Investigation of Changes in Vegetative Growth, Yield and Qualitative Properties of Thomson Navel Orange Fruit with Emphasis on Optimal Consumption of Manure and Chemical Fertilizers

To achieve sustainable agriculture and improve soil quality, it is necessary to use organic sources such as animal manure in combination with suitable amount of chemical fertilizers. The purpose of this study was to investigate the effect of animal and chemical fertilizer on vegetative growth, yield and qualitative properties of Thomson Navel orange (Citrus sinensis L. Osbeck) fruit to reduce chemical fertilizer consumption.

The experiment was carried out as a factorial experiment in a randomized complete block design with three replications. Treatments included three levels of sheep manure (0, 6 and 12 kg per tree) and three levels of macronutrient fertilizers: nitrogen, phosphorus and potassium (0, 30 and 60% recommended amount, based on soil test) on five years old Thomson Navel orange trees (each treatment including two trees) in the Sari Baharestan citrus orchard for four years. Based on the growth of the trees, the above-mentioned values were added 20% annually. At the end of each growth period, vegetative growth including the length of spring and summer shoots and diameter of the collar (top and bottom of the graft union) were measured. Fruit yield and mean weight and diameter, total soluble solids (TSS), total acidity (TA), pH and vitamin C of harvested fruits were also measured.

The results showed that the highest spring growth (shoot length) of Thomson Navel oranges was obtained as a result of the combined application of chemical fertilizer and manure (60% of the recommended amount) and manure (12 kg/tree). The lowest spring vegetative growth of 21.6 cm was observed in the control. The Interaction between different levels of chemical fertilizer and manure on summer growth was not significant. Collar diameter (at the bottom of the graft union) reached its maximum level (132 mm) in 60% recommended fertilizer and 6 kg of manure per tree treatment. The highest fruit yield of Thomson Navel trees was obtained with an average of 49.6 kg/tree, in a combination of chemical fertilizers (60% of the recommended amount) and manure (12 kg/tree). The fruit TA in different levels of chemical fertilizer and manure increased and decreased, respectively. The Interaction between chemical fertilizers and manure on TSS was significant and the highest (11.08%) was observed at 60% of the recommended amount of chemical fertilizers and 6 kg/tree of manure. This treatment and the treatment of 60% of the recommended amount of chemical fertilizer and 12 kg/tree of manure were located in the same group. The 60% recommended chemical fertilizer treatment + 6 kg of manure produced the highest amount of vitamin C in the fruit extract (80.8 mg/100 ml of extract), however the combination of high levels of chemical fertilizer and animal manure reduced the vitamin. The Interaction of chemical fertilizer and manure levels was not significant on fruit diameter but it was significant on average weight and volume of extract. The highest mean weight (352g) and fruit volume (176cc) was obtained in the treatment of 60% chemical fertilizer + 12 kg of manure per tree.

The results showed that 60% of the recommended amount of chemical fertilizers + 12 kg of rotten sheep manure per tree is recommendable to achieve the highest quality and quantity of Thomson Navel orange fruit.