Influence of organic manure amendments on water repellency, water entry value, and water retention of soil samples from a tropical Ultisol

Abstract

Lowered stability of soil aggregates governed by insufficient organic matter levels has become a major concern in Sri Lanka. Although the use of organic manure with water repellent properties lowers the wetting rates and improves the stability of soil aggregates, its effects on soil hydrophysical properties are still not characterized. Therefore, the objective of this study was to examine the relation of water repellency induced by organic manure amendments to the water entry value and water retention of a Sri Lankan Ultisol. The soil was mixed with ground powders of cattle manure (CM), goat manure (GM), Gliricidia maculata (GL) and hydrophobic Casuarina equisetifolia (CE) leaves to obtain samples ranging from non-repellent to extremely water repellent, in two series. Series I was prepared by mixing GL and CE with soil (5, 10, 25, 50%). Series II consisted of 5% CM, GM, and GL, with (set A) and without (set B) intermixed 2% CE. Water repellency, water entry value, and water retention of samples were determined in the laboratory. Soilwater contact angle increased with increasing organic matter content in all the samples showing positive linear correlations. Although the samples amended with CE showed high soil-water contact angles in series I, set A (without 2% CE) and set B (with 2% CE) in series II did not show a noticeable difference, where >80% of the samples had soilwater contact angles <90°. Water entry value (R2 = 0.83–0.92) and the water retention at 150 cm suction (R2 = 0.69–0.8) of all the samples increased with increasing soil-water contact angles showing moderate to strong positive linear correlations. However, set A (without 2% CE) and set B (with 2% CE) in series II did not differ noticeably. Water entry value of about 60% the samples was <2.5 cm. Mixing of a small amount (2%) of hydrophobic organic matter with commonly used organic manures slightly increased the water repellency of sample soils, however not up to detrimental levels. It did not generate adverse effects on water entry and increased the water retention. It was clear that intermixing of small quantities of hydrophobic organic manure with organic manures commonly used in Sri Lankan agriculture, would not generate unfavorable impacts on soils.