282 SCIENCE AND FRUIT GROWING 



heavier soils, such as those at Ridgmont or Harpenden, the 

 difference caused by grass is much less, and may even be in 

 the opposite direction ; for, whilst the grass by its physiological 

 action removes water from the soil, it affords a covering to 

 this same soil, protecting it from the drying action of sun 

 and wind. 



The numerous series of experiments made at Woburn wherein 

 plants have been grown in pots, with and without a surface crop, 

 afford evidence as to the effect of this surface crop on the water- 

 content of the soil, for these pots were weighed at frequent 

 intervals, thus supplying exact information as to the loss of water 

 occurring. When the pots were under glass, the loss due to 

 physical causes was, naturally, very small, and that due to the 

 physiological action of the surface crop was the predominant 

 factor; consequently, it was found that, during the summer 

 months, 30 per cent, more water was lost from the pots where 

 there was a surface crop than from those where there was none : 

 but when the pots were in the open, exposed to the sun and wind, 

 the reverse was often not always the case, and the evaporation 

 from the pots with the surface crop might, during the season, 

 be even less than half of that from those without a surface 

 crop. Whether the difference was in the one direction or the 

 other depended, as may readily be conceived, on the character 

 of the seasons (XVII). 1 



An examination of the water-conditions in grassed and un- 

 grassed ground at .Harpenden was carried out in 1905 to 1907, 

 so as to give a view of the conditions obtaining at different 

 times of the year. Three depths of nine inches each were 

 examined separately, but the results may be sufficiently illus- 

 trated by setting out those for the total depth of twenty-seven 

 inches (Fig. 38) : the horizontal line represents the water- 

 contents of the tilled soil, and the curved line that of the 

 grassed soil, showing that during the winter the latter contains 

 the most water, to the extent of about i per cent., whereas 

 during the summer it contains less water, the deficiency extend- 

 ing to about 5 per cent. The variations in the top spit were, 

 naturally, greater and more irregular, the excess under grass in 

 the winter reaching 3*4 per cent., and the deficit in the summer, 

 6*3 per cent. But the result of most importance was that this 

 deficiency never reached the point at which it could possibly 

 affect the trees growing under the grass and there were trees 

 growing in these very grass plots which were evidently suffering 

 1 The seventeenth Report is still in the Press. 



