220 



weak organic acids which may be contained in secretions from the roots 

 of plants are not capable of exercising a very important solvent influence 

 on the soil particles. 



In fact, one of the organic acids which may be found in the secretions 

 of plants, viz , oxalic acid, is capable of exerting an influence which is 

 unfavourable to the decomposition of mineral matters containing lime. 

 A mineral which is composed in part of lime, when exposed to the ac- 

 tion of oxalic acid, becomes coated with a film of lime oxalate which pre- 

 vents any further decomposing action. The influence of nitric acid, 

 which is due to the activity of soil ferments, is exerted in this case in 

 the most beneficial way, attacking and dissolving the film of lime oxalate, 

 and exposing fresh portions of the mineral substance to decay. Phos- 

 phoric acid, especially, which is so often found in combination with lime 

 may be released by this action and made available. It must not be for- 

 gotten also that lime itself is an essential plant food, and must be sup- 

 plied in appropriate quantities to secure a normal growth of the plants. 



The " vital activity" of the rootlet, a phrase often used, has an indefi- 

 nite meaning and conveys absolutely no comprehensive idea of solvent 

 action. On the other hand, it is known that soil ferments are found in 

 particularly large numbers, clustering about the rootlets of plants, and 

 in fact existing in symbiotic union therewith. This signifies that the 

 relation existing between them is so intimate as to make their vitality 

 mutually dependent. It, is therefore quite probable, as has already been 

 intimated, that the preparation of soil particles for plant food is due 

 quite largely to bacterial activity. 



KINDS OF ORGANISMS. 



The nitric organisms in the soil exist in common with hundreds of 

 others, many of which are doubtless active in the solvent work. The 

 nitrifying organisms themselves, as will be mentioned further on, have 

 such important relation in the supply of nitrogenous food as to have 

 escaped consideration in their more purely solvent action. The atten- 

 tion of bacteriologists has been devoted almost exclusively to a study of 

 the nitrifying organisms in respect of their relation to albuminoid and 

 ammoniacal bodies. For this reason the action of these organisms and 

 others relating thereto as a solvent for mineral particles in preparing 

 them for plant absorption has not received the consideration which it 

 merits. 



THE NITRIFYING FERMENTS. 



The micro-organisms of most importance to agriculture, and those to 

 which attention is particularly called in this article, are the bacteria 

 which act upon nitrogenous matters and oxidise them to nitric acid, or 

 which exert a reducing effect on nitric acid, bringing it to lower forms 

 of oxidation or even to free nitrogen. These organisms belong to many 

 different species, and act in very many different ways. The general 

 group to which these organisms belong is known as nitro-bacteria. The 

 classification of these organisms by genera and species would prove ot ! 

 little interest to readers of this article. In general it may be said that 

 there are three distinct genera, comprising, in the first place, those 

 organisms which form ammonia or carbonate of ammonia from organic 

 nitrogenous compound*, such as albumen ; in the second place, the or- 



