38 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



which have not been withdrawn from the vegetable matter 

 before its death. 



In the second place, there is the nitrogenous food-supply of 

 the plant. It is customary to consider the soil of itself as a 

 dead thing, but such is not the case. The soil is never without 

 a living, swarming population of millions of bacteria and fungi. 

 Since these, being without chlorophyll, cannot utilise the energy 

 of the sun directly as a source of vitality, they must, like 

 animals, make use of organic material as a source of energy ; 

 this organic food-supply is obtained in the dead litter and 

 humus which is itself an intermediate product of the total 

 bacterial activity of the soil. The principal final results of 

 their "digestive" action on vegetable litter are, on the one hand, 

 carbon dioxide which is of no direct importance in the edaphic 

 nutrition of plants, and on the other, simple compounds of 

 nitrogen such as ammonia and nitrates, which, especially 

 the latter, are the only forms of nitrogenous food of any 

 immediate value for most typical green plants. Not only so, 

 but with humus as a source of energy some of these bacteria 

 utilise the nitrogen of the atmosphere, and thus make available 

 to plants in general this additional supply of combined nitrogen 

 without which the available stock would soon be totally 

 depleted, and plant life become extinct. 



It will thus be obvious that the continual addition of supplies 

 of humus through the medium of dead vegetable matter is of 

 the first importance in cultural soil management, and that apart 

 from its structural stability the actual mineral soil is of much 

 less importance for plant nutrition than is the humus, which 

 is to a special degree the medium through which plants receive 

 all the elements of nutrition they require from the soil. 



It is of the highest importance, however, that the bacterial 

 formation of humus should take place under conditions of 

 moisture and aeration similar to those necessary for the growth 

 of higher plants, and that during its formation and afterwards 

 it should have absorbed in it a minimum amount of mineral 

 salts, otherwise, if waterlogging, drought, or leaching occur, 

 types of vegetable decay are formed which not only inhibit plant 

 growth of themselves but reduce the general nutritive efficiency 

 of the soil by "puddling" or deflocculation, accompanied by 

 waterlogging, excessive leaching, and the production of " plant 

 toxins," so that finally only specially adapted plants will occupy 



