16 ECOLOGICAL RELATIONS OF SOIL' 



Consideration of soil as a habitat of animals 

 and plants brings together many of the 

 principles presented throughout this long 

 section, and helps serve as a transition to 

 the discussion of biotic aspects of the en- 

 vironment (p. 227) and to the considera- 

 tion of some phases of the biota itself (p. 

 225). Soil is a highly generalized name for 

 the shallow upper layers of the land surface 

 of the earth that by weathering of underly- 

 ing rocks, intimate association with organic 

 matter and with living organisms, has be- 

 come a suitable habitat for the root sys- 

 tems of plants and for many burrowing or 

 permanently indwelling animals. 



Soil science is a meeting ground for phys- 

 iography (and geology in general) and the 

 physics and chemistry of an involved col- 

 loidal system. Its relations are complicated 

 by constant ecological interactions with a 

 complex biota that forms a normal, integral 

 part of the soil. This soil complex in itself 

 is a bridge between the inorganic, organic, 

 and living worlds. It is a dynamic system 

 and is a unit of such inherent strength that 

 the artificial character of the dissection of 

 nonliving nature into the separate factors is 

 again strongly emphasized. 



SOIL CYCLES 



Soil pulsates with many rhythms ranging 

 from fairly simple daily changes in super- 

 ficial temperature, through the deeper, 

 slower-moving seasonal temperature varia- 

 tions, to longer temperature and rainfall 

 cycles (p. 85) and to geophysical-chemical 

 rhythms that may extend through geolosi- 

 cal epochs. A long-run calcium cycle will 

 illustrate the last. Calcium compounds are 



• It is difficult to summarize the role of soil 

 as an element in the environment of animals, 

 in part because so much is known on the sub- 

 ject. Of books devoted to soils, we have con- 

 sulted Lyon and Buckman (1927), Waksman 

 (1932), Paul Emerson (1930). Robinson 

 (1936). Russell, (1937), and Soils and Men. 

 the 1938 yearbook of agriculture of the U. S. 

 Department of Agriculture. We have profited 

 particularly from reading the treatment of soil 

 in Weaver and Clements (1929), in Newbigin 

 (1936), and Nikiforofi^s summarizing essay in 

 the Sis;ma Xi Quarterly for 1942. The in- 

 terested student of ecology will need to con- 

 sult these and many more to secure full in- 

 formation on the role of soil in ecology. 



weathered from underlying bedrock lime 

 stone and, either in situ or after transport 

 by glacier or travelling water, or both, be- 

 come a part of the soil. As such, they may 

 pass in and out of plant and animal bodies 

 again and again. Finally, the calcium is 

 carried to the sea, where, after possible 

 exposure to other biological cycles, it is 

 fixed bv marine animals, perhaps as coral 

 reef or bottom deposit. In either case it mav 

 again be consolidated to limestone and re- 

 main as such until diastrophism elevates the 

 shallow sea bottom and starts the calcium 

 on another geological cycle. Carbon, nitro- 

 gen, phosphorus, and silicon, to name no 

 more, may also exhibit these long geologi- 

 cal soil cycles as well as shorter ones, and 

 all those mentioned here exhibit biological 

 phases in which the different elements are 

 intimately bound in with living organisms 

 Cycles of abundance of the more impor 

 tant plant-nutrient salts occur in both sea 

 and soil. The cycles of carbon, nitrogen, 

 phosphorus, and sulfur are primarily bio- 

 chemical and show decided similarities in 

 the hydrosphere and pedosphere (the soil) 

 despite the physical diflFerence between 

 these two great storehouses of the non- 

 living environment. Cycles of hydrolysate 

 elements, like iron and manganese, are less 

 easily compared. These substances are not 

 very soluble in water, and plants can secure 

 them more readilv from the soil than from 

 sea water. The alkalies— sodium, potassium 

 rubidium, and caesium— and the alkali 

 earths— magnesium, calcium, strontium, and 

 barium— show highly individual diflFerences 

 in their behavior in the ocean as compared 

 with that in the soil. There is a tendency 

 toward a reciorocity of behavior in these 

 two media, shown especially by calcium 

 and barium in one grour) and by sodium 

 and potassium in the other. The recipro- 

 city is related to the relative solubility of 

 the substances in water and their relative 

 enerev of adsorption, or other fixation, on 

 soil colloids. These relations are outlined at 

 greater length by Hutchinson (1943, p. 388) . 



SOIL FORMATION 



Soil-forming (pedogenic) processes are 

 initiated and continued primarily by energy 

 from the sun and secondarily by the poten- 

 tial energy bound up in crystals, molecules. 



216 



