Visual evaluation of soil structure is a reliable method to detect changes in the soil quality of Colombian Amazon pasturelands

Abstract

The visual evaluation of soil structure (VESS) is an affordable and easy-to-use method for assessing soil quality that could help in the early detection of soil quality changes in pasturelands of less developed countries where ranchers cannot afford quantitative soil studies. Here, we assessed the soil quality of three pasture areas in the Colombian Amazon region using the VESS method and tested its suitability through a correlation analysis with key physical and biochemical soil indicators of quality measured at the same study locations. Moreover, by integrating all assessed soil indicators, we determined a soil quality index (SQI) to correlate with VESS scores. A forest area was used as a reference to evaluate changes in soil indicators and soil quality due to pasture use for >25 years. Our results revealed high VESS scores, indicating poor soil quality in pasture areas and suggesting a compaction process that starts at 6.5 cm soil depth, corroborated by increases in soil bulk density, soil resistance to penetration, and reduction in soil porosity. Soil C and N contents were 35% and 33% lower in pasture than forest. This same pattern was observed in the geometric mean of the enzymatic activity. The VESS scores were significantly correlated with most of physical and biochemical soil indicators and with the overall SQI, demonstrating the ability of VESS to integrate and reflect attributes related to the essential physical, chemical, and biological functioning of soils from the Colombian Amazon region, becoming a useful tool for detecting signs of soil quality degradation in pasturelands.