82 



THE AGRICDLTUKAL NEWS 



March 18, 1911. 



the form of decayed remains such as trash, or as with- 

 ered green dressings. Another matter, to which a pass- 

 ing reference only need be made here, on account ofits 

 comparative unimportance in the tropics, is the effect 

 of humus in darkening the colour of soils and thus 

 increasing their ability to absorb heat. 



It is recognized by the agriculturist that all soils 

 contain mineral plant food in an unavailable condi- 

 tion. He knows, too, that some of this may be 

 freed in a state in which it can be used by plants, 

 by the employment of artificial dressings such as lime. 

 Humus probably plays the greatest ])art, however, in 

 this mattei-, both artificially and in nature. This is 

 because of its power to form bodies, commonly called 

 humic acids, which have a dissolving action on some of 

 the mineral constituents of the soil. The process is 

 naturally slow but is continuous and, where tho soil is 

 well supi)lied with humus, its eventual importance in 

 relation to the provision of mineral plant food is obvious, 

 especially when consideration is given to the compar- 

 atively small bulk of this food that is necessary to 

 plants. 



A final ueneral matter in connexion with humus 

 in a soil is probably of the greatest concern as 

 regards its ultimate effect in enabling that soil to pro- 

 duce quantities of vegetation. This is the circumstance 

 that it provides ideal surroundings to, and a certain 

 amount of fo'xj for, the bacteria which effect improve- 

 ments in the soil, fi'oni the agriculturist's point of view. 

 This appears particularly to be the case in relation to 

 the nitiogen-fi.xing organisms. Greater recognition of 

 the importance of the extent to which these affect the 

 nitrogen content of the soil is being attained rapidly, 

 and the matter is brought particularly to the notice of 

 agriculturists on account of the comparative difficulty 

 and expense of sustaining an adequate nitrogen content 

 in the land from which they raise their crops. 



The next matter to consider is the value uf hinnus 

 in relation to the plant, more directly. In the first 

 place, humus itself contains nitrogen, potash and 

 phosphates, and this is a fact that becomes of im])or- 

 tance when the suggestion is made to remove plant 

 waste from the land, or on the other hand, to bring in 

 supplies of vegetabh' material from othei- areas on 

 which it has been raised. The matter will be consider- 

 ed further, in the formei' aspect, when the question is 

 raised of the destruction of vegetable matter for the 

 control of pests. 



The most imporfexnt and most fully recognized 



direct function of humus in relation to the plant is the 

 effect, that it possesses in increasing the capacity of 

 soils to absorb water, as well as their ability to hold 

 it when it is supplied to them. It has been 

 considered already that the power of a plant to gi'ow 

 depends upon the existence of several limiting factors, 

 the absence of, or deficiency in, any one of which will 

 interfere with its proper development. * Of these 

 factors, as is well known, the one whose absence or 

 insufficiency most quickly shows itself is the water- 

 supply, and the importance of humus in keeping this 

 at an adequate level cannot be underestimated. 



i'he consideration may now be undertaken of some 

 of the more particularized facts in relation to humus. 

 One that has been indicated already is the quick 

 rate at which vegetable matter is lost from sandy 

 soils. This is on account of the easy access of air to 

 such soils, whereby the bacterial action which would 

 lead to the formation of humus is prevented, and 

 oxidation takes place; so that the material is quickly 

 lost in the atmosphere. It is in such soils that the 

 agriculturist requires to exercise the greatest care in 

 regard to the supply of humus, especially as if this 

 is deficient, any water that they receive passes away 

 almost immediately, through drainage. 



The burning of waste vegetable matter, particu- 

 larly of trash in cane fields, for the eradication of pests, 

 is often recommended. It ap])eais, however, that this 

 can only be done continuouslj', with impunity, where 

 there is irrigation and the plentiful supply of artificial 

 manures, as in Hawaii: and it remains to be seen, even 

 there, what will be the ultimate efi'ects of the procedure. 



Other matters that remain for short consideration 

 in the present relation, are these dealing with what is 

 called ' soluble humus ', and the connexion between 

 carbohydrates in the soil and the increase of efficiency 

 of the nitrogen-fixing organisms. Soluble humus, it 

 may be explained, is the product obtained by the 

 action of alkalis such as ammonia and soda on the 

 humic acids; there is a large amount of this matter 

 dissolved in the dark licpiid that drains away from 

 manure heaps, because of the alkalinity of the contents 

 of such heaps, through the production of ammonia, 

 Much remains to be disco\-erod concerning soluble 

 humus, and it is not yet known with certainty if similar 

 bodies are of direct use to green plants. 



The effect of the presence of carbohydrates in the soil, 

 in increasing the power of the nitrogen-fixing bacteria 



* Agricultural New, Vol. IX, p. 257. 



