Optimizing Planting Methods for Sustainable Pomegranate Cultivation in the Shallow Gravelly Land of Central Deccan Plateau Region, India

1. Introduction

Pomegranate is a significant fruit tree cultivated extensively in India’s Deccan plateau region, particularly in Maharashtra, Karnataka, and Andhra Pradesh [1]. The Deccan plateau contributes significantly to India’s total pomegranate production, which amounts to nearly 2.85 million tonnes, with Maharashtra alone contributing over 65% ([2]. The cultivation area and production of pomegranates has been steadily increasing over the past two decades, driven by favourable agroclimatic conditions such as adaptability to harsh climates, three-season flower production, short juvenile period, and staggered cultivation ([2]. Notably, districts like Nasik, Solapur, Sangali, Pune, Osmanabad, and Ahmednagar in Maharashtra’s Central Deccan plateau region account for a significant portion of pomegranate cultivation, primarily on marginal and sub-marginal lands totalling approximately 0.19 mha [3]. These lands exhibit characteristics such as sloping terrain, high coarse fragments, shallow soil depth, elevated bulk density, low water retention, and poor soil fertility with low organic carbon and nutrient content, posing significant challenges to fruit tree growth and development [4].

Furthermore, the growth and development of fruit trees face substantial limitations in slopping, shallow, and gravelly barren land. These soils typically exhibit poor soil structure, leading to restricted root penetration and hindered absorption of nutrients [5]. Additionally, their low water retention capacity results in frequent drought stress and limited moisture availability for fruit trees [6]. The constrained root space in shallow and gravelly soils further impedes nutrient uptake and overall plant vigour [7]. Furthermore, the erosion susceptibility of slopping soils poses risks to root stability and nutrient cycling. These combined challenges underscore the need for targeted soil management and cultivation practices to improve fruit tree performance in such environments.

Planting techniques significantly influence fruit tree growth and sustainable production [3,8], with pit size, shape, and filler materials being crucial factors. Larger pits with increased soil volume enhance root contact, aeration, and water infiltration [9]. Various planting methods have been devised for arid and semi-arid environments, such as trench planting promoting horizontal root growth [10]. Sunken pit planting conserves subsoil water, crucial for tree growth during dry periods [11]. Pit planting enhances crown growth, stem diameter, and shoot length in fruit trees [12]. As trees mature, adjustments to the pit size become necessary for sufficient water and nutrient supply [12]. Neglecting pit size can cause root coiling, impacting tree health and fruit production negatively [13]. Seedling health directly affects fruit quality and quantity, emphasizing the link between tree vigour and sustainable production. Yet, the growth and development of a tree are equally influenced by the soil and other organic materials used to fill the pits, particularly in stony, marginal lands

Soil is another critical component of planting technique that is required for sustainable pomegranate cultivation under skeletal land situations. The ideal soil for is one should meet specific criteria, including high water retention, nutrient supply capacity, and adequate drainage to support robust tree growth and development [3,14]. Soil composition and nutrient availability directly affect the nutritional content and taste of the fruit [15]. Balanced nutrition can result in higher-quality fruits with better flavour and nutritional value [16]. Limited studies have shown that some soil types, such as loamy soil, clay soil, and clay-sand mixtures at specific ratios, can have a significant influence on fruit yield [3,17]. Additionally, the use of organic sources to supplement nutrient requirements has been suggested by Nadeem Shah et al. [18].

When it comes to optimizing planting procedures for plantation crops under skeletal land conditions and with shallow soil depth, insufficient attention has been given to pomegranate trees. This research seeks to address critical questions related to the sustainable cultivation of pomegranates in shallow skeletal soils, specifically: 1. Which planting techniques, pit sizes (varying in length and depth) and soil types are most effective in promoting robust growth in well-developed pomegranate trees in shallow skeletal soils? 2.To what extent do these planting approaches influence leaf nutrient concentrations, yield, and fruit quality of pomegranate trees in shallow skeletal soils? 3.How does increasing the width or depth of the planting pit affect the growth and development of pomegranate trees under conditions of shallow gravelly land? 4. What is the ideal pit volume required to achieve higher and sustainable fruit production from 8-year-old pomegranate trees under condition of shallow skeletal soil? 5. Does the application of spent wash at the time of planting influence the growth, development, and fruit yield of 8-year-old pomegranate trees in shallow skeletal soils

The research aims to harness the untapped potential of shallow skeletal soils by optimizing planting practices and customizing soil mixtures to these unique soil conditions, thereby enhancing their productivity, resilience, and environmental sustainability. This study strives to set the stage for improved fruit quality, economic growth for farmers, minimized environmental footprints, and a robust agricultural sector, ultimately contributing to a sustainable and secure food future.

2. Material and Methods

2.1. Description of the Study Area

The research field, positioned at 18° 09′ 30.62”N, 74° 30′ 03.08”E and elevated 570 meters above sea level, is nestled in the tropical climate of Baramati, a region within the Pune district of Maharashtra, India. This region experiences an average yearly temperature of 26.3 °C, peaking at 32.5 °C in April and dipping to 23.3 °C in December. The majority of the annual rainfall, constituting over 70%, occurs during the southwest monsoon season from June to September. Given the mean annual precipitation of 572 mm, the research area falls within a rain shadow dryland region, situated behind the western ghat ranges [19].

2.2. Establishment of the Pomegranate Orchard

The study area was characterized by sparse vegetation including native grasses, bushes, and trees, situated within a trapezoidal landscape. To facilitate orchard development, elevation points were utilized to create isoline maps, aiding in the establishment of terraces along a 6% slope. These terraces, varying in dimensions from 90-200 m in length and 33-35 m in width, were organized into three blocks, each comprising six contour terraces. Land preparation involved the use of heavy machinery to break rocky layers, while acidic raw spent wash was applied at a rate of 50,000 L ha-1 to soften the rocks. The site underwent micro blasting, ripping, and chaining multiple times to optimize land conditions. Subsequently, one research plot was used for planting the Bagwa variety pomegranate (Punica granatum L.) trees at intervals of 4.5 x 3.0 m2.

2.3. Planting Techniques

The experiment employed a three-factorial planting method, involving four different pits, three soil types, and two soil depths, alongside two farmer methods currently used by farmers. The treatment details for planting pits (I), soil types, and soil depths are provided below: 1. Trench method: A trench was dug on both sides of trees with a length of 3m, a height of 1m, and a width of 1m, resulting in a soil volume of 3m³. 2. Wider pit method: A rectangular pit, 2m in length, 1m in width, and 1m in height, was dug, resulting in a soil volume of 2m³. 3. Pit method: A narrow pit in a square shape, 1m in length, 1m in width, and 1m in height, was dug with a soil volume of 1m³. 4. Auger: A dug pit with an inner diameter of 0.6m and a height of 1m was created, resulting in a volume of 0.28m³. Farmers used a 0.45 m³ pit volume, with and without sugarcane spent wash, for planting of the pomegranate seedlings.

For soil (II), the following were used for planting: 1. Soil mixture at a 1:1 ratio of black and native soil types. 2. Native soil saturated with sugarcane liquid spent wash of one molar volume (450 ml) and 3. Native soil prepared by separating the murrum with a 2mm sieve. The characteristics of spent wash used for experiments given in Table S1. Two soil depths (III) were utilized for planting tree seedlings: normal depth of 1m and enhanced depth of 2m. In both depths, the soil was filled up to a depth of 1m. The enhanced soil depth to 2m was achieved by loosening native coarse fragments with the help of minor blasting techniques, with no external filler material added beyond 1m soil depth (Figure 1). All pits were gradually filled with their respective soil filler treatments up to a 0.3 m depth. Tree seedlings were planted uniformly at a depth of 0.3m and then carefully filled with the designated soil types, firmly pressed by hand.

2.4. Crop Husbandry

Farmyard manure (FYM) was applied at 20 kg per pit during transplantation of six-month-old pomegranate saplings (cv. Bhagwa) in March 2013, following standard practices for the region (Marathe & Jadhav, 2010). The FYM had nutrient contents including N (0.52%), P (0.28%), K (0.62%), Ca (0.21%), Mg (0.15%), Mn (610 ppm), Zn (73 ppm), Cu (76 ppm), Fe (860 ppm), organic carbon (38.5%), pH (6.4), and electrical conductivity (3.2 dS m-1). Initially, each seedling received 10 kg FYM, 300 g N, 150 g P, and 200 g K for two years. From the third year onwards, Hastha bahar pruning was carried out post-southwest monsoon (September-October). After 45 days, an ethereal (0.25%) and DAP (0.5%) spray was applied for leaf shedding, followed by light pruning and additional nutrient application (20 kg FYM, 500 g N, 200 g P, 200 g K per seedling). Full P and K doses were given in November, with N doses split and applied monthly. Drip irrigation was maintained at 7–10-day intervals at the orchard’s periphery.

2.5. Collection, Processing and Analysis of Samples

Soil: Soil was collected at the beginning of the experiment and the distribution of sand, silt, and clay particles of soil used for plantings was assessed with the help of hydrometer [20]. The specific volume of soil was determined through bulk density analysis [21]. The soil available water was determined using the pressure plate apparatus [22]. Soil pH and electrical conductivity (EC) were measured at saturated paste conditions [23]. According to the wet chromic acid oxidation method, the soil organic carbon density was assessed [24]. According to the instruction of Hussain & Malik, [25] instructions, 0.32% potassium permanganate extractant was used for the appraisal of the soil available nitrogen (N) fraction. The Olsen reagent was used to evaluate the soil available phosphorous (P) fraction [26]. The plant available potassium fraction (K) was appraised using the 1N NH4OAC [27].

Above-ground portion of trees: The above-ground biomass of standing trees was evaluated using conventional destructive techniques, which included pruned materials and litter falls collected over the period of 8 years [28]. Tree spread was measured in both the north-south and east-west directions with tape until the last leaf of the tree [29]. The height of the trees was measured from the base to the maximum top of the canopy using a 3m-wooden ruler [29].

Roots: Soil profile method was adopted for root sample collection, where lines were drawn at 30 cm increments surrounding the tree up to 1.5 m away. Three trees from each treatment were harvested and the roots along with the surrounding soil within a 30 cm radius were excavated. Root and soil samples were collected at 20 cm depth intervals up to 1 m depth, separated using wet sieving techniques, and then oven-dried at 65°C for four days to determine root biomass. The lengths of larger roots (> 5mm dia) were measured using a meter scale, the intersection approach was used for smaller roots with diameters up to 5mm for assessing root length and their distribution [30](Habib, 1988). Additionally, composite samples of twenty cores (6 cm height, 5 cm diameter) were collected at the intervals of 30 cm horizontal distances (1.5 m spread) and 20 cm depth (up to 0.6 m depth) for the assessment of root length density (RLD) [31](Pierret et al., 2000). The RLD value for each planting technique was calculated by averaging the RLD values across the sample width and depth of the pits.

$$RLD\left(mm-3\right)=\frac{Totalrootlength}{Soilcorevolume}$$

Leaf nutrients content: At the pre-flowering stage, composite samples of 50 fully matured leaves were randomly collected from each tree. They were dried and the powdered leaf samples were digested with diacid mixture and the nitrogen content was estimated with distillation system. The phosphorous and potassium from the leaves brought into solution by digestion with a triacid procedure. The solution phosphorus was determined by the vanadomolybdate yellow colour method and the solution potassium was appraised with the flame photometer following the standard procedure given in the textbook by Tandon [32].

Fruit yield and quality: Fruit yield was measured from staggered harvest (t ha^-1). A total of 45 fruit samples were used for assessing the fruit length and width using a digital Vernier scale (Mitutoyo, Japan; Least count: 0.1 mm). Total soluble solids (°) in juice were measured with a refractometer (Atago, Tokyo, Japan). After the arils were removed, the juice was extracted with an extractor (Maharaja White line-Smart chef FP-100) and the juice yield was assessed as given below [33].

$$Juiceyield(\%)=\frac{WeightofjuiceX100}{Weightoffruit}$$

2.6. Statistical Analysis

The growth parameters of trees, including above-ground biomass, crown spread, root biomass, root length, root length distribution, leaf nutrient content (N, P, K), and fruit quality parameters (length, width, juice content, total soluble solids), were subjected to the Kolmogorov-Smirnov and Shapiro-Wilk tests to assess normal distribution. Furthermore, a three-factorial ANOVA test (p > 0.05) was conducted to analyse variance attributed to pit techniques, soil depth, filler soil materials, and their interaction effects. Post hoc analysis using Duncan multiple range test (DMRT) (p > 0.05) was employed to identify significant differences between treatments. Multiple regression analysis was performed to identify key variables influencing leaf nutrient content. The liner regression analysis was also carried out to establish functional relationship between pit volume used for planting and fruit yield. Data analysis was conducted using SPSS software version 16.0.

3. Results

3.1. Basic Properties of Soil Types Used for Pomegranate Plantings in the Shallow and Gravelly Barren Land

Table 1 presented herein delineates the foundational soil properties utilized across various planting methodologies for pomegranate cultivation within shallow and gravelly barren land conditions. It is observed that the traditional farmer techniques, represented by barren land, exhibited a significantly elevated coarse fraction content of 73.4% (w/w) as opposed to both the native soil and the black and native soil mixture filled pits. The incorporation of the soil mixture notably transitioned the soil texture from sandy loam, characteristic of the native soil, to loam soil with increased silt and clay particles content. Moreover, the soil mixture showcased augmented soil specific volume and increased plant-available water in comparison to the native soil. The liquid spent wash saturated native soil had a low soil pH of 4.5; conversely, all other soil types maintained a neutral pH range. Furthermore, the soil mixture exhibited a higher soil electrical conductivity (0.41 dS m-1), soil organic carbon content (0.43%), and available nutrients such as N at 155.5 kg ha-1, P at 8.3 kg ha-1, and K at 222.1 kg ha-1 as compared to the remaining soil types (Table 1). Yet, the carbon and available N (< 280 kg ha-1) and P (<10 kg ha-1) of the soil mixture were in low category while K came in medium fertility category (180-350 kg ha-1).

3.2. Effect of Planting Techniques on Growth Parameters of Pomegranate Trees

Table 2 distinctly illustrates that both the pits and soil types, integral components of planting techniques, exerted significant influence on above-ground biomass production. There were notable differences among all the pits in terms of biomass production, with the trench and wider pit methods demonstrating higher production (>7.03 t ha-1) compared to the other pits. When the width of the pit increased by 1 m, the wider pit method showed a significant increase in biomass production, from 6.57 to 7.03 t ha-1. Similarly, the soil types exhibited variations among themselves, and the soil mixture displayed greater biomass production at 10.03 t ha-1 compared to the other soil types (Table 3).

The crown spread of the tree demonstrated substantial variations contingent upon the types of pits and the soil medium utilized for planting pomegranate seedlings. Among the various tree pits, the trench and wider pit exerted a more pronounced influence on the spread, encompassing an area of 3.3 m2. With every 1 m increase in the width of the pit, there was a significant augmentation in the crown spread. Conversely, the auger and pit exhibited a lesser spread, exceeding 2.2 m2, yet still outperforming the existing planting techniques which typically result in a spread greater than 1.5 m2 (Table 2 and Table 3).

The Table 2 and Table 3 also indicate that both pit types and soil mediums significantly influenced the production of root biomass and root length in the planting of pomegranate seedlings. Among the pit types, the trench and wider pit methods resulted in a greater biomass production of 2.45 t ha-1 compared to other pit types. The wider pit method, in particular, produced significantly higher biomass than the standard pit method, which in turn was higher than both the auger planting and traditional farming methods. The root length varied significantly across all pit types, with the longest root (1058 m) produced under the trench method. An increase in pit width from 793.2 to 950 m2 also corresponded to an increase in root length. As with pit types, the type of soil used also significantly influenced biomass production and root length. The soil mixture displayed greater biomass and root length (2.1 t ha-1 and 899 m, respectively) compared to the other two soil types. However, the soil’s effect on biomass production and root length varied slightly with pit size. For all soil types under the auger method, the biomass and root length remained the same. However, black soil exhibited greater values compared to the other soil types in the remaining pits. Both the native soil with and without spent wash showed lower and almost identical biomass and root lengths in the pit, wider pit, and trench methods (Figure 2).

Figure 3 illustrates the distribution of root length at both vertical intervals and horizontal distances from the tree trunk. The length of the roots declined significantly with increasing soil depths. For vertical distribution, the sum of root length across the lateral distances in all the pits demonstrated that the surface depth (0-20 cm) and subsurface depths (20-40 cm and 40-60 cm) significantly differed from each other and also from the 60-80 cm and 80-100 cm depths. However, there was no clear difference between the latter two depths. The interaction effect of root length and soil depth revealed that planting methods also influenced root length, especially at depths of 20-40 cm. The trench, wider pit, and pit methods resulted in longer roots compared to the auger planting method. At a depth of 40-60 cm, the trench and wider pit methods were similar, but all methods were comparable beyond that depth. In terms of horizontal distribution, the root length near the trunk was consistent across all pits but was significantly higher compared to distances at 30-60 cm, 60-90 cm, 90-120 cm, and 120-150 cm. At 30-60 cm and 60-90 cm, both the trench and wider pit methods showed similar and higher root lengths compared to the pit and auger planting methods. However, at distances of 90-120 cm and 120-150 cm from the trunk, the trench method exhibited significantly higher root lengths, while the wider pit, pit, and auger methods showed similar lengths.

Root length density, another crucial parameter of root growth, exhibited significant variations across the adapted tree pits and soil types used in the establishment techniques. Both the trench and wider pit methods had the same and lowest root length density (0.28) in comparison to the other pits. Similarly, the soil mixture presented the lowest density (0.36) when compared to other soil types, which had a density value of 0.41. The auger pit method, on the other hand, had a greater density value of 0.57 over the other methods (Table 2).

However, the application of spent wash to the native soil during tree establishment and the increase of soil depth from 1m to 2m without additional soil did not influence any growth parameters of the pomegranate tree (Table 2).

3.3. Effect of Planting Techniques on Nutrients Content of Leaves of Pomegranate Trees

Table 2 and Table 3 confirm that the concentrations of nutrients in leaves were significantly influenced by both the types of planting pits and soil. The trench method exhibited higher nutrient contents of P and K, at 0.58% and 1.78% respectively, while the trench and wider pit methods had similar N content, exceeding 1.56%. Similarly, the soil mixture had higher leaf N and P content, at 1.48% and 0.21% respectively, while there was no significant difference among soil types in terms of Potassium content. Moreover, both the native soil with and without spent wash saturation exhibited the same N and P content, at 1.43% and 0.19% respectively.

3.4. Effect of Planting Techniques on Fruit Quality Parameters of Pomegranate

The data presented in Table 4 reveals a significant difference in fruit yield between various planting pits and soil types, specifically adapted for pomegranate cultivation in shallow, gravelly land. The trench method yielded a higher fruit output (9.2 t ha-1) in comparison to other methods. Similarly, the soil mixture demonstrated a superior yield (8.6 t ha-1) relative to other soil types. Moreover, the variation in yield due to soil effect was marginal for different pit types. All pit types exhibited significant differences under black soil conditions, with the trench method recording the highest yield of 9.5 t ha-1. Conversely, under native soil conditions, both with and without spent wash saturation, the pit and wider pit methods produced nearly identical yields (Figure 4).

An in-depth analysis of fruit quality parameters has revealed that the type of planting pit significantly impacts both fruit length and width. Fruits produced from the trench and wider pits were of equal or greater size, measuring 65 cm in length and 63.5 cm in width. Soil types also had a significant influence on fruit size, with the soil mixture yielding the largest fruit, measuring 62 cm in both dimensions (Table 2 and Table 4). The width of the fruit exhibited notable variation among different pit types and soil conditions. Under black soil conditions, the trench and wider pit methods resulted in larger fruit sizes, while other methods yielded similar widths across all soil types. The fruit juice content and TSS content also varied significantly based on the type of planting pit and soil used. The trench method and the soil mixture yielded fruits with higher juice content, 48.5% and 44.5% respectively. Interestingly, the interactive effects of soil types and pits showed varying impacts on juice and TSS contents. The combination of the trench method and soil mixture yielded the highest juice content (50.2%) and TSS (16.1%) (Figure 4).

4. Discussion

4.1. Effectiveness of Planting Techniques on Growth and Development of Pomegranate Trees

The marginal land characterized by shallow and gravelly soil types presents limitations due to a higher proportion of coarser fractions, which replace fertile soil and hinder both above-ground and below-ground biomass growth and development. This is primarily due to severe water and nutrient limitations that restrict root growth and consequently affect above-ground plant parts[4,34,35]. The study revealed that the trench and wider pit methods were most effective in promoting robust growth in pomegranate trees in shallow skeletal soils. Irrespective of soil types, the larger pits with increase of width by 2-3 m could store water and nutrients to the maximum required for 8-year-old pomegranate trees in the case of regular irrigation as well as during high intensity rainfall during the off-season [9]. These methods resulted in production of more than 87.4% above-ground biomass, 94.1% more tree crown spread, 87.6% more root biomass, and 62.3% more root length compared to farmer methods (Table 3). Further, increase of width of the planting pit positively influenced growth parameters, with wider pits leading to higher biomass production and root length [3,8]. This indicates the importance of pit size in tree development under shallow gravelly land conditions.

Soil types used for filling pits also played a significant role, with the soil mixture demonstrating superior growth outcomes compared to other soil types [3]. All pits filled with the soil mixture of loam texture had higher soil volume given unit of land, plant available water, soil available N, P and K content compared to native soils and the farmer method (Table 3) highlights that the soil mixture is more important for achieving success in tree development [36].

4.2. Influence of Planting Approaches on Leaf Nutrients Concentrations, Yield, and Fruit Quality

The study found that planting techniques and soil types had a significant impact on leaf nutrient concentrations, fruit yield, and quality parameters of pomegranate trees [37,38]. The trench method and soil mixture of black and native soil at 1:1 ratio resulted in higher leaf nutrient concentrations of P and K. These soil immobile nutrients showed a 64.7% and 44.7% increase in leaves over farmer method and this is a relatively higher increase compared to the leaf nitrogen (37.3%) content (Table 3) Based on multiple regression analysis, it was confirmed that the leaf nutrients increase was attributed mainly for root growth parameters such as root length, root length density, root biomass and pit volume (Figure S1) [39]. When the pit volume changed from 0.1 m3 to the 3 m3 volume under trench method, the 8-year-old pomegranate tree showed 80.6% and 90.8% increase in root length and root biomass, respectively [40]. Further, the greater root length distribution at 90-150 cm distance away from trees and up to 60 cm soil depth under trench method also displayed the influence of planting method on root length distribution [3,10,41]. As pit volume increased, root length density decreased, which could indicate more spares root development and more root exploration away from the root especially under the larger pit of trench method [7]. These all support the uptake of more soil immobile nutrient uptake under the trench method of planting in the shallow and gravelly barren land situation. Further, the black soil highly favoured for uptake of the N and P by fruit trees since their content was higher compared to the native soil [17,42](Newman et al., 1973; Marathe et al., 2018). In contrast, the leaf K content was same to all the soil had soil available potassium ranged between 192.5 and 225.2 kg ha-1 [43]. This findings emphasis soil type use for planting with respect to nutrient uptake by aged pomegranate tree.

The planting method also played a crucial role in achieving fruit yield with superior fruit quality under the challenging land situation. The trench method that the 8-year-old pomegranate showed higher productivity with superior fruit quality and the yield increase was 62.1% more compared to currently exist planting practice [17]. Similarly, the loam soil mixture showed greater fruit yield (56.2%) and fruit quality (>15% of fruit dimension, >29% fruit juice and >21% TSS content compared to other soil types [44]. The interaction effect of soil types and pit size indicated that when smaller pits up to 0.3 m3 used for planting of pomegranate seedling in the shallow and gravelly land, the soil type could not affect fruit quality parameters [45]. This finding emphasises the selection of appropriate soil type when different pits used for planting pomegranate tree under the marginal land situation for archiving higher and sustainable fruit production with better fruit quality.

4.3. Impact of Pit Volume and Spent Wash Application on Fruit Production

The investigation delved into determining the optimal pit volume necessary for sustainable fruit production from 8-year-old pomegranate trees in shallow skeletal soils. A linear regression analysis corroborated that each 1 m3 increase in pit volume led to a corresponding increase of 0.69 tons ha-1 in fruit yield production (Figure 5). Notably, the research revealed that larger pit sizes, particularly those with increased width rather than depth, such as trenches and wider pits (ranging from 2-3 m3 in volume), contributed significantly to higher fruit yield and quality when compared to smaller pits within the context of 8-year-old pomegranate trees grown in marginal land showed a shallow soil depth and a higher gravel stone content de [46].

Spent wash is a liquid waste produced from the sugar industry, which is rich in nutrient source and its application brings improvement to soil physical properties and chemical properties. This is the cheapest nutrient source for small and marginal farmers and is widely used in agriculture in the study area ([47]. However, in our study, the application of spent wash during seedling planting could not have contributed to fruit production since effect of spent wash are mostly short term in nature and the effect might have gotten diluted with tree growth over the period [48]. The deepening of pits beyond 1 m without soil filling also did not yield any improvements in fruit production. This lack of improvement can be attributed to the fact that the majority of the roots are concentrated within the upper 0.6 m of soil, indicating that the pomegranate tree is a shallow-rooted tree [3,49].

5. Conclusion

Trench and wider pit methods which involve digging pits of 2-3 meters in width, have been found to promote an 87.4% increase in above-ground biomass, a 94.1% increase in tree crown spread, an 87.6% increase in root biomass, and a 62.3% increase in root length in pomegranate trees grown in shallow gravelly land. These larger pits with increased width play a crucial role in storing water and nutrients, leading to significant improvements in biomass, root length, and tree crown spread. The loam soil mixture with higher soil specific volume, plant-available water, and higher nutrients content has shown optimal tree development and efficient nutrient uptake and fruit production with superior fruit quality. Moreover, the trench method with soil mixtures showed higher uptake of soil immobile nutrients (P & K). These nutrient improvements directly impact the achievement of higher fruit yield and superior quality, emphasizing the critical role of planting strategies in attaining desirable fruit characteristics. However, the adoption of trench or wider pit methods in pomegranate cultivation requires heavy machinery for pit formation, adding to cultivation costs. This aspect may discourage small and marginal farmers from adopting these techniques. Future research should focus on analysing the cost of cultivation for pomegranates using different planting techniques and the corresponding yield benefits. Additionally, assessing carbon sequestration and potential income from trading sequestered carbon resulting from restoring challenging terrains for planting techniques should be explored in future studies.

The planting volume of 2-3 m³ pit or trench and filling of loam soil to 1 m depth can be recommended for achieving sustainable fruit production from 8-year-old pomegranate trees under the shallow skeletal soil condition. This ensures improving food and nutritional security and enhancing the livelihoods of pomegranate farmers who cultivate rocky barren land.