222 SOILzCONDITIONS AND PLANT GROWTH 



plant, but also the water which would enter the roots if only 

 it could move quickly enough. Shull has shown by a method 

 described later that the water at the wilting coefficient is held 

 by the soil with a force of about 4 atmospheres only, while 

 the pull exerted by the plant root is approximately equa 

 to its osmotic pressure, which, according to Hannig's^ 

 measurements, is about 7 or 8 atmospheres, and according 

 to Dixon and Atkins it may exceed 20 atmospheres.^ 



Other investigators adopt more rapid methods. W. B. 

 Crump (72) assumes that water left in soil air-dried at 1 5° C. 

 is unavailable : he deducts this from the total moisture lost on 

 drying at 100° C. in order to obtain the available water. This 

 method is open to criticism in that air drying at 15° C. is a 

 very indefinite process, and that water left in the soil at 1 5° 

 may, as Shull has shown for water at the wilting coefficient, be 

 obtainable by the plant. 



Much less work has been done on the available water. 



Crump in studying peat soils, found large variations in 



the total water content of different layers, although the root 



development failed to indicate any corresponding difference. 



water 



But when instead of taking total water he took , 5 he 



humus " 



obtained a much less variable result : he therefore assumed 



that it is not the total water which must be regarded as free, 



but the water per unit weight of humus : — 



Soil No. 13 (September 1905) Vaccinium myrtillus and 



Deschampsiaflexuosa, growing in a shallow hollow on a Calluna 



Moor, Yorks : — 



^ E. Hannig, Ber. Deut. hot. Gesell., 1912, 30, 194-204. 



^Proc. Roy. Dublin Soc, 1912, 13, 229. Some of their results are: Roots 

 oi Beta vulgaris, up to 2i*i8 atmos. ; of Ilex aquifolium, 7*64 to io'38 atmos. ; 

 of Helianthus tuberosus, X2"j6 to i8"67 atmos. 



2 The water being weight when sampled minus weight after drying at 15° : 

 and the humus being loss on ignition of oven-dry soil. 



