Global extent, development and economic impact of acid soils
Acid soils occupy approximately 30% or 3950 m ha of the world’s ice free land area and occur mainly in two global belts where they have developed under udic or ustic moisture regimes. The northern belt (cold and temperate climate) is dominated by Spodosols, Alfisols, Inceptisols and Histosols and the southern tropical belt consists largely of Ultisols and Oxisols. Sixty-seven percent of the acid soils support forests and woodlands and approximately 18% are covered by savanna, prairie and steppe vegetation. Only 4.5% (179 m ha) of the acid soil area is used for arable crops. A further 33 m ha is utilized for perennial tropical crops. The value of the annual production in these areas is approximately US$ 129 billion. Value of products from forests, woodlands and permanent pastures on acid soils is difficult to evaluate.
 Mechanisms of adaptation of plants to acid soils
Major constraints for plant growth on acid mineral soils are toxic concentrations of mineral elements like Al, of H+, and/or low mineral nutrient availability either as a result of solubility (e.g. P and Mo), low reserves, and impaired uptake (e.g. Mg2+) at high H+ concentrations. Inhibition of root growth particularly by Al leads to more shallow root systems, which may affect the capacity for mineral nutrient acquisition and increase the risk of drought stress. Of the two principal strategies (tolerance and avoidance) of plants for adaptation to adverse soil conditions, the strategy of avoidance is more common for adaptation to acid mineral soils. Examples are (i) root-induced changes in the rhizosphere such as pH increase, (ii) release of chelators for Al, higher activity of ectoenzymes (acid phosphatases), and (iii) increase in root surface area via mycorrhizae. In order to have a better understanding of the principles of the mechanisms by which plants adapt to acid mineral soils more attention should thus be given to conditions at the root-soil interface.
 HOW DO CROP PLANTS TOLERATE ACID SOILS? MECHANISMS OF ALUMINUM TOLERANCE AND PHOSPHOROUS EFFICIENCY
Acid soils significantly limit crop production worldwide because approximately 50% of the world’s potentially arable soils are acidic. Because acid soils are such an important constraint to agriculture, understanding the mechanisms and genes conferring tolerance to acid soil stress has been a focus of intense research interest over the past decade. The primary limitations on acid soils are toxic levels of aluminum (Al) and manganese (Mn), as well as suboptimal levels of phosphorous (P). This review examines our current understanding of the physiological, genetic, and molecular basis for crop Al tolerance, as well as reviews the emerging area of P efficiency, which involves the genetically based ability of some crop genotypes to tolerate P deficiency stress on acid soils. These are interesting times for this field because researchers are on the verge of identifying some of the genes that confer Al tolerance in crop plants; these discoveries will open up new avenues of molecular/physiological inquiry that should greatly advance our understanding of these tolerance mechanisms. Additionally, these breakthroughs will provide new molecular resources for improving crop Al tolerance via both molecular-assisted breeding and biotechnology.
 Lime-Aluminium- Phosphorus Interactions in Acid Soils of the Kenya Highlands
Liming and phosphorus (P) applications are common practices for improving crop production in acid soils of the tropics. Although considerable work has been done to establish liming rates for acid soils in many parts of the world, information on the role of the lime-Al-P interactions on P fertility management is minimal. A green house pot experiment was conducted at Waruhiu Farmers Training Centre, Githunguri to evaluate the lime-Al-P interactions in acid soils of the Kenya highlands. Extremely acidic (pH 4.48) and strongly acidic (pH 4.59) soils were used for the study. Four lime (CaO) rates and phosphorus (Ca (H2PO4)2 rates were used. The liming rates were: 0, 2.2, 5.2 and 7.4 tonnes ha-1 for extremely acidic soil and 0, 1.4, 3.2, and 4.5 tonnes ha-1 for the strongly acidic soil. Phosphorus applications rates were: 0, 0.15, 0.30 and 0.59 g P kg-1 soil for the extremely acidic soil and 0, 0.13, 0.26, and 0.51 g P kg-1 for the strongly acidic soils. The experiments were a 42 factorial laid in a randomized complete block design (RCBD) and replicated three times. Data collected included: soil chemical properties and P adsorption. The soils had high exchangeable Al (>2 cmol Al kg-1), Al saturation of (>20% Al) and low P. Lime-Al-P interaction significantly (P≤0.05) increased soil pH, extractable P, reduced exchangeable Al, Al saturation, P adsorption and standard phosphorus requirements (SPR).
 Extraction Methods for Estimating Available Manganese to Maize (Zea mays L.) in Acid Soils
Twenty surface soil samples were used to evaluate status of soil Mn using five extraction procedures (Coca-cola, EDTA, HCl, EDTA + NH4OAC and NH4OAC methods). The results show that Coca-cola method extracted the highest amount of the Mn while NH4OAC extracted the least amount of Mn. The results also showed that among the five extractants examined, the highest regression coefficients were found between Coca-Cola and HCl, HCl and EDTA+NH4OAc and, EDTA and EDTA+NH4OAc-extractable Mn for Mn uptake, respectively. Accordingly, the study indicates that, the comparative extraction capacity of these extractants followed the order: Coca-cola> HCl> EDTA> EDTA+NH4OAc> NH4OAC.
 Von Uexküll, H.R. and Mutert, E., 1995. Global extent, development and economic impact of acid soils. Plant and soil, 171(1), pp.1-15.
 Marschner, H., 1991. Mechanisms of adaptation of plants to acid soils. Plant and soil, 134(1), pp.1-20.
 Kochian, L.V., Hoekenga, O.A. and Pineros, M.A., 2004. How do crop plants tolerate acid soils? Mechanisms of aluminum tolerance and phosphorous efficiency. Annu. Rev. Plant Biol., 55, pp.459-493.
 Muindi, E.M., Mrema, J.P., Semu, E., Mtakwa, P.W., Gachene, C.K. and Njogu, M.K., 2015. Lime-aluminium-phosphorus interactions in acid soils of the Kenya Highlands. Journal of Experimental Agriculture International, pp.1-10.
 Eteng, E.U. and Asawalam, D.O., 2016. Extraction Methods for Estimating Available Manganese to Maize (Zea mays L.) in Acid Soils. Current Journal of Applied Science and Technology, pp.1-18.