critical levels of iron, zinc and boron for cotton in varamin rigion

cotton (gossypium hirsutum l. cv. varamin ) an important industrial crop of iran, is grown in 300,000 ha, with an average seed -cotton yield of 1750 kg.ha-1. the main obstacle in the way of increasing average cotton yield is imbalanced fertilizer use. along with urea and triple super phosphate, cotton growers also need k, mg, zn, fe, mn, cu, and b, so these elements’ critical levels should be determined in cotton fields. field experiments were conducted in 12 different fields in 1997 to determine critical levels of fe, zn, and b in soil, and to study the effects of these micronutrients on the yield and quality of cotton in varamin region. the experiment design was a randomized complete block one (rcb) with four treatments and three replications. treatments were; npk, npk+fe, npk+zn, and npk+b. iron was applied as feeddha (20 kg.ha-1), zn as zinc sulfate (40 kg.ha-1), and b as boric acid (20 kg. ha-1) prior to planting. average seed cotton yield, lint yield, seed index, and boll weight increased significantly by adding fe, zn, and b. in a cotton field with higher soil b concentrations, cotton yield decreased with b fertilizer. critical levels of fe, zn, and b were determined based on cate-nelson graphical and analysis of variance methods. in graphical method critical levels of fe, zn, and b were 4.8, 1.1, and 1.0 mg.kg-1 soil, respectively, while on the basis of analysis of variance method, these levels were 5.5, 1.1, and 1.3 mg.kg-1 soil, respectively. in one field, boll shedding decreased by zn and fe treatments but was not affected by b addition. iron to zn ratio in cotton leaves increased at fe treatment and decreased in zn treatments. boron treatment increased b content of leaves. furthermore, zinc sulfate decreased b toxicity in the cotton leaves. in short, applying micronutrients increased quality and yield of cotton in varamin region.