THE SOLVENT ACTION OF ROOTS 55 



Here the culture medium, which represents the soil and its 

 water, has gained alkalinity, nor did it show any indication 

 of an organic acid excreted to neutralise the excess of base. A 

 large bulk of the culture liquid on evaporation and heating 

 gave no signs of charring; the residue also possessed the same 

 reaction before and after ignition. The fact that water cultures 

 will become alkaline has been noticed from time to time ; some 

 books assert that they become acid, but this is only likely 

 to happen when ammonium salts are used as a source of 

 nitrogen and the plant takes in the ammonia as such, without 

 previous nitrification. 



The analyses, of which many exist, of our chief farm crops 

 show that the normal action of plants upon the soil is to 

 leave behind a basic residue from the neutral salts on which 

 they feed, and that, so far from being excretors of acid, their 

 action upon the soil is precisely the contrary. They must 

 leave behind in the soil after their growth quantities of base 

 equivalent to 100 — 300 lb. of calcium carbonate per acre ; and 

 this affords an explanation of several facts in the field hitherto 

 difficult to understand. For example, many soils possess but 

 traces of base (calcium carbonate, etc.) available for the neu- 

 tralisation of the acids produced during nitrification, a process 

 which is always going on in nature, and is indeed the normal 

 preliminary to the supply of nitrogen to the plant. Despite 

 this constant draft upon the small amount of base in the soil, 

 these soils maintain their neutral character when in arable 

 cultivation, and show no signs of becoming sour and infertile. 

 There must be some recuperative process at work, and this 

 we may now attribute to the growth of the crop, which annually 

 takes from the soil such an excess of acid as will leave behind 

 an amount of base of the same order of magnitude as that 

 consumed in the nitrification process. 



Again, it has been noticed that the use as manures of neutral 

 salts, like sodium nitrate and potassium sulphate, induces 

 certain injurious changes in the texture of heavy soils ; the 

 clay lands become more sticky when wet, and dry into harder 

 and more intractable clods ; the water running from the land 

 drains is more turbid and carries a comparative excess of 

 suspended matter {Trans. Chetn. Soc. 1904, 85, 964). Effects 

 of this kind are due to the " deflocculation" of the clay in 

 the soil — i.e. its resolution into the ultimate particles of extreme 



