124 FUNDAMENTALS OF FRUIT PRODUCTION 



Most deciduous fruit crops occupy the same soil for a considerable num- 

 ber of years and consequently are subject to the influence of any toxins 

 that arise from the disintegration of their own leaves, rootlets or other 

 dead tissues. In addition they are subject to the action of toxins that 

 may arise from the growth or decay of intercrops or cover crops that are 

 grown between them. 



It has been shown^^ that ordinary crop plants exert an important 

 influence upon those which follow them and that this influence "seems 

 not to be attributable, at least primarily, to differences in the amount of 

 fertilizer nutrients removed by the crops grown before." Thus the yield 

 of buckwheat following redtop, rye, buckwheat and onions was as 7 : 30 : 

 45 : 88, in a nutrient medium deficient in nitrogen but well supplied with 

 other plant nutrients, even though the nitrogen removal of the preceding 

 redtop, rye and buckwheat crops was as 1.00:2.72:2.42.^^ The pre- 

 sumption is that the differences in the yields of the second crops were due 

 to the effect of toxins, 



Pickering ^^^ has been able by means of various field trials and pot 

 experiments to eliminate the influence of one plant upon another through 

 its effect on moisture and nutrient supply, soil temperature, soil reac- 

 tion, texture, carbon dioxide and bacterial content and thus to deter- 

 mine both quantitatively and qualitatively their mutual influence through 

 •toxic substances. 



He comments as follows on the results of his investigations: 



"It has now been established with a reasonable amount of certainty that the 

 deleterious effect of one growing plant on another is a general phenomenon. 

 By means chiefly of pot experiments . . . the following plants have been found 

 susceptible to such influence: apples, pears, plums, cherries, six kinds of forest 

 trees, mustard, tobacco, tomatoes, barley, clover, and two varieties of grasses, 

 whilst the plants exercising this baleful influence have been apple seedlings, 

 mustard, tobacco, tomatoes, two varieties of clover, and 16 varieties of grasses. 

 In no case have negative results been obtained. The extent of the effect varies 

 very greatly: in pot experiments the maximum reduction in growth of the plants 

 affected has been 97 per cent., the minimum 6 per cent., whilst in field experiments 

 with trees the effect may vary from a small quantity up to that sufficient to 

 cause the death of the tree. The average effect in pot experiments may be 

 roughly placed at a reduction of one-half to two-thirds of the normal growth 

 of the plant, but no sufficient evidence has yet been obtained to justify the con- 

 clusion that any particular kinds of plants are more susceptible than others, or 

 any particular surface crop is more toxic than another; that such differences 

 exist is highly probable, but all the variations observed so far may be explained 

 by the greater or lesser vigour of the plants in the particular experiments in 

 question. Similarly as regards the effect of grass on fruit trees, though the extent 

 of it varies very greatly, and in many soils is certainly small, we must hesitate 

 to attribute this to any specific properties of the soils in question ; for when soils 

 from different localities (including those from places where the grass effect is 



