The diverse elements composing soil include minerals, organic matter, microorganisms, and water content. Each of these factors plays a crucial role in soil fertility and ecosystem health. Understanding what are the components of soil aids in agricultural practices, environmental management, and ecosystem conservation.
What are the Components of Soil?
The chief components of soil are as follows.
- Mineral matter
- Organic matter
- Water
- Air and temperature
- Microorganisms
The typical soil has approximately 5% organic matter, 45% mineral, 20-30% air, and 20-30% water.
Mineral Matter of Soil
This is derived from rock, sand, gravel, and clay. The particles formed directly from the rocks are called primary minerals. Mineral components formed due to the weathering of primary particles are called secondary minerals. Sand and soil are examples of primary minerals and silica and clay are examples of secondary minerals.
The soil that contains more sand is called sandy soil, where the water-holding capacity is less. If clay predominates, the soil will be clayey soil that can retain water within it.
The particles of clayey soil are very small and swell on hydration. The shrinkage is followed by the root system of plants. The aeration is maximum in sandy soil while it is minimal in clayey soil. The clay particles in soil form the small particles in soil called colloidal particles which are usually negatively charged hence they bind on the positively charged ions.
The sandy and clayey soils are very different in their texture and fertility, so the plants growing in them are greatly varied.
A soil in which the sand, silt, and clay particles are more or less the same amount is called loam soil. Loam soil is well-suited for plant growth. The water holding capacity and soil aeration are moderate in this soil.
Humus or Organic Matter of Soil
The organic matter present in the soil, derived from the decomposition of dead remains of plants and animals is called humus. The putrefying bacteria, fungi, and protozoa bring about this decomposition.
The humus is dark in colour and amorphous in nature and the chemical constitution always changes depending on the nature of organic matter. But usually, it contains nitrogen, calcium, magnesium, iron, etc.
Humus is always seen mixed with other components such as clay, sand, etc. This improves the texture of the soil. It binds the sand and clay particles to form aggregates of soil particles. This makes the soil porous. Hence the soil is well aerated and percolation of water becomes possible.
The humus acts as a colloidal particle and hence together with clay forms the colloidal complex of the soil. Together with sand, hummus can increase the water-holding capacity of the sand.
When humus is mixed with clay, it reduces the water-holding capacity of clay so that the soil can hold a moderate amount of water. Thus humus is predominant in soil and it not only makes the soil nutritious but also porous, aerated, moderately moist, etc.
Soil Water
Water forms an important factor in the soil. In ordinary garden soil, it occupies 25% of water. Rain forms the chief source of water for the soil.
- During heavy rain, a major portion of water flows through the surface of the soil and is not available for plants. This is called the runaway water.
- Some of the water percolates down through the pores of the soil. Since this water is under the influence of gravitational force it is called gravitational water which is not available to the plants.
- The small portion of water that is retained in the space between soil particles on the upper surface is called capillary water. This is the main source of water for plants.
- When the atmosphere is dry, plants absorb small quantities of moisture from the atmosphere since roots cannot absorb any water.
- Certain water molecules are held strongly between the soil particles called hygroscopic water. Hygroscopic and capillary water together form the maximum water-holding capacity of the soil.
Soil Air
An important factor for the respiration of roots and other underground organs and also of microorganisms is the soil air. Since most types of soil are porous, air is always present unless these pores get water-logged. The more the size of the soil particles, the better the aeration.
Due to the continued respiration of fauna and flora of the soil, the soil air contains greater carbon dioxide and lesser oxygen, unlike the atmospheric air. The oxygen content of the soil is also necessary for humus formation and nitrogen fixation by nitrifying bacteria.
Oxygen is used by earthworms and other soil organisms which helps soil fertility. Germination of seed, root growth, and formation of root hairs are all in need of oxygen. Since oxygen is less soluble in water, the waterlogged soil cannot supply oxygen to the plants of such habitat (marshy habitat).
In such a situation, anaerobic respiration of roots and microorganisms produces poisonous substances in the soil which is harmful to the plants.
Soil Temperature
Soil temperature is an important factor since the root activity decreases below a certain optimum temperature. If the temperature falls below minimum, absorption of water by the root system seizes and such soil becomes physiologically dry.
However, the aerial parts of the plants can withstand cold temperatures. In mountain tops, the temperature is less than that of the soil. The low soil temperature produces limited growth of plants and thus plant characteristics of such soil conditions are dwarf.
The temperature of soil has a great influence on the growth of microorganisms as well. Hence it is indirectly concerned with soil fertility as well.
Soil Microorganisms
Several bacteria, fungi, and protozoa in a habitat are present in the superficial layer of the soil. They take organic matter in their nutrition and convert it into simpler products. These decomposed products form the humus which is later absorbed by the root system.
Thus vegetation and microorganisms are linked. Since these organisms remove the dead remains of plants and animals from the earth’s surface, they are called scavengers of nature. Some of the bacteria are concerned with the formation of nitrogen compounds in soil.
Ammonifying and nitrifying bacteria are examples. A type of bacteria, Rhizobium resides in the root nodules of pea plants that are capable of fixing atmospheric nitrogen in soil. Thus nitrogen is available to other plants as well.
Certain fungi and roots of high plants such as Pinus, orchids like Monotropa, etc form a symbiotic association known as mycorrhiza. Here, the fungal hyphae increase the absorption of roots while the root provides nutrition to the fungus.
Larger organisms like earthworms inhabit superficial soil layers. They consume organic food and excrete decomposed products that increase soil fertility. Moreover, these soil organisms facilitate the aeration of the soil.
Chemical Characters Of Soil
There are several types of elements, chemical compounds, and minerals present in soil. Their quantity mainly depends on the nature of the rock from which the soil is derived.
The common elements found in soil are oxygen, aluminum, silicone calcium, magnesium, sodium, potassium, and iron. The major trace elements found in soil are copper, zinc, arsenic, barium, strontium, fluorine, cadmium, lithium, etc.
In the limestone region, the soil is rich in calcium. It helps retard the leaching of magnesium and potassium to the deeper layers and reduce the acidity of the soil.
The chemical nature of soil has a great effect on the flora and fauna of that region. The trace elements also affect the growth of the plants.
pH Value of Soil
The soil solution may be mostly acidic or alkaline. Sometimes it is neutral as well. Soil becomes acidic mainly due to the absence of calcium, magnesium, potassium, and sodium ions. The use of certain manures like ammonium sulfate can lead to acidic soil.
Most of the plants grow well in neutral or slightly acidic soil. Plants such as wheat, barley, tobacco, etc will be injured in acidic soil. On the other hand, plants like rice, oats, cotton, tomato, etc can tolerate acidic soil.
In regions of low rainfall, soluble salts accumulate in the superficial layers of soil. Carbonates of sodium and potassium, sulfates, nitrates of sodium, potassium, and magnesium, etc usually increase the soil alkalinity.
Soil rich in any of the alkaline compounds is called saline soil. Saline and alkaline soil practically do not contain any vegetation. Since these plants will be unable to absorb water from this concentrated soil solution, it leads to physiological dryness.
The acidity and alkalinity depend on the hydroxyl ions and thus are referred to as pH value. In a neutral solution of water pH value >7 is alkaline and the pH will be < 7 in acidic soil.
References
- Soil Management. (n.d.). https://www.ctahr.hawaii.edu/mauisoil/a_comp.aspx
- C. W. (2019, May 16). Basic Soil Components – Climate, Forests, and Woodlands. https://climate-woodlands.extension.org/basic-soil-components/
- Shukla, R.S. and Chandel, P.S. (2001) Plant Ecology. S. Chand and Company Ltd., New Delhi.
- Verma, P.S., Agarwal, V.K. (1999). Cell Biology Genetics Molecular Biology Evolution and Ecology. New Delhi: S.Chand Co.(Pvt) Ltd.
