Practical assessment of soil degradation on smallholder farmers' fields in Zimbabwe: Integrating local knowledge and scientific diagnostic indicators

NEZOMBA, H.; MTAMBANENGWE, F; TITTONELL, P.; MAPFUMO, P,

CATENA

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2017

0341-8162

Crop production on smallholder farms in Southern Africa is increasingly being constrained by deteriorating landquality. Assessing soil degradation at farm-level is therefore important to enable formulation and better targetingof rehabilitation options. In this study, farmers' local indicators and scientific diagnostic parameters werecombined to develop criteria for assessing soil degradation on croplands in Hwedza smallholder farming area inEastern Zimbabwe. Farmers used common weed species, crop performance aspects and soil physical attributes tocategorize arable soils into four main productivity classes: productive, moderately productive, degraded andseverely degraded. Broad-leafed weeds, that included Commelina benghalensis L., Bidens pilosa L. and Leucasmartinicensis (Jacq.) Ait. f., were considered as main indicators of productive soils, while grass weeds, Melinisrepens (Willd.) Zizka and Eragrostis minor Host, were perceived as typifying severely degraded soils. Maize grainyields of 6 to 8 scotch carts ha−1 (3 to 4 t ha−1) and< 2 scotch carts ha−1 (< 1 t ha−1) were indicative ofproductive and degraded soils, respectively. Consistent with farmer criteria, broad-leafed weeds contributedmost of the weed biomass on productive soils, with diversity higher on productive and moderately productivethan on degraded soils. Properties of soils sampled from field productivity classes differed significantly(p < 0.05) and the magnitude of the differences was also influenced by farmer resource endowment. Soilorganic carbon (SOC), mineralizable nitrogen (N), available phosphorus (P), exchangeable calcium (Ca) andeffective cation exchange capacity (ECEC) decreased from productive to severely degraded soils, and fromresource-endowed (RG1) to resource-constrained (RG3) farms. Most of the soil properties showed a decline(negative degradation indices) when compared with uncultivated lands. Exchangeable Ca and mineralizable Nshowed a strong relationship with SOC (R2 > 0.5) across the soil productivity classes and farmer resourcegroups. Principal components analysis of 16 soil physico-chemical and biological properties revealed mineralizableN, microbial biomass N, exchangeable bases and available P as the most important parametersinfluencing soil productivity. Drawing on the relationship between weed species populations and soil propertiesobserved in this study, a soil degradation assessment scheme is proposed for enhancing decision-making onoptions for rehabilitation of croplands by smallholder farmers in similar agro-ecologies in Southern Africa.