The Effectiveness of Humus Acids from Different Sources in Improving Phosphorus Availability in Soils with Varying Gypsum Content

• Aswad. H. Aswad1, Ahmed Mahmood Ibrahim2*, Naeem A. Mutlag3

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This study evaluated the effectiveness of humic substances extracted from cow manure and date palm fronds in improving phosphorus availability in two soils with different gypsum contents. The soils contained 98 and 445 g kg⁻¹ gypsum and were designated S1 and S2, respectively. Total humic substances, humic acid, and fulvic acid were applied at 0, 20, 40, and 60 L ha⁻¹, together with phosphorus supplied as triple superphosphate at 0, 50, and 100 mg P kg⁻¹. An experiment was conducted between October 2025 and March 2026 using a completely randomized design (CRD) consisting of three replications and 144 treatments in total. Soil samples were incubated at 25 ± 1 °C for 30 days while moisture was maintained at 60-80% field capacity, and available phosphorus was subsequently determined. Phosphorus availability was consistently lower in S2 than in S1 and increased with extract application rate. Total humic substances generally produced higher available phosphorus than the separated humic and fulvic acid fractions. Without mineral phosphorus addition, mean available phosphorus was 5.596 and 4.944 mg kg⁻¹ for cow-manure extracts in S1 and S2, compared with 5.390 and 4.708 mg kg⁻¹ for palm-frond extracts. At 100 mg P kg⁻¹, the corresponding means were 13.066 and 10.073 mg kg⁻¹ for cow manure and 12.004 and 9.753 mg kg⁻¹ for palm fronds. The highest reported concentration, 18.073 mg kg⁻¹, occurred in S1 receiving total cow-manure humic substances at 60 L ha⁻¹ and 100 mg P kg⁻¹. These findings support the combined use of organic extracts and mineral phosphorus to improve phosphorus availability in gypsiferous soils. Further testing is required to establish field application rates and agronomic optima.

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