Effects of Organic Waste Amendments, Application Methods, and Irrigation Levels on Soil Organic Matter in Two Contrasting Soil Textures Under Maize Cultivation

• Najlla M. Abad-Alhaleam1*, Salah M. S. Al-Atab2, Amen H. J. Muzan3

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Soil organic matter (SOM) is a key determinant of soil fertility and productivity in arid agricultural systems, yet SOM levels in southern Iraq remain low because of intensive cultivation and limited organic inputs. A two-site field experiment was conducted in Basrah Governorate, Iraq, to evaluate the effects of organic amendment type (composted cattle manure, composted date palm residues, and an unamended control), application method (uniform mixing and placement at 10 or 20 cm depth), irrigation regime (100% and 80% of available water), and soil depth (0–15 and 15–30 cm) on SOM accumulation in a silty clay soil (Site 1) and a sandy loam soil (Site 2) under maize cultivation. A four-factor factorial experiment was arranged in a randomised complete block design with three replicates, and SOM was determined by the Walkley–Black method at mid-season and end-of-season. All four factors exerted highly significant effects on SOM (P ≤ 0.01), with organic amendment type showing the strongest influence: composted cattle manure produced the highest SOM at both sites, reaching 27.0 g kg⁻¹ in the silty clay soil (35.0% higher than the unamended control) and 20.0 g kg⁻¹ in the sandy loam soil (42.9% higher than the control) by season end, which may partly reflect its lower C:N ratio (14:1) and higher nitrogen content. Uniform mixing outperformed depth-placement methods by up to 18.5%, full irrigation exceeded deficit irrigation by 4–6%, and the surface layer (0–15 cm) retained 2–3% more SOM than the subsurface layer. No statistically significant interaction effects were detected among the tested factors. These findings indicate that composted cattle manure, uniform incorporation, and adequate irrigation were each the most effective individual management levels for restoring soil organic matter in degraded arid soils.

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