CHAP. XVI HUMUS AND PEAT 6i 



constituents. Humus is black or brown, and rich in carbon, also some- 

 times in nitrogen the Russian ' Black Earth ' contains, according to 

 Kostytscheff, as much as from four to six per cent. In the production 

 of humus a prominent part is played by micro-organisms (bacteria, 

 monerae, and the hke), and by larger animals, particularly earthworms. 



Humus substances combine with nutritive bodies that are soluble 

 with difficulty to form easily soluble compounds, and thus they increase 

 the nutritive value of soil. They also change the physical characters 

 of soil ; when mixed with mineral soil they increase its absorbent faculty, 

 its specific heat, its water-capacity, and so forth. 



There exist wide distinctions among humus soils, according to the 

 degree of decomposition, and according to the species of plants and 

 animals engaged in producing the humus. The first of these soils that 

 we shall deal with is that which is richest in humus : 



(a) Peat soil. If water containing oxygen come into contact with 

 organic bodies it is robbed of its oxygen by these. If then the admission 

 of oxygen be prevented and the activity of micro-organisms excluded, 

 or at least restricted, in many cases an incomplete decomposition and 

 change of the organic substances ensues ; as a result, carbon will accumu- 

 late in larger quantities the more the supply of air is restricted, and free 

 humous acids make their appearance : there is a production of j)eat. 

 Peat is humus rich in carbon, brown (from a light to a black-brown) in 

 colour, and contains jnany free humous acids, and other acids which are 

 contained in the remains of organisms buried in the peat. The organic 

 portions (50 to 90 per cent.) of peat are chiefly plant-remains, which are 

 readily recognizable ; animal-remains, on the other hand, are quite 

 subordinate. By the removal of water and admission of air, peat can be 

 changed into a humus that is well suited to plants. Peat contains 1-2 

 (-3) per cent, of nitrogen and 0-4 per cent, of lime (certain peats, for 

 instance that of moss in Gothland, are stated to have up to 3.21 per cent, 

 of nitrogen and much hme), but peat contains very little potash and still 

 less phosphoric acid. The amount of these important nutritive substances 

 is so small because the acids in peat combine with alkalis to form soluble 

 salts, which are washed away. 



Peat soil has the following characters : 



Of all soils it has the greatest water-capacity, so that it can take up 

 much more water than its solid parts weigh ; air-dried peat contains 

 only 15 to 20 per cent, of water ; peat swells on the addition of water 

 to a far greater size, but contracts on drying and becomes cracked without 

 however crumbling to pieces. When it is completely dried it becomes 

 extremely loose, almost powdery (peat-dust ; driving peat-dust may be 

 compared with driving sand). If one reckons the tenacity of clay as 

 one hundred, that of peat is only nine. It is almost impermeable to 

 water, and its power of raising water exceeds that of all other soils. It 

 is powerfully hygroscopic (absorbing up to ten per cent, of aqueous 

 vapour). In regard to their power of conducting water, different peats 

 (for example those of heath-moors and meadow-moors) behave very 

 differently. Sphagnum-peat conducts water rapidly and is therefore 

 uniformly moist in all cases, but meadow-peat may be dry above and 

 wet beneath. On account of its dark colour peat-soil is strongly heated 

 by the sun, but is intensely cooled at night. Despite its dark colour peat 



