Factors

Hans Jenny suggested a slightly different way of considering the factors of soil formation and their effects, in his 1941 book "Factors of Soil Formation". Jenny's model (idea) is consistent with others in that it indicates five factors of soil formation: (1) climate (cl); (2) organisms (o); (3) topography (r); (4) parent material (p); and (5) time (t). Because the factors define the state and the history of soil systems, they are referred to as state factors, and the whole idea is called the state factor approach. Jenny considered that soil systems could be described mathematically by the following expression:

S = f (cl, o, r, p, t....)

where the....(dots) are additional unspecified factors that may be unique to a particular soil, S= a measureable soil property, and f means "is a factor of".

Later, Jenny suggests that the model is best used in a conceptual way to understand and study soil formation by considering that changes in the soil system that are the result of regular variations in one factor, with all other factors more or less constant. For example, the effect of topography can be evaluated by studying related groups of soils where topography varies, as in a hillslope, and the other variables (parent material, organisms, climate and time) are similar.

Climate as a soil-forming factor

• Water is the solvent, reaction environment, and transport medium for nearly all reactions/processes in soil.
• Temperature determines the rate of chemical reactions and the intensity of biological activity. Also freeze-thaw processes.
• Wind influences soil directly (erosion or deposition) and by influencing the effectiveness of precipitation.

Organisms/vegetation as a soil-forming factor

• Vegetation helps to hold parent material in place, allowing time for soil formation to occur. Plant roots bind soil particles together and increase the entry of water (infiltration) into the soils, reducing runoff and erosion.
• Plant roots growing in cracks and fissures break apart rocks, speeding up soil formation. Similarly, lichen on rock surfaces increases weathering.
• Plants produce weathering agents that increase rates of chemical weathering of soil minerals by releasing acidic components such as organic acids and carbon dioxide. The result is faster breakdown of the minerals and release of nutrients required by the plants and other biota.
• Vegetation is the initial source of the carbon fixed by photosynthesis that becomes organic matter in the soil. Plants that fix atmospheric nitrogen in symbiotic association with Rhizobacter bacteria are an important means by which nitrogen is added to the soil system.
• Vegetation modifies microclimates by: slowing wind speeds, shading the soil surface, and retaining snow, resulting in cooler and more moist soil environments, as well as less variation in soil-forming environments with topography.

Topography (relief) as a soil-forming factor

• The shape of the land surface influences the redistribution of the water received as precipitation. As a general rule land surfaces that are higher in landscapes, particularly sloping or convex surfaces lose water by runoff: and lower surfaces, particularly those that are concave or depressional receive extra water. The net result, is drier, less developed soils on the convex and sloping surfaces, and deeper, more strongly developed soil profiles in the more moist lower areas. Poorly drained or Gleysolic soils often occur where the amount of water received results in water ponded on the soil surface for a significant period of time.
• The climate becomes cooler, and more moist with increase in elevation, which coupled with related changes in vegetation results in regular changes in the soil. This is a good example of the interrelations among factors.
• In the northern hemisphere, slopes with south-facing aspects receive more solar radiation (insolation) than north-facing slopes, so south-facing slopes are warmer and less moist. The differences in soil temperature and soil moisture are not great (perhaps about 2 degrees cooler on average, and just a bit more soil moisture) but the net result over time is deeper, more strongly leached, more acidic soils on the north slopes, and drier, shallower and less well-developed profiles on the south-facing slope.

Parent material as a soil-forming factor

The initial stage of soil formation is the accumulation of the parent materials - the sediments or rocks in which the soils will form. The vast majority of the Canadian land mass was glaciated during the last glacial episode, and hence the majority of parent materials in Canada are of glacial origin and (by the standards of geological time) are relatively young. The nature and properties of the parent materials exert a very strong subsequent control on the pathways of soil genesis. The categories of major parent materials in Canada are shown in the table below. The initial stage of soil formation is the accumulation of the parent materials - the sediments or rocks in which the soils will form. The vast majority of the Canadian land mass was glaciated during the last glacial episode, and hence the majority of parent materials in Canada are of glacial origin and (by the standards of geological time) are relatively young. The nature and properties of the parent materials exert a very strong subsequent control on the pathways of soil genesis. The categories of major parent materials in Canada are shown in the table below.

Parent material influences:

• Soil texture, which influences the entry of water (infiltration) into the soil and its transmission in the soil are related to texture. The depth of leaching is related to the average depth to which water penetrates the soil. Other factors being equal, clayey soils take in water more slowly and, because the moisture storage capacity for a given depth is greater, are less leached, resulting in shallower soil profiles, whereas sandy soils are leached to greater depth
  • Clay content also affects the soil's ability to retain cations, or the cation exchange capacity (CEC) and organic matter content generally increases with clay content - due to the higher plant production on the more clayey soils, and the formation of clay humus complexes that stabilize humus and slow its decomposition.
• Soil mineralogy: minerals vary in their resistance to weathering, and therefore the degree to which elements are made soluble and soils change during soil formation. Some minerals are important stores of nutrients (such as phosphorus, potassium and calcium), which are released slowly as soils weather. Some minerals are characteristic of an Order (for example, the smectite content in Vertisolic soils).
• Buffering capacity, which is the ability of a soil to resist changes in pH. The content of calcium carbonate is important to buffering, in that CaCO_3 is able to neutralize soil acidity. Clay and organic matter contents are also important to buffering capacity.