A degraded soil typically loses its ability to supply food and habitation to living organisms, in its surrounding. When this happens, effort is made to restore the soil back to its natural state. Characteristics of a degraded soil include: high salinity, decline in fertility, decline in organic matter (leading to decline in soil structure), soil erodibility, increase in alkalinity and acidity.
Soil degradation can be caused by man: for example, agricultural activities can disturb the soil structure and its drainage capacity; chemical use can increase soil salinity or alkalinity. It can also be of a natural cause like salinization (when soils originate from salty parent materials) or erosion.
Some ways of restoring damaged soil include:
Use of organic farming techniques: Organic farming involves the application of natural means in farming, to reduce harm to the environment. Some organic farming techniques that help restore the soil include use of green manure (uprooted or sown crop parts incorporated or left on topsoil), cover crops, crop rotation and organic compost.
• Green manure and cover crops: Green manures and cover crops serve as mulch to the soil preventing the soil from wind/water erosion and moisture loss. They also increase the soil organic matter content as they decompose in the soil. Green manure and cover crops that are legumes (plants which produce seeds in pods) have nitrogen fixing ability. The nitrogen fixing bacteria in their root nodules help capture nitrogen from the atmosphere. Green manure and cover crops suppress weed growth. It is a cheap and natural (no herbicide use) method of controlling weeds.
• Organic compost: Organic compost is a generally cheaper method of fertilizing the soils compared to inorganic fertilizers. Compost is a mixture of decomposed plant parts and animal waste. The key benefit of composting is that it increases soil organic matter content. Organic matter improves the soil fertility, the soil structure and its water holding capacity. It also sequesters carbon in the soil. The use of compost reduces use of chemical fertilizers which if applied inappropriately can contaminate neighbouring streams or ground water.
• Crop rotation: This is a farming practice which involves growing different types of crops in one location sequentially. This practice reduces soil erosion, increases the soil fertility and subsequently crop yield.
Soil remediation: Soil remediation involves the removal of harmful contaminants such as, heavy metals, sewage sludge, coal tar, carcinogenic hydrocarbons, liquors and petroleum from soils. Soil remediation can be achieved using biological techniques. This method is called bioremediation. Some examples of bioremediation techniques include:
• Phytoremediation: The use of plants to remove contaminants from soils or to degrade contaminants to a lesser toxic form. Some plants have the ability to extract contaminants from soils. This process is called phytoextraction. The willow (Salix viminalis) is a shrub credited for its ability to extract cadmium from soils1. Phytoextraction is one technique in phytoremediation. Some other techniques are phytostabilization, phytotransformation and phytostimulation.
• Bioaugmentation: This is the introduction of genetically modified micro organisms into contaminated soils with the aim of degrading contaminants. The efficiency of this technique depends on a number of factors, some of which are the physico-chemical properties of the soil and the ability of the introduced micro organisms to compete successfully with the indigenous soil microflora2.
• Land-based treatments: This includes techniques like land farming and composting. In land farming, contaminated soils are taken to land farming sites and continuously overturned and tilled to allow aeration. In composting, micro organisms present in organic material are used to biodegrade soil contaminants.
When soils are contaminated they constitute an environmental hazard. Soil remediation is always an expensive large scale project; requiring funding or approval by the government or local authority involved. A good example of a soil remediation project was the restoration of soils contaminated with stockpiles of obsolete pesticides in two African countries (Mali and Mauritania)3. These soils were restored using composting and land-farming. This project was run by collaboration between the FAO* Pesticide Management Programme, the Wageningen University and Research Centre and the National Institutions of the countries involved.
Desalinization: Soil salinization occurs when high levels of soluble salts accumulate in the root zone. Saline soils frustrate crop growth and reduce crop yield. Soil salinization is encouraged by:
• Formation from parent materials with high salt content
• Low rainfall in arid regions where there is insufficient water to leach salts
• Poor soil drainage system
• Excessive exposure of soil to salty irrigation water or chemicals
Some methods used to restore saline soils are:
• Installing drainage systems to wash salts down the soil profile (this method is expensive and complicated).
• Leaching out saline soils by applying water to contaminated soils to wash salts beyond the root zone.
• Use of salt tolerant plants (halophytes) as bioremediants. Halophytes accumulate salts in their shoots and other aerial plant parts. Examples include Allenrolfea occidentalis (iodine bush)4, Salicornia bigelovii (dwarf saltwort), Panicum virgatum (switch grass), Sesuvium portulacastrum (sea purslane)5.
• Application of gypsum (calcium sulphate dehydrate) to sodic soils. Sodic soils have high content of sodium chloride. Gypsum mixed into the layers of sodic soils replaces sodium with calcium, reducing the sodium level.
References and useful links
1. Klang-Westin E, Eriksson J (2003) Potential of Salix as phytoextractor for Cd on moderately contaminated soils. Plant and Soil 249: 127-137.
2. Mrozik A, Piotrowska-Seget Z (2010) Bioaugmentation as a strategy for cleaning up of soils contaminated with aromatic compounds. Microbiological Research 165:363-375.
3. Harmsen et al. (2009) An African approach for risk reduction of soil contaminated by obsolete pesticides. Retrieved from: https://www.fao.org/agriculture/crops/obsolete-pesticides/what-now/soil-remediation.
4. Zerai DB (2007) Halophytes for Bioremediation of Salt Affected Lands. PhD thesis, University of Arizona.
5. Rabhi et al. (2009) Evaluation of the capacity of three halophytes to desalinize their rhizosphere as grown on saline soils under nonleaching conditions. African Journal of Ecology 47:463-468.
6. Roland Bunch (2012) Restoring the soil. A guide for using green manure/cover crops to improve the food security of smallholder farmers. Retrieved from https://www.fao.org/ag/ca/CA-Publications/Restoring_the_Soil.pdf.
7. Bioremediation Technologies University of Hawaii