lo] ENVIRONMENTAL FACTORS 303 



and breakdown of the dead plant and other remains, Earthworms 

 may be important in dragging down and partly digesting the material. 

 After this, soil Fungi and Bacteria cause further disintegration and 

 ultimately decomposition into more or less simple salts (such as 

 ones containing nitrogen and phosphorus which are essential to 

 plants), carbon dioxide, and water. These are the fundamental 

 plant foods, and the organic matter is the chief seat of the activities 

 of the microorganisms liberating (and sometimes producing) them. 

 Such decomposition and disappearance takes place relatively rapidly 

 in warm, moist, and well-aerated soils, these conditions being 

 favourable to the activity of the ' scavenging ' organisms. On the 

 other hand, in cold and wet soils that are poor in salts and acidic 

 in reaction, the debris may long remain on the surface as scarcely 

 decomposed ' raw humus ' {mor). 



Ordinary humus, having become structurally unrecognizable and 

 largely colloidal, improves both heavy clay and light sandy soils, 

 lightening the former and giving consistency and water-holding 

 capacity to the latter, as well as adding plant food in each case. The 

 * reaction ' of a soil — its degree of acidity or basicity — is also import- 

 ant to many plants and may affect their distribution, as particular 

 species are apt to show a distinct preference for soils whose reaction 

 lies within certain limits. This reaction again is largely bound up 

 with humus content, humus being the main source of acidity in 

 soils. 



(5) Living organisms, together with the roots and other under- 

 ground parts of aerial plants, comprise the other widely essential 

 constituent of the soil. They include, as we have already seen, 

 the mainly microscopic soil flora and fauna whose life is usually 

 centred on the soil's humus content and which are often very 

 sensitive to changes in the conditions of their limited environment. 

 They also include the main mixing and ' scavenging ' animals and 

 above all saprophvtic Fungi and Bacteria, etc., which are so vitally 

 important in the maintenance of ecological balance and indeed of 

 life in the world, f^or higher plants, as we have already noted, 

 these organisms make available various essential food substances, 

 including nitrogen in usable form. 



In highly acid soils. Fungi largely replace the decomposing and 

 other useful Bacteria — such replacement in itself involving significant 

 phytogeographical changes. Indeed these soil communities and 

 their component organisms have their distributions and other 

 geographical implications in much the same manner as higher types. 



