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Weathering and Leaching as a Cause for Soil Acidity Review Paper

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Introduction

Soil degradation is recognized every bit a key factor underlying poor agricultural productivity in sub-Saharan Africa (SSA) affecting the livelihoods of farmers and their surround (Sanchez 2002; Amede and Whitbread 2020). About 65% of the agricultural land (2.three billion ha) is degraded in Africa, mainly due to poor soil fertility direction practices, soil erosion, and soil acidification (Zingore et al. 2015). Severely degraded soils account for about 350 meg ha or twenty–25% of the full land area, of which nearly 100 million ha is estimated to be acutely degraded mainly due to improper agronomical activities. According to Sanchez (2002), soil degradation costs SSA nearly U.S. $68 billion per year and reduces the annual agricultural GDP of SSA by 3%. The productivity of country in Africa ranks among the lowest in the world. Around ii-thirds of the land in SSA are considered unfavourable for agriculture compared to i-third for South America and 40% for Asia (Vlek et al. 2008). As a result, the average yields of a good indicator ingather such as maize stood beneath 2 t ha−one in Africa compared to 5.5 t ha−ane for Asia and 8.0 t ha−1 for Americas (FAO 2019). The sustainable utilize of these soils requires adequate nutrient inputs such as fertilisers, inorganic and organic amendments such as lime, compost, manure and biochar, as soil health or fertility is a manageable soil property, and its management is of utmost importance for optimising crop nutrition to attain sustainable ingather product (Zeleke et al. 2010; Zingore et al. 2015; Agegnehu and Amede 2017).

Soil degradation processes vary according to state-utilise types. The human touch on the productive capacity of agricultural state in SSA is largely related to unsustainable soil management, such every bit removal of crop residues, awarding of very low external inputs, monocropping and shifts to more enervating crops (Vlek et al. 2010). The consequences are soil acidification, loss of soil organic matter (SOM), nutrient mining and soil erosion. Soil acidity increases with the build-upwardly of hydrogen (H+) and aluminum (Al3+) cations in the soil or when base cations such every bit potassium (K+), calcium (Caii+), magnesium (Mg2+) and sodium (Na+) are leached and replaced by H+ or Aliii+ (Von Uexküll and Mutert 1995). According to Sanchez and Logan (1992), one-third of the torrid zone (1.7 billion ha) is acid enough for soluble Al to be toxic for most crops. Similarly, acid soils cover more than a tertiary of SSA (Pauw 1994) and the productivity of these soils is low and declines rapidly due to their poor fertility, Al toxicity and frail structure (Aviles et al. 2020). These soils are mostly common on old, stable surfaces that accept been exposed to tropical weathering. According to Vlek et al. (2008), the SSA was divided into iii zones, i.e. dry [mean almanac precipitation, MAP<800 mm yr−i, sub-humid (800 mm yr−1<MAP < 1300 mm yr−1), and boiling areas (MAP>1300 mm year−1)]. Based on this nomenclature, acid soils predominantly occur in the boiling and sub-humid regions of east and central Africa, west and S Africa every bit almost of the acid soils in SSA accept been influenced by a wet tropical climate, and slightly by the vegetation encompass. High rainfall results in more acid soils, including Ferralsols, Alisols, Plinthisols, Acrisols, and Podzols (FAO and ITPS 2015).

Increasing expanse of agricultural land in high rainfall areas of Sub-Saharan Africa (SSA), where crop production used to be reliable, is affected past soil acidity and the productivity of these soils is low and declines rapidly. At that place is unprecedented overlap of high rainfall areas with soil acidity. Rain-fed farming system is mainly practiced in humid and sub-humid areas where acrid soils are common, using ascendant almanac and perennial crops (Dixon et al. 2019). Most of the acid soil areas, which used to be covered by woodlands and forests, have been rapidly diminishing and replaced by various smallholder farmers varying from shifting cultivation to more intensive dormant systems. Where population pressure is most intense, permanent cultivation systems accept evolved as in parts of central and East Africa. Hence, to reduce the impacts of agriculture on the environment, cracking effort needs to exist fabricated on underperforming lands to decrease agricultural country expansion by closing yield gaps through sustainable intensification, increasing cropping efficiency and reducing food waste (Foley et al. 2011). Soil acidity has besides been affecting large-calibration farms by reducing yield and quality of cash crops such as coffee, tea, pineapple, oil palm, safe and sisal, which are important sources of strange exchange for several African economies (Vlek et al. 2010). The central issue of management of acid soils is that the intensification of land apply necessitated by the increment in Africa's population is severely hindered past the inherent fragile nature of these soils. Sustainable increases in productivity tin only exist achieved by a gradual transformation of traditional farming systems through development pathways that consider socioeconomic and agroecological diversity (Pauw 1994; Fageria and Nascente 2014).

Acid soil management is becoming one of the major strategies to reach food and nutrition security in SSA. On the other paw, the direction of acid soils is still a major challenge that calls for investment. An in-depth analysis and cognition are required to design, adopt and scale up a suitable acid soil management approach. Several studies have been conducted on acrid soils in South America (Fageria and Baligar 2008), Africa (Eswaran et al. 1997a; Tully et al. 2015), Asia and Australia (Eswaran et al. 1997b; Bai et al. 2008). Still, there is limited understanding of the scope of the problems and the management of acid soils in many SSA countries. The gap in feel related to the management of acidic soils tin can be bridged by learning from other success stories countries. For instance, the experience of Cerrado region in Brazil in converting large areas of acrid soils to productive apply through employing integrated solutions could be adapted in SSA through south–due south collaborations, though there is express knowledge transfer and technologies to farmers or scientific communities. Brazil has been able to develop over 60 meg ha of the Cerrado with crops and improved pasture with the implementation of appropriate technologies and inputs, infrastructure, and policy back up (Klink 2014). This review attempts to capture lessons learned in other continents in managing acid soils and how these best practices could be integrated into the African context. The objectives of this review are, therefore, (1) to provide a synthesis of the current extent, distribution, causes and furnishings of soil acidity in SSA and (2) inventory and characterise existing differing acid soil management options and their integration modalities in SSA.

Literature search and information processing

Literature search was conducted through the Web of Science (apps.webofknowledge.com), Google Scholar (scholar.google.com), Scopus (www.scopus.com), AGRIS (agris.fao.org) and ResearchGate (https://world wide web.researchgate.internet). We searched the literature published upwardly to 2020, using 'soil acidity', 'acid soil management', 'integrated acid soil direction', and 'liming' as key terms. Although over 900 publications were retrieved, near 112 publications that provide empirical show on problems and management of soil acidity were reviewed in this paper.

First, the papers were grouped relating to their inquiry objectives and experimental types. These were further categorised into studies focusing on organic and inorganic food sources, including lime and the employ of acid-tolerant ingather species and varieties. Crops tested for soil acidity tolerance in the field were cereals (grain crops, such equally wheat, maize, and barley), food legumes (faba bean, soybean, etc.), and root crops (potato). For some data, statistical analysis was performed using SAS-STAT software and some results were graphically presented using Microsoft Excel 2016. In the literature reviewed soil types were given in different soil classifications (USDA, FAO, and WRB). To ensure uniformity, nosotros converted soil types into the World Reference Base (WRB) for soil resources classification and correlation system (IUSS Working Group WRB 2015). Soil types were verified by referring to the dominant soil types in the harmonised soil atlas of Africa (Dewitte et al. 2013).

Synthesis and discussions

Extent and distribution of acidic soils

Acidic soils belongs to Ferralsols and Acrisols and to a smaller extent the Plinthisols, Alisols and Nitisols (IUSS Working Grouping WRB 2015). Acrisols cover 87.viii meg ha or 2.nine% and Ferralsols virtually 312.4 meg ha or 10.3% of the full country area of Africa (Tully et al. 2015). Nearly 10.viii million km2 or 35% of the total expanse of land in Africa is characterised past slight to high phosphorus (P) fixation, of which 8.2 million kmii is typified past high P fixation due to soil acidity (Eswaran et al. 1997b).

Soil acidity is expanding both in area and level of acerbity in SSA (Effigy 1). Major areas of SSA afflicted past soil acerbity include Eastward and Central Africa (Ethiopia, Republic of kenya, Tanzania, Uganda, Rwanda, Brundi, Malawi, Central African Republic, Democratic republic of Congo), Westward Africa (Ghana, Nigeria, Ivory Coast, Liberia, Sierra Leone, Republic of guinea) and southern Africa (south Africa, Zimbabwe, Mozambique) (Leenaars et al. 2014). For example, in Ethiopia, ∼43% of the cultivated land is afflicted by soil acidity (ATA 2014), of which nigh 28% is strongly acidic (pH 4.one–5.5) (Abebe 2007). Soils adult on non-calcareous parent materials such as Nitisols and Acrisols are inherently acidic in Federal democratic republic of ethiopia. The predominant acidic soil associations in Ethiopia are Dystric Nitisols and Orthic Acrisols with inclusions of Leptosols (Abebe 2007), unlike many African countries where Acrisols and Ferralsols dominate the acidic soils. Nitisols take very good potential for agriculture; they have a stable construction and a loftier-water storage capacity. Workability of these soils is not a trouble even soon after precipitation or in the dry season (IUSS Working Grouping WRB 2015). They accept low CEC and available P is usually very depression (Sertsu and Ali 1983; Abebe 2007; Regassa and Agegnehu 2011). Strongly acidic soils are infertile considering of Ca, Mg, P and Mo deficiencies and the possible Al and Mn toxicities (Barber 1984; Fageria and Nascente 2014).

Figure i. Extent and distribution of soil acidity in Sub-Saharan Africa extracted from Horneck et al. (2011); Leenaars et al. (2014).

Acidic soils contain loftier concentration of aluminum (Al), manganese (Mn), and iron (Iron). At pH below 5.0, Al is soluble in water and becomes the dominant ion in the soil solution. In acid soils, excess Al primarily injures the root apex and inhibits root elongation (Sivaguru and Horst 1998). The poor root growth leads to reduced water and nutrient uptake, and as a consequence crops grown on acid soils are constrained with poor nutrients and water availability leading to reduced growth and yield of crops (Wang et al. 2006; Marschner 2011). Acidic soils such as Acrisols and Ferralsols contain toxic levels of Al and Mn, which have depression availability of nutrients, merely with good physical backdrop (IUSS Working Group WRB 2015). The low nutrient content of soils such every bit Ferralsols and Acrisols is due to the predominance of 1:1 clay minerals, and Fe and Al oxides in the fraction (de Sant-Anna et al. 2017).

Causes of soil acerbity

Generally, there are two types of soil acidity: (1) Active acidity which occurs considering of high H+ ion concentration in the soil solution which is attributable to carbonic acid (HtwoCO3), water-soluble organic acids and hydrolytically acid salts; and (ii) exchangeable acerbity which refers to those H and Al ions adsorbed on soil colloids. There exists an equilibrium between the adsorbed and soil solution ions (i.e. active and exchange acerbity), permitting the ready movement from one form to some other. Such an equilibrium country is of great practical significance since it provides the basis for the soil buffering capacity or its resistance to change in pH. Since the adsorbed H and Al ions motility into the soil solution and then its acerbity is too referred to equally adsorbed or potential or reserve acidity. Reactions of bases (due east.thou. lime) added to the soil occur first with the active acerbity in the soil solution. Afterwards, the puddle of reserve acerbity gradually releases acerbity into the active grade. Co-ordinate to Somani (1996), the equilibrium human relationship between exchange (reserve) and solution (active) acerbity, and acid or base of operations inputs has been illustrated equally follows:

Soil acidification is a circuitous process and there are several causes of soil acidity. Generally, it tin be considered as the summation of natural and anthropogenic processes that decrease the pH of the soil solution (Krug and Frink 1983). Naturally, soil acidification takes place due to carbonic acrid-triggered leaching of basic cations, weathering of acidic parent materials, decomposition of organic matter, and deposition of atmospheric gases such as SO2, NH3, HNO3, and HCl (Goulding 2016; Rahman et al. 2018). Anthropogenic activities such as continuous application of acid-forming fertilisers including sulfur or ammonium salt and contact substitution between exchangeable hydrogen on root surfaces and the bases in exchangeable form on soils, microbial production of nitric and sulfuric acids advance the process of soil acidification that could lead to increased soluble Al+3 concentrations in the soil solution (Fageria and Nascente 2014; Behera and Shukla 2015). The decomposition of organic matter produces H+ ions, which are responsible for acidity (Kochian et al. 2004; Paul 2014), just the development of soil acidity from the decomposition of organic affair is insignificant in the short term. Low buffer chapters from clay and organic thing is another source of soil acerbity, i.e. contact commutation between exchangeable H+ on root surfaces and the bases in exchangeable form on soils. Microbial product of nitric and sulfuric acids also occurs, where leaching is express. The buffering or CEC is related to the amount of dirt and organic matter present, the larger the amount, the greater the buffer chapters.

Removal of cations, peculiarly from soils with modest reservoir of bases due to the harvest of loftier-yielding crops is responsible for soil acidity (Von Uexküll and Mutert 1995; Eswaran et al. 1997b; Vitousek et al. 2009;). Harvest of high-yielding crops plays the virtually significant role in increasing soil acidity. During growth, crops absorb basic elements such as Ca, Mg, and Thousand to satisfy their nutritional requirements. As ingather yields increase, more of these lime-like nutrients are removed from the field. Compared to the leafage and stem portions of the plant, grain contains lower amounts of basic nutrients. Thus, harvesting high-yielding forages affects soil acidity more than harvesting grain does (Fageria and Baligar 2008; Rengel 2011). Soil acidification continues until a balance is reached betwixt removal and replacement of the basic cations such every bit Ca and Mg that are removed through leaching and crop harvest and replaced due to organic affair decomposition and from weathering of minerals (Abebe 2007; Regassa and Agegnehu 2011). With further increase in rainfall, a indicate is reached at which the rate of removal of bases exceeds the rate of their liberation from non-exchangeable forms. Hence, wet climates have a greater potential for acidic soils (Sanchez and Logan 1992). Over time, excessive rainfall leaches soluble nutrients such as Ca, Mg and Yard that prevent soil acerbity which are replaced by Al from the exchange sites (Thomas and William 1984; Brady and Weil 2016).

Long-term application of loftier rates of N fertilisers, loss of cations via leaching, and land use change, i.e. continuous cropping without organic inputs are amid the anthropogenic factors that increment soil acidity (Scheffer et al. 2001; Tully et al. 2015). Hydrogen is added in the form of ammonia-based fertilisers (NH4), urea-based fertilisers CO(NHtwo)2, and proteins (amino acid) in organic fertilisers. Transformation of such sources of N fertilisers into nitrate (NOiii -) releases hydrogen ions (H+) to create soil acerbity. Besides, N fertiliser increases soil acidity past increasing crop yields, thereby increasing the extent of basic elements being removed. Hence, application of fertilisers containing NHfour + to soil can ultimately increase soil acidity and lower pH (Hue 1992; Guo et al. 2010). Inefficient apply of N is some other cause of soil acidification, followed past the export of alkalinity (Guo et al. 2010).

Changes in land use and management practices often modify most soil physicochemical and biological backdrop to the extent reflected in agronomical productivity (Gebrekidan and Negassa 2006). Soil properties such as bulk density, soil organic matter (SOM) content and CEC deteriorate (Conant et al. 2003) due to the conversion of native forest and range lands into cultivated land (Lemenih et al. 2005). For example, the amount of SOM in grazing and cultivated lands has depleted past 42.half-dozen and 76.5%, respectively, compared to the forest soil. Similarly, Agoumé and Birang (2009) reported the negative effect of land apply or country cover modify on some physicochemical properties of Ferralsols, such as clay, silt and sand fractions in the humid forest zone of southern Cameroon. Sand and silt decreased with soil depth, but clay increased. Soil pH, full N, organic C, available P, exchangeable cations, exchangeable Al, effective CEC, and Al saturation varied across land-utilize systems. Aluminum saturation increased with soil depth, and the top soils presented acidity issues while the sub-soils exhibited Al toxicity. Chimdi et al. (2012) also indicated that a decline in full porosity in soils of grazing and cultivated land in comparison to soils of woods land was attributed to a reduction in pore size distribution and the magnitude of SOM loss, in turn, depends on the intensity of soil direction practices.

Effect of soil acidity on availability of plant nutrients

Soil acidity and associated depression nutrient availability is the major constraint to crop production on acid soils. Ane of the detrimental effects of soil acidity is P sorption, which is afflicted by dirt mineralogy, pH, oxides and hydroxides of Iron and Al content of baggy materials. The machinery of P sorption is considered to be mainly through replacement of hydroxyl ions on crystal lattices, and hydrated Fe and Al by phosphate ions (Adams 1990; Abebe 2007). The P sorption capacity increases with the increase in acidity. For instance, soils from the rift valley of Ethiopia (e.g. Melkassa with a pH value of seven.8) had the lowest P sorption, which are the least weathered (Table 1). All the same, in the case of highly weathered soils, where the dominant minerals are Gibbsite, Goethite, Kaolinite and desilicated amorphous materials, P sorption is high to very high (Sertsu and Ali 1983). According to Duffera and Robarge (1999), 70–75% of Nitisols in Ethiopian are highly deficient in phosphorus.

Table 1. Corporeality of P sorbed by some Ethiopian soils at the standard solution P of 0.two ppm.

The solubility and availability of nutrients to plants is closely related to the pH of the soil (Marschner 2011). Soil acidity converts available soil nutrients into unavailable forms. High soil acidity is related to shortage of bachelor Ca, 1000, Mg, P and Mo on the one hand (Somani 1996; Agegnehu and Sommer 2000) and excess of soluble Al, Mn and other metallic ions on the other (Rahman et al. 2018). Soil acidity and Al toxicity limit soil enzyme activities, resulting in suppressed microbially mediated nutrient cycling, and that Al toxicity and a reduced availability of organic matter due to Al and Fe binding, may protect a substantial puddle of organic C from microbial deposition in acidic soils (Kunito et al. 2016). Soil acidity too affects the movement of soil organisms that are important for plants health. If pH of a soil is less than five.5, phosphate tin can readily be rendered unavailable to plants as it is the most immobile of the major plant nutrients (Sanchez and Logan 1992; Agegnehu and Sommer 2000), which results in low crop yield. The quantity of P in soil solution needed for optimum growth of crops ranges betwixt 0.xiii and 1.31 kg P ha−1 as growing crops absorb virtually 0.44 kg P ha−one per day. The labile fraction in the topsoil layer is in the range of 65–218 kg P ha−i, which could replenish P in soil solution (Mengel et al. 2001). Phosphate sorption takes place by specific adsorption and precipitation reactions (Fageria and Baligar 2008). Specific adsorption occurs when P anions replace the hydroxyl groups on the surface of Al and Atomic number 26 oxides and hydrous oxides, while precipitation reaction occurs when insoluble P compounds form and precipitate. At very low soil pH (≤4.5–5.0), add-on of P to soils tin event in atmospheric precipitation of Al and Fe phosphates, whereas at high pH (>half dozen.5) insoluble calcium phosphates can be formed (Abebe 2007).

Effect of soil acidity on ingather growth, yield and grain quality

Soil acidity and depression CEC are major constraints for crop production on highly weathered tropical soils (Von Uexküll and Mutert 1995). Crops differ in their susceptibility to soil acidity. Several adverse effects such as loss of ingather multifariousness, decline in crop yield, lack of response to North and P fertilisers, and complete failure of crops were reported. For instance, yields of barley, wheat, and beans were extremely low, even under application of optimum charge per unit of NPK fertilisers on acrid soils of Bedi (Beyene 1987) and Chencha (Haile and Boke 2011) due to low pH. Some N-fixing strains of the bacteria practice non thrive at pH values below 6, thus pH half dozen or above is all-time for the legumes that crave particular strains of bacteria. Although potato can abound well at higher pH, the recommended soil pH for its optimum growth is 5.0–five.5 since potato scab affliction is more prevalent when soil pH is in a higher place 5.5. In dissimilarity, plants such as azalea and camelia abound well only at pH values below 5.5 in a higher place which they suffer from Fe and Mn deficiencies. The pH of soils for all-time nutrient availability and ingather yields is considered to be between six.0 and vii.0, which is the most preferred range past mutual field crops (Duncan 2002). As indicated in Figure 2 barley grain yield and faba bean seed yield have shown strong positive relationship with soil pH level as both crops are sensitive to soil acidity, implying that an platonic soil pH is a prerequisite for attaining optimum yield of both crops, but with the application of other crop management practices.

Effigy two. Relationship of soil pH status with barley grain yield and faba bean seed yield, data extracted and synthesised from different experimental findings over locations and years.

Soil pH is the most of import chemical belongings of the soil, which plays a meaning role in constitute growth. Soil acidity, at pH 5.five or lower, can inhibit the growth of sensitive plant species, though information technology has petty effect on insensitive species fifty-fifty at pH lower than four. Crops such equally cotton, alfalfa, oats, and cabbage (Brassica oleracea) practice not tolerate acid soils and are considered suitable to neutral soils with a pH range of 7–eight. Wheat, barley, maize, clover, and beans grow well on neutral to mildly acrid to neutral soils (pH six–7). Grasses tend to tolerate acrid soils better than legumes, so liming to pH v.5 may command acidity without limiting yield. Legumes, all the same, need more than Ca and perform best between pH vi.five and 7.5. Among crops tolerant to acrid soils are millet, sorghum, sweet murphy, potato, lycopersicon esculentum, flax, tea, rye, carrot and lupine (Somani 1996). Poor establish vigour, uneven crop growth, poor nodulation of legumes, stunted root growth, persistence of acid-tolerant weeds, increased incidence of diseases and aberrant leaf colours are major symptoms of increased soil acidity which may lead to reduced yields (Somani 1996; Marschner 2011). Increased acidity is likely to lead to poor water use efficiency due to nutrient deficiencies and imbalance and/or Al and Mn toxicity. High Al concentration also affects uptake and translocation of nutrients, especially immobilisation of P in the roots (Fageria and Baligar 2008; Baquy et al. 2017), jail cell partition, respiration, N mobilisation and glucose phosphorylation of plants (Play a trick on 1979; Haynes and Mokolobate 2001). At elevated Al concentrations in the soil solution, root tips and lateral roots become thickened and turn brown, and P uptake is reduced (Syers et al. 2008).

The pH upshot is compounded and often surpassed by Al and Mn toxicity, Ca and Mo deficiency (Somani 1996; Baquy et al. 2017). Roots are commonly the first organs to show injury attributable to acid due to Al toxicity; they go stunted, stubbly. With stunted roots, constitute'southward ability to extract water and nutrients, particularly immobile nutrients such as P, is severely reduced (Pull a fast one on 1979). Consequently, plants become susceptible to drought and decumbent to food deficiencies. Magnesium deficiency symptoms provide a valuable indicator of acerbity problems (Marschner 2011). Exchangeable Al is the dominant cation associated with soil acidity. The impairment of the root growth of sensitive ingather species is caused when Al in the soil solution exceeds one mg kg−1. This frequently happens when 60% or more than of the exchangeable capacity of the soil is occupied past Al (Fox 1979; Somani 1996). Damage may also be caused by Mn, which becomes very soluble at pH less than 5.five.

Consumption of protein is essential for normal growth and maintenance, however, the recommendation of 0.8 1000 of protein/kg/day is based on the poly peptide source existence 'high quality'- typically derived from an animal source. Plant proteins are usually limiting in one or more amino acids compared to man requirements and contain multiple anti-nutritive factors that further inhibit food availability and assimilation. In the context of acrid soils, at that place take been several investigations into the impact of soil acidity on crop yield, yet an additional consideration regarding population nutrient status is the protein content of crops grown under acid soil atmospheric condition.

Low pH soil can be detrimental to the health of crops, and and then many studies highlight the furnishings of soil treatments on nutritional parameters. Ane written report investigated the effect of boron and sulfur fertilisation on yield and quality of soybean grown on acid soil in India (Longkumer et al. 2017). This group identified that higher quantities of boron (0–1.5 kg ha−1) and sulfur (0–threescore kg ha−1) in the fertiliser resulted in an increase in protein content of soybean seed from 31.7 to 40.6%. A like study in alfalfa institute that liming would increment crude protein content in shoots by upwards to nine% depending on flavor (Dugalić et al. 2012). In addition to the utilisation of agronomic treatments to increase protein content, the genetic background of the crop can play an important office. Investigations have been performed linking resistance to soil acidity and protein content in the grain of wheat (Mesdag et al. 1970) and corn (Halimi 2011), and seed of soybean (Ginting et al. 2018). This has the benefit of identifying cultivars with high poly peptide content likewise as those capable of growing in otherwise detrimental conditions, necessary data for breeders looking to optimise crops for a particular geographic region. It is worth noting that while protein content is an important parameter, there is picayune understanding of the affect of soil acidity on indices of protein quality such as amino acrid composition or protein digestibility.

Improved acid soil management practices

The main causes and subsequent effects of soil acidity on soil properties, nutrient availability and plant growth and different acid soil management options are illustrated in Figure iii.

Figure 3. Illustration of causes, furnishings and management of acid soils in the humid tropical agroecosystems.

Liming

Management of acid soils should aim at improving the production potential of soils past applying amendments to correct the acidity and obtain optimum crop yield. Liming is the application of calcium- and magnesium-rich materials to soils in various forms. The most economical liming materials and relatively piece of cake to manage are calcitic or dolomitic agronomical limestone (Pilbeam and Morley 2007; Rengel 2011). Calcitic limestone is by and large calcium carbonate (CaCO3), while dolomitic limestone is a mixture of Ca and Mg carbonates (CaCOiii + MgCO3) which is usually more desirable as it contains both Ca and Mg. Other liming materials (byproducts or industrial products) include burned lime (CaO), hydrated lime, Ca(OH)ii, and woods ashes (Adams 1990).

The master effects of liming are increasing the available P through inactivation or atmospheric precipitation of exchangeable and soluble Al and Iron hydroxides, increase in pH, bachelor P, exchangeable cations and per centum base saturation, and enhancing the density and length of root hairs for P uptake (Kamprath 1984; Upjohn et al. 2005). Hence, toxicity arising from excess soluble Al, Fe and Mn is corrected through liming and then that root growth is promoted, and uptake of nutrients is improved. Liming also stimulates microbial activities and enhances fixation and mineralisation of Northward, thereby legumes benefit highly from liming (Pilbeam and Morley 2007; Fageria and Baligar 2008), as Al toxicity and acidity suppresses microbial activities and nutrient cycling (Kunito et al. 2016). It should ever be noted that when soils are limed plants should be sufficiently fertilised. Liming materials are normally expressed in CaCOthree equivalent values (CCE) (Yang et al. 2018), and the CCE value of CaCO3 is considered as a standard (100%). The acid-neutralising value of Ca(OH)ii is estimated to exist 135%. The college neutralising chapters of Ca(OH)ii, expressed in CCE as 135%, means that when CaCO3 is used in place of Ca(OH)2, the weight of Ca(OH)2 has to be multiplied past 135%, indicating the need for higher rates of CaCO3 (Table 2).

Table 2. Common liming materials and their calcium carbonate equivalent.

The effectiveness of lime material is expressed past the chemical guarantee as CaCO3, CaO or elemental Ca and by the particle size of the liming materials. The less the particle sizes of the liming material the higher the contact surface of the particle to react with the soil (Somani 1996). The reaction of lime with an acidic soil is described beneath in equation one, which shows acidity (H+) on the surface of the soil particles. As lime dissolves in the soil, Ca moves to the surface of soil particles, reducing soluble Al and Mn to nontoxic levels for plants. The acidity reacts with the carbonate (CO3) to form carbon dioxide (CO2) and h2o (H2O); the result is a soil that is less acidic, with a higher pH (Adams 1990; Somani et al. 1996). The ascension in soil pH is associated with the presence of basic cations (Ca2+) and anions (CO3 −2) in lime that are able to commutation H+ from substitution sites to form H2O + CO2. Cations occupy the space left behind past H+ on the exchange leading to the rising in pH (Abebe 2007; Fageria et al. 2007). (i) Acid soil + CaC O iii = H 2 O + C O 2 + A l 2 O 3 (1)

Determination of soil acidity and amount of lime requirement is associated not only to the soil pH but too to the buffer or CEC (Nelson and Su 2010). The buffering is associated with the corporeality of clay and organic matter present, the larger the corporeality, the greater the buffer capacity. Although harvested crops remove abundant lime-like elements, mainly Ca each year, the soil pH does not modify much from yr-to-year, implying the soil is buffered, or resistant to modify. As the ingather removes these elements from the soil solution, fastened elements move from the soil particles to replenish the solution and over time, reserve elements are depleted enough to cause acidity. Typically clay soils and peats have a larger reservoir and higher buffer capacity than sandy ones, which means that they require more lime to achieve a suitable pH (Fageria and Baligar 2008; Rengel 2011). Coarse textured soils with little or no organic matter have low buffer capacity and, even if acid, crave low lime rate. Hence, to avoid over-liming injury on coarse-textured soils, the relationship between pH and percentage base of operations saturation is important for soils with 1:1 and 2:one clays as a much higher base saturation was required to enhance the pH to 6 for montmorillonite than for kaolinite (Somani et al. 1996). Lime requirement (LR) for crops grown on acid soils is determined by the quality of liming textile, condition of soil acidity, ingather species and varieties and their responses to lime charge per unit, crop management practices, and economical considerations (Fageria and Nascente 2014; Li et al. 2019). Soil pH, base saturation, and Al saturation are important acerbity indices which are used as a basis for decision of lime rates (Fageria and Baligar 2008; Rengel 2011). Lime requirements are expressed in terms of effective calcium carbonate equivalent (ECCE), which is established on the basis of two components: the purity of the lime, adamant chemically by the calcium carbonate content in the lime material, and the fineness of the lime material, adamant by how much it is ground (Ritchey et al. 2016). The more than calcium carbonate and the effectively the material size, the college the ECCE, because the ECCE of lime is not always 100%, the amount of material required to provide that percentage should exist calculated: (2) ECCE lime required × 100 = Lime required ECCE % (two)

Buffering capacity (BC) of soils is amidst the alternative methods for estimating lime requirement. The soil lime buffer chapters is a primal soil property that has many useful applications. It is the measure of the amount of soil acerbity that must be neutralised to raise soil pH by one unit. According to Nelson and Su (2010), awarding of the sigmoid role could facilitate more than accurate assessments of acidification risks, acidification rates, and potential management interventions, particularly every bit soils become increasingly acidic. Use of buffer curves to determine the BC of soil groups is an alternative approach to determine the LR of soil samples. It is the amount of lime required to enhance the pH of an acid soil past one unit of measurement. Buffering chapters is the reciprocal of the slope of the buffer curve. Hence, the LR is determined based on the BC value, target pH, and initial soil pH using the following formula: (3) LR = Target pH − Initial pH of soil sample × BC (3)

The slope of the curve is adamant from the role of the bend that can approximate a straight line. The intercept of the bend on the y-axis is taken equally the first signal to determine the changes in the pH values per unit of lime applied. The equation can provide a very proficient estimate of the lime requirement for the range of soil pH classes. The formula can be valid if it is applied inside the ranges of soil pH values indicated in the equation. The use of BC method for the decision of lime requirement can exist cryptic for some users. To avoid confusion arising from the subjective nature of BC method, Tabular array three tin can serve as a guide.

Table 3. Estimation of lime requirements for unlike soil pH ranges using BC method.

Frequency of liming and length of fourth dimension for lime to work is vital to plan liming. Agricultural lime is not easily soluble in water as it is a natural product, even with acceptable soil moisture, it may take a twelvemonth or more for a measurable change in pH (Adams 1990). Ordinarily, calcium carbonate takes more than fourth dimension to exist soluble in water than slaked lime which consists of by and large calcium hydroxide (Somani 1996). Since neutralisation involves a reaction betwixt soil and lime particles, mixing lime with soil increases the efficiency of acidity neutralisation (Somani 1996; Ritchey et al. 2016;). Short-term effects of lime (less than one year) are likely to be the event of physicochemical effects. On highly-weathered acidic tropical soils, where relatively low lime rates are applied to neutralise exchangeable Al (usually to raise pH-HiiO to 5.iii–five.6), atmospheric precipitation of exchangeable Al every bit hydroxyl-Al species will be the main cistron for improving soil structural condition (Haynes and Naidu 1998).

The residual furnishings of liming are usually expected to last for 5 to seven years. For case, the highest yield of barley was obtained in the 3rd yr later on awarding of lime in Ethiopia, implying that the efficiency of lime was more than in the subsequent years than the get-go and 2d year of its application (Beyene 1987). Application of 200–500 kg lime ha−one year−1 has been reported to be acceptable to maintain the level of Ca and Mg in the soil while keeping a check on the release of exchangeable Al (Somani 1996). Footing limestone may take liming action for several years while hydrated lime and quick lime which are normally composed of fine particles and react quickly in the soil may have to be applied more frequently and at lower rates. Maintaining a favourable pH is extremely important in a soil fertility management plan, where periodical soil testing reveals soil pH levels and provides liming recommendations (Fageria and Nascente 2014; Ritchey et al. 2016). Inspections at intervals non greater than two to three years are appropriate to economise the process of amelioration and to avoid over-liming injury to plants (Rengel 2011). The risk of over-liming is that continued increase in pH can cause Mo to become toxic. In addition, over-liming tin can substantially reduce the bioavailability of micronutrients such every bit Cu, Zn, Iron and Mn due to the low solubility of these nutrients at higher pH levels (Fageria and Baligar 2008).

Assessing the furnishings of lime on soil acidity and crop performance is very crucial to maximise the significance of liming. Researchers reported that soil pH increased from 5.03 to 6.72 and exchangeable acidity (EA) was significantly reduced due to the application of 3.75 t lime ha−1 on Nitisols with an inherent property of high P fixation in southern Ethiopia (Buni 2014). Moreover, liming significantly increased CEC and available P and decreased available micronutrients except Cu. The highest (33.34 cmol kg−i) and lowest (19.18 cmol kg−1) CEC values were obtained from the highest lime charge per unit and the control treatment, respectively (Table iv). Soil pH consistently increased from iv.37 to 5.91 every bit lime rate increased, while the EA was significantly reduced from 1.32 to 0.12 cmol (+) kg−1 due to liming. Desalegn et al. (2017) showed that application of 0.55, 1.1, 1.65 and 2.ii t lime ha−1 decreased Althree+ past 0.88, 1.11, 1.twenty and 1.19 cmol (+) kg−ane, and increased soil pH by 0.48, 0.71, 0.85 and ane.ane units, respectively, in Ethiopia. Besides, Anetor and Akinrinde (2007) reported that liming raised soil pH (half-dozen.one–6.6), and resulted in maximum P release (15.one–17.3 mg kg−one) compared to un-amended soil (4.2–7.1 mg P kg−ane) with pH value of 4.8. Application of lime and P also increased plant tissue P, Ca and Mg concentrations (Agegnehu and Sommer 2000).

Tabular array iv. The effect of lime on chemic properties of soils in Sodo commune, southern Ethiopia.

Liming has significantly improved the response of barley and faba bean to P fertiliser application which is otherwise immobilised due to P fixation in acidic soils (Agegnehu et al. 2006; Alemu et al. 2017). For example, Desalegn et al. (2017) reported that the combined application of one.65 t lime ha−1 and thirty kg P ha−1 resulted in 133% more barley grain yield than the control (without P and lime). According to Hillard et al. (1992), decreasing winter pasture productivity in un-limed Ultisols has been associated with increased soil acidity due to N fertiliser awarding. Thus, over three harvest years, rye grass yields increased 90–750% and 25–eighty% at the highest lime and P rates, respectively. In the second year, yield response to practical P was significantly less at the highest lime charge per unit, indicating that liming made the soil P more establish available. Agegnehu et al. (2006) reported that addition of lime at the rates of i–v t ha−ane resulted in faba edible bean seed yield increments of 45–81% over the control. Mahler et al. (1988) also found that seed yields of legumes were optimal between soil pH values of 5.7 and 7.two and yields of pea could be increased by 30% due to lime application to soils with pH values less than 5.four.

Awarding of NPK fertilisers with lime significantly increased white potato tuber yield in Chencha (20.v t ha−1) than in Hagereselam (13.8 t ha−1) (Figure iv), with 42%–279% higher yield in Chencha than in Hagereselam (Haile and Boke 2009). Notwithstanding, lime application alone did non significantly improve potato tuber yield, indicating that the soil in Chencha is better in fertility and more responsive to the treatments than the soil in Hagereselam which is low in soil pH, nutrient content, and yield. Liming improves the yield of crops if an acidic soil has essential nutrients rendered unavailable to crops due to low pH, just if the soils are already depleted of nutrients, crops respond to lime awarding only (Marschner 2011). Haile and Boke (2009) reported that application of NPK + lime resulted in the highest spud tuber yield of 30.67 in Chencha with yield increments of 332 and 73% over NP and NPK fertiliser treatments alone, respectively. Even so, in Hagereselam, the same treatment resulted in the highest tuber yield of 10.03 t ha−1 with yield increments of 82 and 59% over NP and NPK fertiliser treatments alone, respectively (Figure 4). These marked increases in potato yield is due to K application with NP, suggesting that balanced application of NPK is more than efficient than applying NP solitary in K deficient soils. Thus, to enhance crop product in acidic soils, a sustainable solution should consider a balanced application of nutrients.

Figure 4. Lime and NPK fertilisers effects on tuber yield of Irish spud at Chencha and Hagereselam, southern Ethiopia, 2007-2009. Data synthesised and analysed from Haile and Boke (2009). Error confined stand for ±one SE. Units of lime and fertiliser are in t ha-one and kg ha-1, respectively.

Improving soil organic matter

Enhancement of soil organic affair (SOM) is one of the cardinal approaches of current agronomical research and development is to motivate African farmers to brand improve utilise of organic resources to heighten fertility, convalesce elemental toxicity and protect the soil (Pauw 1994; Vanlauwe et al. 2015; Amede et al. 2021). Green manuring, awarding of compost, farmyard manure (FYM), biochar and retention of ingather residues are the major organic inputs to enhance soil organic affair and recycle nutrients to the soil. Studies have shown that organic matter promotes microbial action, improves soil structure, aeration, nutrient memory and water property capacity (Vanlauwe et al. 2015; Agegnehu and Amede 2017; Amede et al. 2021). Crop residues can recycle nutrients removed from the soil by crops, while green manures contribute substantial amount of North to subsequent crops and at the same time protect the soil against erosion (Xu et al. 2002; Kumar and Sukul 2020). Use of organic matter in the course of ingather residues, light-green manures, FYM, compost or biochar could too reduce the furnishings of toxic elements by extracting them from the soil solution and incorporating them into organic compounds (Sharma et al. 1990; Luo et al. 2017; Cornelissen et al. 2018). When organic inputs are practical, the quality, quantity and types of nutrients supplied by them should be considered every bit they are beefy.

Integrated acid soil management (IASM)

IASM is one of the components of the direction of acid soils. In acid soils, application of FYM releases a range of organic acids that can form stable complexes with Al and Fe thereby blocking the P retentivity sites, and thus the availability and use efficiency of P is improved (Sharma et al. 1990; Prasad and Power 1997; Agegnehu and Amede 2017;). Organic sources such as crop residues, manures, compost and biochar can partially or wholly substitute lime (Sharma et al. 1990; Agegnehu and Amede 2017). The addition of organic amendments to acid soils has been effective in reducing phytotoxic levels of Al, thus resulting in yield increases (Haynes and Mokolobate 2001). Studies in SSA likewise indicated that the incorporation of biochar from dissimilar feedstock to acidic soils significantly increased soil pH, organic carbon, available P, N, exchangeable bases, reduced exchangeable acerbity, and improved yield of crops (Abewa et al. 2014; Mensah and Frimpong 2018).

The practical implication of these processes is that organic residues may exist used as a strategic tool to reduce the rates of lime and fertiliser P required for optimum crop production on acid soils. The application of xxx/ten kg NP ha−1 with 50% manure and compost as N equivalence on acidic Nitisols of Ethiopian highlands increased grain yield of wheat past 129 and 68% compared to the control and 23/10 kg NP ha−1 (50% the recommended rate of NP fertiliser), respectively (Agegnehu et al. 2014). The same rate increased soil pH from 5.0 to 5.6, OC from 1.three to 2.3%, total N from 0.12 to 0.eighteen%, bachelor P from 7.7 to 11.2 mg kg−1, NO3-N and NHfour-Northward 6.2–10.7 and five.9–12.9 mg kg−1, respectively. Similar studies showed that the rest effects of manure and compost applications significantly increased electrical conductivity (EC), soil pH, plant-available P and NO3-N concentrations (Eghball et al. 2004; Walker et al. 2004). Bereft application of one nutrient may cause the loss or the imbalance of other essential nutrients. For example, insufficient awarding of M fertiliser increases leaching losses of Ca, Mg and N (Poss and Saragoni 1992). Therefore, aapplication of organic residues not just increment crop yield through the release of nutrients but besides improve the physicochemical and biological properties of soils.

Awarding of lime with other complementary agricultural practices/inputs offers substantial yield improvements. Every bit indicated in Table 5, yield improvements ranging from 34% to over 252% in wheat, barley, and tef (Abewa et al. 2014; Asrat et al. 2014; Desalegn et al. 2017), 29-53% in faba bean and soybean (Agegnehu et al. 2006; Bekere et al. 2013), and 134–217% in tater (Haile and Boke 2011) in Ethiopia, 111–182% in maize in Republic of kenya (Opala et al. 2018), and 45–103% in Mucuna in Nigeria (Agba et al. 2017) have been reported nether moderate to astringent acid soil conditions. In most cases, P, K and N fertilisers should be applied together with lime and other improved management practices to achieve pregnant yield increases. Moreover, a unmarried application of lime has five to 7 years of productivity benefits. In the longer term, lime-induced increases in crop yields will result in greater input of organic cloth and a buildup in soil organic matter and soil biological activity both of which favour improved aggregate stability and increased porosity (Haynes and Naidu 1998).

Tabular array 5. Effect of lime and other soil fertility management practices on yield of selected crops and soil backdrop.

Sustainable cropping systems

Sustainable cropping system is the approach how unlike crops are grown considering the biophysical and socio-economic conditions to sustainably increase soil quality and ingather yield. Cropping arrangement practices are dictated by ingather variety, farming organization, soil types, agricultural ecology, and market place. Ingather rotation and intercropping systems are major components of a cropping system that are practiced by farmers in diverse ways across SSA. Improved intercropping organization could maximise the complementarity between almanac legume and cereal crops or almanac and perennial crops (Pauw 1994). The rotation of ingather species in time has long been known to increase the productivity of state and sustainability of crop yields (Xu et al. 2002). Rotations that include legumes can host North-fixing bacteria in their roots, contributing to optimum establish growth without increased GHG emissions. Crop rotations improve soil fertility, which play a fundamental role in raising healthy and productive crops. For instance, rotating barley with legumes increased grain yield of barley by 26-93% on acidic Nitisols of Ethiopian highlands, compared to barley monocropping (Agegnehu and Amede 2017). Overall, without improving the overall condition of acid soils full intensification would be unlikely.

Matching of certain trees with specific crops every bit an agroforestry system can provide benefits that exceed the sum of the individual organization components due to the food pumping effect of agroforestry trees which bring nutrients upward from the lower parts of the soil. Agroforestry enhances SOM, agricultural productivity, carbon sequestration, water retention, agrobiodiversity and farmers' income (Paul et al. 2017; Haile et al. 2021). Buresh and Tian (1998) also reported that copse could increase the supply of nutrients inside the rooting zone of crops through biological Nii fixation, call up nutrients from below the rooting zone of crops and reduces nutrient losses through leaching and erosion. Agroforestry is increasingly widespread for restoration of degraded lands to contribute to food security and for economical development. Where trees are central components of the system, they tin greatly help restore landscapes that contribute to improve state productivity, thereby deliver multiple benefits for humans and ecosystem services (Paul et al. 2017; Haile et al. 2021).

The choice and application of the right fertiliser types at the right rates and time is 1 of the major acid soil direction practices. Ammonium-based fertilisers increase acidity as they generate H+ ions when NHiv molecules are nitrified. Ammonium fertilisers can also pb to soil acerbity when nitrate leaching exceeds establish uptake (Goulding 2016; Rahman et al. 2018). The fertiliser rates to exist applied should exist adjusted to recoup for the nutrient removals based on the target yield levels. Application of lime is important to tackle the immobilisation of P due to soil acerbity. Use of cheap P sources such as rock phosphate has also additional benefit of a liming upshot, increasing available Ca, raising pH and reducing exchangeable Al.

Acid-tolerant crops and varieties

Over the past decade, several researchers have focused their efforts on identifying and characterising the mechanisms employed past ingather plants that enable them to tolerate Al toxic levels in acid soils. The ii singled-out classes of Al tolerance mechanisms are those that operate to exclude Al from the root apex and those that allow the establish to tolerate Al accumulation in the root and shoot symplasm (Ma et al. 2001; Kochian et al. 2004). Despite considerable assumptions about some unlike Al tolerance mechanisms, several inquiry findings have focused on root Al exclusion based on Al-activated organic acid exudation from the root apex. Testify is likewise increasing for a second tolerance mechanism based on internal detoxification of symplastic Al via complexation with organic ligands, over again primarily organic acids (Barcelo and Poschenrieder 2002; Garvin and Carver 2003; Poschenrieder et al. 2008).

Several plant species of economic importance are more often than not regarded as tolerant to acid soil conditions. Many of them have their centre of origin in acid soil regions, suggesting that adaptation to soil constraints is office of the evolutionary process (Somani 1996). While the species by and large does not tolerate, some varieties of certain species possess acid soil tolerance. Quantitative assessments of institute tolerance to acid soil stress include tolerance to high levels of Al or Mn, and to deficiencies of Ca, Mg, P, etc. Species and genotypes within a species take been reported to have considerable variation in their tolerance to Al and Mn (Somani 1996; Kochian et al. 2004). The option of varieties or species that perform well at high Al saturation levels and thus need just a fraction of the normal lime requirement is of great applied importance (Table 6). Enquiry on the development of acid-tolerant ingather varieties, such every bit barley, maize, soybean, potato, etc., has been undertaken since the last decade in some SSA countries. For case, in Cameroon, a maize yield increase of 51% was obtained with some P-efficient varieties nether depression soil pH, simply a yield reduction of 37% was observed in other varieties (Tandzi et al. 2018). Likewise, some unmarried cantankerous hybrids expressing superior tolerance to Al toxicity were identified in Kenya (Tandzi et al. 2018).

Table 6. Crops and pasture species suitable for acid soils with minimum lime requirements.

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Source: https://www.tandfonline.com/doi/full/10.1080/09064710.2021.1954239

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