Impact of organic manure on fruit set, fruit retention, yield, and nutritional status in pomegranate (Punica granatum L. “Wonderful”) under water and mineral fertilization deficits

Experimental site and plant materials

This study was carried out over two continuous growing seasons (2018 and 2019) using 6-year-old pomegranate trees of the Wonderful cultivar planted in a private orchard in North Cost, Matrouh Governorate, Egypt. The trees were planted at a spacing of 2.5 × 4 m and supplied with water via a drip irrigation system that had two lines per tree and four drippers (8 L/h) per line. Before conducting the experiment, three soil samples were collected at a depth at 0–90 cm for analysis of various physicochemical properties, as shown in Tables 1 and 2. The soil texture in the experimental site is shown in Table 2.

Mineral fertilizer (100%) containing 240 units of N, 60 units of P₂O₅, 192 units of K₂O, 71.65 units of CaO, and 50 units of MgO was applied with the irrigation water weekly from March 1 through to mid-September in both growing seasons. Organic manure (100%) containing 0.24 units of N, 0.07 units of P₂O₅, 0.27 units of K₂O, 0.13 units of CaO, and 0.07 units of MgO was added at a rate of 19.35 kg/tree in November of both years. Organic manure analysis was performed using the standard procedures described by Bertran & Andrease (1994) and showed that the organic fertilizer applied in this study contained 44% organic matter, 22% organic carbon, 1.24% N, 0.38% P₂O₅, 1.40% K₂O, 0.68% CaO, 0.36% MgO, 4500 ppm Fe, 450 ppm Mn, 125 ppm Zn, and 44 ppm Cu and had a moisture content of 11.4 d.b (dry basis) and a C/N ratio of 11:1 (values are averages of both growing seasons).

All pomegranate trees were cultivated using the normal agricultural practices applied in this pomegranate orchards which follow the recommendations of the Ministry of Agriculture, Egypt. The experimental site included a total of 240 trees, and a split-plot design was used in both growing seasons, according to Snedecor & Cochran (1990). The main plots included three levels of irrigation treatments: full irrigation equivalent to 100% of the reference evapotranspiration rate (100% ETo) as a control (I1), 80% ETo (I2), and 60% ETo (I3). The sub-plots included five fertilizer regimes: 100% mineral fertilizer (T1), 100% organic manure (T2), 75% mineral fertilizer + 25% organic manure (T3), 50% mineral fertilizer + 50% organic manure (T4), and 25% mineral fertilizer + 75% organic manure (T5). There were four replications per treatment combination and four trees per replicate.

The necessary climatic data for estimating the daily ETo were collected from the nearest weather station, and the ETo (mm/day) was calculated according to the FAO 56 Penman–Monteith equation (Allen et al., 1998): (1)${\displaystyle {\mathrm{ET}}_{\mathrm{O}}=\frac{0.408\Delta \left(\mathrm{Rn}-\mathrm{G}\right)+\gamma \left(\frac{900}{\mathrm{T}+273}\right){\mathrm{U}}_2\left({\mathrm{e}}_{\mathrm{s}}-{\mathrm{e}}_{\mathrm{a}}\right)}{\Delta +\gamma \left(1+0.34{\mathrm{U}}_2\right)}}$ where Δ is the slope of the saturated vapor pressure/temperature curve (kPa/°C), γ is the psychometric constant (kPa/°C), U₂ is the wind velocity at 2 m height (m/s), Rn is the total net radiation at the crop surface (MJ/m²/day), G is the density of the soil heat flux (MJ/m²/day), T is the mean daily air temperature at 2 m height (°C), e_s is the saturation vapor pressure (kPa), and e_a is the actual vapor pressure (kPa).

Crop evapotranspiration (ETc, mm/day) was then determined as follows: (2)${\displaystyle ETc=Kc\times ETo}$ where Kc (dimensionless) is the crop coefficient, which increased from an initial value of 0.32 in March to a peak value of 0.74 in July, August, and September, according to Intrigliolo et al. (2011). Deficit irrigation was tested at 80% ETo and 60% ETo. The amount of irrigation water applied (L/tree) and the number of irrigations with each irrigation regime during the two growing seasons are shown in Table 3. During the period from January to December seasons 2018 and 2019 under the field experiment, the rainfall was with an annual mean 157.2 and 168.9 mm, respectively. The data were obtained from weather station near the experimental site (Table 3).

Measurement of nutritional status

A total of 30 leaves were collected manually from non-fruiting shoots in the middle part of each tree in the first week of August each year. The nutritional status of the pomegranate trees was estimated by determining the leaf mineral constituents and total leaf chlorophyll content. The total leaf chlorophyll content was determined using fresh leaf samples, according to the method described by Yadava (1986), using the Chlorophyll Meter SPAD-502 (Minolta Camera Co., Ltd., Japan). The SPAD-502 measurements were conducted on fresh leaf samples and the adaxial side of the leaves was always placed toward the emitting window of the instrument and major veins were avoided.

To determine the leaf mineral contents, the leaves were washed with tap water and distilled water and then dried at 65−70 °C for 72 h (Reisenauer, 1976). A 1-g sample of dried ground leaf material from each tree was then digested with sulfuric acid and hydrogen peroxide according to Evenhuis & De Waard (1980). The nitrogen (N) content (%) of the digested solution was determined by the micro-Kjeldahl method following the methods described by Chapman & Pratt (1961), and total N and P were colorimetrically determined according to the methods described by Evenhuis (1976) & Murphy & Riley (1962), respectively. K was determined by flame photometry, as described by Jackson (1967), and the Ca and Mg contents (%) were determined using an atomic absorption spectrophotometer (model 305B) according to Jones (1977).

Percentages of fruit set, drop, and retention

At the time of flowering (April in both years), two main branches growing in different directions were selected and tagged on each tree and the percentages of fruit set (FSP), fruit drop (FDP), and fruit retention (FRP) were determined as follows:

(3)${\displaystyle \mathrm{FSP}=\frac{\text{TNDF}}{\text{TNPF}}\times 100}$ (4)${\displaystyle \mathrm{FDP}=\frac{\text{TNFS-TNFH}}{\text{TNFS}}\times 100}$ (5)${\displaystyle \mathrm{FRP}=\frac{\text{TNFH}}{\text{TNFS}}\times 100}$ where TNDF is the total number of developed fruitlets, TNPF is the total number of perfect flowers, TNFS is the total number of fruit set, and TNFH is the total number of fruit at harvest.

Tree yield and percentages of fruit sunburn, fruit cracking, and marketable fruit

Trees were harvested manually in the second week of October in both years when the fruits reached the ripening stage and became fully collared. The number of sunburned and cracked fruit were determined for each treatment and the percentages of fruit sunburn (FBP), fruit cracking (FCP), and marketable fruit (MFP) were determined as follows:

(6)${\displaystyle \mathrm{FBP}=\frac{\mathrm{NBF}}{\mathrm{TNF}}\times 100}$ (7)${\displaystyle \mathrm{FCP}=\frac{\mathrm{NCF}}{\mathrm{TNF}}\times 100}$ (8)${\displaystyle \mathrm{MFP}=\left(\frac{\text{TNF-NCF}-\mathrm{NBF}}{\mathrm{TNF}}\right)\times 100}$ where NBF is the number of sunburned fruit, TNF is the total number of fruit, and NCF is the number of cracked fruit.

At the time of harvest, five fruit from each tree were collected and weighed, and the average weight was calculated. In addition, the total number of fruit was counted and the average total yield per tree (kg/tree) was calculated.

Irrigation water use efficiency

The irrigation water use efficiency (kg/m³) was determined by dividing the crop yield (kg/ha) by the total amount of irrigation water applied (m³/ha), as reported by Zhang (2003).

Statistical analysis

The data were analyzed using analysis of variance (ANOVA), as reported by Gomez & Gomez (1984), and the means of the various treatments were compared using the least significant difference (LSD) test. All analyses were performed in SAS version 9.13 (SAS Institute, Cary, NC, USA) using the 5% level of significance.

Effects of the irrigation and fertilizer regimes on nutritional status

The nutritional status of the Wonderful pomegranate leaves in terms of the P, N, K, Ca and Mg were significantly affected by the irrigation regimes (P < 0.05) in both seasons (Fig. 1). Moreover, chlorophyll content was significantly affected by the irrigation regimes (P < 0.05) in both seasons (Fig. 2). All of these parameters were fluctuated in both growing seasons and tended to significantly increase with an increasing irrigation level. These findings support those obtained by Khattab et al. (2011), who also showed that the P, N, K, and chlorophyll contents of pomegranate leaves could be significantly increased by increasing the amount of irrigation water applied. This effect may result from an increased accessibility of such nutrients under excess soil moisture conditions, which would improve the uptake rate by a tree and also increase the amount of photosynthesis, thereby increasing the leaf area (El-Kassas, 1983).

Reducing the irrigation rate from 5,720 L/tree (100%ETo, control) to 4,576 L/tree (80%ETo) and 3,432 L/tree (60%ETo) resulted in moderate P, N, Ca, K, Mg, and chlorophyll contents in 2018 and will have had benefits for the environment, as Ren et al. (2014) argued that the huge amounts of water applied by farmers could cause many environmental issues wherever the leaching of fertilizer away from the root zone and into groundwater could occur. Furthermore, under treatment 80%ETo, the pomegranate trees had moderate nutritional values during both seasons, suggesting an absence of limiting factors for ETc to be satisfied (Rodríguez et al., 2012; Galindo et al., 2013; Galindo et al., 2014).

All nutritional parameters of P, N, Ca, K, Mg, and chlorophyll contents were significantly affected by the fertilizer regime (P < 0.05), with obvious differences between the two growing seasons (Table 4). In both seasons, integrated uses of fertilizer sources of the 75% mineral fertilizer + 25% organic manure regime gave the greatest improvement in nutritional status, with higher percentages of N, K, P, Mg, and chlorophyll than 100% mineral fertilizer or 100% organic manure. Furthermore, lower levels of mineral fertilizer in combination with organic manure (i.e., 50% mineral fertilizer + 50% organic manure or 25% mineral fertilizer + 75% organic manure) also gave reasonable values for the percentage contents of N, K, P, Mg, and chlorophyll compared with 100% mineral fertilizer or 100% organic manure. This may be because each fertilizer contained different essential elements or due to the fact that the organic fertilizer helped to facilitate the accessibility and uptake of most nutrients to the trees (El-Shazly et al., 2015). The addition of organic manure can also enhance soil texture by aggregating the soil particles through the use of various organic molecules, such as polysaccharides, and increasing the activities of microorganisms in the soil, which enhances the biochemical cycling, resulting in an increased availability of elements (El-Shazly et al., 2015). Furthermore, organic manure also improves the physical characteristics of soil by increasing the nutrient and water holding capacity, total pore space, aggregate stability, erosion resistance, and temperature insulation and decreasing the apparent soil density (Shiralipour, McConnell & Smith, 1992). Moreover, integrated uses of fertilizer sources help maintain the fertility status of the soil (Baghdadi et al., 2018).

Effects of the irrigation and fertilizer regimes on the percentages of fruit set, drop, retention, sunburn, and cracking, the percentage of marketable fruit, and yield

Irrigation is known to modify processes related to fruit trees (Martínez-Nicolás et al., 2019). However, in the present study, there were no consistent trends in the percentages of fruit cracking, fruit set, fruit sunburn, fruit drop, fruit retention, or marketable fruit or the yield of Wonderful pomegranate trees due to the different irrigation regimes in either season.

The availability of water for agricultural use is the main challenge for optimal fruit tree cultivation under Mediterranean conditions (Martínez-Nicolás et al., 2019). Lower percentages of fruit cracking, fruit sunburn, and fruit drop can be considered advantageous and were observed under an irrigation deficit, with values of 7.66% and 7.27% fruit cracking under 80%ETo, 7.35% and 7.05% fruit sunburn under 60%ETo, and 22.35% and 23.33% fruit drop under 80%ETo in the 2018 and 2019 seasons, respectively. By contrast, the 100%ETo treatment increased the percentage of fruit cracking. Furthermore, the 80%ETo treatment also increased the percentages of fruit set, fruit retention, and marketable fruit and the yield, whereas reducing the irrigation level from 100%ETo to 60%ETo decreased fruit retention from 76.54% to 64.24% in 2018 and from 76.24% to 64.93% in 2019 (Table 5). These findings are in agreement with those obtained by Khattab et al. (2011) and will be useful for informing irrigation management to enhance Wonderful pomegranate production in regions with similar soils and climates. In field experiments on mature drip-irrigated pomegranate trees in Egypt during the 2016 and 2017 production seasons, Taha (2018) found that average fruit yields of 40.2, 38.6, 36.9, 23.8, and 31.8 t/ha were obtained from trees supplied with 13,520, 11,270, 9,020, 6,760, and 18,075 m³/ha irrigation water, which were equivalent to 120%, 100%, 80%, and 60% ETo and local farmer practice, respectively. However, the local farmer practice was done by applying irrigation and fertilizer quantities without consulting from specialist in agriculture guidance and irrigation started in the second week of February and stopped after harvesting in September and minimum quantities of irrigation water were applied during the rest of the growing season.

In terms of the fertilizer regime, the application of lower amounts of mineral fertilizer in combination with organic manure (i.e., 50% mineral fertilizer + 50% organic manure or 25% mineral fertilizer + 75% organic manure) gave reasonable values for the percentages of fruit cracking, fruit set, fruit sunburn, fruit drop, fruit retention, and marketable fruit and the yield of Wonderful pomegranate trees in the 2018 and 2019 seasons compared with 100% mineral fertilizer or 100% organic manure (Table 6). The maximum percentages of fruit set (2018 and 2019: 86.40% and 85.91%, respectively) and fruit retention (77.27% and 77.37%, respectively) and the maximum yield (53.03 kg/tree and 52.72 kg/tree, respectively) were obtained under treatment fertilization regime of 75% mineral fertilizer + 25% organic manure , while the maximum percentage of marketable fruit (74.59% and 74.34%, respectively) was obtained under fertilization regime of 50% mineral fertilizer + 50% organic manure in both seasons (Table 6). These results can be attributed to the fact that organic manure helped to facilitate the availability and uptake of most nutrients to the trees, resulting in an increased fruit yield, and support reports by Rabeh, El-Koumey & Akasem (1993) and Huang, Zhang & Qian (1995) that treating Balady mandarin (Citrus deliciosa Ten.) and Satsuma mandarin (Citrus unshiu Marc.) trees with biofertilizers alone or in combination with organic manure stimulated plant root growth, nutrient absorption, and photosynthesis, leading to an increased yield. Similarly, in a field experiment conducted by Mansour (2018) on 8-year-old Wonderful pomegranate trees in Egypt during the 2015–2017 production seasons. The experiment comprised of two levels of humic acid (0 and 50 g/tree/season) and five nitrogen fertilizers form chicken manure, compost, cattle manure, mineral nitrogen as experiment control 40 kg actual N/fed (fed = 4,200 m²) and mineral nitrogen as orchard control 80 kg actual N/fed. He found that the trees could be fertilized at a rate of 40 kg N/fed/year instead of 80 kg N/fed/year and that the application of compost or chicken manure at a rate of 40 g N/tree/year with or without the addition of humic acid at a rate of 50 g/tree/year could improve the physical and chemical properties of the fruit and yield while decreasing environmental pollution.

Effect of the interaction between the irrigation treatment and fertilizer regime on nutritional status

In both seasons, the percentage contents of N, K, P, Mg, and chlorophyll were highest when the 100% ETo irrigation treatment was combined with the fertilizer regime of 75% mineral fertilizer + 25% organic manure, with values of 1.99%, 0.61%, 1.72%, 0.51%, and 51.25 SPAD, respectively, for 2018 and 2.08%, 0.67%, 1.76%, 0.52%, and 54.33 SPAD, respectively, for 2019 (Table 7). By contrast, the Ca content was highest when 100% ETo was combined with fertilizer regime of 100% organic manure, which gave values of 0.99% in 2018 and 1.05% in 2019 (Table 7). Irrigation regime of 100% ETo in combination with fertilizer regime of 75% mineral fertilizer + 25% organic manure instead of 100% mineral fertilizer improved the concentration of N by 5.99%, P by 14.29%, K by 4.50%, Mg by 13.19%, and chlorophyll contents by 12.04% as a mean of the two growing seasons, while irrigation regime of 100% ETo combined with 100% organic manure instead of 100% mineral fertilizer increased the content of Ca by 20.71%.

Effect of the interaction between the irrigation and fertilizer regimes on fruit characteristics and yield

All of the fruit characteristics measured were significantly affected by the interaction between the irrigation treatment and fertilizer regime (Table 8). The lowest levels of fruit cracking, fruit sunburn, and fruit drop observed were 7.01%, 5.32%, and 15.87%, respectively, in 2018 and 7.15%, 5.18%, and 16.81%, respectively, in 2019. Decreasing the amount of irrigation water to 60% ETo and adding 100% organic manure to each tree reduced the percentage of fruit cracking by 28.67% as an average of the two growing seasons compared with the control treatment (100% ETo and 100% mineral fertilizer), while decreasing the amount of irrigation water to 80% ETo and adding 75% mineral fertilizer + 25% organic manure to each tree reduced the percentage of fruit sunburn by 26.52%, and decreasing the amount of irrigation water to 60% ETo and adding 100% mineral fertilizer to each tree decreased the percentage of fruit drop by 8.13% compared with the control. The other treatments gave intermediate values.

Reduced percentages of fruit cracking, fruit sunburn, and fruit drop were observed under deficit irrigation when both mineral and organic fertilizers were used. By contrast, in a study on pomegranate trees of the Manfalouty cultivar in 2007 and 2008, Khattab et al. (2011) found that irrigation at a rate of 15 m³/tree/year gave the lowest fruit drop percentages (8.75% and 9.09%, respectively), while 7 m³/tree/year gave the highest fruit drop percentages (17.15% and 17.88%, respectively) in both seasons, and that 13 m³/tree/year gave the lowest fruit cracking percentage (6.02% and 6.09%, respectively) followed by 11 m³/tree/year (6.85% and 6.72%, respectively) and 9 m³/tree/year (7.27% and 8.22%, respectively) in both seasons. The reduction in fruit cracking, fruit sunburn, and fruit drop with a decreasing amount of irrigation water observed in the present study may be attributed to the pomegranate fruit being taken from trees that were growing under no water stress due to the presence of organic manure, whereas Khattab et al. (2011) illustrated that a decrease in applied irrigation water increases fruit cracking when fruit are taken from trees growing under water stress.

By exploring the data presented in Table 8, it is clear that all of the parameters that describe the characteristics of the studied fruit were significantly affected by the interaction between the irrigation regime and fertilizer regime. The percentages of fruit set, fruit retention, and marketable fruit and the yield reached maximum values of 94.42%, 84.13%, 86.23%, and 59.57 kg/tree, respectively, in 2018 and 95.55%, 83.19%, 86.84%, and 60.10 kg/tree, respectively, in 2019. Decreasing the amount of irrigation water to 80% ETo and applying 75% mineral fertilizer + 25% organic manure increased the yield by 18.23% as an average of the two growing seasons compared with the control (integrated 100% ETo and 100% mineral fertilizer), while decreasing the amount of irrigation water to 80% ETo and adding 100% organic manure increased the percentage of marketable fruit by 21.82%, decreasing the amount of irrigation water to 80% ETo and adding 25% mineral fertilizer + 75% organic manure increased the percentage of fruit set by 17.83%, and combining 100% ETo with 75% mineral fertilizer + 25% organic manure improved the percentage of fruit retention by 3.65% compared with the control. The finding that deficit irrigation in combination with organic manure with mineral fertilizer or organic manure alone gave the highest yield and percentage of marketable fruit may be attributed to the fact that the presence of organic manure reduces water stress for the growing plants and indicates that the yield of Wonderful pomegranate was not sensitive to water stress under the experimental conditions when organic manure fertilizer was added. Similarly, Intrigliolo et al. (2013) suggested that a mild deficit of water during the flowering and fruit set periods, which saved 9%–14% water, was the best strategy for cultivating Mollar de Elche pomegranate trees, as this resulted in minimal negative effects on fruit yield.

Irrigation water use efficiency

The irrigation water use efficiency index can support decision making around on-farm irrigation (Fernández et al., 2020). In this study, the irrigation water use efficiency was defined as the yield (kg/ha) divided by the amount of irrigation water applied (crop evapotranspiration; m³/ha plus the effective rain). By contrast, the irrigation water productivity is defined as the yield (kg/ha) divided by the total amount of water applied by irrigation (irrigation applied + rainfall; m³/ha) (Volschenk, 2020). Seasonal irrigation applied water including the effective rain were depicted in Fig. 3 as affected by integrated irrigation and fertilizer regimes. However, the total amounts of applied irrigation water were 7,292, 6,148, and 5,004 m³/ha with the 100% ETo, 80% ETo, and 60% ETo treatments, respectively, in 2018 and 7,047, 5,975, and 4,904 m³/ha, respectively, in 2019. The maximum water use efficiency (Fig. 4) was obtained with the 80% ETo treatment combined with 75% mineral fertilizer + 25% organic manure in both seasons with values of 9.69 and 10.06 kg/m³ applied water, respectively. By contrast, Martínez-Nicolás et al. (2019) demonstrated that withholding irrigation during the flowering and fruit set periods for the pomegranate cultivars Mollar de Elche and Wonderful gave irrigation water productivities of 5.6 and 7.1 kg/m³, respectively, whereas Parvizi, Sepaskhah & Ahmadi (2014) found that deficit irrigation applied at 75% and 50% ETc increased the irrigation water productivity of the cultivar Rabab on average over two seasons compared with trees that were irrigated at 100% ETc (irrigation water productivity, 4.2 kg/m³).

The application of 60% ETo in combination with 75% mineral fertilizer + 25% organic manure gave yields of 44.89 and 44.23 t/ha in 2018 and 2019, respectively, giving irrigation water use efficiencies of 8.97 and 9.02 kg/m³ applied water, respectively. Thus, this irrigation and fertilizer regime can be recommended in situations where water is scarce or where production is focused on the delivery of pomegranate fruit to the industry (Intrigliolo et al., 2012). The other treatments provided intermediate values of irrigation water use efficiency, although deficit irrigation is expected to increase water use efficiency due to an increase in irrigation efficiency, as described by Intrigliolo et al. (2012) and Tavousi et al. (2015).