Competitive Adsorption of Phosphate and Humic Acid in Calcareous Soils with Variable Caco3 Content | ||||
| Egyptian Journal of Soil Science | ||||
| Volume 65, Issue 2, June 2025, Page 909-928 PDF (684.75 K) | ||||
| Document Type: Original Article | ||||
| DOI: 10.21608/ejss.2025.362778.2015 | ||||
 View on SCiNiTO | ||||
| Author | ||||
Mohamed Salah kotb  
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| Soil Science Department, Faculty of Agriculture, Cairo University, Giza 12613, Egypt | ||||
| Abstract | ||||
| Calcareous soil is susceptible to phosphorus deficit due to its high phosphate adsorption and fixing rate, which is mostly caused by a high concentration of phosphate adsorbent. Amorphous Al, Fe, and Mn oxides, together with CaCO3, are the main phosphate adsorbents in calcareous soil. They essentially serve as the same binding or adsorbent for humic acid products. The availability and adsorption of P in agricultural soils following phosphate fertilizer treatments may therefore be hampered by humic acid (HA) in calcareous soil. The current study sought to investigate the effects of humic acid (HA) on phosphate competitive adsorption in calcareous soils with different CaCO3 contents by administering HA at a rate of 2.5 g kg−1 soil. To reflect the calcareous soils of Egypt, five soil samples with varying calcium carbonate levels (10.6%, 22.1%, 31.7%, 42.5%, and 51.3%, respectively) were gathered. These samples are referred to as S1, S2, S3, S4, and S5. This soil's phosphate adsorption isotherm was calculated using phosphorus concentrations of 0.5, 1.0, 2.5, 5.0, 10.0, 20.0, and 40 mg P L−1 at its natural pH values. The Langmuir and Freundlich adsorption isotherms models were used to numerically characterize the phosphate adsorption isotherm of the soil samples. To study the competitive sorption of phosphate and humic acid (HA) (order of addition), Three experiments have been carried out to predict the amount of adsorbed-P on soils in the presence of humic acid (HA) 2.5 g kg−1 soil, amounts of P added (5, 10, 25, 50, 100, 200, and 400 mg kg−1), and order of P and HA additions. The order of addition were (i) P before humic acid (P/HA), (ii) P together humic acid (P+HA), and (iii) P after humic acid (HA/P). The results show that the effect of HA was limited at the low initial amounts of added P (25 and 50 mg kg−1 soil). In all soils a maximum reduction in the amount of adsorbed P occurred when HA was added before P (HA/P system) especially at the high rate of added P (400 mg kg−1 soil). The compared means by Least Significant Differences analysis Test (LSD0.05), showed that the amount of P adsorbed on soils in the presence of HA (2.5 g kg−1 soil), significantly decreased the amount of adsorbed-P for all studied soil samples, especially when phosphors added after humic acid (HA/P) system. | ||||
| Keywords | ||||
| Phosphate adsorption isotherm; Competitive adsorption; humic acid; Calcareous soils | ||||
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