THE CONSTITUTION OF THE SOIL 



63 



changes into the soluble bicarbonate and washes out into the drainage 

 water ; the average loss per acre per annum throughout England and 

 Wales has been estimated at 500 lb., and at Rothamsted on the arable 

 land at 800 to 1000 lb. (118). The rate of loss is influenced by the 

 treatment, being increased by the use of ammonium sulphate and 

 decreased by dung and by the crop ; it is much less on pasture than 

 on arable land. Repeated additions of calcium carbonate to the soil 

 are, therefore, necessary : indeed chalk and lime are among the oldest 

 of manures. Soils lying immediately above chalk and limestone 

 are no exceptions and in wet regions they may become thoroughly 

 decalcified. 



On chalk soils the percentage of calcium carbonate may rise very 

 high, and then a wholly new set of properties comes in. It is im- 

 possible to draw any exact line showing where these properties begin 

 to appear, but they entirely mask the effects of the silica and silicate 

 particles and obliterate the distinctions between sands, loams, clays. 

 Chalk soils, therefore, form a class by themselves to which, the ordinary 

 laboratory methods of analysis and investigation do not apply: un- 

 fortunately, appropriate methods have not yet been worked out. 



The Soil Water. 



The soil retains by absorption and surface attractions some 10 to 

 20 per cent, of its weight of water, distributed as films over its particles. 

 This water is of obvious importance as the medium through which 

 plants and micro-organisms derive their food, indeed the Whitney 

 school regard it as the culture solution for the plant. Its relationship 

 to the mineral matter is discussed by Cameron (68, 69). Notwith- 

 standing its importance, however, but little is known of it, because of 

 the difficulty of getting it away from the soil. No pressure method 

 has proved successful, but a centrifugal method which, however, has 

 not come into general use, gave the following results (Whitney & 

 Cameron (304)) : — 



