Abstract:
If a solid is not completely wettable by water, it is considered to be water repellent or
hydrophobic. Organic matter in soils is found be highjy related to the soil hydrophobic
conditions. Several studies showed that hydrophobicity is positively correlated with the
soil organic matter (SOM) content while some others have found more complex
relationships. The objective of this study was to examine the relation of SOM content at
different stages of decomposition to soil hydrophobicity under laboratory conditions.
Soils were taken from the Wilpita Natural Forest in Matara District, low country wet
zone of Sri Lanka. The Great Soil Group was Red Yellow Podzolic soils (USDA Soil
Taxonomy classification: Udults). Soil was air dried under the laboratory conditions
(74±5% relative humidity at 28±1°C) and passed through 2 mm sieve. Two different
types of organic manure, namely, cattle manure (CM), goat manure (GM), and leaves of
Casuarina (Casuarinaequisetifolia) (CE), were thoroughly air dried, grinded usirig^a
mechanical grinder, sieved with 1 mm sieve, and mixed with soil in 5 different
percentages (0, 5, 10, 25, and 50%). The hydrophobicity of samples was measured using
the water drop penetration time (WDPT) test. About 5 g of air-dried samples, in
triplicates, was taken to weighing bottles. One drop (50±1 pL) distilled water was placed
on the soil surface. Time taken for penetration of the water drop was measured. The
SOM content of samples was measured colorimetrically and loss on ignition method. All
the measurements were taken, in triplicates, at 1, 3, 7, 14, and 30 d. Hydrophobicity of
all the soil samples amended with different types of organic manures (CE, CM, and
GM) showed positive correlation with SOM content at all stages of the decomposition
process. Initial WDPT increased with increasing SOM content. The slope of the linear
regression line was 28^44% steeper for CE amended samples compared with those
amended with CM and GM. The CE amended samples showed significantly higher
(p<0.05) hydrophobicities at the same SOM content than those with CM and GM (at 15-
lb g 100 g~' SOM, the WDPT of CE samples = 1400 s, WDPTof CM, GM= 20^30 s).
Results revealed that soils amended with CE acquired high initial hydrophobicity
compared with those amended with CM and GM. This can be considered as governed by
the original hydrophobicities of added manures (WDPT>3 h for 100% CE; WDPT=3-5
min for 100% CM and GM). Intermediary and ultimate hydrophobicities showed a
positive linear correlation (R2= 0.85-0.95) with SOM content of samples with no
significant differences among type of the amended manure.Difference in the slopes of
the linear regression lineswere<20% and <10% for intermediary and ultimate
relationships, respectively. Therefore, it is clear that intermediary or ultimate
hydrophobicities of the samples are related more to the SOM content than to the
composition of SOM, while initial hydrophobicity related more to the composition of
SOM.