Response of Different Citrus Rootstocks to Sulfur Granular Compost and Nitrogen

The consumption of inorganic fertilizers in agricultural ecosystems led to the destruction of soil physical, chemical, and biological characteristics and severely affected the quality of the products. Therefore, the application of compost fertilizers in agricultural lands has been widely considered, and it has been mentioned as the best environmental measure. Rootstock plays a crucial role in the growth of citrus trees. Among all the nutrients needed by citrus, nitrogen has a vital role, and a large amount of this macronutrient is supplied in the soil every year. The release of nutrients from organic matter, especially nitrogen, is slower than the nitrogen release from chemical fertilizers. Rootstock affects the photosynthetic capacity of the transplanted cultivar, which is related to the annual carbohydrate redistribution and is a determining factor for vegetative growth and reproductive development. The growth, yield and fruit quality of cultivars are, therefore, strongly regulated by rootstock. In the recent study, the effects of compost and nitrogen fertilizers on common rootstocks in Mazandaran province, including citrange, citrumelo, and sour orange, were investigated. An experiment was conducted as a completely randomized design at Qaemshahr Horticultural Research Station. The treatments included sulfur granular compost (0, 2.5, 5, and 7.5%), produced by Mazandaran Wood and Paper Industries Company, and pure nitrogen (0, 20, 40, and 80 mg kg-1) in the form of ammonium sulfate. Vegetative growth, including plant height, crown diameter, and the number of leaves, were measured. Also, to study the changes in leaf nutrient concentration, leaf samples were collected, in July and the concentration of nutrients in leaf tissue was measured. A randomized complete block design was conducted at Qaemshahr Horticultural Research Station in 2017. The treatments included sulfur granular compost (0, 2.5, 5, and 7.5%), produced by Mazandaran Wood and Paper Industries Company, and pure nitrogen (0, 20, 40, and 80 mg kg-1) in the form of ammonium sulfate. One-year-old seedlings of the same size from citrange, citrumelo, and sour orange were planted in 7- kg pots containing arable soil and the treatments. The applied soil was analyzed, and its physical and chemical properties were determined. Irrigation of all pots until reaching the field capacity (weight method) was performed uniformly for all treatments. At the end of the growth period, vegetative growth parameters, including plant height, crown diameter, and the number of leaves, were measured. Also, to study the changes in leaf nutrient concentration, fully developed leaves samples were collected, in July and the concentration of nutrients was analyzed. Nitrogen was determined by the Kjeldahl method with a sulfuric acid-hydrogen peroxide mixture. Phosphorus by calorimetric, potassium using a flame photometer, and calcium, magnesium, iron, manganese, zinc, and copper were measured using the flame atomic absorption method. Results showed that the highest nitrogen concentrations in citrange and citrumelo seedlings were 2.92 and 2.97% due to 40 and 80 mg kg-1 N and 2.5% compost, respectively. In citrumelo rootstock, plant height increased with enhancing nitrogen application levels at different compost levels. Compost levels did not show a significant increase in citrange height, but the highest height growth was observed at 40 and 80 mg kg-1 nitrogen. In the sour orange rootstock, consumption of high levels of compost and nitrogen significantly reduced plant height. In all three rootstocks, the highest concentrations of potassium were observed in high levels of compost and no nitrogen application. The concentration of iron and zinc in citrumelo leaves increased with increasing compost levels. The trend was different in citrumelo so that consumption of 2.5% compost and 40 mg nitrogen fertilizer showed the highest concentration of leaf iron (151 μg g-1), while the application of 5% compost without nitrogen reduced the concentration of iron to 62.2 μg g-1. Changes in citrange zinc concentration were similar to citrumelo. In sour orange, iron concentration changed limitedly and ranged from 83.8 μg g-1 in 2.5% compost and 20 mg kg-1 nitrogen fertilizer to 61 μg g-1 in control. In this rootstock, the highest concentration of zinc was obtained from no compost application and 40 mg kg-1 nitrogen (28.14 μg g-1), whereas the lowest concentration of iron was observed in control. Citrange seedlings performed better in nutrient uptake than citrumelo and sour orange seedlings. Consumption of 5 and 7.5% levels of compost in sour orange rootstock reduced plant height and crown diameter and caused rosette, led to an increase in the number of leaves. The most effective level of nitrogen in sour oranges was 80 mg kg-1 nitrogen. Application of 80 mg kg-1 nitrogen and 2.5% compost, and 80 mg kg-1 nitrogen and 5% compost led to the highest number of leaves in citrumelo and citrange rootstocks, respectively. According to vegetative growth and concentration of nutrients data, especially from the nitrogen concentration point of view, maximum vegetative growth was obtained in citrumelo rootstock from 2.5% compost and 40 mg kg-1 nitrogen treatment and in citrange and sour orange rootstocks from 2.5% compost and 80 mg kg-1 nitrogen treatment.