and porous. Loam is a mixture of sand and clay and 

 makes the best soil. The presence of iron oxides and 

 silicates produces the red and yellow colors of some 

 soils. Humus produces black soils. Soil from swamps 

 or bogs and very rich in organic material is called 

 muck. 



Residual soils are formed in situ from under- 

 lying bedrock. Soils may, however, be formed in one 

 locality and moved considerable distances. Soils trans- 

 ported and deposited by wind are called loess; by 

 water, alluvium; by glaciers, ////. 



Biota 



Plants and animals have a highly important role 

 in the formation of soil, both as they affect its struc- 

 ture (Jacot 1936, 1940) and as they aid in the pro- 

 duction of humus. Plants contribute to the mechani- 

 cal and chemical weathering of rock. Plant roots, 

 especially those of trees, can split large rocks. Lichens, 

 mosses, and even bacteria and fungi excrete acids, in 

 the course of metabolism, which dissolve the sub- 

 stances that cement rock granules together. When 

 plant roots die, fungi convert them to dry, soft, 

 spongy material (punk), used as food by saprophytic 

 micro-arthropods. Usually the bark of the root re- 

 mains intact the longest. Hollow tubes are thus 

 formed that permit water and air to penetrate con- 

 siderable depths into the soil. These channels gradu- 

 ally become filled with silt and animal excreta. 



The addition of plant and animal organic matter to 

 heavy compact soils or clay tends to open the soil, 

 making it more porous. Addition of organic matter 

 to sandy soils binds the particles closer together, 

 making the soil less porous. 



Earthworms may be divided into deep- and shal- 

 low-working species (Kevan 1955). Deep-working 

 species dig narrow tube-like channels which may reach 

 2-3 m down through overlying soil to parent rock. 

 Earthworms ingest soil while burrowing, digest and 

 absorb organic matter from it, and egest the residue 

 in a semi-liquid form which is used to cement the 

 walls of the burrow or else is deposited at the surface 

 as castings. Earthworms prefer easily digested suc- 

 culent vegetation and dung for the purpose, but in the 

 autumn may pull the freshly fallen leaves down into 

 their burrows to use as food or nest linings. Ejected 

 petioles may form midden piles around burrow en- 

 trances. In an undisturbed virgin prairie in Texas, 

 earthworm casts made a layer 2-3 mm thick over the 

 entire ground surface and when air-dried weighed 

 about 2400 g/m- (10.7 tons/acre) (Dyksterhuis and 

 Schmutz 1947). Earthworms are not, however, im- 

 portant soil builders in disturbed grassland ; they may 

 be absent altogether in arid regions. In other studies 



(Evans and Guild 1947), the dry weight of casts 

 brought to the surface annually by earthworms varied 

 from 475 g/m^ (2.1 tons/acre) in a moderately hot 

 dry climate, to 24,000 g/m^ (107 tons/acre) in the 

 White River valley of the Sudan, during the rainy 

 season. Earthworm casts compared with the sur- 

 rounding soil show higher total nitrogen, organic 

 carbon, exchangeable calcium, exchangeable magne- 

 sium, available phosphorus, exchangeable potassium, 

 organic matter, base capacity, pH, and moisture 

 equivalent (Lunt and Jacobson 1944). Only certain 

 species make these surface castings ; other species void 

 the ingested soil into subterranean spaces. 



The ant Lasiiis niger neonigcr spends most of its 

 time in its underground burrows and deposits ex- 

 cavated soil upon the ground surface around burrow 

 entrances. In an old field community in Michigan 

 such deposits amounted, at one sampling, to 85.5 

 g/m^ (750 lb/acre). However, entrances are aban- 

 doned and new ones made, so that in the course of a 

 few weeks a much larger quantity of soil is brought 

 up (Talbot 1953). 



In the semi-arid Great Plains of western North 

 America there is at least one species of ant that ex- 

 cavates extensively underground and builds a conical 

 mound of this excavated material above the surface. 

 A single such mound weighs approximately 77 kg 

 (170 lb) : there are as many as 50 such mounds per 

 hectare (20 per acre) in some localities. Plainly, 

 these little excavators move prodigious amounts of 

 soil. The relatively sterile subsoil is gradually mixed 

 with organic material and spread over the surface of 

 the ground, thus increasing the depth of the fertile 

 top-soil. Scarabeid beetles, bees, wasps, and in tropi- 

 cal regions mound-building termites also move con- 

 siderable subsoil to the surface (Thorp 1949). 



The crayfish Cambartis dio genes often occurs in 

 poorly drained fields ; it burrows down to the water 

 table, sometimes a depth of three meters. Excavated 

 material is brought to the surface and built into chim- 

 ney-like affairs which may be 20 cm high and almost 

 that much in diameter. Where crayfish are abundant, 

 as much as 600 to 2000 g/m^ (2.7 to 8.9 tons/acre) of 

 soil per year may thus be moved (Thorp 1949). 



The burrows of prairie dogs and badgers may ex- 

 tend 2 to 3 meters below the surface, and a single 

 mound of excavated dirt weigh from 100 to 10,000 kg. 

 Mounds made by pocket gophers and ground squirrels 

 weigh from 7 to 180 kg each ; it is not unusual to find 

 42 such mounds per hectare (17 per acre). These 

 animals thus move from 7 to 9 kg of subsoil for each 

 square meter of surface (30^0 tons/acre) in a period 

 of several months (Taylor 1935, Thorp 1949). 



Large terrestrial animals trample the soil into 

 greater compaction and destroy vegetation at sites 

 where numerous individuals foregather ; around water 

 holes in grassland where bison and antelope come to 



164 Ecological processes and dynamics 



