Critical Level of Magnesium for Cowpea [Vigna unguiculata (L.) Walp.] in Ultisols of Kerala

Background: Ultisols are characterized by low pH, low cation exchange capacity and low base saturation due to dominance of kaolinite and oxides and hydrous oxides of iron and aluminium. Among the basic cations, magnesium has smaller ionic size and larger hydrated radius which limits binding of Mg to soil exchange sites. As a result the deficiency of magnesium is a common nutritional disorder in these soils due to leaching of bases under humid tropical conditions. The determination of critical level of magnesium in soil and plant helps to manage the nutrient deficiency and will help to avoid crop loss. Methods: A pot culture experiment was conducted to determine the critical level of magnesium for cowpea in Ultisols of Kerala during January 2019. Graded doses of magnesium were applied @ 5 mg kg(-1) to 80 mg kg(-1) of soil as magnesium carbonate along with recommended dose of fertilizers to evaluate the effect of applied magnesium on soil pH, available magnesium in soil, magnesium content in plant, crop uptake and yield. Result: Soil pH and available magnesium in soil recorded at flowering and harvest was highest where magnesium was applied @ 80 mg kg(-1) of soil. Magnesium content in stem and leaves was found to increase with the levels of magnesium added. No significant variation in the content of magnesium in pods was recorded. Crop uptake (66.62 mg g(-1)) of magnesium and yield per plant (79.33 g plant(-1)) was highest in the treatment (T-5) where magnesium was applied @ 10 mg kg(-1) of soil. The critical level of magnesium determined by Cate and Nelson graphical method was found to be 75 mg kg(-1) of available magnesium in soil and 0.38% in leaves of cowpea during flowering.