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Research Article

Legume Research, volume 44 issue 9 (september 2021) : 1060-1065

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

V.P. Soniya1,*, P.S. Bhindhu1, P. Sureshkumar1
1Radiotracer Laboratory, College of Horticulture, Vellanikkara, Thrissur-680 656, Kerala, India.

  • Submitted24-10-2019|

  • Accepted08-01-2021|

  • First Online 08-03-2021|

  • doi 10.18805/LR-4267

Cite article:- Soniya V.P., Bhindhu P.S., Sureshkumar P. (2021). Critical Level of Magnesium for Cowpea [Vigna unguiculata (L.) Walp.] in Ultisols of Kerala . Legume Research. 44(9): 1060-1065. doi: 10.18805/LR-4267.

ABSTRACT

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 (T5) 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.

INTRODUCTION

Magnesium is an essential macronutrient with respect to plant nutrition. It is required by plants in quantities lesser than nitrogen, potassium and calcium. Magnesium plays a number of key functions like photophosphorylation, photosynthetic carbon dioxide fixation, protein synthesis, chlorophyll formation, phloem loading, partitioning and utilization of photo assimilates, generation of reactive oxygen species and photo oxidation in leaf tissues. Magnesium deficiency results in severe inhibition of the phloem transport of sucrose (Cakmak and Yazici, 2010). The build-up of sucrose and starch in the leaves creates a congenial environment for the occurrence of pathogens and pests and cause ruptures or blockages in phloem. The growth of plant root system increases in response to magnesium-stimulated transport of carbohydrates to roots and decreasing auxin concentration in roots, thereby increasing distribution of fine roots (Cakmak and Kirkby, 2008). Magnesium deficiency has serious impacts on uptake of mineral nutrients and water due to impairment in root growth especially under marginal soil conditions (Mengutay et al., 2013). The most important soil properties governing magnesium availability are soil pH, texture, cation exchange capacity, organic matter and soil moisture (Mayland and Wilkinson, 1989).
       
Ultisols occupying more than fifty per cent of total geographical area of Kerala 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 (Sureshkumar et al., 2018). Crops are found to respond to applied magnesium fertilizers with an increase in quantity and quality of produce. But information on soil magnesium level in different soil types required for optimum crop production is scarce.
       
The determination of critical level of nutrient in soil and plant helps to manage the nutrient deficiency and avoid crop loss. Critical level refers to the level of a nutrient in soil below which crops readily respond to applied nutrient. The critical nutrient level for sufficiency in plant is defined as the concentration in the diagnostic tissue that allows a crop to achieve 90% of its maximum yield (White and Brown, 2010).Determination of critical level in tissue assists in diagnosing nutritional problems which can be circumvented without crop loss.

MATERIALS AND METHODS

A pot culture experiment was conducted at Radiotracer Laboratory, College of Horticulture, Vellanikkara to investigate the response of cowpea to magnesium nutrition and to validate critical level of magnesium in soil and plant using cowpea variety Bhagyalakshmi as the test crop. Top soil (0-15 cm depth) representing Ultisols was collected, air dried, ground with wooden mortar and pestle, sieved through 2 mm sieve and characterized for available nutrient content. The soil was sandy clay in texture and very strongly acidic with pH of 4.70. Organic carbon (1.32%), available nitrogen (476.67 kg ha-1) and potassium (240.18 kg ha-1) were medium in status while phosphorus (98.04 kg ha-1) was high. The secondary and micronutrients except magnesium (64.53 mg kg-1) and boron (0.22 mg kg-1) were sufficient.
       
The experiment consisted of twelve treatments viz., absolute control (T1), organic manure @ 20 t ha-1 (T2), recommended dose of fertilizers (T3), T3 + magnesium @ 5 mg kg-1 of soil (T4), T3 + magnesium @ 10 mg kg-1 of soil (T5), T3 + magnesium @ 15 mg kg-1 of soil (T6), T3 + magnesium @ 20 mg kg-1 of soil (T7), T3 + magnesium @ 30 mg kg-1 of soil (T8), T3 + magnesium @ 40 mg kg-1 of soil (T9), T3 + magnesium @ 50 mg kg-1 of soil (T10), T3 + magnesium @ 60 mg kg-1of soil  (T11) and T3 + magnesium @ 80 mg kg-1 of soil (T12). The recommended dose of fertilizers as per the package of practices of KAU (KAU, 2016) includes the application of 20 t ha-1 of organic manure, 250 kg ha-1 calcium carbonate and 20:30:10 kg ha-1 of N, P2O5 and K2O. The experiment was laid out in completely randomized design (CRD) with four replications in earthen pots of 5 kg capacity.
       
The quantity of fertilizers were calculated for 5 kg soil. Organic manure in the form of vermi compost was added after one week of application of calcium carbonate (AR grade). Magnesium required for each treatment was supplied through magnesium carbonate (AR grade) two weeks after organic manure application. Three seeds were sown in each pot and one healthy plant retained one week after emergence. Complete dose of phosphorus, potassium and split half of nitrogen was applied after thinning of plant population and second dose of nitrogen was supplied 15 days later. Foliar application of boron (0.05%) was done twice to combat boron deficiency. The nutrients were supplied through water soluble sources. Irrigation with de-ionized water, weed control and plant protection measures were adopted uniformly in each pot.
       
Soil pH, available magnesium in soil, magnesium content in stem, leaves and pods were determined using standard methods and crop uptake of magnesium was computed from the dry weight produced per plant. Critical level of Mg in soil and plant was worked out by the method proposed by Cate and Nelson (1965). The scatter points on a graphical plot of relative yield (Y-axis) and soil or plant nutrient content (X-axis) was used to determine the critical level of the nutrient in soil and plant respectively. Two perpendicular lines were drawn to produce four quadrants so that maximum number of points fall in positive quadrants. The intercept on X-axis gives the critical level of a nutrient in soil or plant. The relative yield was calculated as
  
                                                                                                                                             
 
 
As the nutrient composition of a plant changes with age, the critical levels are defined for a specific stage of maturity. Flower initiation is taken as the diagnostic stage of growth for plant leaf analysis of cowpea (Fageria, 2009).

RESULTS AND DISCUSSION

Twelve treatments of the study were nine doses of magnesium as magnesium carbonate, absolute control, organic manure alone and treatment with application of recommended dose of fertilizers alone. The magnesium content of vermi compost supplied was 0.64 mg kg-1. Treatments with organic manure alone and recommended dose of fertilizers helped to differentiate response of cowpea with magnesium application over normally followed ad hoc practices of Kerala.
 
Effect of treatments on soil pH and available magnesium
 
The application of graded doses of magnesium as magnesium carbonate showed significant influence on soil pH and available magnesium status at both crop stages studied. The results presented in Table 1 reveals that the addition of magnesium through magnesium carbonate has increased soil pH significantly in comparison to application of calcium carbonate alone as per RDF. The highest soil pH at both crop stages was recorded in treatment T12 where magnesium was applied at 80 mg kg-1 of soil. Magnesium carbonate with a neutralizing value of 119% had significant influence on increasing soil pH. The persistence of higher pH till harvest of the crop indicates the slow solubility of carbonate source in soil.  A concurrent increase in available magnesium status in soil was recorded with graded doses of magnesium and significantly higher status of available magnesium was recorded in T12 at both crop stages. A similar trend of increasing pH and available magnesium with increasing dose of magnesium in cowpea was reported by Fageria and De Souza (1991). The increase in available magnesium status towards crop harvest also points to the release of magnesium from magnesium carbonate thus contributing to the available pool. Magnesium is a highly mobile nutrient in soil as they are less strongly bound to soil charges due to higher hydrated radius. Thus the slow release of magnesium from added source is preferable to optimize crop nutrition.
 

Table 1: Effect of treatments on soil pH and available magnesium during flowering and harvest of cowpea (at 5% level of significance).


 
Effect of treatments on plant magnesium content and magnesium uptake
 
Magnesium content in stem and leaf of cowpea at flowering and after harvest was found to increase with the addition of graded dose of magnesium (Table 2). Similar observations on high positive and significant correlation between rates of magnesium applied and magnesium content in leaves were reported by Canizella et al., (2017). However the magnesium content in pods did not differ significantly with treatments. Karley and White (2009) reported that magnesium absorbed in excess is stored in the leaves of plants. Though the magnesium content in leaf and stem was significantly higher in T11 and T12 with highest dose of applied magnesium the data given in Table 2 shows treatment T5 with RDF+ magnesium @10 mg kg-1 of soil to have the highest crop uptake of magnesium. The higher concentration of magnesium must have restricted the absorption of other cations which had led to lower biomass production.  Similar observations on depressive effect of application of higher dose of magnesium on shoot dry weight of common bean varieties in tropical soils was reported by Canizella et al., (2017).
 

Table 2: Effect of treatments on magnesium content in plant and uptake of magnesium (at 5% level of significance).


 
Effect of treatments on biometric parameters of cowpea
 
Significant variations in the biometric parameters of cowpea crop were recorded due to variations in applied quantities of magnesium. The data given in Table 3 showed a significantly higher plant height in T5 (RDF + magnesium @ 10 mg kg-1 of soil) with a mean value of 61.65 cm. The lowest plant height was recorded in absolute control. Analysis of the data showed a significant influence of treatments on root nodule formation. The number of nodules was higher during flowering than at harvest. Among various treatments significantly higher number of root nodules was recorded in treatment T5 (RDF + magnesium @ 10 mg kg-1 of soil) during flowering stage. Similarly after harvest of crop, significantly higher number of nodules was recorded in T5 which was on par with T6, T7, T8 and T10.  Absolute control (T1) and T2 were  at par in recording significantly lower number root nodules at this stage. The treatments differed significantly with respect to number of pods per plant. Significantly higher number of pods per plant was obtained in treatment T10 (RDF + magnesium @ 50 mg kg-1 of soil) and was on par with T8, T6, T7, T4 and T12. Significantly longer pods were observed in T11 (RDF + magnesium @ 60 mg kg-1 of soil) and was on par with T10 and T8. The data on number of seeds in each pod showed that treatments varied significantly and highest number of seeds were obtained in T11 (RDF + magnesium @ 60 mg kg-1 of soil) and was on par with T10, T8 and T4. Number of seeds in the pods obtained from absolute control was the lowest and was at par with T2 (Organic manure @ 20 t ha-1). The treatments differed significantly with respect to the yield per plant. Treatment T5 (RDF + magnesium @ 10 mg kg-1 of soil) recorded significantly higher yield of 79.33 g plant-1 but was at par with T7, T8, T10, T11 and T12 and the absolute control treatment recorded the lowest yield.
 

Table 3: Effect of treatments on biometric parameters (at 5% level of significance).


 
Determination of critical level of magnesium in soil and plant
 
Scattered plot of soil magnesium and relative per cent yield (Table 4) as shown in Fig 1. revealed the critical level of magnesium in soil to be 75 mg kg-1. Similarly from the plots of relative per cent yield and magnesium content in leaves at flowering as shown in Fig 2, the critical level of magnesium in cowpea leaves was identified to be 0.38%. The finding was in line with Kasinath et al., (2014) who reported critical level of magnesium in soil and plant determined using Cate and Nelson graphical method as 74 mg kg-1 and in tomato plant as 0.39% in Alfisols of Karnataka. Similarly, Fageria and De Souza (1991) reported critical level of magnesium as 0.2 cmol kg -1 in kaolinite rich Oxisols. According to Roy et al., (2006),  relative yield of 80-100% can be achieved in soils with cation exchange capacity less than 20 cmol (p+) kg-1 if the available magnesium is 40-80 mg kg-1.
 

Table 4: Parameters for determination of critical level of magnesium in soil and plant.


 

Fig 1: Critical level of Mg in soil.


 

Fig 2: Critical level of Mg in cowpea leaves at flowering.

CONCLUSION

From the study it can be concluded that the critical level for magnesium below which cowpea responds to applied nutrient in Ultisols of Kerala is 75 mg kg-1 of available magnesium in soil and the critical level for sufficiency in leaves at flowering is 0.38%. Addition of magnesium carbonate as source of magnesium can improve soil pH and retain magnesium in available pool to meet crop need through the crop growth period. In a strongly acidic soil with pH 4.7 and sandy clay texture application of magnesium @ 10 mg kg-1 of soil was found to be the optimum dose to maximize crop yield.

REFERENCES


  1. Cate, R.B. and Nelson, L.A. (1965). A rapid method for correlation of soil test analysis with plant response data. International soil testing series Technical. Bull.No.1. North Carolina State University, Agricultural Experiment Station, Releigh. 420p.

  2. Cakmak, I. and Yazici, A.M. (2010). Magnesium- a forgotten element in crop production. Better Crops. 94: 23-25.

  3. Cakmak. I and Kirby, E.A. (2008). Role of magnesium in carbon partitioning and alleviating photo oxidative damage. Physiol. Plant. 133: 692-704.

  4. Canizella, B.T., Moreira, A., Moraes, L.A.C. and Fageria, N.K. (2017). Response of common bean varieties to the magnesium application in the tropical soil. J. Plant Nutr. 40(2): 207-218.

  5. Fageria, N.K., De Souza, C.M.R. (1991). Upland rice, common bean and cowpea response to magnesium application on an Oxisol. Commun. Soil Sci. Plant Anal. 22: 17-18.

  6. Fageria, N.K. (2009). The Use of Nutrients in Crop Plants. CRC Press, Taylor and Francis Group, London, 430p.

  7. Karley, A.J. and White, P.J. (2009). Moving cationic minerals to edible tissues: potassium, magnesium, calcium. Curr. Opin. Plant Biol. 12: 291-298.

  8. Kasinath, B.L., Ganeshmurthy, N.S. and Nagegowda, N.S. (2014). Critical limit of soil and plant magnesium in tomato-growing soils of South Karnataka. J. Hortic. Sci. 9: 209-212.

  9. Mengutay, M., Ceylan, Y., Kutman, U.B. and Cakmak, I. (2013). Adequate magnesium nutrition mitigates adverse effects of heat stress on maize and wheat. Plant Soil 368: 57-72.

  10. Mayland, H.F. and Wilkinson, S.R. (1989). Soil factors affecting magnesium availability in plant-animal systems: AReview. J. Anim. Sci. 67: 3437-3444.

  11. Roy, R.N., Fink, A., Blair, G.J. and Tandon, H.L.S. (2006). Plant nutrition for food security: A guide for integrated nutrient management. FAO Fertilizer and Plant Nutrient Bulletin.16, 348p.

  12. Sureshkumar, P., Geetha, P. and Bhindhu, P.S. (2018). Chemistry and Fertility - Management of Humid Tropical Soils of Kerala as Influenced by Topography and Climate. Indian J. Fertil. 14(3): 30-44.

  13. White, P.J. and Brown, P.H. (2010). Plant nutrition for sustainable development and global health. Ann. Bot. 105(7): 1073-1080.
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