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Stability analysis of aggregates in relation to the hydrophobicity of organic manure for Sri Lankan Red Yellow Podzolic soils

Abstract Aggregate slaking is linked with rapid pressure buildup within aggregates. Soil water repellency may help hamper the pressure buildup within aggregates by reducing their wetting rates. We examined the effects of animal manure in improving aggregate stability, the hydrophobic effects of green manure, and the possibility of using organic manure mixtures to increase the aggregate stability for Sri Lankan red yellow podzolic soils using model aggregates. Almost all the cow dung (CD) added samples showed extremely low percentages of water stable aggregates (%WSA) demonstrating rapid destruction of aggregates. Although the addition of ≥ 10% goat dung (GD) improved the %WSA, aggregate floating occurred, showing the risk of aggregate floating with runoff water. Addition of 5% GD would be an acceptable solution if the %WSA can be improved. Casuarina equisetifolia L. leaves (CE) was found to be a hydrophobic green manure. Although addition of ≥ 5% CE increased the %WSA up to about 90%, aggregate floating occurred. The possibility of improving %WSA using 1–2% hydrophobic green manure in organic manures mixtures was tested. Samples with 5% GD + 2% CE manure mixture showed the highest and the most stable %WSA without showing aggregate floating. Additions of compost and poultry litter were found not to be effective in improving aggregate stability with or without CE. Strong or higher water repellency was not observed in any of the samples with manure mixtures, showing that the addition of 1–2% hydrophobic CE would not induce detrimental effects of water repellency. There was no clear correlation between %WSA and the hydrophobicity of soils. However, the %WSA can be considered to show a tendency to increase with increasing hydrophobicity, because the %WSA was very high in samples with hydrophobic CE, the %WSA increased when mixed with 1–2% CE, and samples with highest water drop penetration time (WDPT) among all the manure mixtures showed the highest %WSA.