absolutely stunt the tree. The leaves become unhealthy 

 and light coloured, the bark also becomes light coloured, while 

 the fruit loses its green matter and becomes waxy yellow, or 

 brilliant red. Where the grassing was done gradually the trees 

 accommodated themselves somewhat to the altering conditions, 

 but never grew so well as when grass was absent. 



This effect might have been due to various causes : changes 

 in aeration, temperature, water supply, food supply, or physical 

 condition of the soil, but careful experiments failed to show 

 that any of these factors came into play. Covering the soil 

 with cement excluded air at least as thoroughly as grass, and 

 yet did not produce the grass effect, nor was it suppressed by 

 wet seasons, liberal watering, or a supply (in pot experiments) 

 of sufficient water or nutrient solution to keep the soils of 

 grassed and ungrassed trees equally moist, or equally well 

 supplied with food. On the other hand, the grass effect was 

 produced when perforated trays of sand containing growing 

 grass were placed on the surface of the soil in which trees 

 were growing, so that the washings from the grass went 

 straight down to the tree roots. There seemed no possibility 



' Some curious problems are thus left unsolved, some of which are discussed 

 more fully by the author in Science Progress, July, 19 n, p. 135. 



