Abstract:
Soil water repellency is caused by low-energy surfaces where the attraction between solid
and liquid phases is weak. Water repellency is positively correlated with soil organic matter
and occurs when mineral particles are hydrophobized by coatings of organic substances.
Repellent soil becomes wettable when soil water content is above a critical value. The
purpose of his study was to determine the water dependent repellency of pre-wetted
soil samples amended with different amounts of organic matter considering the critical
water content, the highest potential water repellency, and the water content at the highest
potential water repellency in relation to the added organic matter content and the water
repellency of the samples. A soil sample taken from the surface (Ultisol: 0-5 cm) was airdried
and passed through 1 mm sieve. Decomposed and dried plant materials (Casuarina
equisetifolia allowed for decomposition for 18 months under air-dried condition) were
used as the organic matter (OM) amendment. Ground OM was passed through 0.5 mm
sieve and mixed with soil at contents of 1, 5,10, 25, and 50%. Soil without OM (0% OM)
was used as the control. All the samples were wetted with distilled water to approximate
saturation. Then the samples were allowed to air drying and the water repellency of the
samples during drying was estimated with water drop penetration time (WDPT) test. All
the soil samples with OM contents ranging from o to 50% showed very low penetration
times at approximate saturation. With gradual drying, WDPT increased gradually,
except in the control (0% OM), up to a maximum level and thereafter decreased with
decreasing water content. The water repellency of the control didn’t show any difference
and the WDPT remained continuously at a very low value. At oven dried condition, water
repellency decreased to very low values. Critical water content and the OM content showed
linear correlation (R2=o.99i4), showing high critical water content at high OM content.
The highest WDPT during drying increased with the increasing OM content (4 s at 1% OM,
and 414 s at 50% OM) showing a power correlation (R2=o.963i). The water content at
which the samples showed the highest WDPT increased with increasing OM content and
the potential water repellency (R2 = 0.9968 and 0.9641, respectively).
