June 8, 1912.] 



THE GARDENERS' CHRONICLE. 



373 







tides of humus, 



these 



year by 



HUMUS. 



Every gardener knows that humus is formed 

 by the decay of animal and vegetable matter, the 

 most familiar forms in which it is seen being 

 the well-rotted heap of leaf-mould and the re- 

 mains of the old hot-bed, which are such valu- supply of it are of such importance that it will be 

 able ingredients of potting composts. The dif- best to deal with the subject under three heads: 



year until, in the course of time, there appears a 

 thin layer of dark soil, and a layer of partiallj 

 ameliorated soil below that, resting upon the 

 original light-coloured soil. The function of 

 humus and the maintenance of an adequate 









\?- 





v. 



I 





Fig. iSo.-i^io-cattleya lustre "lavxngo" in ^^"]^^^ 



PET^8 AND SEPALS DEEP ROSE-PINK, LIP MARKED WITH CRIMSON. 



(Firtt-olau Certificate. See p. xxl. Second Eihl Ition Supplement.) 



warm the soil, in addition to which its darkening 

 of the colour of soils enables them to absorb more 

 of the sun's heat. 



This latter effect accounts for 

 the fact that Corn ©own in the early spring cornea 

 up more quickly on a dark soil — other 



than 



on 



lighter 



soils. 



things 

 In addi- 



being equal 



tion to all this, a good proportion of humus 

 in a clay soil, by facilitating the passage of water 

 through it, lessens the amount evaporated from 

 the surface, the latter being a process which, 

 as everyone knows, absorbs a vast amount of 

 heat, and thus keeps the soil cold. Another 

 great quality of humus, perhaps the greatest, is 

 that it serves as a store of plant-food in the soil, 

 and this brings us to its 



Chemical Activities. 



Humus, like all organic matter, is gradually 

 undergoing decay and being broken up into its 

 original elements. Amongst the products of 

 its decomposition are various organic acids, 

 the presence of which in quantity restricts the 

 further breaking up of its substance, and may 

 stop it altogether, as in the case of peat. When 

 we apply lime or chalk to a rich garden soil it 

 neutralises these acids and accelerates the break- 

 ing down of the humus, thus setting free stores 

 of plant-food. The stock of humus may in this 

 way be rapidly diminished if it is not kept up by 

 the application of animal manures or other or- 

 ganic substances. Farmers in the Fen country 

 find that the application of lime or basic slag 

 to the sour peats promotes such a rapid breaking 

 down of the peat that its thickness above the 



less in the 



ference between the top layer of black garden 

 soil— 1, 2, or 3 feet thick as the case may 

 be— and the lighter-coloured subsoil is almost 

 entirely due to the quantity of humus present in 

 the former and its comparative absence from the 

 latter. Anyone can observe this dark layer of 

 soil in gradual process of formation during a 

 number of years in the case of a heap of soil 

 resulting from the sinking of a well or the making 

 of a railway embankment or cutting. The 

 gravel, clay, chalk, or whatever it may con- 

 sist of, becomes covered with grass and weeds, 

 the decaying residues of which cause the surface 

 soil to become permeated with veins of dark 

 mould. The latter is composed of minute par- 



physical, chemical and biological, and first we 

 will deal with the 



Physical Properties 



of humus. It is the lightest constituent of the soil, 

 a cubic foot of the pure, dry material weighing 

 less than one-quarter of the same quantity of sand. 

 It is spongy in texture, and is capable of absorbing 

 two or three times its own weight of water, while 

 sand can only hold about one-quarter of it* weight 

 of water, in consequence of which properties 

 humus not onlv keeps heavy soil open, but enables 

 licht soils to hold more water. Thus it facilitates 

 the access of air to the roots of plants in i heavy 

 •t_ — wii. +v.« {nrmpntatmn nf the humus helps to 



underlying clay gets 

 course of a few years. 



appreciably 

 As humus contains all the 



elements of plant-food, its decomposition sets 

 them free for the use of plant life. Moreover, the 

 acids produced attack the minute particles of 

 soil and set free a further store of potash and 

 phosphate. The operations above described, 

 however, are not wholly chemical, but of a 



Biological Nature, 



the breaking down of the humus being itself 

 brought about, as we have most of us learned in 

 comparatively recent years, by myriads of soil 

 organisms. We can only just touch upon this 

 large subject here. Though some of these 

 microbes can do their work without air, it is 

 generally held that those which need oxygen 

 for their vital processes are the most beneficial. 

 Some break down organic matter into humus, 

 others break up humus into ammonia, &c, while 

 others convert ammonia by two stages into 

 nitrates which serve for the direct feeding of 

 plants. The important thing to remember is 

 that the acids produced in the early stages of 

 humus decomposition act as poisons and arrest 

 the vital processes, both of the roots of plants 

 and of the beneficent soil-bacteria. So one of 

 the things we have to do is to keep the soil sweet 

 by aeration and the supply of lime when it is 

 deficient. It is only during the present genera- 

 tion that we have seen the full reason for the 

 benefit of the tillage our ancestors have practised 



for ages. 



Thus lime is removed in large quantities from 

 a soil which is rich in humus, as anyone can test 

 for himself by the hardness of the water drain- 

 ing from such a soil. The breaking down of the 

 humus and the formation of nitrate go on very 

 rapidly in hot weather, especially if the soil *s 

 damp ; hence the luxuriant growth of weeds in 

 late summer, when the soil is still warm from 

 the accumulation of the summer's heat, and often 

 damp. If there is no crop on the soil, much of 

 the nitrates may be washed out before the 

 spring comes, and it therefore behoves the gar- 

 dener to have something growing on the soil dur- 

 ing August and September to absorb the plant- 

 food waiting for them, even if it is only Mustard, 

 to be afterwards dug in to increase the store of 

 humus and thereby supply the needs of the soil 

 microbes. The amount of humus present in a 

 soil is not only a measure of its fertility, but it is 

 a great factor in retaining artificial manures in 

 the soil. Alger Petts. 



