224 



ANALYSIS OF THE ENVIRONMENT 



may be designated as the principle of the 

 partial equivalence of different ecological 

 factors. A physically light soil may be equiv- 

 alent to a clay soil that contains lime. Dry 

 limestone hills in central Europe have, on 

 their south slopes, a biota characteristic of 

 the Mediterranean region. The warm, dry 

 soil produces edaphically a warmer southern 

 microclimate. Conversely, far out into the 

 North African desert, the vegetation along 

 water courses is aflFected by the coolness 

 and moisture in the soil and keeps some- 

 thing of the character of a northern mid- 

 European deciduous forest of poplars. 



Sandy soils in humid climates may com- 

 pensate for dryness, and in more arid cli- 

 mates may have the reverse effect. Aridity 

 is introduced into many moist climates by 

 sand dunes. On the other hand, sandy soils 

 have a lowered wilting coefficient, and more 

 of the soil water present is available for use. 

 In the Great Plains of the United States, an 

 annual precipitation of 40 cm. allows 

 the gramma grass community (Boutelona 

 oli^ostachtfa) to grow, but not the bunch 

 grass community (Andropos,on scoparius) . 

 Under ordinary conditions, bunch grass re- 

 quires some 50 to 60 cm. of rainfall. In 

 regions where denser soil is replaced in part 

 by sand, bunch grass grows even when the 

 rainfall does not exceed 40 cm. 



Deep soil retains its water supply; its 

 temperature varies less, and even in mid- 

 continent the climate approaches "oceanity." 

 Contrariwise, light soils tend to produce 

 features characteristic of a continental cli- 

 mate, even when they occur near the sea- 

 shore. 



Manure replaces not only most of the 

 natural soil nutrients, but also, to some ex- 

 tent, water. A properly manured meadow, 

 even though relatively dry, supports a 

 vegetation resembling that of a humid un- 

 manured grassland. Human and other ani- 

 mal activities may replace certain environ- 

 mental factors. A cool, moist climate favors 

 the production of mountain meadows above 

 the timberline, an effect that can be pro- 

 duced by tooth, axe, or sickle under various 

 conditions. The burrowing of many animals 

 —ants, earthworms, or rodents— can be the 

 equivalent of lime in producing a lighter, 

 more porous soil. 



The action of the principle of partial 

 equivalence modifies Liebig's fundamental 

 'law of the minimum" (p. 198), since there 

 is always the possibility that a single factor 



present in minimal concentration in labora- 

 tory cultures may, in nature, be partially 

 replaced by some other available influence 

 or influences. The same end result may be 

 reached by different routes, some of which 

 may allow the by-passing of a factor pres- 

 ent in subminimal quantites; 2 + 2 -f 2 -f 

 2 and -f 1 -f- 2 + 5 both give the sum 8 

 (Riibel, 1935), figuratively as well as 

 literally. 



HUMUS 



Organic matter in soil is mainly amor- 

 phous, dark-colored material (pp. 218, 

 225). It develops from the decay of vege- 

 tation and of animals and the products of 

 both. The profile of organic matter is much 

 affected by the surface and in-soil biota. 

 There is usually an accumulation of humus 

 in the upper soil that is carried into deeper 

 horizons by burrowing earthworms, rodents, 

 and other animals, and in a different way 

 by the root system of plants. 



Decay of organic matter may continue in 

 the soil until oxidation leaves only water 

 and carbon dioxide. Often such destructive 

 processes do not proceed to completion 

 and dark, amorphous, relatively resistant 

 humus remains. This arises (1) by anaer- 

 obic humification, as in water-logged soils, 

 and, under extreme conditions, results in 

 peat formation. (2) Acid humification takes 

 place in dry or moist soils in the absence 

 of calcium and other bases; it may occur in 

 the presence of good aeration and yields 

 acid peat as an end product. (3) Forest 

 and prairie soil humification is more com- 

 plicated. It is affected by moisture, as illus- 

 trated by the decreasing amount of humus 

 deposited as forest conditions become drier. 

 With still less moisture, when grasslands 

 form, there is a marked increase in humus 

 formation that reaches a peak under condi- 

 tions of greater aridity than those found at 

 the forest-grassland margin. The nature of 

 the processes involved are not yet under- 

 stood, but they are thought to be associated 

 with summer drought and its effect on the 

 microbiology of the soil. With still greater 

 aridity, humus formation declines, in part 

 because of the decreased amount of source 

 material. 



Peat formation, favored bv humid cli- 

 mates, may result in a soil that is almost 

 entirelv composed of organic matter. Usu- 

 ally, even grassland soils are 85 per cent or 

 more inorganic and only 15 per cent or 



