Assessing grassland soil degradation through key soil physical and chemical properties in smallholder farms of Western Kenya

In humid Africa, grassland degradation is widespread, with overgrazing as a major factor, affecting soil health and structure, and vegetation composition. Understanding this degradation is vital for targeted restoration. We assessed grassland degradation and its effects on soil properties and plant diversity in western Kenya at two contrasting sites —Kuresoi and Nyando—classified as degraded or non-degraded based on grazing intensity and land-use history. We analysed soil carbon (SOC), nutrient concentrations (TN, available P) and aggregate stability. Field measurements included soil resistance and hydraulic conductivity, alongside vegetation inventory.

The results show higher SOC and total nitrogen (TN) in non-degraded topsoil (SOC: 6.66 ± 2.21% in Kuresoi, 2.41 ± 0.51% in Nyando; TN: 0.56 ± 0.188% in Kuresoi, 0.149 ± 0.027% in Nyando) compared to degraded soils (SOC: 4.38 ± 1.37% in Kuresoi, 1.93 ± 1.22% in Nyando; TN: 0.351 ± 0.123% in Kuresoi, 0.172 ± 0.082% in Nyando); low and variable phosphorus content (Kuresoi: 3.17 ± 5.80 µg/g in degraded, 4.13 ± 8.52 µg/g in non-degraded; Nyando: 2.33 ± 2.76 µg/g in non-degraded and 3.96 ± 6.52 µg/g in degraded) across sites. We observed high aggregate stability, ranging from 61.3%–92.6%, across sites. Infiltration rates were higher in non-degraded Kuresoi (463 ± 913 mm/hr) than degraded (40.3 ± 45.6 mm/hr), with similar rates ((76.9 ± 82.1 mm/hr in non-degraded and 69.6 ±99.3 mm/hr in degraded) in Nyando. The soils were generally compacted (1.07–6.7 MPa in Kuresoi; 1.82–10.1 MPa in Nyando), with no significant differences between degraded and non-degraded soils. Species diversity indices, Shannon (H’= 2.69 ±0.39 in non-degraded Kuresoi, and 2.54 ±0.18 in degraded Kuresoi; H’ = 2.85 ± 0.32 in non-degraded Nyando, and 2.75 ± 0.21) and Simpson (D = 0.92 ± 0.03 in non-degraded Kuresoi, and 0.91 ±0.01; D = 0.93 ±0.01 in non-degraded Nyando and 0.92 ± 0.015), indicated high diversity across sites.

The findings indicate that while overgrazing driven degradation affects key soil properties such as phosphorus, compaction, and infiltration rates, some soil characteristics like aggregate stability and species diversity remain resilient. Proper grazing management, coupled with soil organic matter amendments, could improve nutrient availability, restore soil structure, and strengthen grassland resilience.