Scientific Agriculture. 



130 



[February, 1910. 



it only one crop of grass (that actually 

 growing at any given moment), and the 

 small amount of material required for 

 the stunted growth of the trees ; where- 

 as, from the tilled soil there has been 

 removed material for an annual crop 

 of fruit, and also for the vigorous 

 growth of the trees. Analysis also 

 shows that the grassed soil is the richer 

 of the two, and it also shows that, in 

 this particular case, there is practically 

 no difference between the water con- 

 tents of the grassed and open plots. 



Of the many other experiments on 

 these points, the most conclusive are, 

 perhaps, those made with Apple trees 

 grown in pots. In some of these the 

 grass roots were separated from the 

 tree roots by very fine wire gauze, 

 through which the former could not 

 penetrate ; the pots were weighed and 

 watered every two days, so as to keep 

 the water contents the same, and such 

 water and food as was added was in- 

 troduced from below, so that the tree 

 should have the first pull at it. Yet 

 the trees still suffered badly from the 

 grass, although the soil was actually 

 moister and richer than in the case of 

 similar trees without grass. Correspond- 

 ing experiments have been made with 

 trees planted in the open. Though in- 

 crease of moisture up to a certain point 

 and increase of food in certain cases 

 may benefit the trees, the benefit is 

 much too small to do more than very 

 slightly diminish the deleterious effect 

 of the grass. 



The behaviour of a tree in grass is 

 clearly a case of starvation in a land 

 of plenty, and this cannot be explained 

 by supposing (untenably as such a sup- 

 position is for other reasons) that the 

 grass roots took up whatever nourish- 

 ing solution there is in the soil, leaving 

 none for the tree roots. The pot ex- 

 periments, just quoted, effectively nega- 

 tive this. Nor can we explain the 

 matter by supposing that the tree was 

 only temporarily affected by the grass, 

 but, being in a weak state after trans- 

 planting, this check resulted in its 

 becoming permanently stunted ; for a 

 precisely similar, and even more marked 

 effect has been proved to be produced 

 by grassing over trees which have been 

 established, in one case for four years, 

 and in another case fcr twelve years ; the 

 effect, indeed, was so great that in the 

 first instance, many of the trees have 

 been killed, and, in the second instance, 

 a similar result appears imminent. 



Other explanations which suggested 

 themselves have been investigated, and 

 found equally unacceptable ; these were 

 differences in soil temperature, differen- 



ces in aeration or proportion of carbon 

 dioxide, and differences in the physical 

 condition of the soil. The only other 

 explanation which appears to be possible 

 is that the growth of the grass results 

 in the formation of some substance 

 which is poisonous to the tree. This 

 may be an active poison— a toxin— or 

 the poisonous action may result from 

 an alteration in the proportion of 

 various substances present in the soil. 

 An active poison may be produced in 

 various ways, such as by the decompo- 

 sition of the debris of the grass, actual 

 excretion from the grass roots, or as 

 a product of the bacteria present in 

 the soil. As to the origin of the toxin, 

 no definite evidence has yet been ob- 

 tained, but it has been found that 

 toxins may be formed in soils by heat, 

 and other means, producing effects 

 which are analogous in many respects 

 with those produced by grass on trees. 

 Thus, on heating soil, substances are 

 produced which are toxic towards the 

 germination of seeds, and these have 

 been found to be toxic towards plant 

 growth also. That established plants 

 grow better on heated than in unheated 

 soil, is due to the fact that heating 

 causes a considerable increase in the 

 soluble nitrogen present in the soil, and 

 also in the composition of the bacterial 

 flora of the soil. Moreover, the toxin 

 formed as the result of heating the soil 

 soon becomes oxidised and destroyed, 

 allowing the favourable conditions to 

 assert themselves: If, however, the toxin 

 is present in sufficient quantity, it is not 

 all destroyed before the plant grows, 

 and its deleterious effect becomes appar- 

 ent. It is noticeable that this affect 

 varies greatly in difficult cases, and is 

 very much less in the case of grasses 

 than in that of the other plants which 

 have been examined. Earth from grass- 

 ed ground behaves in the same way as 

 earth which has been slightly heated and 

 which contains only a limited amount 

 of toxic matter, for trees planted in 

 it (the grass being removed) do better 

 than in soil taken from tilled ground, 

 such toxic matter as there was present 

 in it having evidently become destroyed 

 before the tree started into growth ; 

 whether its presence originally in soil 

 can be established in its effect on germin- 

 ating seeds, still remains to be seen. 



If the formation of a toxic substance is 

 the explanation of the grass effect, we 

 might naturally expect great variations 

 in this effect in different soils ; and this 

 is certainly the case. At Ridgmont the 

 effect is, perhaps, greater than in any 

 other instance which has come under 

 the writer's observation, but cases of 

 very nearly the same intensity have 



