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Frontiers in Soil Science ·

Volume 6 - 2026 | https://doi.org/10.3389/fsoil.2026.1950553

Calcium and potassium fertilization improves productivity and forage quality with implications for soil available potassium in Opuntia ficus-indica under hot arid condition

  • Anandkumar Naorem1,2*corresponding author
  • Sawsan Hassan3
  • Mounir Louhaichi3,4
  • Devi Dayal2
  • Deepesh Machiwal1,2
  • Rahul Dev2,5
  • Dheeraj Singh1
  1. 1Indian Council of Agricultural Research (ICAR)-Central Arid Zone Research Institute, Jodhpur, Rajasthan, India
  2. 2ICAR-Central Arid Zone Research Institute, Regional Research Station, Bhuj, Gujarat, India
  3. 3International Center for Agricultural Research in the Dry Areas (ICARDA), CGIAR, Tunis, Tunisia
  4. 4Department of Animal and Rangeland Science, Oregon State University, Corvallis, OR, United States
  5. 5ICAR-National Bureau of Plant Genetic Resources, Regional Station, Bhowali, Uttarakhand, India
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Abstract

Climate change and increasing aridification have intensified the need for nutrient management strategies that sustain productivity while preserving soil fertility in dryland production systems. Opuntia ficus-indica, a drought-tolerant multipurpose crop, has gained importance in arid regions, yet information on the combined effects of calcium (Ca) and potassium (K) fertilization remains limited. A one-season field experiment was conducted under the hot arid conditions of Kutch, India, using a 4 × 4 factorial randomized block design with four replications. Four Ca (0, 20, 40, and 60 kg ha −1) and four K (0, 30, 60, and 90 kg ha −1) application rates were evaluated for their effects on plant growth, forage yield and quality, fertilizer-use efficiency, and post-harvest changes in soil available K. Calcium and potassium fertilization significantly improved plant growth, cladode production, biomass yield, and forage quality (P< 0.05). The combined application of 40 kg Ca ha −1 and 60 kg K ha −1 produced among the highest fresh and dry biomass yields while increasing crude protein concentration and reducing fiber fractions compared with the unfertilized control. Fertilizer-use efficiency declined with increasing nutrient application rates. Greater forage productivity was associated with larger changes in soil available K after harvest, indicating the need for careful K management in perennial cactus production systems. Calcium and potassium fertilization improved productivity and forage quality with implications for post-harvest soil available K. These findings indicate that fertilizer recommendations for O. ficus-indica should integrate forage productivity with fertilizer-use efficiency and post-harvest soil nutrient dynamics to support nutrient management under hot arid conditions.

Keywords

1 Introduction

Arid and semi-arid regions are among the world’s most climate-vulnerable agricultural systems. In addition to this, increasing temperatures, prolonged droughts, and greater rainfall variability are intensifying soil degradation and reducing fertilizer-use efficiency. These changes have increased the need for nutrient management strategies that sustain crop productivity while preserving soil fertility under water-limited conditions. Arid lands occupy approximately 41% of the Earth’s terrestrial surface (1). Crop production in these regions is constrained by inherently poor soil fertility, low organic carbon content, limited water-holding capacity, reduced microbial activity, and the frequent accumulation of soluble salts (2). Soil salinity, alkalinity, nutrient deficiencies, and wind erosion further intensify these constraints, making agricultural production increasingly challenging. Climate change is accelerating aridification and increasing the vulnerability of dryland production systems to declining soil fertility, nutrient imbalances, and reduced fertilizer-use efficiency (3). Soil rehabilitation and integrated nutrient management are increasingly recognized as complementary approaches for improving the resilience of arid production systems (4). Developing resilient nutrient management strategies has therefore become as important as identifying drought-tolerant crops capable of maintaining productivity under water-limited conditions (5, 6). Among these species, Opuntia ficus-indica (L.) Mill. has emerged as a multipurpose horticultural crop cultivated for fruit, young cladodes, forage, and biomass in arid and semi-arid regions (7–9). O. ficus-indica possesses several adaptations that enable survival under prolonged water deficit, including crassulacean acid metabolism photosynthesis, succulent cladodes for water storage, extensive shallow root systems, and high water-use efficiency (10, 11). These adaptations allow the species to maintain biomass production under conditions where many conventional crops perform poorly (12, 13). Beyond its tolerance to environmental stress, O. ficus-indica provides important agricultural and socio-economic benefits. In semi-arid regions of Latin America, Africa, and the Mediterranean, cactus pear serves as a reliable year-round forage resource, supporting livestock production when alternative feed supplies are scarce (14). Its cladodes are widely used as livestock feed during drought because of their high moisture content (85–95%) and favorable digestibility (15, 16). The crop also contributes to soil conservation, carbon sequestration, and the rehabilitation of degraded lands (9), while supporting rural livelihoods in dryland environments (17). A life cycle assessment by Abyar et al. (18) identified O. ficus-indica as a low-input crop with a balanced environmental footprint under water-limited conditions. Therefore, O. ficus-indica is well suited to climate-smart agricultural systems (19).

Although interest in cactus pear cultivation has increased worldwide (9), information on nutrient management in O. ficus-indica remains limited, particularly under arid conditions. Most published studies have concentrated on nitrogen and phosphorus nutrition (15, 20), whereas the contributions of Ca and K have received comparatively little attention. Nkoi et al. (21) reported that increasing N application enhanced oleic and stearic acid contents. Arba et al. (22) found that nitrogen and phosphorus fertilization increased flowering, fruit number, and fruit size but did not influence fruit quality. Ca plays a fundamental role in cell wall stabilization, membrane integrity, signal transduction, and plant responses to environmental stress (23). K is equally important for osmotic regulation, stomatal conductance, enzyme activation, carbohydrate metabolism, and drought tolerance (24). These physiological functions are particularly relevant in O. ficus-indica, where biomass production and forage quality depend on tissue succulence and efficient cellular water regulation under water-limited conditions. The substantial nutrient demand of cactus pear further emphasizes the need to evaluate Ca and K fertilization. For example, the ‘Gigante’ cultivar removes 464.63 kg Ca ha−1 and 421.04 kg K ha−1 from the soil, yet conventional fertilizer recommendations for cactus pear rarely include either nutrient (24). Continued nutrient removal without adequate replenishment may progressively decline soil available K and increase the dependence of crop production on external fertilizer inputs, particularly in low-fertility arid soils. To our knowledge, despite the recognized importance of Ca and K in cactus nutrition, no factorial studies have systematically evaluated their combined effects on the growth, forage quality, and productivity of O. ficus-indica under hot arid conditions. Furthermore, previous studies have not assessed fertilizer-use efficiency or examined the implications of Ca and K fertilization for changes in soil available potassium, leaving important questions regarding nutrient management and long-term soil fertility unresolved. Consequently, fertilizer recommendations remain largely based on crop response without considering their implications for long-term soil fertility. This knowledge gap is particularly relevant in calcareous arid soils, where abundant Ca may alter K availability and uptake through ionic antagonism (25). Understanding these interactions is essential for developing fertilizer strategies that improve forage productivity while maintaining efficient nutrient use under arid conditions.

The objectives of this study were to (i) evaluate the individual and interactive effects of Ca and K fertilization on vegetative growth, biomass production, forage quality, fertilizer-use efficiency, and post-harvest changes in soil available K of O. ficus-indica grown under hot arid conditions, and (ii) identify fertilizer combinations that improve productivity while balancing fertilizer-use efficiency and post-harvest soil available K. We hypothesized that balanced Ca and K fertilization would enhance biomass production and forage quality through their complementary physiological functions, whereas increasing K application beyond crop demand would be associated with larger changes in post-harvest soil available K and lower fertilizer-use efficiency.

2 Materials and methods

2.1 Experimental site and climatic conditions

The field experiment was conducted at the Experimental Farm (23°12′47.3″ N, 69°47′35.8″ E) of the ICAR–Central Arid Zone Research Institute, Regional Research Station, Kukma, Bhuj, Kutch, Gujarat, India. The experimental site is located on an Aridisol within a hot arid ecosystem characterized by high evaporative demand, erratic rainfall, and alkaline sandy soil. The initial soil had a pH of 8.5, an electrical conductivity of 2.78 dS m−1, and a soil organic carbon content of 0.20%. Initial concentrations of KMnO4-extractable nitrogen, Olsen-extractable phosphorus, and NH4OAc-extractable potassium (1 N ammonium acetate, pH 7.0) were 45 ± 5, 8 ± 2, and 210 ± 25 kg ha−1 (mean ± SEM), respectively (26–28).

Monthly mean air temperature and total precipitation recorded during the experimental period are presented in Figure 1. Mean monthly air temperature varied throughout the study period, while total rainfall was 388.8 mm, with most precipitation occurring in August (183.3 mm) and June (89.7 mm). These data characterize the climatic conditions prevailing during the experiment.

Monthly mean air temperature (°C) and total precipitation (mm) recorded during the experimental period (April 2021-May 2022) at the experimental site in Kutch, Gujarat, India. Air temperature is shown as bars (right y- axis), and precipitation as a line (left y-axis).
Figure 1. Monthly mean air temperature (°C) and total precipitation (mm) recorded during the experimental period (April 2021-May 2022) at the experimental site in Kutch, Gujarat, India. Air temperature is shown as bars (right y- axis), and precipitation as a line (left y-axis).

2.2 Experimental design and treatment details

The field experiment was conducted from February 2021 (cladode establishment) to May 2022 (final harvest). Fertilizer treatments were imposed in April 2021 after establishment of the plants. The experiment was arranged in a factorial randomized block design (FRBD) with two fertilizer factors: calcium (Ca) and potassium (K). Ca was applied at four rates (0, 20, 40, and 60 kg ha−1), while K was supplied at 0, 30, 60, and 90 kg ha−1, resulting in 16 treatment combinations with four replications. One healthy and uniform cladode (15–20 cm long, 4–6 cm in diameter, and 8–12 months old) was planted in each pit. Cladodes were collected from healthy mother plants, surface-disinfected with 1% NaOCl, allowed to form a callus, and then planted upright (basal end downward) to a depth of 10 cm. Each experimental plot measured 4 × 4 m (16 m2) and contained nine plants established at a spacing of 2 × 2 m (2,500 plants ha−1). Before treatment application, well-decomposed farmyard manure was incorporated uniformly into all plots at 5 t ha−1. The accession CBG (CAZRI Botanical Garden), routinely used for forage evaluation at the experimental station (7), was selected for the study. Because no fertilizer recommendations are available for Ca and K application in O. ficus-indica under the hot arid conditions of India, the selected application rates represented a gradient from no nutrient addition to relatively high fertilizer inputs to evaluate crop response, fertilizer-use efficiency, and post-harvest soil nutrient dynamics. Calcium was supplied as agricultural gypsum (CaSO4·2H2O; approximately 23% elemental Ca and 18% S), corresponding to 87, 174, and 261 kg gypsum ha−1 for the 20, 40, and 60 kg Ca ha−1 treatments, respectively. Potassium was supplied as sulphate of potash (K2SO4; 50% K2O, equivalent to approximately 41.5% elemental K, and 18% S), corresponding to 72, 145, and 217 kg SOP ha−1 for the 30, 60, and 90 kg K ha−1 treatments, respectively. Agricultural gypsum was used as the Ca source because it supplies readily available Ca without substantially increasing soil pH while also providing sulfur (29). Sulphate of potash (SOP) was chosen as the K source because its chloride-free composition makes it more suitable than muriate of potash for moderately saline arid soils (30). Because both gypsum and SOP supplied sulfur, the contribution of sulfur could not be separated from those of calcium and potassium in the present experiment. This limitation was considered during interpretation of the results. Fertilizers were uniformly broadcast within the root zone of each plant, incorporated into the upper 10–15 cm of soil immediately before irrigation, and lightly mixed with the soil. Plots receiving no Ca or K (Ca0K0) served as the control. The crop was irrigated using groundwater of normal quality (electrical conductivity<1.0 dS m−1). Irrigation was applied at intervals of approximately 10–15 days during summer and 20–30 days during winter, depending on crop water requirements and prevailing weather conditions. Irrigation was withheld or reduced following effective rainfall. Weeds were controlled by periodic manual weeding and intercultural operations. To isolate the effects of Ca and K fertilization, no mineral nitrogen or phosphorus fertilizers were applied during the study.

2.3 Soil and plant analysis

Soil and plant measurements were recorded one year after treatment application. To assess changes in soil nutrient status, representative soil samples (0–15 cm depth) were collected from each experimental plot before fertilizer application and again after harvest. Each composite soil sample consisted of five cores collected from the rooting zone of each plot at a depth of 0–15 cm. The cores were collected midway between adjacent plants, avoiding the immediate stem base, and thoroughly mixed to obtain one representative composite sample per plot. Samples were air-dried under shade, gently crushed, and passed through a 2-mm sieve before analysis. Initial soil properties represent the mean ± SEM of the composite soil samples collected from all experimental plots before treatment imposition (n = 64). Soil pH and electrical conductivity (EC) were measured in a 1:2.5 soil-to-water suspension (31) using digital pH and conductivity meters, respectively. Soil organic carbon was determined by the Walkley and Black wet oxidation method (32). Available nitrogen, phosphorus and potassium were determined using the alkaline permanganate (27), Olsen extraction (26) and neutral normal ammonium acetate extraction methods (28), respectively. Plant measurements and the final harvest were conducted in May 2022, approximately 13 months after fertilizer treatments were applied in April 2021. Biometric observations were recorded from five healthy representative plants in each experimental plot. At final harvest, mature cladodes of harvestable size were removed from the five tagged plants, while the basal mother cladode and young newly emerging cladodes were retained to maintain plant regrowth. The harvested cladodes were used to determine fresh biomass yield. Plant height (cm) was measured from the soil surface to the highest point of the canopy using a measuring tape. Cladode length (cm) was determined from the base to the apex with a digital vernier caliper. Cladode width (cm), measured along the north–south and east–west axes, and cladode circumference (cm) were recorded using a flexible measuring tape. Fresh and dry cladode weights were measured with a digital balance before and after oven drying, respectively. Fresh biomass yield (kg) was determined immediately after harvest, whereas dry matter was measured after oven drying at 65 ± 2 °C until constant weight. Fresh and dry biomass yields were converted to tonnes per hectare using the equivalent planting density of 2,500 plants ha−1.

2.4 Forage quality analysis

Representative cladode samples were collected at harvest from the five tagged plants in each experimental plot for forage quality analysis. Samples were dried and ground following the procedure described in Section 2.3. The ground material was passed through a 1-mm sieve and stored in airtight containers until analysis. Nitrogen content was determined after H2SO4 digestion and distillation using the micro-Kjeldahl method (33). Nitrogen concentration was determined following AOAC Official Method 2001.11 (34). Ground samples were digested with concentrated H2SO4 using K2SO4 and a copper catalyst. Ammonia was liberated by alkaline steam distillation, collected in 4% boric acid, and quantified by titration with standardized acid. Reagent blanks were included with each analytical batch. Crude protein (CP) was calculated as:

Crude protein=Nitrogen (%)×6.25

Neutral detergent fiber (NDF) and acid detergent fiber (ADF) were determined sequentially using the Van Soest detergent method (35). Briefly, 0.5 g of the ground sample was subjected to neutral detergent extraction followed by acid detergent extraction. The fiber residues were filtered, dried, and ashed according to the standard procedure. All analyses were performed in triplicate, and the results are expressed on a dry matter basis (% DM).

2.5 Fertilizer-use efficiency, post-harvest change in soil available K, and partial K balance

To understand the agronomic effectiveness of Ca and K fertilization, agronomic efficiency (AE) and partial factor productivity (PFP) were calculated for both nutrients. Agronomic efficiency (AE, t kg-1) was calculated separately for Ca and K by comparing each fertilized treatment with the corresponding treatment receiving no application of the nutrient being evaluated while holding the rate of the other nutrient constant (36): Agronomic efficiency of Ca was calculated as:

AECa=YCaiKj−YCa0KjCai

where YCaiKj is the fresh cladode yield under the ith Ca rate and jth K rate, YCa0Kj is the fresh cladode yield under zero Ca at the same K rate, and Ca<sub>i</sub> is the applied Ca rate.

Agronomic efficiency of K was calculated as:

AEK=YCaiKj−YCaiK0Kj

where YCaiKj is the fresh cladode yield under the ith Ca rate and jth K rate, YCaiK0 is the fresh cladode yield under zero K at the same Ca rate, and K<sub>j</sub> is the applied K rate. Agronomic efficiency was undefined for treatments receiving no application of the nutrient under evaluation and was therefore reported as not applicable (NA).

The partial factor productivity (PFP, t kg−1), representing fresh cladode yield per unit of applied nutrient, was calculated as follows (36):

PFPCa=YCaiKjCai

PFPK=YCaiKjKj

where YCaiKj is the fresh cladode yield under the corresponding Ca and K treatment combination. PFP was undefined for treatments receiving zero application of the nutrient under evaluation and was therefore reported as not applicable (NA).

Dry matter yield (DMY) and crude protein yield (CPY) were estimated to integrate biomass production with forage nutritional quality. Both of these parameters were calculated using the following formula:

DMY=Fresh cladode yield × DM100

CPY=DMY × CP100

To assess changes in soil available potassium following fertilization, the difference between pre-treatment and post-harvest soil available K was calculated as:

Changes in soil available K=Kinitial−Kfinal

where Kinitial and Kfinal are the NH4OAc-extractable soil K contents (kg ha−1) before fertilizer application and after harvest, respectively.

A partial K balance was calculated as:

Apparent K removal=Kinitial+Kapplied−Kfinal

where Kinitial and Kfinal represent the initial and post-harvest soil available K, respectively, and Kapplied represents fertilizer K added, all expressed as kg K ha−1. Apparent K removal was not interpreted as crop K uptake because it may also include K fixation, leaching, and exchanges with non-exchangeable or subsoil K pools.

2.6 Statistical analysis

Data were analyzed using two-way analysis of variance (ANOVA) for a factorial randomized block design, with Ca and K as fixed effects and block as a random effect. Residuals were examined for normality and homogeneity of variance before conducting ANOVA. When the Ca × K interaction was significant, treatment means were compared using Tukey’s HSD test. When the interaction was not significant, the main effects of Ca and K were interpreted. Results are presented as mean ± standard error of the mean (SEM). Agronomic efficiency and partial factor productivity were analyzed separately for Ca and K. Treatments receiving zero application of the nutrient under evaluation were excluded because these indices are undefined when fertilizer input is zero. Initial soil available K, post-harvest soil available K, change in soil available K, and apparent K removal were analyzed using two-way ANOVA. Principal component analysis (PCA) was performed to examine the multivariate relationships among morphological traits, forage productivity, forage quality, fertilizer-use efficiency, and to visualize treatment grouping. Principal component analysis (PCA) was performed using plot-level observations. The PCA included plant height, cladode number, cladode length, cladode circumference, cladode width (N–S and E–W), cladode thickness, fresh cladode weight, dry cladode weight, fresh cladode yield, dry matter yield, crude protein, crude protein yield, neutral detergent fiber, acid detergent fiber, and change in soil available K. All quantitative variables were standardized to z-scores before PCA to eliminate differences arising from measurement units and scale. Standardization was performed using the equation z = (x−µ)/s, where x is the observed value, µ is the variable mean, and s is the sample standard deviation. The PCA biplot was generated from the standardized dataset. Linear regression analysis was used to evaluate the relationship between fresh cladode yield and post-harvest changes in soil available K. The regression analysis was exploratory and intended to describe the association between variables rather than infer causality. Statistical analyses and graphical visualizations were performed using R (Version 4.5.1) (37), jamovi (Version 2.6) (38), OriginPro (Version 2019b, 9.65) (39), and GraphPad Prism (Version 8.0.0) (40). The two-way ANOVA results were presented using the approach described by Assaad et al. (41).

3 Results

3.1 Morphological characteristics

Potassium application significantly influenced all morphological characteristics of Opuntia ficus-indica (P<0.05). On the other hand, calcium significantly affected cladode circumference, cladode number, cladode thickness, and plant height (P<0.05), but had no significant effect on cladode length or cladode width in either the north–south (N–S) or east–west (E–W) direction. No significant Ca × K interactions were observed for any morphological trait (Table 1; Supplementary Table 1). Cladode length increased with increasing K application, with the highest value recorded under Ca60K90, which was statistically comparable with Ca40K90, Ca20K90, and Ca0K90. Similarly, cladode circumference was greatest under Ca40K90, although it did not differ significantly from Ca60K90. Both cladode width (N–S) and cladode width (E–W) were maximized under Ca60K90, with values statistically comparable to those obtained under other high-K treatments, particularly Ca20K90, Ca40K90, Ca40K60, and Ca60K60. The highest cladode number was observed under Ca40K60, which was statistically comparable with Ca60K60 and Ca40K90. Likewise, the greatest cladode thickness was recorded under Ca40K60, although it did not differ significantly from Ca60K60, Ca20K60, Ca40K90, Ca60K30, and Ca20K90. Plant height was also highest under Ca40K60, remaining statistically comparable with Ca40K30, Ca40K90, Ca60K60, Ca60K30, and Ca20K60. For each response variable, F-values, degrees of freedom, and exact P-values for the main effects and interaction are provided in Supplementary Tables 1, 2.

Table 1. Effects of calcium (Ca) and potassium (K) fertilization on the morphological characteristics of Opuntia ficus-indica grown under the hot arid conditions of Kutch, India.

VariablesCa0Ca20Ca40Ca60Significance
K0K30K60K90K0K30K60K90K0K30K60K90K0K30K60K90CaKC x K1
Cladode length (cm)7.5 ± 0.39c7.6 ± 0.37c8.9 ± 0.34bc9.7 ± 0.51ab7.5 ± 0.28c8.3 ± 0.39bc9.4 ± 0.43ac10 ± 0.36ab7.5 ± 0.41c8.3 ± 0.47bc9.3 ± 0.50ac10.2 ± 0.37ab7.6 ± 0.28c8.3 ± 0.44bc9.1 ± 0.39ac11.1 ± 0.41aNS*NS
Cladode circumference (cm)24 ± 1.89d26 ± 1.44cd31.1 ± 1.77bd35.2 ± 2.03abc24.2 ± 1.32d29.3 ± 1.46bd32.3 ± 2.07bd36.2 ± 2.05ab27.4 ± 1.56bd28.2 ± 1.46bd34.1 ± 2.02abc42.3 ± 2.47a28.2 ± 1.56bd32.1 ± 1.85bd35.1 ± 1.73abc42.1 ± 1.98a**NS
Cladode width (N-S, cm)11.1 ± 0.63e13.3 ± 0.79ce14.4 ± 0.61ace17.2 ± 0.88ac11.1 ± 0.60e14.2 ± 0.85bce15 ± 0.62ace17.4 ± 0.88ab12.4 ± 0.69de14 ± 0.59bce16 ± 0.89acd17.2 ± 0.82ac12 ± 0.64e14.1 ± 0.78bce16.2 ± 0.82acd18.2 ± 1.04aNS*NS
Cladode width (E-W, cm)10.3 ± 0.41e13.2 ± 0.54bce15.4 ± 0.77ac17.1 ± 0.97ab11.2 ± 0.76de12.2 ± 0.62ce15 ± 0.77acd17.2 ± 0.93a11.2 ± 0.46de14.2 ± 0.70ae14.3 ± 0.99acd17.1 ± 1.04ab12.3 ± 0.67ce14.1 ± 0.64ae15.6 ± 0.89ac17.3 ± 0.79aNS*NS
Cladode number (plant−1)8.6 ± 0.70g11.2 ± 0.83eg14.2 ± 1.05cdg12.3 ± 1.03dg10.3 ± 0.91fg15.3 ± 1.21bdef18 ± 1.32ad16.3 ± 1.10adef12.2 ± 0.75dg18 ± 1.25ad22.2 ± 1.97a19.2 ± 1.57abc11.2 ± 0.89eg17.2 ± 1.54ade21.6 ± 1.77ab17.3 ± 1.4ade**NS
Cladode thickness (cm)1.2 ± 0.067f1.5 ± 0.14df1.8 ± 0.11bdf1.6 ± 0.095cdf1.4 ± 0.12ef1.9 ± 0.15abde2.3 ± 0.15ab2.2 ± 0.14abc1.6 ± 0.09cdf2.1 ± 0.14ad2.5 ± 0.15a2.3 ± 0.11ab1.5 ± 0.07df2.2 ± 0.15abc2.4 ± 0.16ab2.1 ± 0.11ad**NS
Plant height (cm)44.3 ± 1.23e50.2 ± 1.86de52.6 ± 1.34ce50.2 ± 2.2de52.2 ± 1.41ce57.1 ± 1.96acd60.2 ± 2.66acd56.2 ± 2.08acd54.3 ± 2.10bce62.2 ± 2.38ac65.2 ± 2.83a62.1 ± 2.44ac53.2 ± 1.80bce59.3 ± 1.70acd63.6 ± 2.78ab57.3 ± 1.83acd**NS

Values are presented as means ± SEM (n = 4). Means within a row followed by different lowercase letters are significantly different at P< 0.05 according to Tukey’s multiple comparison test following two-way ANOVA. Ca × K denotes the interaction between calcium and potassium treatments. The significance of the main effects of Ca and K and their interaction (Ca × K) is indicated as significant (P< 0.05) or not significant (NS). *Significant at P < 0.05.

3.2 Biomass production and fresh cladode yield

Fresh cladode weight and dry cladode weight were significantly influenced by the main effects of calcium and potassium, as well as by their interaction (P< 0.05). In contrast, fresh cladode yield was significantly affected only by potassium application, whereas calcium application and the Ca × K interaction had no significant effect (Table 2; Supplementary Table 2). The highest fresh cladode weight was recorded under Ca40K60, which was statistically comparable with Ca60K60. Similarly, the greatest dry cladode weight was observed under Ca40K60 and remained statistically comparable with Ca60K60. Fresh cladode yield reached its maximum under Ca20K90; however, this treatment was statistically comparable with Ca40K90, Ca60K90, Ca0K90, Ca0K60, Ca20K60, Ca40K60, and Ca60K60 (Table 2).

Table 2. Effects of calcium (Ca) and potassium (K) fertilization on biomass production, forage yield, and nutritional quality of Opuntia ficus-indica.

VariableCa0Ca20Ca40Ca60Significance
K0K30K60K90K0K30K60K90K0K30K60K90K0K30K60K90CaKC×K1
Fresh cladode weight (g cladode−1)82.1 ± 3.12j110 ± 4ij140 ± 3.89gh120 ± 4.67hi100 ± 3.22ij160 ± 6.29fg180 ± 5.96df160 ± 5.89fg120 ± 4.57hi200 ± 7.34bd240 ± 7.85a210 ± 8.28bc120 ± 3.46hi190 ± 7.31cde220 ± 5.87ab171 ± 5.95ef***
Dry cladode weight (g cladode−1)8.2 ± 0.33h11.3 ± 0.47fh14.2 ± 0.47ef12.3 ± 0.61fg10.1 ± 0.33gh16.6 ± 0.66de18.5 ± 0.62cd16.2 ± 0.71de12.3 ± 0.54fg20.2 ± 0.71bc24.2 ± 1.08a21.1 ± 0.99ac12.4 ± 0.57fg19.3 ± 0.81bcd22.5 ± 0.72ab18.5 ± 0.58cd***
Fresh cladode yield (t ha−1)3.82 ± 0.61d7.72 ± 0.96bd12.8 ± 1.33abc13.8 ± 1.75ab6.28 ± 1.09cd9.3 ± 0.98bd12.7 ± 1.20abc16.9 ± 1.58a8.02 ± 1.4bd8.15 ± 0.96bd12.1 ± 1.57abc16.5 ± 1.64a7.5 ± 0.75bd10.7 ± 1.48abc12.4 ± 1.28abc16.4 ± 1.93aNS*NS
Dry matter yield (t ha−1)0.31 ± 0.05f0.86 ± 0.11def1.8 ± 0.25be1.74 ± 0.17be0.64 ± 0.11ef1.49 ± 0.19cdef2.34 ± 0.29abc2.78 ± 0.26ab0.98 ± 0.17def1.63 ± 0.20be2.84 ± 0.35ab3.51 ± 0.49a0.883± 0.11def2.03 ± 0.25bd2.78 ± 0.26ab2.87 ± 0.31ab**NS
Tissue N (% DM)0.768 ± 0.02i0.99 ± 0.02gh1.28 ± 0.04ef1.14 ± 0.02fg0.896 ± 0.02hi1.31 ± 0.03df1.57 ± 0.03bc1.41 ± 0.03cde1.02 ± 0.03gh1.47 ± 0.05bd1.79 ± 0.06a1.63 ± 0.05ab1.01 ± 0.03gh1.46 ± 0.03bde1.79 ± 0.04a1.54 ± 0.03bc***
Crude protein (%)4.8 ± 0.13i6.2 ± 0.15gh8 ± 0.28ef7.1 ± 0.16fg5.6 ± 0.18hi8.2 ± 0.19df9.8 ± 0.22bc8.8 ± 0.21cde6.4 ± 0.19gh9.2 ± 0.29bd11.2 ± 0.36a10.2 ± 0.34ab6.3 ± 0.21gh9.1 ± 0.20bde11.2 ± 0.25a9.6 ± 0.22bc***
Crude protein yield (t ha−1)0.0151 ± 0.002g0.0541 ± 0.008efg0.144 ± 0.021cdg0.123 ± 0.010dg0.0358 ± 0.006fg0.123 ± 0.018dg0.23 ± 0.030ad0.245 ± 0.024ad0.0619 ± 0.009efg0.152 ± 0.023cdf0.319 ± 0.042ab0.363 ± 0.02a0.0556 ± 0.007efg0.186 ± 0.027bcde0.311 ± 0.030ab0.275 ± 0.030ac***
Neutral detergent fiber (NDF, %)42.1 ± 0.65a39.2 ± 0.90ab35.2 ± 0.56cd37.3 ± 0.65bc40.1 ± 0.87ab35.2 ± 0.96cd31.1 ± 0.70ef33.2 ± 0.51de37.2 ± 0.92bc31.3 ± 0.85ef27.2 ± 0.48g30.3 ± 0.63eg38.2 ± 0.62bc32.3 ± 0.83de28.2 ± 0.52fg30.2 ± 0.64eg**NS
Acid detergent fiber (ADF, %)28.3 ± 0.96a26.2 ± 0.69ab23.2 ± 0.81bcd25.3 ± 0.88ac27.2 ± 0.70a23 ± 0.71bcd20 ± 0.45df22.4 ± 0.50cde25.3 ± 0.61ac20 ± 0.54df18.2 ± 0.58f19.2 ± 0.48ef25.3 ± 0.72ac21 ± 0.58df18.3 ± 0.41f20.4 ± 0.68df**NS

Values are presented as means ± SEM (n = 4). Means within a row followed by different lowercase letters are significantly different at P< 0.05 according to Tukey’s multiple comparison test following two-way ANOVA. Ca × K denotes the interaction between calcium and potassium treatments. The significance of the main effects of Ca and K and their interaction (Ca × K) is indicated as: P < 0.05 (*) and not significant (NS).

3.3 Dry matter and crude protein yield

Dry matter yield was significantly affected by the main effects of calcium and potassium (P< 0.05), whereas the Ca × K interaction was not significant. In contrast, crude protein yield was significantly influenced by the main effects of calcium and potassium as well as their interaction (Table 2; Supplementary Table 2). The highest dry matter yield was obtained under Ca40K90, which was statistically comparable with Ca40K60, Ca60K90, Ca60K60, Ca20K90, and Ca20K60. Similarly, the greatest crude protein yield was recorded under Ca40K90, but it did not differ significantly from Ca40K60, Ca60K60, Ca60K90, Ca20K90, and Ca20K60 (Table 2; Figure 2). The marked increase in crude protein yield resulted from the combined increases in dry matter yield and crude protein concentration because crude protein yield was calculated as their product. Therefore, the increase in crude protein yield does not represent a proportional increase in tissue N concentration.

Response surface contour plots illustrating the combined effects of calcium (Ca) and potassium (K) application rates on **(A)** dry matter yield (t ha<sup>−1</sup>) and **(B)** crude protein yield (t ha<sup>−1</sup>) of *Opuntia ficus-indica*. Contour lines depict the variation in yield across the experimental ranges of Ca (0–60 kg ha<sup>−1</sup>) and K (0–90 kg ha<sup>−1</sup>).
Figure 2. Response surface contour plots illustrating the combined effects of calcium (Ca) and potassium (K) application rates on (A) dry matter yield (t ha−1) and (B) crude protein yield (t ha−1) of Opuntia ficus-indica. Contour lines depict the variation in yield across the experimental ranges of Ca (0–60 kg ha−1) and K (0–90 kg ha−1).

3.4 Forage quality

Measured tissue N concentration was significantly affected by Ca, K, and the Ca × K interaction (P<0.05, Table 2). Tissue N concentration ranged from 0.768% under Ca0K0 to 1.79% under Ca40K60 and Ca60K60. The highest tissue N concentrations were recorded under Ca40K60 and Ca60K60, and both treatments were statistically comparable with Ca40K40. Crude protein concentration was significantly affected by the main effects of calcium and potassium as well as their interaction (P< 0.05). In contrast, neutral detergent fiber (NDF) and acid detergent fiber (ADF) were significantly influenced by the main effects of calcium and potassium (P<0.05), whereas the Ca × K interaction was not significant (Table 2; Supplementary Table 2). The highest crude protein concentration was recorded under Ca40K60 and Ca60K60, with both treatments being statistically comparable. These treatments also remained statistically comparable with Ca40K90. Neutral detergent fiber was lowest under Ca40K60, which was statistically comparable with Ca60K60, while acid detergent fiber was lowest under Ca40K60 and remained statistically comparable with Ca60K60, Ca60K90, Ca40K30, Ca20K60, and Ca20K30 (Table 2).

3.5 Fertilizer-use efficiency

Agronomic efficiency of calcium (AECa) and potassium (AEK) was not significantly affected by calcium, potassium, or their interaction (P > 0.05). In contrast, partial factor productivity of calcium (PFPCa) was significantly influenced by the main effects of calcium and potassium as well as their interaction, whereas partial factor productivity of potassium (PFPK) was significantly affected only by potassium application (Table 3; Supplementary Table 3). Among the calcium treatments, the highest PFPCa was recorded under Ca20K90, which was statistically comparable with Ca20K60. The lowest PFPCa values were observed under Ca60K0, Ca60K30, Ca40K0, and Ca40K30, which did not differ significantly from one another. For PFPK, the maximum value was obtained under Ca60K30, which was statistically comparable with Ca20K30, Ca40K30, and Ca0K30. The remaining potassium treatments were statistically comparable and exhibited lower PFPK values than the K30 treatments (Table 3; Figure 3).

Table 3. Effects of calcium (Ca) and potassium (K) fertilization on fertilizer-use efficiency and post-harvest soil nutrient dynamics of Opuntia ficus-indica grown under hot arid conditions.

VariableCa0Ca20Ca40Ca60Significance
K0K30K60K90K0K30K60K90K0K30K60K90K0K30K60K90CaKC×K1
AECa (t kg−1 Ca)NANANANA0.123 ± 0.0610.079 ± 0.057-0.005 ± 0.0540.146 ± 0.1270.044 ± 0.01-0.029 ± 0.037-0.015 ± 0.050-0.01 ± 0.0340.061 ± 0.0020.050 ± 0.040-0.007 ± 0.0280.043 ± 0.045NSNSNS
AEK (t kg−1 K)NA0.13 0± 0.0350.150 ± 0.0220.111 ± 0.014NA0.101 ± 0.0520.107 ± 0.0280.118 ± 0.024NA0.004 ± 0.0790.124 ± 0.0380.094 ± 0.019NA0.107 ± 0.0660.082 ± 0.0320.099 ± 0.026NSNSNS
PFPCa (t kg−1 Ca)NANANANA0.314 ± 0.054ce0.465 ± 0.049bc0.635 ± 0.06ab0.845 ± 0.079a0.200 ± 0.035de0.204 ± 0.024de0.302 ± 0.039ce0.413 ± 0.041cd0.125 ± 0.013e0.178 ± 0.025e0.207 ± 0.021de0.273 ± 0.032ce***
PFPK (t kg−1 K)NA0.257 ± 0.032ac0.213 ± 0.022bc0.153 ± 0.019cNA0.31 ± 0.033ab0.212 ± 0.02bc0.188 ± 0.018bcNA0.272 ± 0.032ac0.202 ± 0.026bc0.183 ± 0.018bcNA0.357 ± 0.050a0.207 ± 0.021bc0.182 ± 0.021bcNS*NS
Change in soil available K (kg ha−1)10.4 ± 1.50e48.3 ± 3.14d92.1 ± 4.60c140 ± 5.60b9.7 ± 1.91e49.7 ± 3.23d100 ± 5.0c153 ± 6.12ab13.1 ± 1.70e50.7 ± 3.30d107 ± 5.35c165 ± 6.60a11.1 ± 1.44e48 ± 3.12d112 ± 5.60c173 ± 6.92a***
Apparent K removal (kg ha-1)10.4 ± 1.5e78.3 ± 3.14d152 ± 4.6c230 ± 5.6b9.7 ± 1.91e79.7 ± 3.23d160 ± 5c243 ± 6.12ab13.1 ± 1.7e80.7 ± 3.3d167 ± 5.35c255 ± 6.6a11.1 ± 1.44e78 ± 3.12d172 ± 5.6c263 ± 6.92a***

Fertilizer-use efficiency was evaluated using agronomic efficiency (AE) and partial factor productivity (PFP) for both calcium and potassium.

Values are means ± SEM, n = 4 per treatment group. a-hMeans in a row without a common superscript letter differ (P< 0.05) as analyzed by two-way ANOVA and the TUKEY test. 1C × K = Ca × K interaction effect. Change in soil available K was calculated as the difference between the pre-treatment and post-harvest NH4OAc-extractable soil K concentrations. Treatments receiving zero application of the nutrient under evaluation were reported as not applicable (NA) because agronomic efficiency (AE) and partial factor productivity (PFP) are undefined when the denominator is zero. Statistical analyses of AECa and PFPCa excluded the Ca0 treatments, whereas analyses of AEK and PFPK excluded the K0 treatments. Apparent K removal was calculated as initial soil available K + fertilizer K applied − post-harvest soil available K. This value represents a partial K balance and should not be interpreted as measured crop K uptake. Asterisks indicate significant effects at P< 0.05; NS indicates a non-significant effect.

Effects of calcium (Ca) and potassium (K) fertilization on fertilizer-use efficiency of *Opuntia ficus-indica*: agronomic efficiency of calcium (AE<sub>Ca,</sub> t kg<sup>-1</sup>), agronomic efficiency of potassium (AE<sub>K,</sub> t kg<sup>-1</sup>), partial factor productivity of calcium (PFP<sub>Ca,</sub> t kg<sup>-1</sup>), and partial factor productivity of potassium (PFP<sub>K,</sub> t kg<sup>-1</sup>). Values are means ± SEM (n = 4).
Figure 3. Effects of calcium (Ca) and potassium (K) fertilization on fertilizer-use efficiency of Opuntia ficus-indica: agronomic efficiency of calcium (AECa, t kg-1), agronomic efficiency of potassium (AEK, t kg-1), partial factor productivity of calcium (PFPCa, t kg-1), and partial factor productivity of potassium (PFPK, t kg-1). Values are means ± SEM (n = 4).

3.6 Change in soil available K and apparent K removal

The change in soil available K was significantly affected by the main effects of calcium and potassium and their interaction (P< 0.05). Initial and post-harvest soil available K values are presented in Supplementary Table 4, whereas changes in soil available K and apparent K removal are presented in Table 3. The smallest changes in soil available K were observed under the unfertilized treatments (Ca0K0, Ca20K0, Ca40K0, and Ca60K0), which were statistically comparable. Among the fertilized treatments, the greatest change in soil available K was recorded under Ca60K90, which was statistically comparable with Ca40K90 and Ca20K90. Similarly, treatments receiving 60 kg K ha−1 formed a statistically comparable group, whereas all 30 kg K ha−1 treatments were also statistically similar to one another (Table 3). Apparent K removal was significantly affected by Ca, K, and the Ca × K interaction (P< 0.05; Table 3). The highest apparent K removal was observed under Ca40K90 and Ca60K90, which were statistically comparable with Ca20K90. The Ca20K90 treatment was also comparable with Ca0K90. All treatments receiving 60 kg K ha−1 were statistically comparable, irrespective of Ca application. Similarly, the treatments receiving 30 kg K ha−1 formed a common statistical group, while all treatments without K application were statistically comparable. Linear regression analysis revealed a significant positive relationship between fresh cladode yield and the change in soil available K (R2 = 0.77, P< 0.05). The fitted regression equation was Y = 11.16X − 41.94 (Figure 4). Because both variables were measured on the same experimental units under the common fertilizer-treatment structure, this relationship should be interpreted as descriptive rather than causal.

Linear relationship between fresh cladode yield and change in soil available potassium in *Opuntia ficus-indica* under different calcium and potassium fertilization treatments. The solid line represents the fitted linear regression and the dotted lines represent the 95% confidence intervals (R<sup>2</sup> = 0.77, *P* < 0.05).
Figure 4. Linear relationship between fresh cladode yield and change in soil available potassium in Opuntia ficus-indica under different calcium and potassium fertilization treatments. The solid line represents the fitted linear regression and the dotted lines represent the 95% confidence intervals (R2 = 0.77, P < 0.05).

3.7 Principal component analysis

Principal component analysis (PCA) separated the Ca and K treatments according to their combined effects on plant growth, forage productivity, forage quality, and changes in soil available K (Figure 5). The first two principal components explained 83.8% of the total variation, with PC1 accounting for 70.0% and PC2 for 13.8%. Variables related to biomass production, including fresh cladode yield, dry matter yield, crude protein yield, plant height, cladode number, cladode length, cladode width, cladode circumference, and the change in soil available K, were positioned along the positive axis of PC1. In contrast, NDF and ADF were oriented along the opposite side of PC1. Treatments receiving moderate to high rates of Ca and K clustered on the positive side of PC1, whereas the unfertilized treatments were grouped on the negative side of PC1.

Principal component analysis (PCA) biplot showing the relationships among morphological traits, forage productivity, forage quality, and change in soil available K in *Opuntia ficus-indica* under different calcium and potassium fertilization treatments. The first two principal components (PC1 and PC2) explained 70.0% and 13.8% of the total variation, respectively. Variables oriented in the same direction indicate positive associations, whereas those oriented in opposite directions indicate negative associations.
Figure 5. Principal component analysis (PCA) biplot showing the relationships among morphological traits, forage productivity, forage quality, and change in soil available K in Opuntia ficus-indica under different calcium and potassium fertilization treatments. The first two principal components (PC1 and PC2) explained 70.0% and 13.8% of the total variation, respectively. Variables oriented in the same direction indicate positive associations, whereas those oriented in opposite directions indicate negative associations.

4 Discussion

4.1 Calcium and potassium nutrition enhanced morphological development

Calcium and potassium fertilization improved the morphological development of O. ficus-indica under hot arid conditions, although their contributions differed among individual growth traits. Potassium significantly influenced all measured morphological characteristics, whereas calcium primarily affected cladode circumference, cladode thickness, cladode number, and plant height. Similar positive effects of Ca and K nutrition on cactus growth and biomass production have been reported by Galizzi et al. (42) and Blanco- Macías et al. (43), who identified these nutrients as important determinants of plant development and tissue nutrient composition. The observed responses may be interpreted in relation to the distinct physiological functions of the two nutrients. Potassium is the principal inorganic osmoticum in plant cells and regulates cell turgor, stomatal movement, enzyme activation, and assimilate transport (44–46). Under the high evaporative demand characteristic of hot arid environments, adequate K availability likely promoted osmotic adjustment, maintained cell turgor, and sustained cladode expansion through improved water uptake (47, 48). Calcium, in contrast, contributes primarily to the structural stability of plant tissues by strengthening cell walls and plasma membranes through calcium-pectate cross-linking within the middle lamella (49–52). These structural functions are consistent with the significant improvements in cladode circumference, cladode thickness, cladode number, and plant height observed following calcium application. The absence of significant Ca × K interactions for the measured morphological traits suggests that the effects of the two nutrients were largely independent rather than synergistic. Potassium may have promoted overall vegetative development, while calcium contributed mainly to structural growth, together supporting vigorous canopy development under hot arid conditions. Similar independent roles of Ca and K in regulating plant growth have been reported in other succulent and drought-adapted species, where potassium primarily regulates physiological processes associated with growth, whereas calcium maintains structural integrity during tissue expansion (50, 51).

4.2 Calcium and potassium fertilization affected biomass production and yield

Calcium and potassium fertilization enhanced biomass production in O. ficus-indica, although their contributions differed among biomass traits. Potassium significantly increased fresh cladode yield, whereas both calcium and potassium significantly influenced fresh and dry cladode weight. Similar improvements in cactus pear productivity following nutrient application have been reported by Galizzi et al. (42), Blanco-Macías et al. (43), and Dubeux et al. (53). The increase in fresh and dry cladode weight observed in the present study may be attributed to the complementary physiological functions of calcium and potassium during cladode development. Potassium promotes photosynthate transport from source tissues to developing cladodes through its role in phloem loading and assimilate translocation, thereby supporting biomass accumulation (54–56). Calcium contributes to membrane stability and cell wall organization, which are essential for maintaining tissue integrity during the rapid expansion of succulent cladodes (50, 57). Together, these functions favor the production of larger and heavier cladodes.

Unlike fresh and dry cladode weight, fresh cladode yield responded primarily to potassium application, indicating that potassium availability was the dominant factor statistically associated with total forage production under the experimental conditions. Potassium plays a central role in stomatal regulation, osmotic adjustment, and the transport of assimilates, thereby enhancing carbon fixation and biomass accumulation, particularly under hot arid environments where water availability frequently limits plant growth (44, 46, 47). The absence of a significant calcium effect on fresh cladode yield suggests that improvements in structural growth did not necessarily translate into greater total biomass production. Similar responses have been reported in other succulent crops, where potassium exerts a stronger influence on biomass accumulation than structural nutrients (43, 53). The present study did not quantify tissue nutrient concentrations; therefore, the extent to which nutrient uptake, partitioning, or ionic balance contributed to the observed biomass responses could not be determined. Nevertheless, the results indicate that potassium was more strongly associated with fresh forage production, whereas calcium contributed mainly to structural biomass development and dry matter accumulation.

4.3 Balanced calcium and potassium nutrition enhanced forage quality

Calcium and potassium fertilization improved the measured forage-quality traits of O. ficus-indica by increasing crude protein concentration and reducing fiber fractions. The highest crude protein concentrations were observed under Ca40K60 and Ca60K60, indicating that these treatments were associated with greater protein concentration. Similar responses to fertilization have been reported previously (53). The maximum crude protein concentration of 11.2% was at the upper end of values reported for O. ficus-indica cladodes and should therefore be interpreted cautiously (53). Nevertheless, the corresponding tissue N concentration was obtained from replicate Kjeldahl determinations rather than inferred independently from biomass production. The approximately 24-fold difference in crude protein yield was driven largely by increased dry matter production combined with a smaller increase in tissue N concentration. Calcium and potassium did not provide additional N; therefore, the N accumulated in the biomass was presumably derived primarily from soil N pools and mineralization of the uniformly applied FYM. Fertilization may have influenced the capacity of plants to acquire these existing N sources, but this mechanism cannot be confirmed because plant N uptake, root traits, soil N mineralization, and nutrient partitioning were not measured. Established roles of K in enzyme and ribosome function and of Ca in nitrate signaling provide possible physiological explanations for the observed crude protein response (58–61); however, these mechanisms were not evaluated in the present study. Furthermore, crude protein was expressed on a dry-matter basis; therefore, variation in tissue water content alone cannot explain the observed increase in crude protein concentration. A direct test of this possibility was not performed because tissue moisture and nitrogen concentration were not determined on the same analytical subsamples. Neutral detergent fiber and acid detergent fiber decreased significantly following Ca and K fertilization, potentially indicating improved forage digestibility. Under nutrient-limited conditions, cactus pear allocates a greater proportion of biomass to structural carbohydrates, increasing cell wall constituents that contribute to tissue rigidity (62, 63). Improved nutrient availability may have favored the development of metabolically active parenchymatous tissues, thereby reducing the relative proportion of structural carbohydrates in the harvested biomass (64). Calcium may also contribute to maintaining cell wall organization without promoting excessive accumulation of indigestible structural components (65). This response indicates that the fertilizer combination producing the highest crude protein concentration was not necessarily the same as that producing the greatest crude protein yield, highlighting the importance of considering both forage nutritive value and protein production when developing nutrient management strategies.

4.4 Increasing fertilizer application reduced partial factor productivity despite continued gains in forage productivity

Partial factor productivity declined with increasing fertilizer application, indicating lower fertilizer-use efficiency at higher nutrient input levels despite continued improvements in forage productivity. Similar responses have been reported in numerous fertilized cropping systems, where progressively larger fertilizer inputs produce proportionally smaller increases in biomass production (66, 67). In the present study, agronomic efficiency of both calcium and potassium was not significantly affected by fertilizer treatments. In contrast, partial factor productivity of calcium was significantly influenced by calcium application, potassium application, and their interaction, whereas partial factor productivity of potassium responded only to potassium application. These results indicate that partial factor productivity was more sensitive than agronomic efficiency in detecting treatment effects under the present experimental conditions. The decline in partial factor productivity with increasing fertilizer rates may be interpreted in relation to the diminishing efficiency with which additional fertilizer inputs were converted into forage production. Such responses are commonly attributed to the progressive reduction in nutrient-use efficiency as nutrient supply exceeds plant demand (66). One possible explanation is that increasing external K concentrations may shift nutrient uptake from high-affinity to low-affinity transport systems, thereby reducing nutrient acquisition efficiency (68). However, because tissue nutrient uptake was not measured, this mechanism remains speculative. Overall, the results indicate that higher fertilizer inputs continued to improve forage production but with progressively lower fertilizer-use efficiency, emphasizing the importance of considering productivity and nutrient-use efficiency simultaneously when developing fertilizer recommendations for hot arid production systems.

4.5 Increased forage productivity was associated with greater changes in soil available potassium and partial K balance

An important outcome of the present study was that higher forage productivity was accompanied by greater changes in soil available K following harvest. Although K fertilization substantially increased biomass production, the measured change in exchangeable soil K also increased with fertilizer application. Tissue K concentration, total plant K uptake, nutrient partitioning, and different soil K fractions were not measured. Therefore, the observed change in NH4OAc-extractable K and the calculated apparent K removal cannot be attributed specifically to crop removal, rhizosphere mobilization, root architecture, leaching, fixation, or release from non-exchangeable or subsoil pools. These processes are discussed only as possible explanations based on previous studies. Moreover, fresh cladode yield showed a significant positive relationship with the change in soil available K, indicating that treatments producing greater forage yields were also associated with larger reductions in soil available K during the experimental period. Because both fresh cladode yield and the change in soil available K responded to the fertilizer treatments, particularly potassium application, the observed association may partly be interpreted in relation to their common response to K fertilization rather than a direct causal relationship between the two variables. These findings highlight the substantial K requirement of O. ficus-indica and emphasize the importance of monitoring soil available K when developing fertilizer management strategies. Although the maximum fertilizer rates applied in the present study (60 kg Ca ha−1 and 90 kg K ha−1) were considerably lower than the seasonal Ca and K removal reported for high-yielding O. ficus-indica (24), the observed changes in soil available K highlight the need to balance fertilizer inputs with crop nutrient demand to sustain long-term soil fertility. The observed changes in soil available K are consistent with the high physiological demand for potassium in cactus pear. Potassium plays essential roles in osmotic regulation, enzyme activation, stomatal function, and assimilate transport, all of which support rapid biomass production in succulent plants (46, 56). Consequently, greater biomass production could hypothetically be associated with increased K uptake. Similar relationships between rapid biomass accumulation and high K demand have been reported for O. ficus-indica (42, 43). The partial balance indicated an apparent K removal of 263 kg K ha−1 under Ca60K90. Given the observed dry-matter yield, this magnitude cannot reasonably be attributed to crop K uptake alone. This value exceeded the initial available K pool because the calculation included the 90 kg K ha−1 fertilizer input. It should not be interpreted as measured crop uptake because it may include crop removal, leaching, K fixation, release from non-exchangeable reserves, and K acquisition below the sampled 0–15 cm layer. The lower post-harvest available K under higher K rates therefore represents an unresolved soil K balance rather than direct evidence of greater crop uptake. Potassium fixation and release in calcareous soils may strongly influence NH4OAc-extractable K following fertilization (25). If crop uptake contributed to the observed changes in soil available K, the extensive shallow root system of cactus pear may have enhanced exploitation of surface soil nutrients under arid conditions (11). Rhizosphere processes may also enhance the availability of less readily exchangeable K fractions (69), although these processes were not evaluated directly in this study. The present findings therefore highlight the importance of evaluating forage productivity together with changes in soil available K when developing nutrient management strategies for perennial cactus production systems. Because O. ficus-indica is harvested repeatedly over multiple years, continued reductions in exchangeable soil K without adequate replenishment may gradually reduce soil K availability during successive production cycles. This highlights the importance of balanced nutrient replenishment and long-term soil fertility management to sustain productivity in resource-limited arid production systems, consistent with broader evidence from dryland restoration and agroforestry studies emphasizing sustainable nutrient management for maintaining soil health and ecosystem resilience (70–72).

4.6 Principal component analysis summarized the coordinated response of O. ficus-indica to calcium and potassium fertilization

Principal component analysis (PCA) provided an integrated summary of the responses of O. ficus-indica to Ca and K fertilization across morphological traits, forage productivity, forage quality, and changes in soil available K. The first two principal components explained 83.8% of the total variation, indicating that the major treatment-related variation was represented by PC1 and PC2. Variables associated with vegetative growth and productivity, including fresh cladode yield, dry matter yield, crude protein concentration, crude protein yield, plant height, cladode number, and cladode morphological traits, were positioned in the same direction along PC1, whereas NDF and ADF were positioned in the opposite direction. The change in soil available K was also located in the same region of the ordination as the productivity-related variables. Treatments receiving intermediate to higher rates of Ca and K were separated from the unfertilized treatments, indicating distinct multivariate responses to fertilization. Because several variables included in the PCA were derived from common measurements (for example, dry matter yield and crude protein yield), the ordination should be interpreted as a multivariate summary of treatment responses rather than evidence of causal relationships among variables.

4.7 Study limitations and future research

This study was conducted at a single location during one growing season using a single O. ficus-indica accession. Consequently, the findings should be validated across multiple growing seasons, different genotypes, soil types, and arid production environments to account for seasonal and environmental variability. In addition, gypsum and sulphate of potash supplied sulfur together with Ca and K; therefore, the individual effects of sulfur could not be separated from those of the applied nutrients. The physiological mechanisms discussed here were inferred from the established functions of Ca and K because tissue Ca, K, and N concentrations, nutrient uptake, and nutrient partitioning were not measured. Quantifying nutrient accumulation, partitioning, and uptake dynamics would provide a more direct understanding of the mechanisms underlying the observed responses. Long-term field studies incorporating plant nutrient uptake together with measurements of exchangeable and non-exchangeable soil K fractions are also needed to distinguish crop uptake from soil K transformations and to determine whether repeated harvesting progressively reduces soil K availability under continuous cactus pear production. Addressing these knowledge gaps will support the development of nutrient management strategies that improve forage productivity while maintaining long-term soil fertility in arid production systems.

5 Conclusion

Calcium and potassium fertilization improved the growth, biomass production, forage quality, and productivity of Opuntia ficus-indica under the hot arid conditions of western India. Potassium was the principal factor influencing fresh cladode yield, whereas both calcium and potassium contributed to improvements in structural growth, dry matter yield, crude protein yield, and forage quality. Partial factor productivity declined with increasing fertilizer application, indicating diminishing fertilizer-use efficiency at higher nutrient inputs despite continued gains in forage production. Greater forage productivity was also associated with larger changes in post-harvest soil available K, highlighting the importance of considering soil nutrient dynamics together with crop productivity in perennial forage systems. Although higher fertilizer rates increased dry matter and crude protein yields, several moderate and high fertilizer combinations produced statistically comparable responses for many agronomic traits. Therefore, fertilizer recommendations should consider productivity, forage quality, fertilizer-use efficiency, and post-harvest changes in soil available K rather than focusing solely on maximizing biomass production. The present study was conducted over a single growing season and did not quantify plant nutrient uptake or soil K fractions. Future research should evaluate the long-term effects of repeated harvesting and fertilization on nutrient cycling, potassium balance, and soil fertility to develop sustainable nutrient management strategies for O. ficus-indica in hot arid environments.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fsoil.2026.1950553/full#supplementary-material

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Footnotes

Edited by: Abdulmannan Rouhani, Jan Evangelista Purkyně University in Ústí nad Labem, Czechia

Reviewed by: Muhammad Madnee, Islamia University of Bahawalpur, Pakistan

Saeed Shojaee Barjoee, St. Petersburg Mining University, Russia

Ghazwa Basma, Jan Evangelista Purkyně University in Ústí nad Labem, Czechia

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was received for this work and/or its publication. This research was supported through the collaborative research program between the Indian Council of Agricultural Research (ICAR), Department of Agricultural Research and Education, Government of India, and the International Center for Agricultural Research in the Dry Areas (ICARDA). The work also contributes to the CGIAR Science Program on Multifunctional Landscapes, funded by contributors to the CGIAR Trust Fund.

Data availability

The original contributions presented in the study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Author contributions

AN: Conceptualization, Data curation, Methodology, Software, Visualization, Writing – original draft. SH: Writing – review & editing. ML: Writing – review & editing. DD: Writing – review & editing. DM: Writing – review & editing. RD: Writing – review & editing. DS: Writing – review & editing.

Acknowledgements

The authors acknowledge the Indian Council of Agricultural Research (ICAR) and the International Center for Agricultural Research in the Dry Areas (ICARDA) for supporting this collaborative research.

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Citation

Naorem, A., et al. (2026). Calcium and potassium fertilization improves productivity and forage quality with implications for soil available potassium in Opuntia ficus-indica under hot arid condition. Frontiers in Soil Science, Volume 6. https://doi.org/
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Copyright © 2026 Naorem, Hassan, Louhaichi, Dayal, Machiwal, Dev and Singh.

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*Correspondence: Anandkumar Naorem

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