Little is known about the effects of the individual components of organic matter (OM) on aggregate stability (AS). We hypothesized that AS of a Typic Haplustalf from which native OM was either removed or retained would be affected by incubation periods and application rates of a hydrophilic polysaccharide gum (G) and a hydrophobic stearic acid (S) with or without pretreatment with a hydrophobic humic acid (HA). Removal of OM reduced AS of unmodified soil by ≈40 and 20% after soil incubation for 7 and 40 d, respectively. In both soil samples, AS was best at the highest rate of G (5.0 g kg−1). Its effect was better on Soil A (where OM was removed) than Soil B (where OM was retained) but diminished rapidly during 40 d. At this rate, G increased AS by 750% in Soil A and by 335% in Soil B compared with no addition. With S, aggregate stability increased more with time in Soil B than in Soil A. Its maximum effect was also at the highest application rate (5.0 g kg−1), where AS increased 100% on Soil A and 131% on Soil B. At the highest rate (0.2 g kg−1), HA increased AS by 73% on Soil B and 27% on Soil A. The effect of HA alone did not vary with time. Soil pretreatment with HA before addition of G reduced significantly both the state of aggregation and AS of both soils. The reverse occurred when HA was applied before S. After 40 d, S+HA increased AS in Soil B by 34%, whereas G and G+HA decreased AS by 14 and 4%, respectively. We found that soil AS was improved and maintained with time more by hydrophobic than by hydrophilic components of organic matter. Long‐lasting aggregate stability of soils can be thus achieved by addition of hydrophobic humic material with hydrophobic organic wastes.

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