Lime and Manure Combo Lifts Maize Yields on Ethiopia’s Acidic Highlands

In the highland district of Meket, in the Amhara region of northeastern Ethiopia, farmers have been watching their maize fields quietly decline for years. The culprit is not drought or pests but chemistry: the soils beneath their feet have grown steadily more acidic, locking away nutrients, stunting roots, and draining productivity from plots that once fed families and generated surplus grain. A new field experiment published in PLOS One by Endalkachew Fekadu, Yirbeb Woretaw, and Mohammed Hussen offers a carefully quantified answer to this slow-motion crisis, and its findings could reshape how acid-affected farmland across the Ethiopian highlands is managed.

Soil acidity is one of the most widespread and underappreciated constraints on crop production in tropical highlands. As soils acidify, aluminum and manganese become increasingly soluble and toxic to plant roots, while essential nutrients such as phosphorus, calcium, and magnesium are rendered less available. The result is a double penalty: roots cannot explore the soil properly, and even the nutrients that remain are chemically trapped. In Ethiopia, the problem has been expanding across the highlands, where decades of continuous cultivation, the removal of crop residues, and the leaching of base cations from abundant rainfall have pushed soil pH below the thresholds at which maize, the country’s most important cereal, can express its full yield potential.

Farmers and researchers have long known two broad remedies. Lime, typically applied as calcium carbonate, neutralizes acidity and reduces toxic aluminum. Organic amendments such as farmyard manure improve soil structure, feed soil microbes, and slowly release nutrients. Yet, as the authors of the new study point out, the appropriate rates and, crucially, the right combinations of these two inputs had not been sufficiently documented for specific crops and locations. Blanket recommendations risk either under-treatment, which wastes a season, or over-application, which wastes scarce resources in farming systems where both lime and manure are costly or in limited supply.

To close that gap, the research team designed a factorial field experiment in Meket District that crossed four manure rates, 0, 4, 8, and 12 tonnes per hectare, with three lime rates, 0, 1.5, and 3 tonnes of calcium carbonate per hectare. Every combination was tested in a randomized complete block design with three replications, a standard but rigorous layout that separates true treatment effects from the natural variability of field soils. The team then measured not only maize grain yield but also stalk yield, total aboveground dry biomass, and a suite of soil chemical properties, including organic carbon and available phosphorus, providing a picture of both productivity and soil health.

The headline result is striking for its nuance. The maximum grain yield, 4,087 kilograms per hectare, was not harvested from the highest rates of either amendment. Instead, it came from the combination of 8 tonnes of manure with 1.5 tonnes of lime per hectare, a moderate pairing that outperformed both the untreated controls and the more aggressive treatments. This matters because it suggests a sweet spot: enough lime to ease the acidity that constrains root growth, and enough manure to supply nutrients and organic matter, without pushing either input past the point where additional quantities stop paying for themselves.

Biomass told a different story. The tallest, heaviest maize plants, measured as stalk yield of 11,155 kilograms per hectare and total dry aboveground biomass of 14,803.7 kilograms per hectare, were significantly higher, at the P < 0.01 level, in plots that received the full combination of 3 tonnes of lime with 12 tonnes of manure. For farmers who grow maize partly for stover, the stalks and leaves that feed livestock through the long dry season, this distinction is economically meaningful. A treatment that maximizes grain may not maximize fodder, and the study’s data allow farmers and extension agents to choose the balance that fits their household needs.

Beneath the surface, the soil itself responded in ways that explain the yield patterns. Plots that received 12 tonnes per hectare of manure alone recorded the highest soil organic carbon content, 1.96 percent, an increase of 54 percent over the untreated soil. Organic carbon is the engine of soil fertility in tropical agriculture: it improves water retention, supports microbial communities, and acts as a reservoir for nutrients. Available phosphorus, often the most chemically starved nutrient in acid soils because it binds tightly with iron and aluminum oxides at low pH, peaked at 14.41 milligrams per kilogram in plots treated with 3 tonnes of lime plus 12 tonnes of manure, again a statistically significant difference. The mechanism is straightforward but powerful: lime raises pH, releasing phosphorus from its chemical bonds, while manure supplies additional phosphorus and chelating compounds that keep it in plant-available forms.

The broader significance of the study lies in its demonstration that integrated soil management, rather than any single silver bullet, is the key to restoring productivity on acidified highland soils. Neither lime nor manure alone achieved the best grain yield. Lime without organic matter risks depleting soil structure and carbon over time, while manure without lime cannot fully overcome the aluminum toxicity and phosphorus fixation that acidity imposes. Together, they address both the chemical and the biological dimensions of soil degradation, and the study’s rates give Ethiopian extension services concrete, locally validated numbers to recommend rather than generic guidelines imported from other regions.

There are also economic and practical implications worth weighing. Lime must be transported, often over long distances from quarries or processing plants to remote highland districts, and its cost can be prohibitive for smallholders. Manure, by contrast, is produced on-farm but competes with its use as household fuel in the form of dung cakes, a common reality in Ethiopian highlands where firewood is scarce. The finding that a moderate rate of 8 tonnes of manure with 1.5 tonnes of lime delivered the best grain yield suggests that farmers may not need to max out either input, easing the burden on household labor and cash. The authors conclude that the integrated application of 3 tonnes of lime with 8 tonnes of manure per hectare could help increase maize yield and profitability while maintaining soil fertility in the acid-affected soils of the Meket area and similar locations.

As acid soils continue their quiet expansion across the Ethiopian highlands and other tropical mountain regions, studies like this one provide the evidence base for a response that is both scientifically grounded and practically achievable. The Meket experiment does not promise a miracle; 4,087 kilograms per hectare remains a modest yield by global standards. But it demonstrates, with the statistical rigor of a replicated field trial, that degraded acid soils can be brought back into productive service with inputs that farmers can realistically access, and that the health of the soil itself, measured in organic carbon and available phosphorus, can be rebuilt at the same time. For millions of smallholder families farming weathered highland soils, that combination of higher harvests and improving fertility may be the most important news of the season.

Subject of Research: Effects of lime and manure application on maize yield and soil chemical properties in acid soils of northeastern Ethiopia

Article Title: Maize (z ea mays l.) yield, and selected chemical properties of soils as influenced by lime and manure application in acid soils of Meket District, northeastern Ethiopia

Article References: Fekadu, E., Woretaw, Y., & Hussen, M. (2026). Maize (zea mays l.) yield, and selected chemical properties of soils as influenced by lime and manure application in acid soils of Meket District, northeastern Ethiopia. PLOS One, 21(10), e0360551. https://doi.org/10.1371/journal.pone.0360551

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0360551

Keywords: soil acidity, maize, lime application, farmyard manure, Ethiopian highlands, soil fertility, organic carbon, available phosphorus, grain yield, integrated soil management, Meket District, PLOS One