Role of potassium and calcium nutrition in improving fruit quality of mango

Introduction

Nearly 40 million tonnes of mango are produced worldwide each year, yet post-harvest losses in South and Southeast Asia routinely reach 25-40% before the fruit arrives at retail [1] . A large share of these losses traces back to poor firmness, uneven ripening, and short shelf life traits strongly influenced by the mineral status of the tree during fruit development [2] . Among mineral nutrients, potassium and calcium stand out for their contrasting but complementary roles in fruit quality [3] . Potassium is the most abundant cation in fruit tissue. It regulates osmotic potential, drives phloem translocation of sucrose, and activates over 60 enzymes involved in carbohydrate metabolism [4] . Deficient K supply during fruit filling reduces sugar content, delays colour development, and increases the incidence of internal breakdown disorders [5] . Calcium, by contrast, is relatively immobile in the phloem and must be supplied during early fruit growth. In the cell wall, Ca²⁺ ions bridge galacturonic acid residues in pectin chains, forming the socalled egg-box structure that maintains tissue rigidity [6] . Low Ca in the fruit predisposes mango to soft nose, jelly seed, and rapid senescence [7] . Despite this understanding, most mango fertilisation schedules in eastern India focus heavily on nitrogen and phosphorus while giving limited attention to K and Ca [8] . This research examined how soil-applied K₂SO₄ combined with foliar CaCl₂ sprays affects physicochemical quality and shelf life of Himsagar mango, a premium cultivar of West Bengal. The work was done at the North Bengal Institute of Tea Research, Jalpaiguri, from March 2021 to July 2022 [9] .

Climatic and environmental conditions

The experimental orchard is located at the North Bengal Institute of Tea Research, Jalpaiguri (26.49°N, 88.73°E), at an elevation of 75 m above sea level. The region has a subtropical humid climate with mean annual rainfall of 3200 mm, most of which falls between June and September. During the fruit development window (March-June 2021 and 2022), mean maximum temperature ranged from 31.4 to 36.8°C, mean minimum from 18.2 to 24.6°C, and relative humidity stayed between 62% and 88%. Soils were alluvial sandy loam (pH 5.6, organic carbon 1.12%, available K 138 mg/kg, exchangeable Ca 3.8 cmol/kg).

Material and Methods

Material

Fifteen-year-old, uniformly sized Himsagar mango trees spaced at 10 × 10 m on alluvial sandy loam were selected. Potassium sulphate (K₂SO₄, 50% K₂O) and calcium chloride (CaCl₂•2H₂O, analytical grade) were the nutrient sources. Eight treatments comprised factorial combinations of three K₂SO₄ rates (0, 200, 400 g/tree applied in two equal splits at panicle emergence and at marble stage) and three CaCl₂ foliar spray concentrations (0, 0.5, 1.0% applied thrice at 15-day intervals beginning at pea stage), minus one redundant combination to maintain a manageable block size. Each treatment was replicated four times in a randomised block design with single-tree plots. Baseline soil and leaf samples were collected before treatment application.

Methods

Fruits were harvested at physiological maturity (shoulder development, specific gravity ≈ 1.02) and analysed within 24 hours. Total Soluble Solids (TSS) were measured with a digital refractometer (Atago PAL-1). Titratable acidity was determined by titrating pulp extract against 0.1 N NaOH to a phenolphthalein endpoint. Fruit firmness was recorded at two equatorial points using a penetrometer fitted with an 8 mm probe. Ascorbic acid was estimated by the 2,6dichlorophenolindophenol method. Reducing sugars and total sugars were measured by the Lane-Eynon volumetric method. Potassium and calcium content in pulp, peel, and seed were determined after wet digestion with a diacid mixture (HNO₃:HClO₄, 9:4) by flame photometry (K) and EDTA titration (Ca). Shelf life was assessed by storing 10 uniform fruits per replicate at 30 ± 2°C and recording the number of days until the fruit became unmarketable (>20% surface shrivelling or decay). Data were analysed by twoway ANOVA and Tukey's HSD at p≤0.05 using SAS 9.4.

Results

Nutrient budget and input-output analysis

In the K400 + Ca1.0% treatment, total K input (soil + foliar) was 208.6 g K per tree, of which an estimated 47.3 g was removed in harvested fruit (average 38.2 kg fruit/tree at 1.24 g K/kg fresh weight). Apparent K recovery in fruit was thus about 22.7%. Calcium input via three foliar sprays at 1.0% CaCl₂ amounted to roughly 14.4 g Ca per tree per season; fruit removal accounted for 6.1 g, giving an apparent recovery of 42.4%. These figures suggest that a meaningful fraction of applied Ca reached the fruit, which is notable given the limited phloem mobility of Ca²⁺.

Comprehensive interpretation

The K × Ca interaction was significant for TSS (p=0.008), firmness (p=0.014), and shelf life (p=0.003), indicating that the two nutrients act synergistically rather than simply additively. Trees given both K and Ca showed 26.8% higher TSS and 62.5% greater firmness than unfertilised controls.

Discussion

The positive response of TSS to potassium agrees with earlier findings in mango cv. Dashehari [10] and citrus [11] , where K promoted sucrose loading into phloem sieve tubes and, in turn, sugar accumulation in sink organs. The 21.3 °Brix recorded here under the highest K + Ca treatment is comparable to the 20-22 °Brix range reported for wellmanaged Alphonso orchards in western India [12] . Calcium's effect on firmness fits the pectin cross-linking model described by Hocking et al. [6] . Pre-harvest Ca sprays have been shown to delay softening in apple, peach, and strawberry by maintaining middle lamella integrity [13] . In mango specifically, Joyce et al. [14] reported that Ca dips after harvest reduced internal breakdown, but pre-harvest foliar application as done here has the advantage of building Ca reserves in the fruit before the onset of ripening. Shelf-life extension from 6.4 to 10.7 days has practical value for the Jalpaiguri mango supply chain, where fruits are often transported 600-800 km to Kolkata wholesale markets without cold storage. Even a 3-4 day improvement could reduce transit losses from the current 30% to below 15% [15] . The K × Ca synergy likely reflects complementary mechanisms: K sustains membrane integrity and osmotic balance during senescence, while Ca physically reinforces the cell wall [16] .

Conclusion

Soil-applied K₂SO₄ at 400 g/tree combined with three foliar sprays of 1.0% CaCl₂ during fruit development significantly improved TSS, firmness, ascorbic acid, and shelf life of Himsagar mango compared to untreated controls. The K × Ca interaction was significant for the most commercially relevant traits. Fruit K recovery was 22.7% and Ca recovery 42.4%, suggesting acceptable nutrient-use efficiency for a perennial crop. These results support the inclusion of K and Ca in mango fertilisation programmes, particularly in eastern India where current schedules neglect both nutrients. On-farm validation across multiple seasons and cultivars is recommended before issuing revised fertiliser guidelines.

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