22 



Mr. A. D. Hall and Dr. N. H. J. Miller. [Mar. 30, 



soils almost devoid of calcium carbonate has long been a problem, but it 

 now seems probable that the calcium carbonate and other bases which are 

 required for nitrification are in some way returned to the soil as bases, and 

 that when a ready-formed nitrate like sodium nitrate is used as a manure 

 there is an addition of available base to the soil or a corresponding diminu- 

 tion in the amount of calcium carbonate removed by the drainage water. 

 Furthermore these or other agencies conservative of calcium carbonate are 

 sufficient to maintain the quantity in the soil at the level for comparatively 

 healthy growth. Of the possible conservative actions, two will be now con- 

 sidered and evidence be brought to show that (1) the normal growth of plants 

 leaves behind a residue of base in the soil, (2) the decay of plant tissues 

 results in the production of calcium carbonate. 



II. — Effect of Plant Growth on the Keaction of the Soil. 



The plant, it is well known, does not take up the salts of the soil water in 

 the proportions in which they are present in the solution, but exercises a 

 selective action in favour of substances necessary to the nutrition processes, 

 such as potash and phosphoric acid. And if the composition of the ashes 

 of the plant be taken into account, it is clear that the selective action is 

 exercised not merely on the salts with which the root is in contact but on 

 their acids and bases considered separately. For example, from a solution of 

 calcium nitrate the plant would withdraw more nitric acid than its equivalent 

 of lime and from a solution of potassium sulphate more potash than its 

 equivalent of sulphuric acid. 



When a plant is burnt the ash is usually alkaline, because the organic acids 

 and any nitrogen present as nitrate in the plant are all driven off, leaving the 

 bases as carbonates. But when a balance is struck between the acids and 

 bases in the ash and when the nitrogen present in the plant before burning is 

 calculated as an acid, since it all entered the plant as nitrate, the acids are 

 generally to be found in excess. Warington,* indeed, has already pointed out 

 with reference to the published analyses, that plants must retain more acids 

 than bases. It does not appear to have been noticed, however, that such a 

 result, by leaving behind in the soil a corresponding excess of base, must 

 have an appreciable effect upon the reaction of the soil, although Knop and 

 other early investigators have observed that the solutions in which plants 

 are grown as water cultures become alkaline after a time. It is hardly 

 possible to decide whether the excess of base is left behind in the soil water 



* 'Agricultural Students' Gazette,' 1899, p. 133 ; see Lawes and Gilbert, 4 Journ. Koy. 

 Agri. Soc.,' vol. 55, 1894, p. 640. 



