Visual field assessment of soil structural quality in tropical soils

Abstract

Visual field assessments have already been tested for ‘temperate’ soils, but there is scant information about their applicability to ‘tropical’ soils. This survey contributes to the validation of the visual field assessments by comparing the performance of three of such methods on ‘tropical’ soils. This study was conducted across six different soils with contrasting soil type and land use, in the central-northern part of Venezuela between November 2011 and January 2012. Scores provided by the soil quality scoring procedure (SQSP), the visual evaluation of soil structure (VESS), and the visual soil assessment (VSA), as well as soil physical properties were measured to assess the soil's structural quality. All methods showed that soil structural quality was unfavourable on a loamy soil (Alfisol) with continuous cereal growth, conventional tillage and low soil organic carbon (SOC), as well as on a silty clay soil (Alfisol) under natural vegetation and cattle production. Where SQSP scores ranged between 1 (extremely firm) and 2 (firm), VESS scores ranged from 4 (compact) to 5 (very compact), and VSA scores were between 0 (poor) and 0.5 (moderately poor). The sandy clay loam (Ultisol) and clay loam (Mollisol) soils under no-tillage and with high SOC had the best soil quality. In our ‘tropical’ Venezuelan soils there was also high correlation (P<0.01) between the visual assessment scores and soil physical properties such as bulk density (BD), porosity, SOC, and saturated hydraulic conductivity (Ksat), as has been reported for ‘temperate’ soils. A visual poor condition of soil structure corresponded to BD values higher than 1.4Mgm−3, porosity lower than 0.5m3m−3, SOC below 25gkg−1, and Ksat (log) values under 0.5cmh−1. In those cases where the rooting system could not be evaluated because of fallow, VSA and VESS appeared to be the most appropriate methods for assessing the soil structure. The rating of the indicator ‘number of earthworms’ should be adjusted for ‘tropical’ soils; this shall improve the accuracy of the VSA method. These methods were capable of distinguishing the different soil structural quality, and are therefore suitable for assessing soil structural quality of ‘tropical’ soils with contrasting soil type and land use.