148 AERATION AND AIR-CONTENT. 



them two or three times a week; but even when the grass-roots were prevented 

 from coming into contact with the tree-roots by a layer of wire gauze, and 

 when the water was supplied from below, so that the tree got all that it wanted 

 first, the effect of the grass on it was nearly as great as ever. 



"As to the food-supply, it is difficult to see how the tree can suffer from want 

 of nourishment so long as the soil is rich and the water-supply is sufficient. 

 The trees in the grassed plots have been manured annually just like those in 

 the tilled plots, and the grass crop is not removed, but is left to rot on the 

 ground: the soil of these grassed plots may be poorer by the amount of 

 material in the one crop which is actually growing on them, but in a series 

 of years this would represent a removal of food-material far smaller than that 

 removed by the vigorously growing and cropping trees in the tilled plots: 

 indeed, it is well known that grass crops, if properly manured, actually 

 enrich the soil, and it has been found by direct experiment that, when trees 

 are grown in soil taken from the grassed plots, they flourish better than in 

 soil taken from the tilled plots. Various other experiments have been made 

 on the subject, of which it is only necessary to allude to some pot experi- 

 ments, similar to those mentioned above, in which nourishment was supplied 

 with the water, without effecting any appreciable reduction in the action 

 of the grass, though the soil was thereby rendered richer than it was in the 

 pots without grass, where the trees were growing vigorously. It is evident, 

 therefore, that the grass-effect cannot be explained by any lack of nourish- 

 ment: if the immediate cause is starvation, it is starvation in a land of 

 plenty, due to some other factor which prevents the roots from availing 

 themselves of the food which is there. 



"Amongst the possible causes of the action of grass, that of a physical 

 alteration in the soil has been examined, but with negative results. The 

 grass might either by mechanical or chemical means cause an accumulation 

 of very fine soil particles at a depth corresponding with that of the tree-roots, 

 and so interfere with the functioning of these. But mechanical analyses of 

 several grassed and tilled plots of ground failed to reveal any alteration in 

 the distribution of small soil particles which would account for the effect of 

 grass. Other experiments in which the soil was made alkaline, showed that 

 the grass-effect could not be attributed to alkalinity produced by the grass 

 in its growth. 



"Incidentally, the physical alteration produced in soil by rendering it 

 alkaline with potassium carbonate was investigated and found to be sur- 

 prisingly small. 



"The question of soil bacteria was also partially examined. The numbers 

 of such bacteria in some grassed soil in which trees had been suffering from 

 the grass-effect was found to be considerably greater than in the neighboring 

 tilled soil; but this could not account for the grass-effect, for such an effect 

 was equally apparent in the case of trees grown in sand, where the number of 

 bacteria present was found to be much less than in tilled soil. 



"In connection with this question trees have been grown in soil which had 

 been partially sterilized by heating to different temperatures, and they have 

 been found to behave in the same way as other plants. The action of heat on 

 a soil results in the destruction of the greater part of the bacteria present in 

 it and the total destruction of certain protozoa, which feed on the bacteria; 

 the result of which is that, after a certain lapse of time, the bacteria left in 

 the soil multiply without check, and the soil becomes richer in bacteria, and 

 in the nitrates formed by them, than it was originally; such soil is specially 

 favourable to plant-growth; at the same time, however, the heating results 

 in the production of some substance which is actively toxic toward plant- 

 growth, and so long as this is present, plants will not flourish in it. But the 



