Lime is the most common and widely used method to ameliorate the impact of Al- toxicity in acid soils of temperate regions (such as Europe and North America) (Rao et al., 1993). In these areas, soil acidity develops in surface soils mainly as a
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consequence of the heavy use of mineral fertilizers, and from environmental pollution (Rao et al., 1993).
In the tropics, significant yield increase may result from the appropriate application of lime. However, due to sub-soil acidity, and their strong buffering capacity of acid soils, such soils need substantial doses of lime to neutralize the acidity (Rao et al., 1993;
The et al., 2006). Most of resource poor farmers in the tropics are constrained by the local unavailability of lime, its high cost, and the costs of transporting a bulky product in the quantities that are needed. Furthermore, these farmers lack the appropriate technology for deep mechanical incorporation, which combines with the inherently slow movement of lime into soils, and especially the acidic sub-soils. Consequently, root development of acid sensitive crops is restricted to the surface soil, leaving these crops vulnerable to even minor droughts (Little, 1989; Foy, 1992). This is particularly important because many acid soils have inherently low water holding capacity (Little, 1989; Haynes and Mokolobate, 2001). Runoff pollution and the adverse effect of lime on calcifuge crops in rotation systems are negative side effects of lime applications (Wang et al., 2006).
The use of organic matter in the form of manure and compost can significantly reduce soil acidity (Wong and Swift, 2003). Many organic compounds are released or synthesized during the decomposition of organic matter by soil microorganisms.
Among these, soluble humic molecules and low molecular weight aliphatic acids are efficient in detoxifying Al3+ by forming various complexes (Haynes and Mokolobate, 2001). In addition, the regular application of organic matter increases soil pH. Under higher pH the toxic species of Al are converted to non-toxic and insoluble hydroxyl-Al compounds. Application of organic matter also improves the availability of deficient soil nutrients such as phosphorus. Use of organic matter seems an applicable strategy to resource poor farmers of the tropics who cannot afford large quantities of lime and fertilizers. However, in countries like Ethiopia, animal manure and crop residues have many uses, including their use as fuel, animal feed, and construction material.
Therefore, regular applications of organic matter to acid soils are not common (Schlede, 1989; IFPRI, 2010).
Di-ammonium phosphate (DAP) and urea are the mineral fertilizers exclusively used on all soil types and agro-ecologies in Ethiopia (Abebe, 2007). Assimilation of these fertilizers into roots produces protons are excreted into the external medium,
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increasing soil acidity (Marschner, 1995; Barak et al., 1997; Bolan and Hedley, 2003).
Hence, these fertilizers are classified as acid-forming fertilizers. Cautious use of these fertilizers in areas with acid soils involves the concurrent application of acid-equivalent quantities of lime that can immediately neutralize acidity as it is released (Bolan and Hedley, 2003). Accordingly, the acidity equivalent or the number of parts of pure lime (calcium carbonate) required to neutralize the acidity caused by 100 parts of DAP and Urea is 74 and 79, respectively. For these two fertilizers, DAP and urea, the number of years required to decrease the pH by one unit varies between 10-33 and 25-78 years, respectively, depending on the buffering capacity of the soil to change in pH (Bolan and Hedley, 2003). Currently, with the objective of improving crop productivity per unit area and bridging the existing yield gap between potential and actual yields, the extension services of the country are promoting the exclusive use of these fertilizers, without concurrent applications of lime The outcome of this practice will be that the less acidic soils will systematically be converted to strongly acidic soils.
As consequence of high human and animal population pressures, farmers in cereal dominated production areas have abandoned traditional fertility management practices such as fallowing. Crop rotation lesson longer widely practiced due to land shortage and the unsuitability of acid soils for Al-intolerant rotation crops. Contending uses of crop residue and animal manure has restricted farmers from using these resources to replenish soil fertility. Overall, utilization of external inputs is unbalanced and very low. Soil erosion is rampant. All these factors combine to ensure that a substantial outflow of mineral nutrients from the soil system is very common.
Consequently, in some areas of Ethiopia affected by acid soils, farmers have changed to growing crops such as oat, triticale, white lupine that are adapted to acid soils, in order to ensure their household food security. However, these crops, do not have a good market demand or value compared to popular cereal crops, especially tef.
Furthermore, areas that were previously under forest, woodland and savannah are being converted into crop production fields in order to increase national production of food, industrial and biofuel crops. These soils, specifically the ones from the southern, south western and north western parts of the country have a strong tendency to become acid because of the parent soils, and the favourable climatic factors (Abebe, 2007).
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Hence, the goal of food security through increased productivity and production needs to employ sound and integrated methods to manage acid soils. From this perspective, the breeding and release of Al-tolerant varieties would be socially, economically and technically acceptable, and environmentally friendly for small-scale farmers of developing countries like Ethiopia. The combined uses of tolerant crop varieties, lime and organic fertilizers have a synergistic effect that will result in the increased productivity and sustainable health of acid soils. For instance, The et al. (2006) reported that maize varieties tolerant of acid soils gave 61% higher grain yields than Al-sensitive varieties. With lime treatments, yield increases of 208% and 82% were obtained for Al-sensitive and Al-tolerant varieties, respectively.