In this work, we investigated phosphorus (P) availability of three soil orders (Entisols, Inceptisols, and Mollisols) in the Kurdistan Region of Iraq. Addition of P fertilizers causes chemical and physical fixation of P due to the high amounts of calcium carbonate often present in the soils of this region. Understanding these fixation processes is important for predicting and managing P availability in this area. Two methods were used to assess P availability in this study: solubility diagrams and the Diagnosis Recommendation Integrated System chart (DRIS chart) method, which utilizes corn P concentration as a plant indicator of available P. A pot experiment was conducted during the spring corn growing season in 2014 (March 15, 2014, to July 1, 2014) to study the effects of five application levels of P (0, 80, 160, 240, and 320 kg P2O5 ha) on P availability. The double function parameters—phosphate potential (logH2PO4–pH) and lime potential (logCa + 2pH)—were used to study P solubility equilibria. The DRIS chart was used to study the balance between P and, separately, nitrogen (N) and potassium (K). The results of both methods indicated that an application of 320 kg P2O5 ha yielded the highest corn dry matter weight and highest P availability; that is, we found the best nutrient balance between P and both N and K at an application rate of 320 kg P2O5 ha. In addition, we found that the P for this treatment was in the most available form (dicalcium phosphate dihydrate) CaHPO4 · 2H2O. Our results indicated low P availability in the Inceptisol and high availability of P in the Mollisols studied. Phosphorus availability followed the following order: Mollisols > Entisols > Inceptisols. The high P availability in Mollisols was due to its relatively high organic matter content. Except the Inceptisols, the increase in levels of applied P caused a decrease in dry matter weight per pot. Both the solubility digram and DRIS chart method indicated low available P in the Bakrajo Inceptisol and adequate available P in the other soil orders studied. This is because of Inceptisols in the region have high calcium carbonate and low organic matter content when compared with Mollisols and Entisols.

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