What are five stages of soil formation?

New Zealand has a wide variety of landscapes. A day’s sightseeing journey might include volcanic plateaus, rolling hills, river basins and coastal lands. It is easy to notice how the scenery changes from one place to the next, but the differences underground are not so obvious. As the landscapes change, so do the types of soils underlying them. These changes in soils are a result of how they were formed. The five factors that influence soil formation are parent material, climate, living organisms, topography and time.

Parent material

The rock from which soil is formed is called parent material. This can come from weathered rock, volcanic ash or sediments moved and deposited by water, wind or glaciers. For example, the central North Island has soils that began as pumice from volcanic eruptions. The Waikato basin is an old flood plain of water-deposited volcanic debris. Soils in the Canterbury Plains have their origins in residue deposited by glaciers – the residue washed out into river systems and was deposited in the low, relatively flat landscape.

Climate

Climate (temperature and rainfall) plays a large part in the weathering (breaking down) of parent materials.

Temperature effects:

• Rocks expand and contract as they heat up or cool, breaking them apart.
• Temperature controls the rates of chemical weathering (when water interacts with minerals in the rocks to create chemical reactions). Chemical weathering happens much faster in warm places like Northland than it does further south.
• Warmer temperatures may also mean more plant growth, soil organisms and litter decomposition.

Rain effects:

• Increased moisture means more plant growth.
• Rainwater washes materials off slopes.
• Rain dissolves minerals and leaches them deeper into the soil.

Living organisms

Living things influence soil formation in many ways. Plants, microorganisms, animals and even humans can make a difference. Once a plant community becomes established, it has a big effect on soil development. Tree roots penetrate deeply into soils, bringing up minerals and incorporating them into organic matter. Grasses penetrate less deeply but have increased biological activity and more rapid nutrient cycling.

New Zealand’s kauri trees have had a unique impact on soil formation. Kauri are large, long-lived trees. Over hundreds of years, rain dissolves strong acids from kauri leaf litter and leaches it underground. The acids strip aluminium and iron from the upper part of the soil, leaving a distinct white layer. The aluminium, iron and organic matter create a ginger-brown layer below. These soils look amazing but have low fertility and are very acidic.

Earthworms and other animals tunnel through and mix the soil. They aerate the soil and allow water to penetrate more deeply. Humans also influence soil formation. For example, Māori modified the soil in their kūmara gardens by adding sand and gravel to improve the drainage.

Topography

Topography refers to the lay of the land – the elevation and steepness of hills. Steep slopes can be more easily eroded, and rainfall tends to run off rather than infiltrate the soil.

Topography also influences climate. The South Island has a wide variety of soils due to the influence of the Southern Alps. Soil formation and the vegetation the soils support differ greatly on the very wet western slopes through to the dry basins of Canterbury and Central Otago.

Time

It takes a long time for soil to develop. For example, once rock is exposed in a warm, humid climate, it may take a hundred years for mosses and lichens to take hold. They trap dust and organic matter and break down the rock. Within a few hundred years, grasses and shrubs become established. Roots begin to penetrate the rocks and accelerate physical and chemical weathering. Over thousands of years, climate, organisms and topography influence how parent materials are turned into soils.

Soil formation and land use

Soil formation plays a part in agricultural land use. Fertilisers may be needed to supplement pumice soils low in certain minerals – like cobalt, selenium and copper – to ensure animal health. Soils in the Pukekohe and Gisborne Plains are ideal for market gardening because of their fertility and structure but must be protected from erosion. Indeed, all soils should be looked after. They take a very long time to form and are the basis of much of the life on Earth.

New Zealand Soil Classification System

Soil scientists use the New Zealand Soil Classification System to identify specific soils. Soils are classified according to their physical, chemical and morphological characteristics (which refer to the soils' appearance). Find out more about soil naming systems in the article Soil names.

When scientists develop explanations about the world, they share data, information and explanations about phenomena they are trying to explain. Because scientists are communicating with other scientists, they need a common vocabulary. Soil scientists have developed a universal soil classification system, one outcome of which is the construction of soil maps used to aid with land use, planning and environmental protection.

Visit Landcare Research’s S-map Online – a digital soil map for New Zealand.

Soil Formation

The transformation of rocks into soil may be termed as soil formation. It starts with the weathering of rocks. The weathering processes are primarily destructive in nature. It helps to change the consolidated rocks into unconsolidated material. The soil forming processes are constructive and result in a soil profile that has been developed from the surface few feet of parent material.

Five factors, at any specific location on the surface of the earth, act simultaneously to produce soil. They are (1) Climate (2) Parent material (3) Relief /Topography (4) Biosphere (Vegetation, organism, man, etc.) (5) Time. Their relationship to the soil properties has been expressed by Jenny in the following equation.

s = f (cl, b, r, p, t)

Where, ‘s’ = Any soil property e.g. clay content. ‘f’ = function of or dependent upon ‘cl’ = climate, ‘b’ = Biosphere, ‘r’ = relief, ‘p’ = parent material, ‘t’ = time (age)

Therefore, any soil property is the function of collective efforts of all the soil forming factors.

i. It primarily regulates the moisture air regime of the soil and determines the leading trends in soil profile depending upon available percolating water.

ii. Rainfall also affects profile development through erosion producing thin soils on steep slopes and deposition of soil material down hill.

iii. The intensity, frequency and distribution of precipitation influence the course of soil formation.

iv. With increasing moisture, nitrogen and carbon content, clay content, aggregation, saturation capacity and exchangeable hydrogen tend to increase.

v. Exchangeable base and pH value tends to decrease with increasing moisture.

b. Temperature: Temperature affects the velocity of chemical reactions, which approximately doubles for every 100C increase in temperature. It influences the organic matter decomposition and microbial activities in soil though the evapotranspiration phenomenon. Temperature also determines the efficiency of rainfall. In general, with increase in temperature the depth of weathering and clay content show increase on the contrary nitrogen, organic matter, silica, alumina and base alumina ratio tends to decrease with rising temperature.

2. Parent material and soil formation: Rock on the surface of earth is weathered until the essential elements become available to support lichen and other lower forms of plant life. As continuing generations of lichens grows, die and decay, they leave increasing amounts of organic matter. Organic acid further has tendency to decay the rock. Different parent material affect profile development and produce different soil especially in initial stages. The nature of the elements released during decaying of rocks has a specific role in soil formation.

3. Relief / Topography and soil formation: The prominent types of topography designations as given in FAO guidelines are:

i. Flat or almost flat: Land surface with slope less than 2%

ii. Undulating: Land surface with slope between 2-8%

iii. Rolling: Land surface with slope between 8-16%

iv. Hilly: Land surface with slope between 16-30%

v. Steepy dissected: Land surface with slope greater than 30%

a. Soil formation on slopy land: The soils on steep slopes are generally shallow, storey and have weakly developed profiles with less distinct horizonation. It is due to

i. Accelerated erosion, which removes surface material before it has time to develop.

ii. The reduced percolation of water through the soil because of surface runoff.

iii. Lack of water for the growth of plants, which are responsible for checking erosion and promoting soil formation.

b. Soil formation on level topographic position: On level topographic position, almost then entire water received through the rain percolates through the soil. Under such condition the soil formed may be considered as representative of the regional climate. They have normal solum with distinct horizons.

4. Biosphere and soil formation: Vegetation, microbes, animals and man all greatly influence the soil formation processes. Vegetation exerts its main influence on soil formation through the amount and nature of O.M. that it adds to the soil. Soil developed under biosphere of forest vegetation have more horizon a more highly leached. Vegetation also aids in control of erosion. Burrowing animals causes constant mixing within soil. Man through his land use cause both beneficial and harmful effect on soil.

5. Time and soil formation: The length of time required for a soil to develop horizons depends upon many unrelated factors such as climate, nature of the parent material, burrowing animal and relief. It has been seen that rocks and mineral disintegrate and /or decomposes at different rates, the coarse particle of limestone are more resistant to disintegration than those of sandstone. There are five stages of weathering that are dependent on minerological features of soil.

Weathering stages in soil formation:

1. Initial – Unweathered parent material.

2. Juvenile – Weathering started but much of the original material still Unweathered.

3. Virile – Easily weatherable mineral fairly decomposes, clay content has increased.

4. Sensile – Decomposition reaches at final decomposition stage only most resistance minerals survive.

5. Final – Soil development completed under prevailing conditions.

Processes of soil formation: The basic processes involved in soil formation are

1. Gains or addition of water, organic and inorganic minerals to the soil.

2. Losses of the above material from the soil.

3. Transformation of mineral and organic substances with the soil

4. Translocation or movement of soil material divided into

i. Movement in solution (leaching).

ii. Movement in suspension (eluviation) of clay, organic matter etc.

A. Fundamental processes of soil formation:

1. Humification: Helps in formation of surface layer, called Ao horizon. Its characteristics depends upon the nature of vegetational residue and the way it becomes decomposes and synthesized into new organic compounds. The percolating water passes through this humus layer dissolves certain organic acids affect the development of lower A and B.

2. Eluviation: Elevation means washing out. It is a process of removal of constituents in suspension or solution by the percolating water from the upper to the lower layer. Mechanical eluviation removes finer suspended fraction of soils, producing textural profiles by a coarse texture.

3. Illuviation: The process of deposition of soil material (Removed from the eluvial horizon) in the lower layer is known as illuviation. The horizons formed by this process are termed illuvial horizons.

B. Specific pedogenic processes:

1. Calcification: It is the process of precipitation and accumulation of calcium carbonate (CaCO3) in some part of the profile. The accumulation of CaCO3 may result in the development of acidic soil. Such soil belonging to group called pedacal.

2. Decalcification: It is the reverse of classification i.e. the process of removal of CaCO3 or Ca ions from soil by leaching.

3. Podsolization: Podzolization is negative of calcification where as calcification tends to concentrate calcium in the lower part ‘B’ horizon podsolization reaches the entire solum. Apart from the calcium the other bases are also removed and the whole soil becomes distinctly acidic. Process is mainly acid leaching.

4. Laterization: In this process, silica is removed while iron and alumina remain behind in the upper surface / layers and usually there are no well-defined horizon. Laterization is favoured by rapid decomposition of parent rocks under climates with high temperature and sufficient moisture for intense leaching. Podsolization and latrization produce soils that belong to the pedalfer (iron accumulating) group.

5. Gleization: It is a process of soil formation resulting in the development of glei (or gley horizon) in the lower part of soil profile above the parent material due to poor drainage condition and where water logged conditions prevail such soils are called hydromorphic soils. Gleying may be observed at any depth depending on the depth of ground water.

6. Pedoturbation: Inversion of soil takes place in deep black cotton soils which contain montmorillonite clay colloid (vertisol) has max swelling shrinkage capacity. As alternate weting and drying, expansion and contraction takes place due to which cracks is formed. Due to crack formation A horizon goes down and soil B horizon comes up. These are also known as self ploughed soils. Therefore in dry farming technique ploughing once in three years is recommended to conserve the soil moisture and since the inversion of soil takes place naturally there is no need of ploughing every year.

7. Salinization: Accumulation of soluble salts under dry climate or no rainfall. Salts accumulate on soil surface. Na+, K++, Ca++, Mg++. Soil becomes saline due to high rate of evaporation, reclaimed by leaching, flooding. (pH8.5) Grow salt tolerant crops, shevri, Dhaincha.

8. Alkalinization: Accumulation of sodium salts only like Na2Co3 and Na2 (CO3)2.