Scientific Agriculture. 



[July, 1911. 



matter of common observation, however, 

 in this part of the world, that the humus 

 content of clay soils is higher than that 

 of sandy soils, because the access of air 

 to the interior of the soil is not as great 

 in the case of the former as in the latter ; 

 so that almost ideal conditions for the 

 production of humus are afforded by 

 clay soil*. 



In dealing with humus more generally, 

 it will be well to treat in turn, of its 

 importance, firstly in relation to the 

 soil, and secondly with reference to the 

 plant. It must not be forgotten, how- 

 ever, that all final effects of humus have 

 a direct or indirect influence on the 

 plant ; for whatever affects the soil 

 must ultimately have its influence on the 

 vegetation which it supports. One of 

 the most useful effects, to the agricul- 

 turist, of the possession by a soil of a 

 good humus content is the circumstance 

 that this improves its texture, so that 

 there is ease as well as economy in till- 

 age operations. The artificial employ- 

 ment of this fact has its largest appli- 

 cation on heavy clay soils, in the im- 

 provement of their condition by the use 

 of vegetable matter either in the form 

 of decayed remains such as trash, or as 

 withered green dressings. Another 

 matter, to which a passing reference 

 only need be made here, on account of 

 its comparative unimportance in the 

 tropics, is the effect of humus in darken- 

 ing the colour of soils and thus increas- 

 ing their ability to absorb heat. 



It is recognized by the agriculturist 

 that all soils coutain mineral plant food 

 in an unavailable condition. 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 part, however, in this 

 matter, 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 con- 

 tinuous, and where the soil is well sup- 

 plied with humus, its eventual import- 

 ance in relation to the provision of 

 mineral plant food is obvious, especially 

 when consideration is given to the com- 

 paratively small bulk of this food that 

 is necessary to plants. 



A final general matter in connexion 

 with humus in a soil is probably of the 

 greatest concern as regards its ultimate 

 effect in enabling that soil to produce 

 quantities of vegetation. This is the 

 circumstance that it provides ideal 

 surroundings to, and a certain amount 

 of food for, the bacteria which effect 



improvements in the soil, from the agri- 

 culturist's point of view. This appears 

 particularly to be the case in relation to 

 the nitrogen-fixing 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 parti- 

 cularly to the notice of agriculturists on 

 account of the comparative difficulty and 

 expense of sustaining an adequate nitro- 

 gen content in the land from which they 

 raise their crops. 



The next matter to consider is the 

 value of humus 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 importance when the sugges- 

 tion is made to remove plant waste from 

 Lhe land, or on the other hand, to bring 

 in supplies of vegetable material from 

 other areas on which it has been raised. 

 The matter will be considered further, 

 in the former aspect, when the question 

 is raised of the destruction of vegetable 

 matter for the control of pests. 



The most important 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 grow 

 depends upon the existence of several 

 limiting factors, the absence of, or 

 deficiency in, any one of which will 

 interfere with its power 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 

 under-estimated. 



The consideration may now be under- 

 taken 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 mat- 

 ter, particularly of trash in cane fields, 

 for the eradication of pests, is often 



