Effect of Potassium and Zinc Nutrition on Growth and Yield of Short Duration Maize (Zea mays L.) under Dryland Vertisols

The prominence of maize is only next to rice and wheat in India and currently contributing 9% of India’s national food basket (Kiran et al., 2018). Maize has been bestowed with the titles of “Queen of cereals” and “poor men’s nutria-cereal” due to higher productivity, low cost of cultivation, higher response toward fertilizer input, a higher level of dietary protein, fat, minerals and vitamins (Alamerew, 2008; Jaliya et al., 2008). However, maize’s higher production is subjected to the adequate supply of plant macro and micronutrients as maize is highly responsive to fertilizer inputs. When it comes to nutrient requirement, maize is susceptible to available soil fractions of potassium and zinc significantly when growing under challenging conditions (Farooq et al., 2015).
       
Raising maize in the dryland vertisols tract of western India faces some unique challenges, including imbalanced nutrient management. The decades’ old researches indicate that better INM coupled with adequate soil moisture can significantly improve crop yield in this region and maize is no exception.
       
The general notion of black soils (vertisols) being rich in potassium has resulted in a lower potassium application rate by the farmers, which could be the growth and yield-limiting factor of maize for this area. However, a significant portion of Indian vertisols is dominated by the beidellite-nontronite type of minerals, which often becomes exhausted of exchangeable and soil solution K, especially in the case of high nutrient demanding crop such as maize. This reduction of K availability is even more manifested when the soil is flooded with water after a long dry season  or in case of excessive (Ca+Mg)/K situation, which leads to reduced K concentration in soil solution, thus high yielding crop fails to uptake required quantity of K desirable for enhanced production (Dobermann et al., 2002). The increased productivity of any crop requires more K and a faster release rate in the soil, which can only be met by external application by fertilizer.
       
On the other hand, zinc is one of the essential micronutrients, yet 50% of the world’s soil is deficient in Zn (Welch, 1993). The Zn deficiency is wide- spread in India (Shivay and Prasad, 2014) and most prevalent in dry calcareous soils (Katyal and Vlek, 1985), prominently found in Maharashtra’s dryland tract. The deficiency of Zn on soil deters sound plant growth (Behera et al., 2015) and causes Zn deficiency in human, which the fifth principal risk factor for disease in emerging countries like India (Guilbert, 2003).
       
The interaction between potassium and zinc is another aspect which is needed to be studied, especially in Indian condition. Considering all these factors, the field experiment was conducted explore potassium and zinc nutrition’s integrated effect on maize under dryland condition.

This experiment was initiated during the Kharif season of 2016 at, Agronomy Research Block of Dr. Panjabrao Deshmukh Krishi Vidyapeeth, Akola, Maharashtra, India. The experimental plot is located between 20.69°N latitude, 77°E longitude and 283.2 m above mean sea level. The summed up meteorological data recorded by the institutional weather station has been presented in Table 1. The physiochemical properties of soil of the experimental site (before Kharif 2016) are presented in Table 2.
 

 

       
Factorial Randomized Block Design (FRBD) with two factors, i.e., potassium (K₂O) and Zinc (ZnSO₄) fertilization and each with three levels was chosen as experimental design; keeping in mind to find out their individual as well as an interaction effect. The three K levels were K₃₀- 30 kg K₂O ha^-1, K₆₀- 60 kg K₂O ha^-1 and K₉₀- 90 kg K₂O ha^-1. In case of Zn; the constituent three levels were Zn₂₀- 20 kg ZnSO₄ ha^-1, Zn₃₀- 30 kg ZnSO₄ ha^-1 and Zn₄₀- 40 kg ZnSO₄ ha^-1 respectively.  Each treatment consists of combinations of K and Zn replicated thrice and applied in a gross plot of 24 m² area. The combination of 30 kg ha^-1 of K₂O and 20 kg ha^-1 of ZnSO₄ (K₃₀:Zn₂₀) is the commonly used dose of K and Zn of this region and widely practised by the farmer hence it was taken as control.
 
A nitrogen dose of 120 kg N ha^-1and phosphorus dose of 60 kg P₂O₅ ha^-1 was applied to all the treatments using urea and SSP. The phosphorus was applied entirely as a basal dose during the sowing, while nitrogen was applied in two split doses; the 1^st was during the sowing and the second one as a top dressing at 30 DAS. Muriate of potash was uniformly broadcasted to the plots as per the assigned treatments. The zinc was applied by dissolving zinc sulphate heptahydrate in water (15 litre of water per kg of ZnSO₄) followed by spraying near the crop rows.
       
The short duration dwarf variety Ravi- 81 was sown with a seed rate of 20 kg ha^-1 by following a spacing of 60 × 20 cm and a uniform sowing depth of 5 cm using dibbling method. Thinning and gap-filling operation was performed following the recommended procedures to keep an equal number of plants per plot.
       
Five plants have been randomly selected from each plot and duly tagged; subsequently, these plants are used to record non-destructive biometric observations such as the plant’s height, the number of functional leaves, leaf area, etcetera. Similarly, five plants were uprooted from the plot for destructive sampling in each interval which were sundried followed by oven dried at a constant temperature of 65°C.
       
The plant height was recorded at 20, 40, 60, 80 DAS and at the final harvesting stage. The number of functional leaves was enumerated on a plant basis while the leaf area was measured using a table top biovis leaf area meter. Leaf area index was estimated by dividing the leaf area per plant by the ground area occupied by that plant (Sestak et al., 1971). The chlorophyll content was measured using a SPAD 502 (Konica, Minolta Sensing line, Japan), Chlorophyll meter, and expressed in SPAD unit.
       
The recorded replicated mean data were analysed for ANOVA, critical difference of means, post-hoc analysis (Duncan multiple range test) and path coefficient analysis by following the standard procedure mentioned by Gomez and Gomez (1984) using R statistical programme (Rstudio, V-1.3.1093, 2020) with Agricole package.

Plant growth attributes and dry matter accumulation
 
The various growth and development parameters of maize were found to be significantly affected by both potassium and zinc application either due to interaction or due to stand alone affect.
       
Plant height (Fig 1A) of maize was well responded and increased under different levels of potassium and zinc application except for the initial stages of 20 DAS. Although the interaction between different levels of potassium and zinc has abortive to produce any significant result; the individual effect was significant. The maximum plant height was recorded at 60 kg K₂O ha^-1 which was 17.42, 5.62, 5.34 and 5.03% more than 30 kg K₂O ha^-1 at 40, 60, 80 DAS and at harvest stage. The increase of plant height due to potassium can be argued due to the enhanced activity of Auxin (Marre, 1977).
 
 

In case of zinc application; the plots which received 30 kg ha^-1 of ZnSO₄ recorded the tallest plant height which were 10.56, 3.64, 4.71 and 4.72% taller than 20 kg/ha of ZnSO₄ however found to at par with 40 kg ha^-1 of ZnSO₄. Such kind of increment due to zinc application is a result of higher nitrogen uptake and enhanced enzymatic activity (Mahdi et al., 2012).
       
Similarly, the number of functional leaves/plants was also found to be higher when potassium was applied at a rate of 60 kg K₂O ha^-1 and zinc was applied at a rate of 30 kg ZnSO₄ ha^-1 at 40 and 60 DAS (Fig 1B). The higher number of leaves is a sign of higher source space formation for photosynthesis (Kubar et al., 2013; Ebrahimi et al., 2011).
       
The SPAD recorded highest greenness index when potassium was applied at 60 kg K₂O ha^-1 as it recorded 29, 21.70 and 21.27% higher SPAD value than 30 kg K₂O ha^-1 at 40, 60 and 80 DAS (Fig 1C). Similarly; application of 30 kg ZnSO₄ ha^-1 recorded 6.5, 5.93, 5.95% more SPAD value than 20 kg ZnSO4 ha^-1.
       
The leaf area index (LAI) has been found to be equally improved by the application of potassium and zinc (interaction effect was significant) during the active growth period of 40 DAS, 60 DAS and 80 DAS (Table 3). The data indicated that increase in K application maximized the LAI up to 60 kg K₂O ha^-1 but at 90 kg K₂O ha^-1 it declined. In case of zinc; the augmentation of ZnSO₄, however, resulted in a positive trend even up to 40 kg of ZnSO₄ ha^-1. At 40 DAS, the highest leaf area index was observed on K₁Zn₃ (30:40), which was found to be at par with K₂Zn₁ (60:20). This is maybe since Vertisols are generally rich in potassium, but only a small pool of soil solution potassium is readily available (Mc Lean et al.,1985). As a result, in case of the fast-growing and heavy feeder crop like maize responded well to the additional amendment of potassium and zinc in early active growth stages (Zhang et al., 2013). 
 

       
Application of both potassium and zinc resulted in higher dry matter accumulation (Table 4). This could be attributed to enhanced plant height, leaf area index and photosynthates accumulation, thereby improving the plant vigour due to source-sink relationship (Hussain et al., 2015). The two-way interaction table (Table 4) depicts how gradually increasing zinc fertilization in the presence of escalating potassium dosage effects the dry matter accumulation in maize. Highest dry matter accumulation was observed when potassium was applied at the rate of 60 kg K₂O ha^-1 along with 20 kg ha^-1 of ZnSO₄.
 

 
Path coefficient analysis of growth attributes
 
Path analysis (Fig 2) of three active vegetative growth stages (40, 60 and 80 DAS) elucidates how collective application of potassium and zinc fertilizer rendered its effect on prominent growth attributes which finally influences the yield. These can be summed up as combined application of potassium and zinc resulted in enhanced plant height in earlier active vegetative growth stages which resulted in accommodation of a higher number of functional leaves; again, on the later stages, both nutrients resulted in larger leaf size which finally helped to attain higher the grain yield.
 

 
Grain and stover yield
 
The interaction between 60 kg K₂O ha^-1 and 30 kg ZnSO₄ ha^-1 also resulted highest grain yields (4708 kg ha^-1) and stover yield stover yield (9783 kg ha^-1) (Table 5 and 5.1). The higher grain yield and straw yield is due to better photosynthate mobilization as well as increase of the number of sink space. The increment of yield due to the stand-alone application of zinc on maize was also earlier reported by Kumar et al., (2017) and Panda et al., (2019).

This study conclusively indicates that combined application of potassium at a rate of 60 kg K₂O ha^-1 along with 30 kg ZnSO₄ ha^-1 improves growth and yield of maize under dryland condition while in some cases positive interaction between potassium and zinc has also been observed.