The Implication of Soil Acidity and Management Options for Sustainable Crop Production in Africa

By Shamie Zingore, Temesgen Desalegn, Asseta Diallo, Tialhun Amede, Samuel Njoroge, Madani Diallo, Lilian Wanjiru, and Øystein Botillen 

Acidic soils are widespread in Africa and present a major crop production challenge in a region faced with multiple climatic and soil constraints. Effective management recommendations in line with the 4Rs of lime management (Right Source, Rate, Time, and Place) are necessary for optimizing the agronomic and economic benefits of lime at the farm-level. In addition, policies and incentives are needed to develop viable lime supply chains. 

The soil degradation challenge in sub- Saharan Africa 
Soil degradation is recognized as a major underlying factor for both low crop productivity and high prevalence of malnutrition in sub-Saharan Africa (SSA) (Sanchez et al., 2002). It affects the majority of the 60% of Africans who directly depend on agriculture for food and income. Soil degradation in cropping systems is driven by suboptimal management practices that induce declines in soil biological, chemical, and physical quality, and reduce soil’s capacity to support production and environmental functions. About 65% of the land area in SSA is classified as degraded (Vlek et al., 2008). Degraded soil accounts for about 350 million (M) ha or 20-25% of the total land area, of which about 100 M ha is estimated to be severely degraded mainly due to agricultural activities. Soil degradation costs SSA approximately $68 billion (B) yr-1 and reduces the regional annual agricultural gross domestic product (GDP) by 3%. 

The major constraints associated with degraded soils in SSA include aluminum (Al) toxicity, low cation exchange capacity, soil erosion, shallow soil depth, high phosphorus (P) fixation, vertic properties, salinity, and sodicity (Fig. 1). It has long been recognized that soil acidity is one of the most serious challenges to agricultural production worldwide in general. Limitations associated with soil acidity, including Al toxicity and high P-fixation, are pervasive in SSA. 

Figure 1. Major soil quality problems in sub-Saharan Africa and their distribution (adapted from Tully et al. 2015). 

The status of soil acidity in SSA 
In SSA, acid soils occupy > 15% of the total land area, and the problem is expanding both in area and severity (Agegnehu et al., 2021). The major drivers of soil acidity are parent material, climate, vegetation, landscape position, atmospheric depositions, nutrient mining, and management. Soil acidity develops on old, stable surfaces exposed to tropical weathering. Acid soils predominantly occur in humid and sub-humid regions in SSA (Fig. 2). In these regions, frequent heavy rains cause rapid nutrient leaching and soil chemical weathering. The main soil types affected by acidity in SSA are Ferralsols (covering 312 M ha), Acrisols (88 M ha), Alisols (20 M ha), and Nitisols (60 M ha). Management-induced acidity is often caused by unbalanced fertilization (e.g., continuous application of high rates of N fertilizers), continuous cropping without organic inputs, and lack of soil conservation measures resulting in soil erosion. Notably, fertilizer-induced acidity is not widespread in SSA due to low fertilizer application. 

Figure 2. Distribution of acidic soils in sub-Saharan Africa (Leenaars et al. 2014). 

Soil acidity is severe in areas where annual precipitation exceeds evapotranspiration and is a leading cause of low agricultural productivity. Crop productivity is constrained in acidic soils due to their poor fertility, P fixation, Al toxicity, and fragile structure. A significant limitation on acidic soils is P fixation caused primarily by a high concentration of Al and Fe oxides and hydroxides, which fix phosphate ions in forms that are unavailable to plants. This causes P deficiency that is difficult to overcome, since added phosphate fertilizers rapidly become fixed in the soil. Over 820 M ha of land area in SSA experiences high P fixation problems. Soil acidity reduces crop yields by about 10% in tropical areas (Sierra et al., 2003), and in Kenya, acidic soils are estimated to reduce yields by 16-28% (Ligeyo, 2007; AGRA, GBD, 2016). Considering the occurrence of acidic soils and potential to increase productivity with liming and other appropriate management practices, acid soils have become a subject of high priority for agricultural research and development in SSA. 

The negative spiral of soil acidity and low crop productivity 
Although crops vary widely in their tolerances to soil acidity, severe soil acidity has been shown to limit even highly acid-tolerant crops. The following are the specific detrimental effects of soil acidity on crop productivity: 

Aluminum and manganese toxicities: Al and manganese (Mn) are toxic to plant roots and result in poor root development. This results in poor water and nutrient uptake. During low rainfall seasons, Al toxicity magnifies the effects of drought. However, the extent of Al toxicity to roots depends on the relative quantities of Al and bases [principally calcium (Ca) and magnesium (Mg)]. For this reason, the soil acid saturation index is a more reliable indicator of the Al toxicity hazard than soil pH. 

Deficiencies of calcium and magnesium: Levels of Ca and Mg in acid soils are often very low and may limit plant growth. Adequate supplies of Ca in the soil are particularly critical for root growth. The combination of high Al and deficient Ca concentrations in subsoil is a common yield-limiting factor. 

Fixation of phosphorus: Acid soils also have high P fixing capacity. Hence, a large proportion of the P applied as fertilizers in acidic soils are not available for crop uptake. Low P availability to crops is cited as a major factor limiting crop production on acid soils (Desalegn et al., 2016). Phosphorus deficiencies and Al toxicities often occur simultaneously in many acid soils and combine to exacerbate poor yields in acid soils. 

Micronutrient deficiencies: Deficiencies of some micronutrients, in particular molybdenum (Mo), frequently limit plant growth on acid soils. In the case of Mo, most soils contain adequate reserves of this nutrient for plant growth, but its availability for plant uptake is severely reduced under acidic conditions. 

Soil biological activity: Acidic soil conditions negatively impact soil biological activity. Al toxicity and acidity suppress microbial activity and nutrient cycling (Kunito et al., 2016). Soil acidity also affects other soil organisms, including most earthworm species, resulting in reduced soil biodiversity and bioactivity. 

Management of soil acidity 
Soil acidification is a natural process that can be amplified by human activity or controlled by appropriate soil management practices. Several agricultural practices have been recommended to overcome the problem of tropical acidic soil infertility worldwide. The main methods include liming, use of organic inputs, planting low pH-tolerant crops, crop rotation, and balanced and effective fertilizer management (Fig. 3). An integrated management approach that uses multiple complementary technologies offers the most effective strategy for correcting soil acidity. 

Figure 3. Using lime on acidic soils, along with other good agricultural practices, can significantly increase crop productivity (Mbakaya et al., 2010; in AGRA, GBD, 2016). 

Applying agricultural lime is one of the most effective ways to reduce soil acidity and increase crop productivity (AGRA, GBD, 2016). The main sources of lime material include ground Ca and/ or Mg carbonates and hydroxides. Applications of lime at appropriate rates and timings can have an immediately positive effect on soil pH and soil physical, chemical, and biological properties. The most economical and relatively easy to manage liming materials are calcitic or dolomitic agricultural limestone. Calcitic limestone is mostly calcium carbonate (CaCO3), while dolomitic limestone is usually more desirable since it has a mixture of Ca and Mg carbonates (CaCO3+MgCO3). Other liming materials include burned lime (CaO), hydrated lime (Ca(OH)2), and wood ashes. 

The main benefits of liming include: (i) increased available P through precipitation of exchangeable and soluble Al and Fe hydroxides; (ii) increased exchangeable cations and percent base saturation; (iii) increased density and length of root systems and enhanced uptake of nutrients and water; and (iv) stimulation of microbial and biological activities that lead to enhanced N-fixation by legumes and greater N mineralization. These benefits of lime depend on various 4Rs of lime management including the following key considerations: 

Right Source: Lime requirements are often expressed in terms of effective calcium carbonate equivalent, which is based upon two criteria: (i) the purity of the lime, determined by the calcium carbonate content in the lime material, and (ii) the fineness of the lime material, determined by how much it is ground (Ritchey et al., 2016). 

Right Rate: Liming rates depend on the site-specific conditions including soil pH, desired pH, and soil texture.

Right Time: Lime should be applied at least three months before planting season for sufficient reaction time. 

Right Place: Reaction of lime is also accelerated when lime in incorporated well mixed in the soil. 

Using organic inputs 
Increasing soil organic matter (SOM) through application of green manure, farmyard manure, compost, biochar, and crop residues buffer soil pH while also enhancing soil fertility (Agegnehu et al., 2021). A growing body of evidence shows that organic inputs promote microbial activity, improves soil structure, nutrient retention, and water holding capacity. The organic acids from these inputs can form stable complexes with Al and Fe, thereby blocking the P retention sites. Therefore, regular application of organic inputs can reduce soil acidification. Application of lime along with improved SOM has been shown to enhance nutrient use efficiency (NUE) of fertilizer applied to cereal crops in Ethiopia (Amede and Diallo, 2022). 

Growing low pH-tolerant crops 
The two distinct classes of Al tolerance mechanisms are those that operate to exclude Al from the root apex and those that allow the plant to tolerate Al accumulation in the roots and shoots. Crops with a high soil acidity tolerance include rice, wheat, potatoes, cowpea, and maize among others (Agegnehu et al., 2021). The Ethiopian Institute of Agricultural Research (EIAR) has successfully released acid-tolerant crop varieties for bread wheat, food oats, sweet lupin, and triticale through its intensive breeding program. 

Using appropriate fertilizers 
Applying the right source of fertilizer at the right rate, time, and place is a critical element of managing acid soils. For example, ammonium-based fertilizers can increase soil acidity as they generate H+ ions when ammonium (NH4+) molecules are oxidized. Nitrate and sulphate-based fertilizers can also acidify soils due to nitrate (NO3-) and sulphate (SO42-) leaching that is accompanied by exchangeable bases. Fertilizers containing Ca and Mg are less acidifying. 

Agegnehu et al. (2021) provides examples of yield improvement in barley, beans, faba beans, potato, soybean, teff, and wheat with liming and complementary inputs over experimental control treatments in Ethiopia. Adequate liming increased maize and wheat yields by about 70% compared to recommended NPK fertilizer rates alone. Another study conducted in the central highlands of Ethiopia using different rates of lime and P fertilizer showed that liming at 1.65 t ha-1 gave a 133% yield advantage over the control (Desalegn, 2010). Similar experiments conducted in southeastern Ethiopia to validate modeled lime requirements found that yield increases due to liming can be very high with incremental lime rates (Fig. 4) if soil nutrients are not limiting and agronomic management is optimized. Similarly, Hijbeek et al. (2021) reported consistent increases in maize yields in Kenya due to liming, but associated profits were only positive if NP fertilizer was included and returns on liming investments were positive only after at least two years. 

Figure 4. Grain yield increase with increased rate of lime (Desalegn and Dawit, 2022).

Achieving impact with lime at scale in SSA 
The critical role of liming to mitigate soil acidity and reduce phytotoxic levels of Al and Mn has been well-researched and widely documented. Liming has achieved substantial yield gains in several countries. However, there is a need for identifying areas where lime application brings significant change and benefit in crop yield. For example, about 43% of the cultivated land in Ethiopia is acidic, of which 28% is strongly acidic. About 9 M t of lime is required to amend only the strongly acidic areas in Ethiopia. Accurate information is lacking in most countries on the area requiring liming. 

Initiatives such as GAIA (Guiding Acid Soil Management Investments in Africa – https://www.cimmyt.org/projects/gaia), implemented in Ethiopia, Kenya, Rwanda, and Tanzania by CIMMYT in partnership with national research institutions and international organizations, is developing data-driven and spatially explicit recommendations for the rehabilitation of acid soils in smallholder farming systems. GAIA’s overall goal is to increase returns on investment for all stakeholders in the lime sector including farmers, private entities, and Government. Initial results show a potential lime requirement of 50-96 M t across the four countries. There are, however, several challenges on the demand side including limited demand from farmers, limited financing, and high costs of production and distribution. Analysis conducted at the regional level show that the ROI for liming at the farm level varies substantially between countries, depending on crop types (Fig. 5). Agronomic trials suggest that legumes are currently more responsive to liming than cereal crops, and that the greatest economic returns to lime are achieved with a relatively low (1 t ha-1) lime rate. 

Figure 5. Returns on investments (ROI) to liming in SSA countries (GAIA project). Crop prices were based on average FAO prices across SSA. Lime requirement was based on Kamprath (1970). Lime price = US$100 t ha-1. ROI = sum of returns from extra production of all crops in a pixel divided by the lime requirement in that pixel times the lime price. 

To develop viable lime value chains, there is need to stimulate a level of demand within the crop production systems that can support high ROIs in the lime value chain to reduce key logistical costs, such as transport. There is also need to build functional markets and enact supportive policies on taxes, land, investments, and standards among others. 

Empowering soil acidity management 
The fertilizer industry is working with stakeholders to remove the barriers to expanding soil acidity management. In Tanzania, lime use is still very limited, and it is a challenge to make it available to farmers in an affordable manner. Private- Public partnerships (PPPs) like the one formed in 2016 between Yara, the Southern Agricultural Growth Corridor of Tanzania (SAGCOT), Tanzania Agricultural Development Bank (TADB), and Uyole Agricultural Research Institute brought together the key stakeholders needed to coordinate the implementation of, for example, field demonstrations and trials in Mbozi and Mbeya Districts. 

At the farm level, it is important to ensure that a lime recommendation is part of a soil analysis report (Fig. 6), and it is further supported by farmer/distributor awareness sessions, digital tools such as AfricaConnect and FarmCare, dependable access to lime and fertilizer supply chains, and policy lobbying and advocacy for widespread adoption of liming. 

Figure 6. Yara fertilizer and lime recommendation in Tanzania based on soil analysis (Megalab, Pocklington). 

In Kenya, the private sector and farmers face challenges that restrict lime market development and use including the lack of clear quality regulations guidelines and standards for agricultural lime, lack of a coordinated nationwide lime promotion program and limited incentives to support the production, distribution, and use of agricultural lime, among others. The government of Kenya recently established its liming flagship program with a first phase of four years. However, further support for private sector participation and promotion of liming, including soil analysis, will be needed for long-term sustainability of the liming program. 

Specific recommendations for improved soil acidity management include: 
1 Building PPPs for long-term coordinated and harmonized lime market development and demand creation at the farm-level, targeting the regions most affected by soil acidity, 
2 Research on liming management options, including source, rate, time, and place to develop innovations for effective lime use on-farm, and 
3 Standards for lime quality and labeling of lime products to help farmers make the right choice.

In Mali, Quarries and Lime of Mali (CCM), established in 2010 to produce and supply dolomitic lime in West Africa, provides a rapid and cost-effective soil acidity testing kit that is used locally by farmers to test for soil pH and site-specific lime requirements. The soil pH test kit uses litmus paper and a cheap container (glass or half plastic bottle) of 100 ml of water. Soil and water are mixed 1:1 and the litmus paper is soaked into the mixture to reveal a color that is compared with a standardized pH scale. 

The litmus paper is available in booklets of 40 to 80 strips at a cost of less than $1 per booklet. This cheap and simple method enables farmers to adapt the test method to their region by first defining the pH of test water, and then determining their lime requirement. 

Trials done in the Sikasso region of Mali in the cotton (CMDT) and cereal [Mali Office Haute Vallée du Niger (OHVN)] production areas with the Institut d’Economie Rurale (IER) show yield improvement with liming compared to fields without lime [e.g., in cotton (1,303-1,818 kg ha-1; +39%), sorghum (704-1,139 kg ha-1; +62%), and maize (1,239- 1,849 kg ha-1; +49%)]. 

Policy, incentives, and potential: Lessons from Brazil 
The Brazilian experience provides an excellent example of acid soil management and valuable lessons for SSA. Brazil has developed over 60 M ha of acid soils (pH of 4.8-5.1) in the Cerrado with the implementation of appropriate technologies and inputs, infrastructure, and policy support. These soils are also deficient in many essential plant nutrients including P, K, Ca, Mg, and sulfur (S). Until the 1970s, the Cerrado was of limited value for agricultural production. Liming for a base saturation of 50% together with corrective and maintenance application of PKS and micronutrients transformed the region into one of the bread baskets of the world today. This was made possible by public investments in agricultural research and development, rural credit, and price support, and land-tenure policies supportive of both large-scale and smallholder farmers. 

It is important for countries to continue to formulate appropriate policies including land tenure policies supportive of long-term investment in lime by commercial as well as smallholder farmers. Fiscal incentives (e.g., income tax exemptions, subsidies) should also be formulated to support private investment in acid soil management. Developing national and regional standards and guidelines will ensure the quality and proper use for better returns on investments.

Dr. Zingore(e-mail: s.zingore@apni. net) is APNI Director of Research & Development, Benguérir, Morocco. Dr. Desalegn is a Soil Scientist, EIAR, Addis Ababa, Ethiopia. Drs. Diallo and Amede are with AGRA, Nairobi, Kenya. Dr. Njoroge is APNI Scientist, Nairobi. Dr. Diallo is Founder and Director of CCM, Karaga, Mali. Dr. Wanjiru is Senior Agronomist, ICL East Africa, Nairobi. Mr. Botillen is Yara’s Stakeholder Relations and Business Development Manager, Oslo, Norway. 

Cite this article 
Zingore, S., Desalegn, T., Diallo, A., Amede, T., Njoroge, S., Diallo, M., Wanjiru, L., Botillen, Ø. 2023. The Implication of Soil Acidity and Management Options for Sustainable Crop Production in Africa, Growing Africa 2(1), 32-38. https://doi.org/10.55693/ga21.IFCZ1970

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