Peri-urban agricultural management impacts the soil nutrient status of Nitisols in the central highlands of Kenya

In sub-Saharan Africa, urban and peri-urban agriculture (UPA) plays an important role in improving food and nutrition security amid rapid urbanization in the region. Poor soil fertility has been suggested as the primary reason for low crop productivity in the region, however UPA is often associated with soil nutrient accumulation in other developed or recently developed nations. There is still a lack of evidence regarding soil nutrient status of UPA in this region. To understand the current soil fertility status and the impacts of soil nutrient management, an evaluation of soil profiles, surface soil conditions, and current nutrient management was conducted in Kiambu County, which is a part of the Greater Nairobi Metropolitan Region of Kenya. Soil samples were collected from both the Tigoni experimental station and 26 surrounding small-scale peri-urban farms. All three soil profiles were classified as Eutric Nitisols with well-developed soil structure, adequate cation exchange capacity, relatively high phosphate retention, and high base saturation. The surface soil nutrient status between the Tigoni experimental station and the surrounding farms showed significant differences except for exchangeable K, which is likely derived from the parent materials. Available phosphate (Truog-P) levels were excessively high, exceeding 1000 mg‑P₂O₅ kg⁻¹ in three commercial farms, and nearly half of the farms were in the excessive range, contrary to the past findings in other counties in Kenya. In contrast, phosphate levels at the experimental station were mostly deficient. Exchangeable Ca levels followed a similar trend to Truog-P, while Mg was deficient relative to K at the experimental station and relative to both K and Ca in the commercial farm fields. The use of animal manure is considered a major factor contributing to the high P and Ca levels observed in the peri-urban farm fields. Future studies are needed to explore the relationship between the current variable soil fertility status and vegetable productivity. Additionally, the introduction of a low-cost soil testing method is necessary to establish a system for optimizing the soil conditions.