soil Discuss salinization, nutrient depletion and soil pollution as causes of soil degradation. Describe the relevance of the soil organic matter (SOM) in preventing soil degradation, and outline its physical and biological functions List common organic soil pollutants and their sources.

salinization Salinization is the result of irrigating soils i.e. watering them. Water used for irrigation usually contains dissolved salts, which are left behind in the soil after the water evaporates. In poorly drained soils, the salts left behind are not washed away and begin to accumulate in the topsoil. Plants cannot grow in soil that is too salty.

Nutrient depletion (1) Plants remove nutrients and minerals from the soil. If the soil is not allowed to recover i.e. allowed time for the removed nutrients to be replaced through natural nutrient cycles the soil becomes depleted in nutrients and this will reduce future productivity. Nutrients can also be replaced by using artificial fertilizers or compost. Harvesting crops stops this natural nutrient cycle as the nutrients are removed permanently. Monoculture, i.e. growing the same crop time after time, accelerates nutrient depletion.

Nutrient depletion (2) Nutrient depletion can be prevented or slowed by: allowing soils to remain fallow (no crops are grown and nutrients can be replaced) for a while use crop rotation i.e. grow different crops which absorb different nutrients are some solutions. using artificial fertilizers or compost.

soil pollution Excessive use of chemicals such as pesticides and fertilizers. These chemicals can: disrupt the soil food web, reduce the soil’s biodiversity ultimately ruin the soil. These chemicals also run off the soil into surface waters and move through the soil, polluting groundwater.

Soil organic matter (SOM) includes Plant and animal tissue such as leaves, twigs, animal parts e.g. skin, hair, ... that have not been decomposed yet. Decomposition products of those animal and plant tissues such as: high-molecular-mass organic materials such as polysaccharides and proteins and simpler organic substances such as sugars, amino acids and other small molecules Humus which is what is left after plant and animal tissues are not being decomposed anymore.

SOM prevents soil degradation The functions of SOM can be broadly classified into two groups: biological: provides nutrients for the plants, in particular nitrogen, as it provides amines and amino acids; physical: improves structural stability, influences water-retention properties: the OH- and NH2 groups on the SOM molecules allow hydrogen bonding between these molecules and water molecules in the soil alters the soil thermal properties.

pollutants Agrichemicals Sources Agrichemicals Pesticides (insecticides, fungicides, herbicides), fertilizers, growth hormones through direct application which is fine if controlled; problem is spills and overuse. Hydrocarbons Transport, solvents, direct spills, leaks (e.g. from landfills or underground storage tanks), Spills/leaks/dumping of dry cleaning solvents, paint thinners, nail polish remover, detergents, … Polynuclear aromatic hydrocarbons (PAHs) Chemicals common in coal, tar e.g. creosote which is used to preserve wood in buildings, fences, ships. PAHs are present in coal and crude oil and are released during combustion e.g. industrial/power station emission and from incinerators.

pollutants Pollutants Sources Polychlorinated biphenyls (PCBs), Leaks from electrical insulators, transformers, circuit breakers, electromagnets, hydraulic oils, paints Organotin compounds Compounds which contain at least one bond between tin and a hydrocarbon chain Used in manufacture of antifouling compounds (coatings or paints) which prevent organisms (e.g. barnacles) from growing on surfaces exposed to water Fungicides Pesticides / biocidal agents; Dioxins Incineration plants, wood burning, herbicides Spread trough the air and settle on soil

Soil pollutants Petroleum hydrocarbons (hydrocarbons from crude oil) Compounds such as hexane, petrol, benzene, diesel oil, jet fuels, lubricating oils. Direct spills, leaks (e.g. from landfills or underground storage tanks), being dumped