Soil Acidity and Liming Handbook for Kenya

Abstract

Soil acidity develops from a combination of natural and anthropogenic processes. Its effects can be evaluated through its impact on soil’s physical, chemical, and biological activities that ultimately affect nutrient availability and uptake by plants. Soil testing and data interpretation are integral parts of soil acidity management. Soil testing provides information on the acidity status of soils that can be used for making lime application recommendations. Soil acidity has adverse impacts that threaten Kenya’s food security and limit agribusiness potential. Acidic soils hamper crop production and are a major cause of crop yield reduction, resulting in reduced agricultural incomes. Currently, 13% (7.5 million hectares) of Kenya’s soils are acidic, which translates to approximately 63% of Kenya’s arable land. Soil acidity is concentrated in the Central, Western, and Rift Valley regions – the main food baskets of Kenya. In addition, soil acidity is also found in some parts of Eastern and Coastal regions. Several strategies have been documented for managing acid soils in Kenya. These include addition of liming materials, use of organic materials, judicious choice and application of fertilizers combining lime with organic materials and inorganic fertilizers (Integrated Soil Fertility Management or ISFM) and growing acid tolerant crop species. However, these options for managing soil acidity are not currently accessible to most farmers. Liming of acid soils is the most common and effective amelioration strategy for improving crop production. It enhances the soil’s physical, chemical, and biological characteristics. The reduction of soil acidity indirectly mobilizes plant nutrients immobilizes toxic aluminium (Al) and manganese (Mn) and improves soil structure. As the term is used in agriculture, liming is the addition of any calcium (Ca) and/or magnesium (Mg) containing compounds that can reduce soil acidity. A range of liming materials, which vary in their ability to neutralize soil acidity, is available. The most used liming materials are carbonates of Ca or Ca and Mg, including ground limestone, dolomitic ground limestone, and ground chalk. The amount of liming material required to achieve a target pH that is favourable for crop production is defined as lime requirement. Many methods are used in different countries to determine lime requirements. Regardless of the method used to determine lime requirement, it is advisable to avoid excessive lime applications because it can significantly depress yields. The efficiency of lime use can be improved by applying the 4R principle that is used in improving fertilizer use i.e., right source, right rate, right placement, and right timing. Although the agronomic benefits of liming are well known, the practice is not yet common in Kenya. As of 2023 adoption of lime by farmers was very low ranging between 1% and 8%. Considerable research on lime use has been conducted in Kenya. However, the range of crops studied is not wide and most focus is on maize, which is the staple food. These studies show that yield response to liming vary from 0 to > 400%. The increase in yields depends on initial soil pH, the crop, soil characteristics, and the lime rate. It is therefore vital to have soils tested for soil pH and exchangeable acidity before liming is recommended. Lime must, however, be applied with other nutrients, particularly macronutrient NP and NPK fertilizer which are usually the most limiting on most smallholder farms. Kenya does not have a fully developed supply chain for agricultural lime (ag-lime). The value chain of ag-lime and policy considerations have not been extensively evaluated, shared and exploited. The ag-lime business in Kenya is still in early development stages. Aspects of ag-lime market dynamics such as market overview, market drivers, supply chain analysis and market challenges/constraints need to be studied. Comprehensive policy, legal and regulatory frameworks that lead to increased use of ag-lime need to be developed to encourage local manufacturing, distribution, and utilisation. Although soil acidity has been identified as a major constraint to crop production by the Government of Kenya and other stakeholders in agriculture, there are no comprehensive plans to correct it. To address acidity, the use of lime is encouraged. However, inefficiencies and challenges arise due to several reasons. A key recommendation is the creation of a national action plan to rehabilitate acidic soils. Various stakeholders should coordinate efforts (national and county governments, development partners, private sector, and other players) to increase awareness and support for soil acidity and liming. Executive Summary XIV Soil Acidity and Liming Handbook for Kenya In conclusion, strategic research is needed to develop and promote liming in addition to integrated crop, soil, water, soil fertility management practices for acidic soils. Manuals for simple field tests, laboratory procedures, and lime requirement recommendations based on the soil test results need to be developed for different crops and varieties. Further, lime response studies need to be conducted to determine economical and optimum rates of liming soils in Kenya. In order to improve lime and liming materials’ adoption, the government should increase its budgetary allocation to support agricultural research, development and knowledge dissemination that addresses soil acidity. There is also a need for a national economic assessment of the benefits of liming agricultural soils in Kenya. The government should put together new regulations as part of a comprehensive policy framework for regulating lime use and application, with an appropriate legal framework that aligns with the current global liming trends. A lime business model for management of acidic soils in Kenya is urgently required. A first step is a detailed action plan, which is a step-by-step approach to develop the lime value chain. This will serve as a basis for donors, the national and county governments to fund activities that develop the value chain.