Changes in the surface charge characteristics of degraded soils in the wet tropics through the addition of beneficiated bentonite

Abstract

In their pristine state, soils of the wet tropics maintain highly productive climax rainforests that have an intrinsically tight nutrient cycling capacity. When these ecosystems are disturbed and placed under agronomic production, soil organic matter is rapidly lost due to continuous stirring of surface soils, and consequently, there is a rapid decline in fertility. In this study a methodology is presented that quantifies the degree of degradation that an agronomic system has undergone since land conversion. In an effort to reverse this degradation, a glasshouse study was undertaken to evaluate the effect of applying beneficiated bentonite clays on the surface charge characteristics of 2 degraded soils and their influence on the growth of sorghum.

The properties of an Oxisol cleared of climax rain forest 53 years previously and currently under tea production were compared with an adjacent undisturbed forest. Soil pH declined by approximately 0.6 unit. Organic carbon levels decreased dramatically under the disturbed site, along with exchangeable basic cations. The degree of degradation associated with changed land use was estimated to be 85% for the surface soil horizon.

In an effort to remediate the aforementioned degraded Oxisol and a similarly degraded light-textured Ultisol currently under sugarcane, varying rates (0–40 t/ha) of beneficiated (Ca ²⁺ , Mg ²⁺ , and K ⁺ saturated in a ratio of 8 : 4 : 1) bentonite were applied. Charge fingerprints were produced for each treatment prior to and after the growing of a sorghum crop. The basic cation exchange capacity at soil pH was increased from 1.15 to 3.00 cmol c /kg on a light-textured Ultisol and from 0.8 to 2.00 cmol _c /kg on the Oxisol through the addition of beneficiated bentonites. This increase in surface charge was found to be permanent. Concomitant with the improved charge characteristics was a significant and sustained increase in forage sorghum biomass production with increasing additions of bentonites on both soil types. The cumulative increase in yield between the control and 40 t/ha bentonite application was a 7.7- and 3.1-fold increase for the Ultisol and Oxisol soil types, respectively.