By Gift Briton

With soil acidity being a major problem in tea and coffee regions mainly in the highlands of Kenya, the issue of how much lime to apply in order to raise the soil pH to the required levels has been a big challenge to most farmers.

According to Prof. Njue Mugai, an Associate Professor of Soil Science and Plant Nutrition of the Department of Horticulture and Food Security, Jomo Kenyatta University of Agriculture and Technology (JKUAT), raising the soil pH above 5.6(over liming) and applying less lime to pH less than 5.5(under liming), may lead to low crop yields.

Furthermore, for a low base soil to realize its full potential in terms of yields, a soil PH of 5.5 is ideal, he noted.

Through his digital lime calculator (, farmers can now calculate the required amount of lime to apply on their farms by just keying in the soil pH in the calculator.

After carrying out field experiments on acid soils in central Kenya, Prof Mugai developed a digital lime calculator that uses a regression equation to determine the amount of lime required to raise soil pH to the recommended levels.

Farmers only need to get their soil pH determined by an extension worker then key in the soil pH value in the digital lime calculator and it estimates the liming amount automatically.

Prof. Mugai further notes that there are three main problems associated with soil acidity; aluminum toxicity, deficiency of calcium, magnesium and phosphorous.

Prof mugai speaking during the interview at JKUAT

However, many methods of estimating lime requirements developed in temperate countries are not relevant to Kenyan soils, while other methods also require laboratory work, a process which is beyond farmers and extension workers, he said.

“This lime calculator is a very timely innovation that will be very helpful to students who are doing their projects on soil acidity, farmers and researchers,” he said.

According to him, liming (anything that when applied to soils raises its pH) precipitates the toxic aluminum which is the major cause of low yields in acid soils. Aluminum toxicity reduces the root growth of crops leading to stunting of the crops.

Even though there are many types of limes used to raise the soil pH, Prof Mugai warns that the use of calcium carbonate lime is not recommended because if used to raise the pH to around 5.5 the yields in long term decline due to lack of magnesium.

Therefore, he recommends a lime that contains both calcium and magnesium like the dolomitic lime.

According to him, factors leading to soil acidity includes weathering of soil, application of inorganic fertilizers especially those with ammonium nitrogen and farming systems that deplete soil organic matter which buffers the toxic aluminum.