1. 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.

2. : soil degradation  Soil Degradation: Occurs where human activity (either directly or indirectly) reduces the capacity of the soil to support life.  Causes: intensive farming, desertification, erosion, and pollution.  Bad: reduces crop production

3. : soil degradation 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.  Another problem caused by irrigation is that over irrigation/water run off can remove vital nutrients such as Ca 2+, K +, NH 4 + and Mg 2+ from the topsoil.

4. : soil degradation Nutrient depletion  Plants remove nutrients and minerals from the soil.  If soil is not allowed to recover i.e. allowed time for the removed nutrients to be replaced through natural processes, the soil becomes depleted in these nutrients and this will effect future plant growth.  Monoculture i.e. growing the same crop time after time increases nutrient depletion.  Allowing soils to remain fallow (no crops are grown and nutrients can be replaced) for a while or use crop rotation i.e. grow different crops which absorb different nutrients are some solutions.

5. : soil pollution Soil pollution  Use of chemicals such as pesticides and fertilizers.  They 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.

6. Soil organic matter (SOM)  SOM includes:  main-constituent: undecayed plant and animal matter which can be:  high-molecular-mass organic materials such as polysaccharides and proteins and  simpler substances such as sugars, amino acids and other small molecules  humus: the residue left of organic material after it has been fully broken down by bacteria in the soil.

7. Soil : SOM  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 NH 2 groups on the SOM molecules allow hydrogen bonding between these molecules and water molecules in the soil  alters the soil thermal properties.

8. : organic pollutants  Many organic compounds end up in soil through direct spills, leaks (e.g. from landfills, underground storage tanks), dumping or washed from the air through rain.  Most organic pollutants are petroleum hydrocarbons which are compounds which originally come from crude oil, coal gas, tar, natural gas; either directly through distillation (e.g. octane) or manufactured.

9. Soil : organic pollutants  These petroleum hydrocarbons are :  volatile organic compounds (VOCs) such as shorter alkanes, petrol, benzene, toluene, xylenes, …or  semi-volatile organic compounds (SVOCs) which are organic compounds with boiling temperatures higher than water but which evaporate at temperatures above room temperature; examples are phenol, naphthalene, plasticizers, many PAHs and PCBs.

10. : pollutants PollutantsSources agrichemicalspesticides (insecticides, fungicides, herbicides), fertilizers, growth hormones through direct application which is fine if controlled; problem is spills and overuse. organic solvents  spills/leaks/dumping of dry cleaning solvents, paint thinners, nail polish remover, detergents, …  many are also VOCs such as dichloromethane, trichloroethane, and trichloroethylene polynuclear aromatic hydrocarbons (PAHs)  chemicals which do not degrade easily  synthetic PAHs banned in 1975  chemicals common in coal tar/creosote which have been 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 emissions

11. : pollutants PollutantsSources polychlorinated biphenyls (PCBs),  stable compounds, can function as fire retardants and plasticizers  transformers, circuit breakers, electromagnets, hydraulic oils, paints  bioaccumulate organotin compounds  compounds which contain at least one bond between tin and a carbon part of a hydrocarbon chain  used in manufacture of: o antifouling compounds (coatings or paints) o fungicides o pesticides / biocidal agents; o catalysts dioxins  incineration plants, wood burning, traffic  spread trough the air and settle on soil