Physiological and yield responses of apple trees to combined organic and phosphate fertilization under sandy loam soil in Babil Governorate, central Iraq

Soil fertility depletion and non-optimal fertilization are key problems in apple orchards established in a semi-arid environment, which directly affect fruit yields and quality. This experiment investigated the effect of organic and phosphorus fertilization, either alone or in combined on yield, fruit quality and physiological properties for apple trees (Malus domestica L.) cultivated in central Iraq. The experiment was carried out using a factorial randomized complete block with three replicates; the factors were three levels of organic fertilizer (0, 2.5 and 5 kg tree⁻¹) and three phosphate doses (0,100 and 180 g tree⁻¹). The total fruit yield was also substantially enhanced under integrated fertilization to 85.0 kg tree⁻¹, higher by 112.5% with respect to control (40.0 kg tree⁻¹). Fruit quality traits were also improved, increasing in diameter from 69.8 to 82.3 mm and firmness increased from 6.2 to 8.7 kg cm⁻² with the highest content of soluble solids (from 12.1 to 15.2 °Brix). Leaf nutrient contents (N, P and K) and some physiological parameters were also enhanced such as chlorophyll content (2.18 mg g⁻¹ FW) and net photosynthetic rate (18.7 μmol CO₂ m⁻² s⁻¹). Activities of key enzymes were highest in combined fertilization treatment, where Rubisco and SOD activities were higher up to 4.2 and 22.4 U mg⁻¹ protein, respectively. In general, combined organic and phosphate fertilization is a promising approach that can improve apple yield and fruit quality in semi-arid ecosystems.