Scientific Agriculture. 



230 



[March, 1912. 



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 water-supply is sufficient. The trees 

 in the grasped 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 ; iudeed, it is well-known that grass 

 crops, if properly manured, actually en- 

 rich 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 the 

 soil taken from the tilled plots. Various 

 other experiments have been made on 

 the subject, of whichitis only necessary 

 to allude to some pot experiments 

 similar to those mentioned above, in 

 which nourishment was supplied with 

 the water, without effecting any appre- 

 ciable reduction in the action or the 

 grass, though the soil was thereby 

 rendered richer than it was in the plots 

 without grass, where the trees were 

 growing vigorously. It is evident, there- 

 fore, that the grass-effect cannot be 

 explained by any lack of nourishment ; 

 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 alter- 

 ation 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 me- 

 chanical 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 investig- 

 ated and found to be surprisingly 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 neighbouring tilled 

 soil ; but this could not account for the 

 grass-effect, for such an effect was equ- 

 ally 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 sterilised 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 result 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 cer- 

 tain 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 and such soil is specially 

 favourable to plant-growth ; at the same 

 time, however, the heating results in 

 the production of some substauce which 

 is actively toxic towards plant-growth, 

 and, so long as this is present plants 

 will not flourish in it. But the toxin is 

 rapidly oxidised by the action of air 

 and moisture, and is destroyed under 

 cultivation in a few weeks. In soil 

 which has been heated, therefore, plants 

 will not thrive at once, especially if the 

 supply of air is restricted, though after 

 a time they grow better in it than in 

 soil which had not been heated at all. 

 Thus plants may behave in diametric- 

 ally opposite ways in heated soil, accord- 

 ing to the conditions under which they 

 are grown. This has been found to be 

 the case with apple trees as well as with 

 grasses and other plants. 



The toxic substauce produced by heat- 

 ing soils was found to be toxic towards 

 the germination of seeds as well as to- 

 wards the growth of plants, retarding 

 the germination and reducing the per- 

 centage of seeds which germinate. In 

 extreme cases seeds may take five or 

 six times as long to germinate in heated 

 as in unheated soil. As experiments on 

 seed-germination can be carried out in 

 a day or two, whereas those on plant- 

 growth require mauy weeks, during 

 which the character of the soil may be- 

 come materially altered, the former 

 offered a promising means for searching 

 for the presence of toxic matter in grass- 

 ed soil. A considerable number of instan- 

 ces were taken in which grassed and 

 tilled soils within a few feet of each 

 other were examined as to their be- 

 haviour towards germinating seeds, and 

 the examination was conducted at three 



