146 AERATION AND AIR-CONTENT. 



Caii vet (1861 : 320) concluded that roots physiologically sound did 

 not excrete poisonous or other substances absorbed by any portion 

 of the plant. He maintained that the theories of Macaire, Chatin, 

 and Bouchardat were not well grounded, and that the theory of 

 rotation advanced by DeCandolle and supported by Macaire and 

 Liebig was based upon error. He declared that the sterility of a 

 field after cultivation was not due to the deposit in the soil of material 

 injurious to plants of the same species. Differences in the amounts 

 of nutrients absorbed were ascribed to the selective power of the roots 

 rather than to the effect of root excretions. 



The Woburn researches. These have been carried on at the Woburn 

 Experimental Fruit Farm and at Ridgmont, England, since 1897, by 

 Bedford and Pickering. The results have been communicated in the 

 first, second, third, fifth, and thirteenth reports, for 1897, 1900, 1903, 

 1905, and 1911, respectively, and in various papers. A resume" of 

 the first four reports has been given by Livingston (1907 : 10). A 

 summary of the investigations for the period of 16 years is given in 

 the report for 1911, and it appears desirable to repeat it in full: 



"The action of grass on fruit trees is often so deleterious that it arrests 

 all growth, and even causes the death of the tree. In none of the experiments 

 on the subject, which have now extended over sixteen years, has any recovery 

 from the effect been noticed, except in cases where the roots began to extend 

 beyond the grassed area. But trees which become grassed over gradually 

 during the course of several years, apparently accommodate themselves to 

 the altering conditions, and suffer much less than when the grass is actually 

 sown over their roots. It is partially due to this circumstance that the effect 

 of grass in commercial orchards is often less than that observed in the exper- 

 imental plots at the farm; whilst another reason for differences in the results 

 is that the effect undoubtedly varies in intensity in different soils, though 

 the instances where the effect appears to have been nil are very rare. The 

 fact that a tree has become well-established in the ground before the land is 

 grassed does not, however, prevent it from suffering from the grass; trees 

 at the farm were grassed over four years after they had been planted, and 

 they were so much affected that many of them were nearly killed ; and other 

 trees standards as well as dwarfs when similarly treated twelve years 

 after planting, are behaving in the same way, though they did not suffer so 

 severely till the third or fourth season after grassing. 



"Some varieties of apples dependent, no doubt, on their vigour of growth 

 evidently suffer less from grass than others, but very little difference has been 

 found between the effect on standards on the free stock, and dwarfs on para- 

 dise, and no explanation of the difference in the grass-effect in different 

 soils can be traced to the depth of good soil available for root-development. 

 The baleful effect of grass is by no means confined to apples; pears, plums, 

 and cherries were found to be affected by it in the same way, and to, probably, 

 nearly the same extent; though in the case of these trees the standards 

 suffered less than the dwarfs. 



"It is possible that in some soils where the effect produced is not great, 

 grass might be advantageous from a commercial point of view, for the check 

 given to the growth of the tree tends to increase its cropping, and grass affects 

 the colouring matter of all parts of the tree, generally resulting in a high 



