Appaeent Infertility of the Soil abound Trees. 257 



of the field, the progressive iucrease in the moisture-content of the 

 soil seemed to tally very closely with the continuous improvement in 

 the appearance and amount of the crop as one came away from tlie 

 trees, and the crop did not reach full normal growth until a distance 

 of 20 yards was reached. This certainly tends to bear out the above 

 contention that the difference in amount of growtli is due chiefly, at 

 any rate, to the difference in the moisture-content of the soil. The 

 deep-rooted trees would be able to obtain the moisture they require 

 from a considerable depth in the sub-soil, but this depletion of the 

 underground supply of water would inevent a sufficient amount being 

 drawn up to near the surface by capillary action for Hie needs of the 

 neighbouring crop. 



Another Aspect. 



There is, however, another aspect of this problem v/hicJi sliould 

 be mentioned. Pickering and some American authorities maintain 

 that some plants have a toxic or poisoiious effect on other plants 

 growing in their immediate vicinity. It was found ut Woburn, for 

 instance, that the effect of sowing grass round apple trees was to 

 arrest all healthy growth and absolutely stunt the trees. In a series 

 of experiments Pickering also showed conclusively that the leachings 

 from the soil in wliicdi certain plants were growing liad a marked 

 deleterious effect when allowed to percolate into a fertile soil in which 

 a liealthy and vigorous crop was growing. The latter crop imme- 

 diately showed the poisonous effect of ihe leachings; it did not con- 

 tinue its former vigorous growth, but became stunted and showed 

 every sign of a toxic action going on in the previously healthy soil. 

 Whether the roots of a plant secrete toxins, or whether these are 

 l)roduced during the decomposition of the vegetable nuitter from the 

 plant, is undecided, but that a deleterious action goes on when some 

 particular plants are growing alongside others seems to be proved 

 by these and other experiments of Pickering. Not only does he main- 

 tain, as a result of his researches, that such a shallow-rooted crop as 

 grass will adversely affect a deep-rooted plant or tree, owing to the 

 fornmtion of toxins by the formei', but that grass and other surface 

 crops may be adversely affected by trees for the same reason. He 

 instances the case of some apple trees having" an injurious action on 

 a crop of brussels sjircmts planted between them, wliich he puts down 

 to some toxic effect of the trees on the vegetable crop, but it is not 

 clear whether he had completely satisfied himself tliat the adverse 

 effect was not due largely, at any rate, to the result of the tiees 

 depriving the surface soil of an adequate supply of moisture. In fact. 

 Hall, a former Director of the Rothamsted Experiment Station, refers, 

 in his 'book on " The Soil," to the experiments at the AVoburn Fruit 

 Farm of planting fruit trees and sowing the seed of meadow-grasses 

 and vegetable crops at the same time, and maintains that the injurious 

 effect of the grass and vegetable crops on the newly planted trees is 

 due to the fact that the quickly gTowing surface crops deplete the soil 

 of moisture at the most critical season for the trees, when they are 

 making their first start in their new quarters, and when they are but 

 indifferently supplied with water-collecting roots. It is obvious that 

 for a season or two these roots are few in number and have a very 

 restricted range, and the trees are consequently very ill-fitted to com- 

 ])ete with a crowd of fibrous grass roots surrounding them for the 



