Land degradation, a decline in land quality caused by human activities, will remain high on the international agenda in the 21
st century (
Akinnagbe and Umukoro, 2011). Land degradation is a global problem affecting an estimated 1.9 billion hectares of land and almost two billion people due to degradation (
Naseer and Pandey, 2018). Forests make up over 47% of degraded land worldwide, whereas cultivated land makes up roughly 18% of all degraded land. Commercial agriculture with natural vegetation replaced over large surfaces by monocultures of cash crops especially in African countries brings severe land degradation processes
(Sonneveld et al., 2016). The term land degradation involves both soil and vegetation degradation. Soil degradation refers to negative changes in the physical, chemical and biological properties of the soil, whereas vegetation degradation is the reduction in the number of species and the vegetational composition
(Abdi et al., 2013). To illustrate, about 1.5 to 2 billion tons of topsoil are lost annually in Ethiopia due to soil erosion and costing the country 3% of its Agricultural Gross Domestic Product (AGDP) (
Kiptoo and Mirzabaev, 2014). If this soil is conserved, it could have produced 1 to 1.5 million tons of grain can be added to the countries harvest.
Usually land degradation is described in terms of the loss in natural resources (soil, water, fauna and flora) or in the biophysical process by which its functions. Soil can be eroded, salinized or impoverished. Water can be lost through evapotranspiration, evaporation, infiltration, run-off, pollution, or overuse. Land degradation occurs when this balance is lost. The causes of land degradation are complex, and may be the product of anthropogenic or climatic factors, such as the impact of drought and desiccation on ecosystems, the overgrazing of these rangelands, unsustainable land-tenure rights, the undervaluation of land resources, and pricing failures and numerous other social and economic processes (
Kiptoo and Mirzabaev, 2014).
Due to temperature constraints, there is a cropping boundary between the upper and lower limits of agro-climatic conditions in Ethiopia. The upper limit of the Ethiopian farm land is continuously a cropping land while the lower limit is defined as dryness and which is not suitable for rain-fed cultivation of crops
(Hurni et al., 2010). Therefore, land degradation is very much severing in the lowlands than in the highlands of Ethiopia. Land is the basic natural resource that provides habitat and sustenance for living organisms. The intensification of cultivation resulting in the opening up of new lands exposes the top soil to the elements of degradation and alters the natural ecological conservatory balances in the landscape (
Akinnagbe and Umukoro, 2011). Improper management of land resources creates low crop productivity and brings poverty. Therefore, the main objective of this paper is to review the major impacts of land degradation on crop yields and to point out the major management options of land degradation problems.
The materials for the compilation of this paper, different literatures on impacts of land degradation on crop yields and the management options were reviewed from books, journals and proceedings from internet sources. The methods of this paper would be organizing the obtained resources from the internet and then it was written by taking the ideas of the resources what the literatures said about the impacts of land degradation on crop yields and its management options.
Effects of land degradation on crop yield productivity
Yields of crops were shown to decline because of soil factors, some associated with erosion, which leads to reduction in effective rooting depth; decrease in available water capacity, decline in soil organic carbon and depletion of other soil nutrients (
Ajayi, 2015). Some information for local and regional scales is available and has been reviewed by
Eswaran (2001). In Canada, for example, on-farm effects of land degradation were estimated to range from US$700 to US$915 million. The economic impact of land degradation is extremely severe in densely populated South Asia, and Sub-Saharan Africa. On plot and field scales, erosion can cause yield reductions of 30 to 90% in some root-restrictive shallow lands of West Africa. Yield reductions of 20 to 40% have been measured for row crops in different countries (
Eswaran, 2001). Land degradation reduces the yield of crops and live stocks productivity in Ethiopia (
Assemu Tesfa and Shigdaf Mekuriaw, 2014).
An annual loss of 30,000 ha due to water erosion with over 2 million ha already severely damaged (
Leonard, 2003). A total loss of 4,000 ha of state farms due to severe salinization and an estimate of 1 billion tonnes of topsoil is lost per year (
Mekonen, 2002) and nutrient depletion at rates of 30 kg/ha of nitrogen and 15-20 kg/ha of phosphorus (
UNDP, 2002) and loss of 62,000 ha of forest and woodland per year (
World Bank, 2001). The estimated range of grain yield losses through soil degradation for two important crops is shown in Table 1. The data in the Table are derived from nutrient studies in areas of high and low nutrient loss. Table 2 shows the monetary losses in those two situations. The total loss per hectare of wheat is about 400 birr (US$ 46) per hectare in areas of low loss and 4,736 birr ($ 544) per hectare in areas of high soil loss. The corresponding figures for maize are $ 31 and $ 379 per ha.
Major forms of land degradation
Generally, there are three types of land degradation. These are physical, chemical and biological land degradations. Due to high variability in climate, relief, soil type, altitude and farming systems, all types of soil degradation take place in Ethiopia. These are physical, chemical and biological soil degradations. The physical soil degradation includes soil erosion (mainly water erosion) and gully erosion. The chemical soil degradation includes soil acidity, soil salinity and sodicity and soil fertility decline. The biological soil degradation includes depletion of vegetation cover and soil organic matter (SOM) and reduction of biological activities (
Tolessa and Beshir, 2009).
Aw-Hassan et al., (2015), describes land degradation types as secondary salinization, soil erosion and overgrazing.
Causes of land degradation
The causes of land degradation can be either natural or human induced activities
(Barman et al., 2013). The natural causes includes earth quakes, tsunamis, droughts, landslides and mud flow, volcanic eruptions, flood, tornado, wild fire. As the natural causes are uncontrollable, the human induced degradation is very important in view of sustainability. Land degradation can lead to many issues like soil erosion, soil acidification, soil alkalinisation, soil salination, soil water logging and destruction of the structure of soil which will directly affect the productivity and yields of crops
(Barman et al., 2013). Generally, there are two ways of causing land degradation. These are proximate and root causes of land degradation.
Proximate causes of land degradation
Multiple interacting forces have been causing land degradation in Ethiopia. The proximate causes of land degradation include clearing of woodlands and forests, unsustainable arable farming techniques, the use of dung and crop residues for fuel and overstocking of grazing lands (
Leonard, 2003).
Most arable land (70%) in the highland is in cereals, with wheat and barley at the higher elevations and teff (a small grain), sorghum and maize in the lower areas. All these crops leave part of the soil surface bare during some or all of the growing season, exposing the soil to erosion. The annual crops are mainly planted after the rains begin, so early rains directly impact the soil, contributing to high soil erosion rates (
Leonard, 2003). Additionally, more fragile marginal lands are used as the population grows. A further result of population growth is the reduction in fallow periods in some areas from a five-year rotation to a two-year or even shorter rotation (
Leonard, 2003;
Bindraban et al., 2000). As rural populations have grown and woodland is converted to cultivation, the use of dung and crop residues for fuel has become much more important. Studies in two upland villages showed maize and sorghum stalks providing 69% of total fuel use in one village and dung providing 50% of energy use in the other. According to
Samuel Benin and Pender (2002), restricting grazing lands management can reduce the negative consequences of land degradation effects.
Root causes of land degradation
The root cause of land degradation is poor land use practices. Land degradation represents a diminished ability of ecosystems or landscapes to support the functions or services required for sustaining livelihoods. When agriculture is introduced in place of natural vegetation and is then intensified to maximize yields, farmers simplify agro-ecosystem structures by limiting the variety of vegetation. Such vegetation changes immediately affect water use and cycling in landscapes and result in biodiversity loss and the development of a less complex network of ecosystem interrelations than occurs naturally
(Bindraban et al., 2000). The major root causes of land degradation are natural conditions
(Nyssen et al., 2014; Kang et al., 2009 and
Leonard, 2003), population growth
(Nyssen et al., 2014), land ownership
(Abate et al., 2012), institutional issues (
Assemu Tesfa and Shigdaf Mekuriaw, 2014), rural markets (
Leonard, 2003) participation and extension (
Leonard, 2003) and low-technology agriculture.
Management options of land degradation
According to
Asrat (2014), land management is a result of a continuous adaptation of the environment to meet the need of the community. These adaptation involves controlled livestock husbandry and irrigated system based on flood harvesting, integrated soil and water conservation practice includes stone terraces, tied ridges, thrash line, agro forestry, intercropping, fallowing, green manuring, shifting, decomposition of debris and crop residues management, minimum tillage and commercial fertilizer. The divisions of soil and water conservation measures include agronomic, vegetative, and structural land management measure is arbitrary.
Potential and research needs for the Ethiopian highlands, all have emphasized in their approvals the need for conservation-based integrated development as a strategy to overcome the degradation of land resources and improve agriculture and forestry development in Ethiopia. According to the study of
Reed et al., (2015) stated that reorienting the land degradation towards sustainable land management through adequate policy support and economic mechanisms. Moreover, priorities indicate that the initial effort be directed to areas where the environmental degradation is high and food production returns are low. To overcome deforestation and land degradation on the Ethiopian highlands and provide the people with food, fuel wood and fodder on sustainable bases the following natural resource management strategies are proposed:
• Implementation of agroforestry and social forestry in the rural areas where subsistence farming is practiced.
• Expansion of plantation forestry both industrial and non-industrial on currently uncultivated and sloping lands.
• Conservation of the remaining natural forests to conserve species and biodiversity.
Agroforestry and social forestry
The role of agroforestry in satisfying the basic needs of the rural peoples of Ethiopia is large, but little research has been initiated to identify suitable agroforestry technologies and appropriate tree species for specific areas of Ethiopia (
Badege Bishaw, 2009). However, based on the work done by the Technical Committee for Agroforestry in Ethiopia, alley cropping, fodder tree planting on unproductive pasture and degraded hillsides, tree planting in home gardens and woodlots, tree planting as living fences on farm boundaries and roadsides and tree planting on contour structures, inside and along gullies are the agroforestry technologies appropriate for the land-use systems in the Ethiopian highlands.
