18 



JOIVRXAL OF THK KDVAL HOK TiCr l/lT HAL SOC'lK^^. 



hns i^-ivon vise to snmll quantities of acid. Tlieso acid soils carry very 

 disliiu'tno l\ [1CS of N electa I icMi, tlic plants of wliicli ai'c often iatiolei'ani 

 of transference to a non-acid soil. 



The reason does not appear to lie in the plant itself, for so far as 

 we can learn from water-culture experiments plants grow freely and 

 well in a medium that is distinctly acid wlien no other disturbing 

 factors are allowed to come into play. In the field, however, the 

 acidity or alkalinity of the soil determines entirely the character of 

 the micro-flora which plays so large a part in the nutrition of the 

 higher plants. 



Under normal conditions plants are dependent for their nutrition 

 upon the work of bacteria in the soil; the complex nitrogenous 

 bodies contained in plant and animal residues are attacked by various 

 groups of bacteria and converted into successively simpler com- 

 pounds until they reach the state of ammonia. Ammonia does not 

 long remain in a soil, but is in its turn converted into nitrates by the 

 action of other bacteria. From these nitrates the normal cultivated 

 plant obtains its nitrogen, though we are beginning to suspect that with 

 the nitrogen supply of many plants ammonia is more concerned than 

 has hitherto been supposed. For the proper working of these 

 groups of bacteria a neutral or evidently alkaline medium is 

 requisite, such as the presence of carbonate of lime ensures, while a 

 very slight acidity is sufficient to suspend the action of nearly all such 

 bacteria. On the other hand, we find the micro-fungi, the moulds, 

 the wild yeasts, and other kindred organisms, which are also common 

 in such a medium as the soil, are best favoured by a slightly acid 

 medium, and may in their turn be repressed and rendered inoperative 

 by making the medium neutral. Thus in an acid soil the normal 

 bacterial actions are comparatively suspended, their place being taken 

 by parallel changes brought about by fungi and moulds. The effects 

 upon a higher vegetation of these differences in the micro-flora are very 

 great. Experimentally it is on some of the plots at Woburn and 

 Eothamsted that these effects can best be studied, because there certain 

 portions of the land have been rendered acid artificially through the 

 long-continued use of ammonium salts as manure. For reasons which 

 need not be enlarged upon here, ammonium salts act as an acid, and 

 unless the soil starts with a considerable proportion of carbonate of 

 lime this acidity eventually accumulates until its effect becomes per- 

 ceptible. On the Woburn plots which have become acid, barley 

 now refuses to grow and the plots become covered with spurrey, a 

 weed which is found only sparsely elsewhere on the same land, but 

 which invades and completely swamps all other vegetation on the 

 plots in question. At Bothamsted the acidity does not occur in the 

 arable land but on certain of the grass plots, and there the surviving 

 grasses have developed a habit of growing in tufts, with bare spaces 

 between on which dead vegetation accumulates in a form resembling 

 peat. It is clear that the formation of peat must be associated with 

 the acid condition and the consequent suspension of bacterial actions. 



