Liming Boosts Millet Yields and Cuts Greenhouse Gas Emissions on Kenya’s Acid Soils

Across the tropics, one of the quietest threats to food security is the slow chemical decay of the soil itself. In western Kenya, decades of intense weathering have stripped the land of its base nutrients, leaving behind acidic Ferralsols and Acrisols with pH values hovering between 4 and 5. On these soils, roughly 63 percent of Kenya’s arable land is now considered acidified, and smallholder farmers pay the price in stunted roots, locked-up phosphorus and disappointing harvests. A new field study published in the journal Plant and Soil offers a strikingly simple countermeasure: ordinary agricultural lime. Researchers found that a single application of calcium hydroxide raised finger millet yields by up to 95 percent at two sites in western Kenya, while simultaneously lowering emissions of nitrous oxide, one of the most potent greenhouse gases in the atmosphere.

The chemistry behind Kenya’s acid soils is unforgiving. Under humid, high-rainfall conditions, essential base cations such as calcium, magnesium, potassium and sodium are leached away and replaced by hydrogen and aluminium ions. Once soil pH drops below 5.5, aluminium hydroxide begins to dissolve, releasing highly mobile and phytotoxic aluminium ions into the soil solution. These ions accumulate in plant tissues and, above all, cripple root development, cutting off the plant’s access to water and nutrients. At the same time, the iron and aluminium oxides that dominate these highly weathered soils carry increasingly positive surface charges under acidic conditions, binding negatively charged phosphate ions through electrostatic adsorption and ligand exchange. The result is a paradox familiar to tropical agronomists: soils may hold considerable total phosphorus, yet almost none of it remains soluble and available for plant uptake.

For many Kenyan smallholders, the standard response has been to pour on mineral fertilizers such as diammonium phosphate or NPK compounds. But earlier work cited in the study showed that phosphorus applications of 26 to 52 kilograms per hectare on Kenyan acid soils achieved recovery rates of only 9.6 to 13.5 percent, with the rest fixed by soil minerals. Achieving adequate yields therefore demands fertilizer quantities that are often simply unaffordable. Liming offers an alternative route: by neutralizing excess hydrogen ions, calcium-rich amendments push soil pH above the critical 5.5 threshold, converting toxic aluminium ions into less soluble and less harmful hydroxide forms while deprotonating iron and aluminium oxide surfaces so that phosphorus is released back into solution. Despite these well-documented benefits, lime remains rarely used in Kenya because of high costs, limited availability and gaps in farmer knowledge.

The research team, led by Eric Scherwietes of the Leibniz Centre for Agricultural Landscape Research in Germany together with colleagues from the Karlsruhe Institute of Technology, Aarhus University, the University of Eldoret and Justus Liebig University Giessen, tested two remedies side by side. At a primary site near Eldoret in Uasin Gishu County, at 2410 metres elevation on soil with a pH of 4.7, they applied calcium hydroxide at a rate equivalent to 5 tonnes per hectare. Alongside the lime, they incorporated sediment collected west of Lake Baringo in the Kenyan Rift Valley at rates of 1 and 3 percent by volume, corresponding to 24 and 72 tonnes per hectare. This sediment, originally applied in 2022, is a calcium-rich, aggregated matrix containing aluminium- and silicon-bearing particles, shaped by volcanic tuffs and tephra that enrich it with base-forming cations, phosphorus and silicon. A secondary site near Malava in Kakamega County, on ferralitic soil with a pH of 4.9, received lime only.

The crop was finger millet, a staple grain of East Africa and a fascinating physiological case in its own right. Unlike barley, which had shown dramatic responses to the same treatments in previous experiments, finger millet is a C4 plant with high water-use efficiency, a deeper root system and a documented tolerance of aluminium, allowing it to maintain active photosynthesis and stable yields even under acidic conditions. The researchers sowed the variety P224 manually at both sites in spring 2024, fertilized all plots uniformly with NPK 17:17:17 at 50 kilograms per hectare before planting and again after germination, and harvested in August and November 2024. At the Eldoret site, whole-plant samples were collected at four growth stages, from tillering through harvest, and soil samples from the top 15 centimetres were analysed for pH, calcium, aluminium, silicon and phosphorus availability using Mehlich III extraction and ICP-OES.

The results were unambiguous for lime. In Eldoret, limed plots produced 32.2 tonnes of dry biomass and 8.12 tonnes of grain per hectare, significantly more than the control plots, which yielded 19.94 tonnes of biomass and 4.32 tonnes of grain, an 88 percent yield increase. At Malava, liming raised yields by 95 percent to 3.27 tonnes per hectare. Underlying these gains were dramatic shifts in soil chemistry: liming lifted soil pH to 6.86, more than doubled calcium availability and cut aluminium availability to 2.65 milligrams per gram, all highly significant changes. The Baringo sediment at 3 percent produced comparable chemical effects, raising pH to 6.52 and, uniquely among the treatments, significantly increasing silicon availability, yet its yield gains of roughly 45 percent fell short of statistical significance. The researchers attribute this muted response to finger millet’s own resilience: a crop already adapted to acid Ferralsols simply has less to gain from soil improvement than a sensitive species like barley, which had responded to the same sediment with yield increases exceeding 1000 percent in earlier work.

The greenhouse gas findings may prove even more consequential. Nitrous oxide has a global warming potential far higher than carbon dioxide and is a major contributor to stratospheric ozone depletion, and agricultural soils are among its dominant anthropogenic sources. Using a portable laser-based LI-7820 analyser connected to static chambers, the team measured fluxes once or twice weekly for 13 weeks at Eldoret. Limed soils emitted the lowest mean fluxes, 39.3 micrograms per square metre per hour, compared with 47.5 for controls and 42.5 to 49.6 for sediment treatments. Yield-scaled emissions showed significant negative relationships with both soil pH and calcium availability. The mechanism is microbial: higher pH improves the abundance and stability of nitrogen-cycling microbes, particularly nitrous oxide reducers, which convert the gas all the way to inert nitrogen, while improved plant growth and nitrogen uptake leave less mineral nitrogen available for microbial production of the gas in the first place.

The measurements also captured dramatic hot moments. Fluxes remained stable during the dry pre-planting phase, then surged after the first rains and sowing coincided with fertilization, peaking up to two orders of magnitude above background levels. Limed soils responded earliest, likely because acidity had already been alleviated and re-wetting rapidly activated microbial nitrogen transformations, while control plots recorded the highest peaks the following week. Emissions subsided within a week despite continued rainfall. The authors caution that measurements taken once or twice weekly, with a gap in April, may underestimate seasonal totals, and that high-frequency automatic chambers would be needed to confirm the treatment differences, since rainfall-driven pulses are notoriously easy to miss.

For the millions of smallholder farmers working Kenya’s acidified Ferralsols, the study distils into a clear message: liming acidic soils is a rare double win, boosting staple grain yields while trimming a powerful greenhouse gas. The Baringo sediments, though less effective for a tolerant crop like finger millet, still improved soil chemistry three years after a single application, suggesting they could serve as an affordable, long-lasting fertility amendment, particularly for acidity-sensitive crops or severely degraded soils. As climate pressures intensify across East Africa, practices that simultaneously raise harvests and lower emissions may become not just agronomically attractive but essential.

Subject of Research: Effects of liming and local sediment amendment on millet yield and soil nitrous oxide emissions in acidified Kenyan soils

Article Title: Effects of liming and local sediment amendment on millet production and soil N2O emission in western Kenya

Article References: Scherwietes, E., Wangari, E. G., Mwanake, R. M., Gettel, G., Bawen, T. K., Butterbach-Bahl, K., & Schaller, J. (2026). Effects of liming and local sediment amendment on millet production and soil N2O emission in western Kenya. Plant and Soil. https://doi.org/10.1007/s11104-026-08856-2

Image Credits: AI Generated

DOI: 10.1007/s11104-026-08856-2

Keywords: liming, soil acidification, finger millet, nitrous oxide, Kenya, Ferralsols, aluminium toxicity, phosphorus fixation, greenhouse gas emissions, soil pH, Rift Valley sediment, smallholder agriculture