Role of hydrophobic components of soil organic matter in soil aggregate stability

Little is known about the effects of the individual components of organic m atter (OM) on aggregate stability (AS), We hypothesized that AS of a Typic Haplustalf from which native OM was either removed or retained would be aff ected by incubation periods and application rates of a hydrophilic polysacc haride gum (G) and a hydrophobic stearic acid (S) with or without pretreatm ent with a hydrophobic humic acid (HA). Removal of OM reduced AS of unmodif ied soil by approximate to 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 rat e (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% o n 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 aggreg ation 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 decre ased 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 achie ved by addition of hydrophobic humic material with hydrophobic organic wast es.