278 SCIENCE AND FRUIT GROWING 



That the effect of grass on trees can in any way be attributable 

 to there being less carbon dioxide in the soil than there is in 

 ungrassed ground is, of course, highly improbable, and was 

 definitely negatived by growing trees in soil wherein the pro- 

 portion of that gas was artificially increased by directing a 

 continuous stream of it into the ground throughout every alter- 

 nate week during the growing season, and finding that this had 

 no stimulating effect on the trees. It was also found that in the 

 case of the trees grown in the cemented enclosures, as described 

 above, there was a much greater accumulation of carbon dioxide 

 in the soil than in tilled ground 31* 3 vols. compared with 21*4 

 vols. without there being any corresponding increase in vigour 

 of the trees, for the cement-covered trees were at that time 

 considerably less vigorous than those in tilled soil (III, 42). 



Differences in soil-temperature suggested another explanation 

 of the effect of grass on trees. This was examined by deter- 

 mining the temperature of the soil six inches below the surface 

 at several different stations in the farm where there were 

 grassed and tilled plots adjoining each other. During the day- 

 time the tilled ground w r as the hotter, and during the night it 

 was the cooler. Under ordinary summer conditions the average 

 excess of temperature in the tilled ground was 2 to 3 F. during 

 the day, though on occasions it might rise to 10, as, for instance, 

 when the air was dry and the temperature was high ; but as an 

 average difference of 2 or 3 is less than differences occurring 

 in consecutive summers, it is clear that they can afford no 

 explanation of the grass-effect, for this is in evidence just as 

 much in hot seasons as in cold ones (III, 45). 



The bacterial conditions of grassed and tilled soil are, no doubt, 

 very different, and still require much investigation. So far as 

 experiments have gone at present, no connection can be observed 

 between the mere number of bacteria in the soil and the be- 

 haviour of trees growing in it. Thus, trees were grown in earth, 

 and also in sand, with and without a surface crop of grass, and 

 the number of bacteria per gram determined. The results 

 were 



In earth, no grass . . 2*3 millions 



grass . . 9-0 



In sand, no grass . . . 0'5 

 grass . . 2-5 



so that, though in both media the presence of grass largely 

 increases the number of bacteria, there are practically the same 



