247 



hand, represent land of almost perpetual fertility ; on the other, it 

 may constitute wastes of any degree of sterility. 



LIME ESSENTIAL. 

 It is possible also that on some of the newer lands this and 

 kindred bacteria are absent because the conditions are not entirely 

 suitable to their development. A. Koch has shown that the pre- 

 sence of calcium carbonate is necessary to the action of Azotohacter, 

 and determinations of the power of soils from the various Rotham- 

 sted fields to induce fixation confirm his results, the development 

 of the organism in question being feeble when the soil was de- 

 rived from some of the fields that had escaped the "chalking" 

 process to which the calcium carbonate of the Rothamsted soils is 

 due. The value of calcium carbonate in this connexion only adds 

 to the many actions which are brought about by the presence of 

 lime in the soil — lime, that is, in the form of calcium carbonate, 

 which will behave as a base towards the acids produced by bacte- 

 rial activity. The experimental fields at Rothamsted afford a 

 singular opportunity of studying the action of lime, since the soil, 

 a stiff, flinty loam, almost a clay, is naturally devoid of calcium 

 carbonate, though most of the cultivated fields contain now from 

 2 to 5 per cent, in the surface soil, due to the repeated applications 

 of chalk, which used to be so integral a part of farming practice 

 up to the middle of the 19th century. Where this chalking process 

 has been omitted, as is the case in one or two fields, the whole 

 agricultural character of the field, is changed ; the soil works so 

 heavily that it is difficult to keep the land under the plough, and 

 as grass land it carries a very different and altogether inferior 

 class of vegetation. On the experimental fields it has been pos- 

 sible to measure the rate at which natural agencies, chiefly the 

 carbonic acid and water in the soil, are removing the calcium car- 

 bonate that has been introduced into the surface soil, and it is 

 found to be disappearing from the unmanured plots under arable 

 cultivation at an approximate rate of l,ooolbs. per acre per annum 

 — a rate which is increased by the use of manures like sulphate of 

 ammonia, but diminished by the use of nitrate of soda and of 

 dung. Failing the renewal of the custom of chalking or liming — 

 and its disuse is now very general — the continuous removal of 

 calcium carbonate thus indicated must eventually result in the de- 

 terioration of the land to the level of that which has never been 

 chalked at all, and even a state of sterility will ensue if much use 

 is made of acid artificial manures. That many soils containing 

 naturally only a trace of calcium carbonate remain fairly fertile 

 under ordinary farming conditions is due on the one hand to an 

 action of the plant itself, which restores to the soil a large propor- 

 tion of the bases of the neutral salts upon which it feeds, and 

 partly to the action of certain bacteria in the soil, which ferment 

 organic salts like calcium oxalate existing in plant residues down 

 to the state of carbonate. Were it not for these two agencies 

 restoring bases the soil must naturally lose its neutral reaction, 

 since the process of nitrification is continuously withdrawing some 

 base to combine with the nitric and nitrous acids it sets free. 



