E7 Soil

Soil is composed of inorganic and organic material including living organisms, and its composition varies considerably. Soil degradation is when the quality of soil has been affected in some way so that crop production is lowered, and can be caused by changes in weather pattern or manmade factors (i.e. acidification, desertification, contamination, erosion and salinization, timber cutting, overgrazing, industrailization).

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E.7.1 Discuss salinization, nutrient depletion and soil pollution as causes of soil degradation.

Salinization: Constant or excess irrigation using water that contains salts, which upon evaporation, remain in the soil and accumulate in the fertile topsoil. These salts can either build up to toxic levels in the plant or dehydrate plants by preventing the uptake of water, both ways resulting in the death of crops.

Nutrient depletion: Intensive farming, without proper management practises results in nutrients being depleted without replacement. Plants require minerals and nutrients for healthy growth and by harvesting these plants, the normal cycling of nutrients through the soil food web is unable to occur. As a result, there is a amelioration of nutrient depletion. Soil should be left to fallow for some periods and organic material like manure and compost should be added to maintain the levels of nutrients.

Soil pollution: Caused by a variety of factors- industrial discharge and improper dumping of toxic waste material ( long term soil pollution), organic soil pollution from the transport and illegal dumping of spent engine oil (short term effect), use of pesticides and fertilizers disrupt the food chain and reduces soil's biodiversity, pollutants can run into surface waters and move through the soil through throughflow and percolation, polluting groundwater.

E. 7.2 Describe the relevance of the soil organic matter (SOM) in preventing soil degradation, and outline its physical and biological functions.

Soil requires not only minerals to allow healthy plant growth, but also organic matter (e.g. in compost). SOM represents the organic constituents of the soil including undecayed plant and animal tissues, their partial decomposition products (i.e. polysaccharides, proteins, sugers, amino acids) and the soil biomass. Humus is a complex mixture of simple and more complex organic chemicals of plant, animal or microbial origin.

SOM can be used in three main ways:

-biological

-chemical

-physical

BIOLOGICAL: Humus provides a source of energy and essential nutrient elements (N, P, S); contributes to resilience of soil/plant system.

PHYSICAL: Humus is involved in structural stability and water-retention and thermal properties. It helps soil to retain moisture and therefore increases capacity to withstand drought, and encourages formation of good soil structure. Its dark colour causes it to absorb more heat and this hence helps warm the soil in colder seasons.

CHEMICAL: Cation exchange capacity helps it act like clay. Contains active sites that bind to nutrient cations to prevent them from being washed away and making them more available to plants. Toxic cations also bind to humus and this stops them from entering the wider ecosystem. Humus acts as an acid-base buffer and enhances the ability of the soil to maintain a constant pH.

E.7.3 List common organic soil pollutants and their sources.

1) Hydrocarbons, VOCs, SVOC (semi-volatile organic compounds): from transport, solvents and industrial processes.

2) Agrichemicals: pesticides, herbicides, fungicides

3) Polyaromatic hydrocarbons: incomplete combustion of coal, oil, gas, wood and garbage.

4) Polychlorinated biphenyls (PCBs): coolant and insulater in electrical equipment (e.g. transformers, generators)

5) Organotin compounds: Bactericides and fungicides used in paper, wood, textile and anti-fouling paints.