152 SOIL CONDITIONS AND PLANT GROWTH 



magnesium sulphate has furnished it with an extra 320 Ib. 

 over and above what the crop on Plot 1 1 can get. 



On the other hand, gypsum has no such effect. The super- 

 phosphate applied to Plot 1 1 contains a considerable propor- 

 tion of calcium sulphate, but it does not increase the weight 

 of potassium in the crop. This appears to be the general 

 rule : Briggs and Breazeale ] obtained the same result on 

 Californian soils. The result is at variance with a statement 

 commonly made that potassium is dissolved out when soil is 

 shaken with calcium sulphate solution. 2 



Some ions are not precipitated in the soil, including CO 3 , 

 SO 4 , NO 3 , Cl, Mg, Ca, Na; 8 these are, therefore, the chief 

 constituents of drainage water (see p. 128). 



Organic substances, particularly those of high molecular 

 weight, are also withdrawn from their solutions, but the 

 reaction is apparently of a different type, since nothing appears 

 to be given up from the soil in exchange. The result is of 

 extreme importance ; practically the whole of the organic 

 matter added to the soil by plant residues or manure remains 

 near the surface unless carried down mechanically by some 

 agency such as earthworms. Even when heavy dressings of 

 dung are annually supplied at Rothamsted there is after fifty 

 years no appreciable enrichment of the subsoil in nitrogen 

 (Table XLII.). The purification of sewage by land treat- 

 ment affords further illustrations of the absorptive power of 

 soil for organic matter. In English experience a sewage farm 

 on a good loam can deal with 30,000 to 40,000 gallons of 

 sewage per acre per day (i.e. I -3 to I '8 inches per day). 



l journ. Ag. Res., 1917, 8, 21. 



2 E.g. Hilgard, Soils, 1906, p. 43 ; probably based on experiments by 

 E. Heiden, Jahresber. Agrik. Chem., 1868, p. 59; Annal. Landw. Preussen., 

 1868, 50, 29; Kalmann and Bocker, Landw. Versuchs-Stat., 1878, 21, 349. See 

 also G. Andre", Compt. Rend., 1913, 157, 856. 



3 From the time of Aristotle it has been known that sea water could be 

 '* desalted " by filtering through sand or soil. But it has been shown by Von 

 Lipman and Erdmann (Chem. Zeit., 1911, xxxv., 629) that the water first 

 running through the sand filter is not desalted sea water, but displaced water. 

 When this has all gone the salt water runs through unchanged. 



