Soil - properties and contamination

Soil is a living system that has to remain in a state of dynamic balance to be healthy. It is a complex ecosystem in which the substances move in cycles from plants to animals, to soil bacteria and back again to plants. Solar energy is the natural fuel that drives the soil cycles and living organisms of all kinds are necessary to sustain the whole system and keep it in balance. Soil is an essential base of human life.It is a  site for agricultural and forest production, a place for storing raw materials and wastes, a constituent element of landscape and a mirror of the history of civilizations and cultures.

Soil is a biologically active, complex mixture of minerals, organic materials, living organisms, air and water. Soil is an interface between biosphere, lithosphere, hydrosphere and atmosphere. Land occupies 38% of the total space. Soil is the outer most layer of the earth’s crust (Earth’s living skin-1/3 of surface). About 95% of human food is derived from the earth. Only 10% of the world’s land area is suitable for growing crops. Soil quality is degrading worldwide. Soil biota depend on the soil environment for their energy and nutrient supply.

 Soil functions 

Soil is an essential natural resource. It is an integrator of all parts of ecosystems. It is a medium for plant growth, a home for organisms and a storehouse of water, heat and chemicals.It is a decomposing medium for wastes, a source material for construction of shelter and a buffer system  to neutralize harsh environmental changes. Soil is a dynamic universal ecological system and a complex heterogeneous medium. A vital resource that provides food, feed, fuel and fiber. Soil is a material which nourishes and supports growing plants. Soil is a mixture of mineral matter, organic matter, water and air. (Example: Loam soil = 45% mineral matter, 5% organic matter, 25% water, and 25% air). Soil is a three phase system of a solid phase (Mineral matter,organic matter), a liquid phase (soil water) and a  gaseous phase( soil air).

Major components of soil

 Soils have four major components: Mineral matter contains three fractions, sand, silt, and clay. Organic matter contains appreciable quantities of nitrogen, phosphorus and sulfur. Air and water  occupy the pore spaces in soils.

Soil volume -Soil consists of organic particles and inorganic matter with pore spaces between and within them. Pore spaces contain soil air, and soil solution.  In other words, soil volume consists of solid, liquid and gaseous phases.

Types of soil water (i.e. water in the soil) 

 Gravitational water:  Gravitational water fills all the pore-space, and leaves no room for oxygen and gaseous exchange.

Capillary water: This water which is held with the force of surface tension by the soil particles, and is resistant to the forces of gravity. 

Hygroscopic water: This water is held so tightly (by surface tension) to the soil particles that the plant roots can't take it up.

Physical Properties of Soils 

 Soil Texture –the relative proportions of sand, silt, and clay in the soil. e.g. sandy soil, silty soil,and clay soil.

Soil Structure – Structure refers to the arrangement of soil particles. E.g., Granular, Platy, Wedge, Blocky, Prismatic, and Columnar. Soil structure is of particular importance in the absorption of water and the circulation of air.

Soil Color - Color in soils is due primarily to two factors, humus content and the chemical nature of the iron compounds. Humus has a dark brown or black color. Iron is an important color material which stains mineral particles. Ferrous oxide gives gray color. Ferric oxide gives red color. Hydrated ferric oxide gives yellow color.

Soil pH is primarily controlled by the concentration of free hydrogen ions in the soil matrix. Acidic soils have a relatively large concentration of hydrogen ions. Alkaline soils have a relatively low concentration of hydrogen ions.

Soil Types - Depending on the size of the particles in the soil, it can be classified as:

sandy soil, silty soil, clay soil, loamy soil, peaty soil and chalky soil.

Soil contamination

It is the presence of man-made chemicals or other alteration to the natural soil environment. Contaminants bind tightly to the soil. Contaminants evaporate into the air and end up in soil/ground water. The common chemicals of soil contamination include petroleum hydrocarbons, pesticides, heavy metals and solvents.Sources of soil contaminants comprise industrial wastes, active mine wastes, solid wastes and waste waters.

 Petrochemicals- Consumption, transportation and extraction of fossil fuels. 
Agricultural chemicals- Pesticides, fertilizers.
Solid wastes - Wastes from agriculture – crop and farm residues, animal manure.
Wastes from mining – coal wastes, metal ore wastes.
Industrial wastes- solvents chemicals, paints.
Solids from sewage treatments -biomass sludge, settled solids.
Ashes – residues from solid fuels
Garbage – glass, metals, clothes, plastics, wood, papers.

The intensive use of agrochemicals reduces soil fertility, soil biodiversity, nitrogen fixation and crop yield. Pesticides may infiltrate soil, carried away by wind, spread by runoff, leach out into the groundwater and finally reach rivers, lakes and ocean.

Human health effects

Exposure to heavy metals in soil results in nervous system disorders, kidney damage and liver toxicity. Exposure to agricultural chemicals causes cancer and infertility. Chronic exposure to other industrial toxins may causes birth defects, nervous system disorders and kidney diseases.

Quote for reflection

"We abuse land because we regard it as a commodity belonging to us. When we see land as a community to which we belong, we may begin to use it with love and respect." 

-Aldo  Leopold, A sand county Almanac.