MSc

Ultisols constitute the major soil type occupying more than fifty per cent of the total geographical area of Kerala. They 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. The deficiency of magnesium is a common nutritional disorder in these soils due to leaching of bases under humid tropical conditions. Hence, the present investigation entitled “Optimization of soil environment and crop response for magnesium nutrition in Ultisol” was undertaken at Radiotracer Laboratory, College of Horticulture, Vellanikkara during 2017-2019. The objectives of the study were: (i) to evaluate the modifications of the acidic soil environment through added magnesium source and (ii) to validate the critical level of magnesium in soil and plant thereby optimizing magnesium nutrition for cowpea.
Top soil (0-15 cm depth) representing Ultisols was collected from Water Management Research Unit, Vellanikkara to conduct an incubation study and a pot culture experiment. Initial characterization of physico-chemical properties and total population of Rhizobium/ BradyRhizobium, free-living nitrogen fixing bacteria and total spore count of arbuscular mycorrhizal fungi was done using representative soil sample. The incubation experiment was conducted to study the release pattern of magnesium from added source. Magnesium carbonate required to theoretically raise the available magnesium status in soil to 120 mg kg-1 was taken as the optimum dose and one level above (150%) and below (50%) the optimum dose was added to soil with and without the addition of recommended dose of calcium carbonate (250 kg ha-1) and organic manure (20 t ha-1).
Soil pH, electrical conductivity (EC) and available magnesium was analysed at weekly intervals for four months of incubation. The increase in soil pH, EC and available magnesium was concomitant to the added doses of magnesium carbonate. Though significantly higher soil pH was recorded after one week of incubation, pH in



all the treatments at the end of the experiment was higher than the initial value. This indicated the persistence of magnesium carbonate throughout the incubation period. Available magnesium was significantly higher after eight weeks of incubation whereas electrical conductivity in soil was found to increase over the period of incubation. The fractionation of soil magnesium after incubation period showed all fractions to be significantly influenced by the treatnment. The variations in acid soluble and mineral fraction between treatments substantiates the presence of magnesium carbonate as a solid phase in soil.

A pot culture experiment to investigate the crop response to magnesium nutrition and to validate critical level of magnesium in soil and plant was conducted using cowpea variety Bhagyalakshmi as the test crop. The experiment consisted of twelve treatments with 4 replications viz., absolute control (T1), organic manure @ 20 t ha-1 (T2), POP recommendation with calcium carbonate @ 250 kg ha-1 (T3), POP recommendation with dolomite @ 400 kg ha-1 (T4), T3 + magnesium carbonate required to theoretically raise available magnesium in soil to 120 mg kg-1 (T5) and treatments T6 to T12 comprised of T3+ graded doses of magnesium carbonate at 10% of T5 (T6), 20% of T5 (T7), 40% of T5 (T8), 60% of T5 (T9), 80% of T5 (T10), 125% of T5 (T11) and 150% of T5 (T12).
Physico-chemical properties of soil, microbial population in rhizosphere and nutrient content in stem, leaf and pods were analyzed during flowering and after crop harvest. An increase in soil pH and available magnesium content in soil could be achieved at both crop stages in response to added doses of magnesium carbonate. A significantly higher population of Rhizobium and freeliving nitrogen fixers were observed in rhizosphere soil of T7 that was supplied with 20% optimum dose of magnesium. Though the magnesium content in stem and leaf of cowpea increased with the graded doses of magnesium, the data on yield per plant (74.33 g plant-1) showed



treatment T7 (T3+ MgCO3 @ 20% of T5) to be the optimum level with maximum crop response.
Scattered plot of relative per cent yield with available magnesium and magnesium content in leaves revealed the critical level of available magnesium in soil to be 75 mg kg-1 and that in leaf tissue at flowering to be 0.38 %. Further studies are to be conducted on the efficacy of magnesium carbonate or dolomite under field conditions to optimize crop nutritison.