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XL. 



A METHOD FOR THE ESTIMATION OF HYGEOSCOPIO 

 MOISTURE IN SOILS. 



By W. D. HAIGH, B.Sc, A.R.C.Sc.I., 

 Demonstrator of Geology, Eoyal College of Science, Dublin. 



[communicated by prot'essok g. a. j. cole, f.g.s.] 



[Read June 22. Published July 6, 1915.] 



When an ordinary moist soil is spread out and allowed to lose water by 

 evaporation, a point is at last reached when the soil becomes " air-dry " and 

 dusty. It still contains some moisture, however, the amount of which will 

 depend, to a large extent, on the temperature and amount of water-vapour 

 present in the atmosphere. Tiiis is known as the hygroscopic moisture, and 

 is held very closely by the soil particles. The amount of this moisture 

 retained by soils will vary greatly with their nature. Broadly speaking, 

 sandy soils retain the least ; while clayey soils and those containing much 

 humus material will retain the largest proportion. 



It was considered by the earlier experimenters on soils that the hygro- 

 scopic moisture was very important for the welfare of plants ; but within the 

 last few years it has been shown that this is not the case. The pot experi- 

 ments by Sach, and F. J. Al way's studies of soil moisture conditions in 

 the " Great Plains " region of north-western America, have sliown that it is 

 only the moisture present in excess of the hygroscopic moisture tliat can be 

 of direct service to crops. Plants will begin to wilt in a clay soil containing 

 as much as 8 per cent, of water, but will flourish in a sandy soil containing 

 only 5 per cent. It has also been shown that wilting begins even before the 

 hygroscopic moisture limit has been reached. Yet it plays au important 

 part, though indirectly, in the life of a crop, since soils of higli hygroscopic 

 power can absorb from moist air sufHcient moisture to sustain the life of 

 a crop in time of drought, although it cannot maintain normal growtli. It 

 is of enormous importance in some regions where hot, dry winds are a feature 



SCIENT. PROC. R.D.S., VOL. XIV., NO. XL. 4p 



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' DEC U 1916 



