CHAP. XVI HUMUS 63 



almost completely, become poor in nutriment, and thus there is formed 

 under the raw humus a light-grey or black ' bleached sand '.^ As raw 

 humus dries, some of the humous substances that were originally freely 

 soluble become scarcely soluble, and are precipitated as carbonized humus. 



Movements of the water also carry down particles of clay, ferric oxide, 

 and humus, which are soluble only in water containing but httle salt, 

 and convey these through the layer of sand that is poor in salt until, 

 at the lower limit of this washed out sand, they meet with the particles 

 of soil which are still undergoing disintegration, and which therefore 

 still contain soluble salts. The water takes these salts into solution, and 

 the humous acids are precipitated as a gelatinous mass, which at a definite 

 degree of dryness becomes solid, possibly by chemical changes, and 

 insoluble in water. The grains of sand are cemented together, and there 

 is formed a reddish-brown or brow^n layer of earth known as hard pan or 

 moor-pan, which may be half a metre in thickness, and when completed 

 is impervious to plant-roots. 



The change from ordinary humus soil to raw humus is brought about 

 by the following means : 



(i) Plants with densely interlaced roots occur ; 



(2) Animals, particularly earthworms, vanish, so that the soil is not 

 worked ; 



(3) Particles of soil, particularly grains of sand, sink down and leave 

 the soil more compact and poorer in air. 



(c) Ordinary humus (leaf-mould, garden-mould, vegetable mould-) is 

 an intimate mixture of sand and clay with completely disintegrated 

 organic ingredients rarely more than ten per cent. The mixture comes 

 about mainly through the agency of animals and water.^ It is devoid 

 of free soluble humous acids, and is neutral or alkaline in reaction. It 

 contains many fungal mycelia, earthworms, insects, and so forth. That 

 ordinary humus soil is so excellent a nutritive substratum for plants is 

 due to 



Its physical attributes (loose, having compound particles, aerated). 



Its chemical characters, as it contains many compounds of carbon 

 and nitrogen. 



The humous substances, forming freely soluble compounds with 

 nutritive substances that are otherwise scarcely soluble. 



The production of humus in forest partially replaces the manuring 

 and amelioration of the soil in agricultural practice. 



Factors that strongly promote decomposition of organic rnatter 

 hinder the production of humus ; according to Wollny, heat and moisture 

 are the factors of greatest import. In connexion with both these, as 

 in all other physiological processes, there exist a minimum, an optimum, 

 and a maximum. Temperatures above the maximum are almost devoid 

 of significance ; moisture beyond a moderate amount may exclude air 

 from clay and humus soils, and thereby exert a restrictive action upon 

 decomposition even before the soil is saturated with water. 



In lower latitudes decomposition proceeds very slowly during the 

 dry season, but at the wet season it is greatly accelerated, and in most 

 spots is so complete that a soil very poor in humus results.' In the 



See Albert, 1907. * Darwin, 1887. ' See Chapter XX. 



* Hilgard, 189.-. 



