(Iritik, for instance, or winter yards of deer and moose, 

 trails on hillsides, wallowing places, and so on. These 

 reactions are usually very local, however. 



As animals burrow and bring large ([uantities of 

 loose soil to ground surface exposure, the likelihood 

 of water and wind erosion destruction is greatly in- 

 creased, especially true if the burrowing is done on 

 liillsides where the How of water is faster and where 

 the animals always tend to deposit the soil on the 

 downslope side of burrow entrances. On the other 

 hand, the very same activities may decrease erosion 

 where a soil is, in conseiiuence, made more porous so 

 that there is less water runoff. 



Hur 



In soil, organic matter that is partly or entirely 

 decom|)osed is called humus. The amount of humus 

 varies from less than one per cent to as much as 20 

 per cent of the soil ; peat soil may be largely organic 

 material, but much of it resists decomposition, and 

 hence is not true humus. Decomposition breaks down 

 complex organic compounds into simpler ones that 

 are washed back into the soil, thus becoming available 

 again as nutrients. On virgin prairie in Texas 

 the ground litter of dead grasses and herbs amounted 

 to over 300 g/m- when measured in April ( Dykster- 

 huis and Schmutz 1947). The annual dry weight of 

 leaves that fall to the ground in deciduous and conifer- 

 ous forests varies from year to year, from site to site, 

 and with the density of the trees, but is in the range 

 of 100 to 900 g/m-. In mature climax forests the rate 

 of decomposition of the litter and re-absorption by 

 plants of the nutrients thus yielded keeps pace with 

 the annual accumulation so that an equilibrium is 

 established. In serai stages, decomposition and utiliza- 

 tion do not keep up wath the annual accumulation so 

 that the organic content of the forest floor increases 

 with time. Under spruce, sugar maple, and birch in 

 Xew Hampshire, the organic ground matter equals 

 3 kg/m-, but in Florida where high temperatures and 

 rainfall favor rapid decomposition and leaching, there 

 may be only 0.4 kg/m- under old growth longleaf 

 pine (Kittredge 1948, Ovington 1954). 



The thick organic layer on the ground moderates 

 extremes in the daily and seasonal rhythms of soil 

 temperature, retards freezing of the ground in the 

 autumn and thawing in the spring, and retains soil 

 moisture. Because of humus formation (involving 

 oxidation ) and the respiration of plant parts and ani- 

 mals underground, soil air contains little oxygen but 

 much carbon dioxide, and it possesses a higher mois- 

 ture content than does the general atmosphere above 

 ground. This is especially marked in warm summer 

 months when these processes go on more rapidly. 

 The decay of organic matter usually makes the top 



soil somewhat acid (most commonly \)\\ 5 to 7), but 

 in the mineral subsoil, the acids are often neutralized 

 by the basic salts commonly present. 



The mineral content of leaf fall varies according 

 to the species of tree, but in the northern United 

 -States it averages about as follows (Chandler 1941, 

 1944) : 



Silicon, copper, manganese, carbon, and zinc are al.so 

 present in the leaves of hardwood trees. Carbon is 

 relatively more abundant and nitrogen less abundant 

 in coniferous than in deciduous leaves ; commonly the 

 carbon/nitrogen ratio is 55:20 (Ovington 1954). 



Both plants and animals are important agents 

 effecting the decomposition of organic matter and the 

 formation of humus. An animal digests and metab- 

 olizes plant foods, the total quantity of which is re- 

 turned to tile soil, in part as the excreta of the living 

 animal, the rest as the body of the dead animal. Fully 

 formed humus is, in fact, derived mostly of fecal ma- 

 terial. The larger herbivorous and carnivorous ani- 

 mals pass urine and feces containing simple nitrog- 

 enous compounds and compounds of phosphorus, 

 potassium, and traces of calcium, magnesium, sulfur, 

 and other elements. Humus is but one point in a con- 

 tinuous cycle of decomposition of plant and animal 

 organic matter, absorption of decomposition products 

 by plants, ingestion and metabolization of plant mat- 

 ter by animals, decomposition of plant and animal or- 

 ganic matter, — ad infinitum. The consumption by 

 saprovores and herbivores of living and dead plant 

 matter and the consumption of herbivores by carni- 

 vores, neither add nor subtract from the total nutrient 

 supply of an ecosystem. The chemical elements avail- 

 able in the air, water, and soil of an ecosystem pass, 

 in one compound and another, from one organism to 

 another, and through one stage in the cycle after 

 another, and they continue thus to circulate within 

 the ecosystem unless and until they are physically 

 withdrawn from it. To remove plant and animal 

 crops from an ecosystem is to withdraw nutrients 

 from it, and thus to reduce the fertility of the system. 

 Fertility can then be maintained only if the nutrient 

 supply is kept replenished by artificial fertilization. 



Kangaroo rats defecate promiscuously throughout 

 their underground burrow systems. The soluble ni- 

 trate content of the soil in the region of one burrow 

 system averaged 221 ppm and in another one 570 

 ppm, compared with a maximum of 15 ppm in the 



Reactions, soil formation, and cycles 165 



