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.
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].
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.
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.