Phosphorus Availability in Entisols, Inceptisols, and Mollisols of Iraqi Kurdistan

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.