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Both plants and animals profoundly influence soil development. The plant kingdom consists of the macroflora (trees, shreds, and herbs) and microflora (bacteria and fungi).

Grasses and trees require somewhat different chemical substances for growth. Trees, particularly the conifers, use little calcium and magnesium. Hence they thrive well in the pedalfer soils from which these substances have been leached and which are usually acid. Grasses and small grains (wheat, oats, barley) need abundant calcium and magnesium and do well in the pedocal soils of the semi-arid and marginal lands. For grasses to grow well in acid pedalfers, calcium must be added to the soil in the form of lime or crushed limestone. Plants tend to maintain the fertility of soil by bringing the bases (calcium, magnesium, potassium) from lower layers of the soil into the plant stems and leaves, then releasing them to the soil surfaces as the plant decomposes.

Dead plants provide humus, the finely-divided organic matter of the soil. Humus gives a dark brown or black colour to the soil. Humus particles of colloidal size act in the same way as mineral colloids in holding ions in the soil. The process of humus development, or humification, is essentially the slow oxidation, or burning, of the vegetative matter. Acids, known as the organic acids, are formed during humification. They aid in decomposing the minerals of the parent soil material. The hydrogen ions of the acid solution tend to replace the ions of potassium, calcium, magnesium, and sodium, which are removed in the leaching process. Soils of the cold humid climates are therefore deficient in the bases and are consequently of low fertility for crop farming. The deficiency of nutrients is remedied by applications of fertilizers rich in nitrogen, phosphorus, and potassium.

Turning now to the microflora, or bacteria and fungi, we find that bacteria consume humus. In cold climates bacterial growth is slow, hence humus may accumulate on and in the soil. Soils of the sub arctic and tundra climates have much undecomposed organic matter, which locally forms layers of peat, but in humid tropical and equatorial climates, bacterial action is intense and most dead vegetation is rapidly oxidized by bacteria. Here humus content of the soil is low. The organic acids formed by humus are therefore also lacking, and certain bases such as aluminum, iron, and manganese accumulate in a large proportion relative to silica. In this way the fundamental differences in soils of cold and warm climates can be traced back to intensity of bacterial activity. Another function of some bacteria and other soil organisms (algae) is to take gaseous nitrogen from the air and convert it into a chemical form that can be used by plants. This process is known as nitrogen fixation .

The influence of animals in the soil is largely mechanical, but nevertheless important. Earthworms are a particularly important agent in humid regions. They not only continually rework the soil by burrowing, but also change the texture and chemical composition of the soil as it passes through their digestive systems. Ants and termites bring large quantities of soil from lower horizons to the surface. Such burrowing animals as prairie dogs, gophers, ground squirrels, moles, and field mice disturb and rearrange the soil. Digging of burrows brings soil of lower horizons to the surface; collapse of burrows carries surface soil into lower horizons.

The Pedogenic Regimes

To bring the study of soils into a more unified perspective we may concentrate upon several basic trends in soil development, each leading to formation of a distinctive major soil group under the control of a particular climatic regime. Such basic trends may be referred to as pedogenic regimes.

The regime of podzolization dominates in climates having sufficient cold to inhibit bacterial action, but sufficient moisture to permit larger green plants (macroflora) to thrive. Such conditions exist only in middle and high latitudes, and high altitudes. The corresponding climatic regime may be equable, provided that it is prevailingly cool (marine west coast climates, poleward of 40° latitude), or it may be a continental regime, which has cold winters and adequate precipitation distributed throughout the year (humid continental climate; continental sub arctic climate). In its extreme development podzolization is associated with coniferous trees (spruce, fir, hemlock, pine). These plants do not require the bases (calcium, magnesium, and potassium) and hence do not restore them to the soil surface.

The result is that humic acids, produced from the abundant leaf mold and humus, leach the upper soil strongly of bases, colloids, and the oxides of iron and aluminum leaving a characteristic ashgray A2 soil horizon composed largely of silica (SiO2). Colloids, humus, and oxides of iron carried out of the A2 horizon accumulate in the B-horizon, which may be dark in colour, dense in structure, and in some cases hardened to rocklike consistency (ortstein).

The pedogenic regime of laterization is in some respects a warm-climate relative of podzolization, in that both are associated with climatic regimes of ample precipitation and with forests. Laterization takes place in a warm climate having copious rainfall well distributed throughout the year (equatorial rainforest climate; tropical wet-dry climate with long wet season; humid subtropical climate). A high mean annual temperature and a lack of severe winter season permit sustained bacterial action, which destroys dead vegetation as rapidly as it is produced. Consequently little or no humus is found upon or in the soil. In the absence of humic acids the sesquioxides of iron (Fe₂O₃) are insoluble and accumulate in the soil as red clays, nodules, and rocklike layers (laterite). Silica, on the other hand, is leached out of the soil and disposed of eventually by stream flow in the process of desilication. No distinctive soil horizons are developed. In the absence of silicate colloids the soil tends to be firm and porous rather than sticky and plastic, and will transmit water readily. Laterization results in very low soil fertility because bases are not held in the soil and humus is lacking.

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