HUMIC SUBSTANCES AND SURFACTANTS EFFECTS ON THE STABILITY OF 2 TROPICAL SOILS

Soil conservation in the tropics is influenced by soil management practices that include addition of organic matter and agrochemicals, which often contain surfactants. Changes in the structural stability of a weakly aggregated Entisol and a strongly aggregated Ultisol from southern Nigeria following additions of humic acids (HA) and anionic (AS) or nonionic (NS) surfactants were evaluated in a laboratory study. Four rates of HA and four rates of either AS or NS (0, 0.1, 1.0, and 10 g kg-1) were used. At the macroaggregation level, the percentage of water-stable aggregates (WSA) increased with increasing rates of RA and NS, whereas the AS reduced WSA at atl application rates on both soils. At high application rates, the combination of RA and NS increased WSA more than either of the soil additives acting alone. Humic acids also significantly reduced the magnitude of disaggregation caused by AS additions on the soils. For example at zero HA addition, the 10 g kg-1 AS rate reduced WSA by 13.8% on the Ultisol and 23.8% on the Entisol, whereas at the highest combined rates of both additives WSA increased by 5.3 and 47.6% on the Ultisol and Entisol, respectively. At the microaggregation level the combination of HA and NS significantly decreased the amount of dispersible clay (i.e., increased stability) especially at higher application rates. On the Ultisol each RA and AS combination rate decreased stability but on the Entisol addition of HA significantly reduced the amount of water-dispersible clay induced by AS application. At moderate to high application rates, therefore, humic acids appear to have the potential to positively counteract the negative effects of anionic surfactants on the structural stability of fragile soils.