Soil Formation

Soil is something you touch, feel, and play in every day, yet describing it precisely is surprisingly hard. It is a dynamic medium — a living, breathing body in which chemical, physical, and biological activities go on constantly. Soil is at once a result of decay and the medium for growth. It is a changing, developing body whose characteristics fluctuate with the seasons: it may be alternately cold and warm, dry and moist. When soil becomes too cold or too dry, biological activity slows down or stops altogether. Organic matter increases when leaves fall or grasses die.

Process of Soil Formation

Soil formation, or pedogenesis, depends first on weathering. The weathering mantle — the depth of weathered material — is the basic input from which soil forms. The process unfolds in stages:

• The weathered material or transported deposits are first colonised by bacteria and inferior plant bodies like mosses and lichens.
• Several minor organisms may take shelter within the mantle and deposits.
• The dead remains of organisms and plants help in the accumulation of humus.
• Minor grasses and ferns may grow; later, bushes and trees start growing from seeds brought in by birds and wind.
• Plant roots penetrate downward, burrowing animals bring particles up, and the mass of material becomes porous and sponge-like — capable of retaining water and permitting the passage of air.
• Finally, a mature soil forms: a complex mixture of mineral and organic products.

Factors Controlling Soil Formation

The book identifies five controlling factors: parent material, topography, climate, biological activity, and time. Of these, time, topography, and parent material are considered passive control factors, while climate and biological activity are active factors.

Parent Material

Parent material determines the colour, texture, chemical properties, and mineral content of the soil. It also influences the rate of weathering — some parent materials weather faster than others. The texture of the parent material affects the percolation of water and the depth of the soil profile.

Topography

Topography influences soil formation through the altitude and slope of the land. Soils on steep slopes are generally shallow and poorly developed, since materials keep moving downslope and never accumulate long enough to develop a deep profile. In low-lying flat areas, drainage is poor and waterlogging is common, which slows down the decomposition of organic matter. Such conditions are very favourable for the accumulation of organic matter. Soils over flat areas may develop a thick layer of clay with good accumulation of organic matter, giving the soil a dark colour.

Climate

Climate is an important active factor in soil formation. The climatic elements involved are:

• Moisture — in terms of the intensity, frequency, and duration of precipitation, evaporation, and humidity.
• Temperature — in terms of seasonal and diurnal variations.

Precipitation gives soil its moisture content, which makes chemical and biological activities possible. Excess water helps in the downward transportation of soil components through the soil — a process called eluviation — and deposits them down below, called illuviation.

In wet equatorial rainy areas with high rainfall, not only calcium, sodium, magnesium, and potassium but also a major part of silica is removed from the soil. The removal of silica from the soil is known as desilication.

In dry climates, high temperature makes evaporation exceed precipitation. Groundwater is brought up to the surface by capillary action, and as the water evaporates, it leaves behind salts in the soil. These salts form into a crust known as hardpans. In tropical climates and in areas with intermediate precipitation conditions, calcium carbonate nodules, called kanker, are formed.

Temperature acts in two ways — increasing or reducing chemical and biological activity. Chemical activity increases at higher temperatures, reduces at cooler temperatures (with the exception of carbonation), and stops in freezing conditions. That is why tropical soils with higher temperatures show deeper profiles, while soils in frozen tundra regions contain largely mechanically broken materials.

Biological Activity

The vegetative cover and organisms that occupy the parent materials from the beginning, and also at later stages, help in adding organic matter, moisture retention, and nitrogen. Dead plants provide humus, the finely divided organic matter of the soil. Some organic acids formed during humification aid in decomposing the minerals of the soil parent materials.

The intensity of bacterial activity shows up differences between soils of cold and warm climates:

• Cold climates: Humus accumulates because bacterial growth is slow. With undecomposed organic matter due to low bacterial activity, layers of peat develop in sub-arctic and tundra climates.
• Humid tropical and equatorial climates: Bacterial growth and action are intense, and dead vegetation is rapidly oxidised, leaving very low humus content in the soil.

Bacteria and other soil organisms take gaseous nitrogen from the air and convert it into a chemical form usable by plants. This process is known as nitrogen fixation. Rhizobium, a type of bacteria, lives in the root nodules of leguminous plants and fixes nitrogen beneficial to the host plant.

The influence of large animals like ants, termites, earthworms, and rodents is mechanical, but it is nevertheless important in soil formation, as they rework the soil up and down. In the case of earthworms, as they feed on soil, the texture and chemistry of the soil that comes out of their body changes.

Time …