Asian Journal of Soil Science and Plant Nutrition

Declining soil fertility and soil acidity are major limitations to sustainable crop production in the Kenyan highlands, where Nitisols are intensively cultivated for maize production. Excessive exchangeable acidity, particularly in the form of aluminium and hydrogen ions, can restrict root growth, reduce nutrient availability, promote phosphorus fixation and limit the uptake of exchangeable bases. Conventional soil assessments often focus on the upper 0-30 cm of soil and may therefore fail to identify constraints that vary with depth. Comprehensive soil profile characterization is consequently important for understanding the vertical distribution of acidity, nutrient availability, clay accumulation, compaction and biological activity, as well as for developing appropriate soil-management practices. The study characterized a representative Nitisol profile at Kariene highland in Meru County, Kenya to examine the vertical distribution of its morphological, physical, chemical and biological properties. A profile pit 150 cm deep was excavated and the soil was described according to the FAO Guidelines for Soil Description. Because the study focused on the characterization of a single representative soil profile, the results were analyzed descriptively without inferential statistical analysis. The profile consisted of Ap, AB, Bt₁, Bt₂ and Bt₃ horizons and had developed from basaltic volcanic parent material. It showed a gradual increase in clay content and well developed subangular blocky structures with depth. The soil was classified as a Humic Nitisol and assigned to suitability class S2, indicating moderate suitability for agricultural production. Bulk density increased from 1.12 to 1.34 g cm⁻³, whereas total porosity decreased from 56.07 to 49.43%, indicating increasing soil compactness with depth. Soil pH declined from 4.77 in the Ap horizon to 4.71 in the Bt₁ horizon before increasing to 5.40 in the Bt₃ horizon. Exchangeable acidity increased from 2.93 to 3.20 cmol (+) kg⁻¹ in the upper and middle horizons before declining to 2.88 cmol (+) kg⁻¹ in the deepest horizon. The initial increase in acidity and decline in pH may reflect the accumulation of acidic cations and organic derived acidity in the upper and middle horizons. The subsequent increase in pH and decline in exchangeable acidity at depth may be associated with reduced organic matter inputs, changes in mineral composition and the redistribution or leaching of acidic ions. Available phosphorus declined markedly from 38.19 mg kg⁻¹ in the Ap horizon to 8.21 mg kg⁻¹ in the Bt₃ horizon. Exchangeable calcium, magnesium and potassium also generally declined with depth, from 1.70 - 1.08 cmol (+) kg⁻¹, 0.82 to 0.71 cmol (+) kg⁻¹ and 0.81 to 0.67 cmol (+) kg⁻¹, respectively. Iron increased from 123 to 135 mg kg⁻¹ down the profile, suggesting a greater potential for phosphorus fixation in the deeper, iron rich horizons. Total organic carbon and total nitrogen declined with depth, from 2.14 to 0.91% and from 0.18 to 0.07%, respectively. Roots, pores and other biological features were concentrated mainly in the Ap and AB horizons and were scarce or absent in the deeper horizons. The profile exhibited strong vertical differentiation, with acidity and nutrient constraints concentrated mainly in the upper and middle horizons and with reduced nutrient availability and biological activity at depth. These findings demonstrate the importance of whole profile assessment for diagnosing soil constraints and informing appropriate liming, organic matter management and balanced fertilization strategies in maize based production systems.