POSSIBLE CAUSES OF THE GRASS EFFECT 283 



for it has been shown x that the optimum proportion of water in 

 a soil, so far as the flourishing of plants is concerned, is from 

 40 to 50 per cent, of the maximum which that soil will hold : 

 but the maximum water .capacity of this soil was found to be 

 35 per cent., so that the optimum water-content would be from 

 14 to 17*5 per cent. ; yet, during the period of plant-growth 

 (June and July) the water-contents of this grassed soil taking 

 even the top spit where there was the greatest lack of water 

 varied between 13-8 and 177 per cent., so that it was practically 

 in the best condition as regards moisture for plant-growth; yet 

 in spite of that, the grass effect was manifest (XIII, 54). 



+ 5 



I -5 



0. 



Jan I" 100 200 300 Jarrl*' 



Days. 



FIG. 38. DIFFERENCE BETWEEN THE WATER OF 

 TILLED (T) AND GRASSED (G) SOIL. 



When a comparison is made between the grassed and tilled 

 soil during the months of May, June and July it is found that, 

 according to the results obtained at Harpenden, there is an 

 average difference of 4 per cent, of water in favour of the tilled 

 ground, and this would be equivalent to about 120 tons of water 

 per acre in a layer of soil twenty-seven inches thick, a weight 

 which corresponds to a rainfall of i'2 inch : if a difference such as 

 that could account for the action of grass on trees, it is evident 

 that a similar action, but one of much greater magnitude, should 

 occur in a dry season ; for the differences in rainfall in dry and 

 wet seasons often extend to four or five inches of rain. As a 

 matter of fact, no differences in the deleterious effect of grass on 

 trees can be noticed in wet and dry years, and a comparison of 

 the rainfalls in March to July for the years 1896 to 1908 with 

 the behaviour of trees under grass, showed that the two years 

 of heaviest rainfall (ten to twelve inches during these months) 

 did not in any single case correspond with an y exceptional 

 vigour of the grass-grown trees, nor did the three driest years 



1 Hellriegel, Beitrage zu den N aturwissenschaftlichen Grundlagen des 

 Ackerbaues, Braunschweig, 1883. 



