Abstract
Indiana cultivates approximately 7000 acres of watermelons (Citrullus lanatus) every year, with the majority of production concentrated in southern Indiana, thus making this region a key area for watermelon production in the United States. Diverse irrigation and fertilization practices are used for watermelon production in the region, yet their effects on production outcomes remain poorly understood. To address this knowledge gap, this study investigated the impact of existing practices on watermelon yield to optimize irrigation and fertilization practices for improved production. The experiment was conducted at the Southwest Purdue Agricultural Center, Vincennes, IN, USA, during the 2022 and 2023 watermelon seasons. The following four treatments were applied: high irrigation, low irrigation, no irrigation, and fertigation. The fertigation treatment received the same water application as the high irrigation treatment, but with frequent fertilizer application with irrigation; however, fertilizers were applied before planting in the high, low, and no irrigation treatments. Although soil moisture levels at the different depths varied notably among treatments, no significant differences in yield by weight were observed. The minimal impact of irrigation on watermelon yield suggested that sufficient water is stored in the soil to prevent yield-reducing stress during dry periods. However, the high irrigation and fertilization treatments produced more fruit than the low irrigation and no irrigation treatments. The dry periods during both years coincided with the watermelon fruit-setting stages, potentially contributing to the lower fruit set in the low irrigation and no irrigation treatments. Fertigation showed a higher early yield in 2022 than that of the other treatments. An analysis of soil and tissue nitrogen levels indicated that solely applying nitrogen before planting could lead to excessive soil nitrogen during vegetative growth. This excess nitrogen might delay flowering and harvest. This project offers insights into enhancing irrigation and fertilization practices for watermelon production in southern Indiana, provides recommendations, and discusses future research directions.
The history of watermelon (Citrullus lanatus) production dates back over a century in southern Indiana (Troop and Woodbury 1908). Indiana cultivates approximately 7000 acres of watermelons every year, ranking sixth in the nation (US Department of Agriculture 2024). More than 70% of this production is concentrated in and around Knox County, making southern Indiana a key region for watermelon production in the United States (Illiana Watermelon Association 2024).
Despite its significance, watermelon production faces many challenges, including erratic rainfall patterns exacerbated by climate change. Irrigation has emerged as a critical strategy in mitigating negative environmental effects. Watermelons, although well-adapted to arid and semi-arid regions worldwide, require an estimated 10 to 20 inches of water per season (Doorenbos and Pruitt 1977). Summer rainfall typically meets these needs in southern Indiana, but prolonged dry periods can negatively affect yields.
Since the 1980s, using black plastic mulch has become common in watermelon farming. This practice not only effectively controls weeds near the row but also warms the soil in the early season, resulting in higher and earlier yields (Abdelkhalik et al. 2019; Bhella 1988; Smajstrla et al. 2002). Because plastic mulch prevents water from infiltrating mulch-covered soils, drip irrigation is often used in conjunction with plastic mulch (Bhella 1988). Drip irrigation improves water use efficiency and also allows for effective fertilizer application through fertigation (Gioia et al. 2009).
Despite the widespread adoption of drip irrigation and fertigation in major watermelon-growing regions across the country (Kemble et al. 2024), this system is less commonly used in southern Indiana. In this area, although nearly all watermelons are grown on beds covered with plastic mulch, a significant portion of the acreage is not currently irrigated. For the irrigated fields, both drip irrigation and overhead irrigation are used. For fields equipped with drip tapes, irrigation frequency varies greatly from farm to farm, ranging from multiple events per day to only during extremely dry conditions. Fertilization practices also vary significantly. Some farms adopt weekly fertigation practices through drip irrigation, and others apply most or all fertilizers before planting. Additionally, some farms complement fertilizers used before planting by applying fertilizers through overhead irrigation or spray together with fungicides.
The wide variations in irrigation and fertilization practices in the region are partially attributed to the diverse soil types used for watermelon production in southern Indiana. Princeton loamy fine sand (fine-loamy, mixed, active, mesic Typic Hapludalfs) (US Department of Agriculture 2011a), which is found along the sandy belt of the Wabash Valley in Gibson County and from Decker northward in Knox County, is considered the most superior soil for melon production in the region (McWilliams 1989). The soil is locally known as deep sand soils. These soils feature a loose, light-brown loamy fine sand surface layer and are underlain by gray marl at a depth of 5 to 7 ft. Other sandy soils used for melon cultivation in the region share similar surface soil characteristics but lack the marly subsoil. In response to the need to expand watermelon production and the accumulation of soilborne pathogens in some deep sand soils that make the soil no longer suitable for watermelon cultivation, farmers in southern Indiana are also planting watermelons in heavier soils, like fine sandy loam or sandy clay loam soils (McWilliams 1989).
Another factor that contributes to the varied irrigation practices is a vine decline known as mature watermelon vine decline, which has plagued watermelon production in southern Indiana since the 1980s (Egel and Martyn 2010; Martyn 2007). The vines often wilt and decline soon before harvest because of heavy rains. Although the definitive cause of the plant collapse has not yet been determined, encouraging good root development through judicious irrigation is recommended.
Although southern Indiana watermelon growers use diverse irrigation and fertilization management practices, the effects of the varied practices on production outcomes remain poorly understood. To bridge this gap in knowledge and facilitate evidence-based recommendations, this study aimed to explore the effects of existing practices on watermelon yield and develop irrigation guidelines for optimal production in the region.
Materials and methods
Experiment location and soil characteristics
The experiment was conducted at the Southwest Purdue Agricultural Center in Vincennes, IN, USA (lat. 38.739°N, long. 87.487°W) in 2022 and 2023. The soil at the experiment site in 2022 was a Bloomfield sandy loam (sandy, mixed, mesic Lamellic Hapludalfs) (US Department of Agriculture 2007); in 2023, the soil was a Petrolia silty clay loam (fine-silty, mixed, superactive, nonacid, mesic Fluvaquentic Endoaquepts) (US Department of Agriculture 2011b). The soil water contents at field capacity, permanent wilting point, and available water are presented in Table 1.
Soil series and water contents at field capacity, permanent wilting point, and the available water of the soil in the experiment fields at the Southwest Purdue Agricultural Center, Vincennes, IN, USA, in 2022 and 2023.
Watermelon production
The seedless watermelon and pollenizer cultivars used in the experiment were Fascination and SP-7. Seeds were planted in propagation trays with 50 round cells with a depth of approximately 2.3 inches depth. The trays were filled with peat-based substrate (Metro-Mix 830; Sungro Professional Growing Mix, Agawam, MA, USA). Seeds were sowed on 20 Apr 2022 and 19 Apr 2023, and germinated on heating mats at 86 °F for the first 48 h. Seedlings were grown in a greenhouse at 77 °F for 3 weeks; then, they were moved outdoors for 1 week to acclimate before planting.
Watermelons were transplanted on 15 May 2022 and 19 May 2023. The seedlings were planted on 4-ft black plastic mulch-covered beds with an in-row spacing of 4 ft for the seedless watermelons. The beds had a width of approximately 28 inches and a height of 3 inches. A randomized complete block design with four replications was used for the experiment. Each experimental unit included 12 seedless plants. A pollenizer plant was planted between every two seedless watermelons in 2022 and between every three seedless watermelons in 2023. Pest management followed the Midwest recommendations for watermelons (Phillips et al. 2024).
Irrigation and fertilization treatments
The experiment included three irrigation treatments, high irrigation, low irrigation, and no irrigation, and a fertigation treatment. In 2022, the irrigation thresholds were set at 15% water depletion at a depth of 1 ft for high irrigation and a depth of 2 ft for low irrigation. In 2023, the irrigation threshold for low irrigation was adjusted to 40% water depletion at a depth of 1 ft. The high irrigation treatment maintained the same irrigation threshold as those in 2022. The no irrigation treatment received irrigation once in 2023 after transplanting to ensure transplant establishment. There was no water applied for the no irrigation treatment in 2022. The fertigation treatment received the same water treatment as that for the high irrigation treatment. Irrigation was applied using drip tapes (Ro-Drip; Rivulis, Chandler, AZ, USA) with 12-inch emitter spacing and a flow rate of 0.22 gallons per minute per 100 ft.
The irrigation was performed for 3 to 3.5 h at each event for the high irrigation and fertigation treatments in 2022, and for 2.5 to 3 h in 2023. For the low irrigation treatment, the event was performed for 10.7 h in 2022 and 5 h in 2023.
The three irrigation treatments received the same amount of granular fertilizers: 150 lb/acre nitrogen (N) from urea, 1 lb/acre boron from boron 15% (Winfield Solutions LLC, St. Paul, MN, USA), 1 lb/acre zinc from zinc sulfate (0N–0P–0K–7S–10Zn; Winfield Solutions), 150 lb/acre potassium, and 20 lb/acre magnesium from 0N–0P–60K (potash; Knox Fertilizer Co., Knox, IN, USA) and potassium magnesium sulfate (0N–0P–22K–11Mg–22S; K-Mag). The granular fertilizers were broadcast-applied before bed preparation.
Fertilizer was applied before planting at a rate of 30 lb/acre N from urea with the fertigation treatment in both years. In 2022, the same amount of K and other micronutrients as those used in the other treatments were applied to the fertigation treatment before planting. In 2023, only N fertilizers were applied before planting to the fertigation treatment. The fertigation treatment received injections of liquid fertilizers at a rate of 10 lb/acre/week N using urea ammonium nitrate 32N–0P–0K in 2022 and 4N–1P–8K (Liberty Acres Fertilizer, Darlington, SC, USA) in 2023. The fertilizers were injected during one of the irrigation events each week starting the third week after transplanting. Although the fertigation treatment is expected to receive weekly fertigation for 12 weeks, no fertilizer was applied during weeks when no irrigation occurred. As a result, different amounts of fertilizers were applied to the fertigation treatment compared with those of the other treatments.
In 2022, liquid fertilizers were injected six times, with the last application conducted during the first week of harvest. In 2023, fertigation was conducted nine times, with the last two applications performed during the first 2 weeks of harvest. The fertigation treatment received a total of 90 lb/acre N and 150 lb/acre K in 2022, and a total of 120 lb/acre N and 150 lb/acre K in 2023.
Data collection
Fruit harvesting
Mature watermelons were harvested once per week. The first harvests were conducted on 28 Jul 2022 and 25 Jul 2023. A combination of fruit characteristics, including a yellow belly under the fruit, a dead or brown tendril, and diminished gloss or brightness of the watermelon surface, were used to identify ripe fruit. Harvest lasted 6 weeks in 2022 and 7 weeks in 2023. Fruit was separated into marketable and unmarketable. Unmarketable fruit was not counted as part of the total yield.
Vine growth
During the early season, vine growth was evaluated by measuring the longest vines of individual plants. Three plants from each experimental unit were measured, and the average of these three readings was used for statistical analysis. When the vines of adjacent plants began growing together, the normalized difference vegetation index (NDVI) was used to estimate vine coverage. The NDVI was measured using a handheld sensor (Greenseeker; Trimble Inc., CA, USA) held 3 ft above the foliage. Each experimental unit was measured three times, and the average of these measurements was used for statistical analysis. The NDVI measurements were conducted at 5 and 8 weeks after transplanting (WAT) in both years. Vine length was only measured in 2023 at 4 WAT.
Soil nitrogen analysis
Soil samples were collected biweekly throughout the 2023 season. A total of seven samples were collected from 2 WAT to 14 WAT. A soil probe with a 0.75-inch diameter was used to collect the soil samples. Eight soil cores, at a depth of 8 to 10 inches, were randomly collected from each experimental unit. The eight soil cores were combined and mixed. A subsample was used to measure nitrate nitrogen (NO3-N) and ammonium nitrogen (NH4-N). The measurement was conducted at Waters Agricultural Laboratories, Inc. (Owensboro, KY, USA).
Leaf tissue analysis
Plant tissue samples were collected biweekly throughout the seasons in both years. The most recently mature leaves (the fifth leaf counting from the tip of the vine back toward the stem) were sampled. If there was a large flower or fruit on the same node as the fifth leaf, then another leaf on the same vine was chosen. Approximately 14 to 16 leaves per experimental unit were collected. The leaves from each experimental unit were treated as one sample. A standard plant analysis, including measurements N, phosphorus, potassium, magnesium, calcium, sulfur, boron, zinc, manganese, iron, and copper contents, of the tissue samples was conducted at Waters Agricultural Laboratories Inc.
Experiment design and statistical analysis
A randomized complete block design with four replications was used for the experiment. The data were analyzed separately by year using an analysis of variance (RStudio 4.2; PBC, Boston, MA, USA). Differences among means were tested by Fisher’s protected least significant difference (α = 0.05).
Results
Precipitation
Figure 1 displays the precipitation patterns and quantities during the 2022 and 2023 watermelon seasons. In 2022, 2.35 inches of rain fell from 29 May to 7 Jul, but 7.95 inches fell on 25 Jul and 26 Jul. In 2023, May and June also received little rain; the dry condition was relieved by 3.85 inches of rain on 29 Jun and 30 Jun. Total precipitation measurements from May to August were 24.87 inches in 2022 and 15.01 inches in 2023. Although 2022 had 9.86 inches more rain than 2023, 32% of the rainfall during the time period in 2022 occurred during 2 d in July.
Precipitation distribution (inches) in 2022 and 2023 at Southwest Purdue Agricultural Center, IN, USA, from May through August. The information was adapted from Indiana State Climate Office, Purdue Mesonet (https://ag.purdue.edu/indiana-state-climate/purdue-mesonet/purdue-mesonet-data-hub/).
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05483-24
Irrigation and soil water content
The high irrigation and fertilization treatments received 21 irrigation events in 2022 and 28 events in 2023. The low irrigation treatment received one irrigation event in 2022 and eight events in 2023 (Table 2, Figures 2 and 3).
Irrigation threshold, irrigation amount, and the number of irrigation events for irrigation and fertigation treatments in the watermelon experiment conducted at the Southwest Purdue Agriculture Center, Vincennes, IN, USA, in 2022 and 2023.
Soil volumetric moisture content (m3/m3) of the beds at depths of 1 ft and 2 ft of watermelons grown under high irrigation, low irrigation, and no irrigation treatments at the Southwest Purdue Agriculture Center, Vincennes, IN, USA in 2022. Blue arrows indicate irrigation events for the high irrigation treatment during the time period when the soil volumetric moisture content was measured. The red arrow indicates the irrigation event for the low irrigation treatment during the time period.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05483-24
Soil volumetric moisture content (m3/m3) of the beds at depths of 1 ft and 2 ft of watermelons grown under high irrigation, low irrigation, and no irrigation treatments at the Southwest Purdue Agriculture Center, Vincennes, IN, USA, in 2023. Blue arrows indicate irrigation events for the high irrigation treatment during the time period when the soil volumetric moisture content was measured. The red arrow indicates the irrigation event for the low irrigation treatment during the time period.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05483-24
In 2022, the irrigation threshold for the low irrigation treatment was set at 15% water depletion at a depth of 2 ft, assuming plants develop roots to access water at this depth when it was limited in the first 1 ft. When the 15% depletion threshold was reached at the depth of 2 ft, the water depletion at a depth of 1 ft was approximately 75%, which was drier than the 50% assumed for the purposes of calculating the volume of water to apply. The low irrigation threshold was reached only once during the 2022 watermelon season (Fig. 2). For the high irrigation treatment, the soil water content at a depth of 2 ft was maintained at a constant level and responded to the rainfall that occurred in the middle of July. After the excessive rains at the end of July, the soil water content at the depth of 2 ft under high irrigation and low irrigation treatments remained above field capacity for an extended period. In contrast, the water content at the depth of 2 ft declined more rapidly under the no irrigation treatment following the excessive rainfall.
In 2023, the irrigation threshold for the low irrigation treatment was set at 40% water depletion at a depth of 1 ft instead of the 15% water depletion at a depth of 2 ft used in 2022. As a result, the water content at the depth of 1 ft under the low irrigation treatment exhibited less variation compared with that in 2022. At the depth of 2 ft, soil water contents were similar across all treatments and declined slowly until rain occurred at the end of June. Despite the minimal rainfall in June, the water content at a depth of 2 ft for the no irrigation treatment barely declined in June, in contrast to observations in 2022, when the water content dramatically decreased until July rainfall (Fig. 3). This disparity may be attributed to the higher water-holding capacity of the soils at the 2023 experimental site compared with that in 2022.
Yield
Across the treatments, the average yield was 47,693 lb/acre in 2022 and 60,935 lb/acre in 2023 (Table 3), which falls within the typical range of watermelon yield in Indiana (Guan and Nowaskie 2023). When comparing treatments, the total marketable yield by weight was similar, but the number of fruit varied. In 2022, the high irrigation treatment produced more fruit than the low irrigation and no irrigation treatments. In 2023, the fertigation treatment produced the highest fruit count, which was significantly greater than that of the low irrigation and no irrigation treatments.
Mean and standard deviation (SD) of total and early yields and the average fruit weight of watermelons grown under high irrigation, low irrigation, no irrigation and fertigation treatments at the Southwest Purdue Agriculture Center, Vincennes, IN, USA, in 2022 and 2023.
Early yield, defined as the yield during the first 2 weeks of harvest, differed among treatments in 2022. The fertigation treatment had a significantly higher early yield in both weight and fruit count compared with the low irrigation and no irrigation treatments.
Plant growth
Watermelons grown under high irrigation and fertigation treatments had significantly higher NDVI values compared with those under the low irrigation and no irrigation treatments at 5 WAT in 2022 (Table 4). Differences among treatments were not detected at 8 WAT. In 2023, watermelons under fertigation treatment exhibited greater vine length at 4 WAT and higher NDVI values at 5 WAT compared with the other treatments. The no irrigation treatment had the shortest vines and the lowest NDVI at 4 WAT and 5 WAT, respectively. As in 2022, the difference among treatments diminished by 8 WAT.
Mean and standard deviation (SD) of vine length at 4 weeks after transplanting (WAT) and normalized difference vegetation index (NDVI) at 5 and 8 WAT of watermelons grown under high irrigation, low irrigation, no irrigation, and fertigation treatments at the Southwest Purdue Agriculture Center, Vincennes, IN, USA, in 2022 and 2023.
Soil nitrogen
In 2023, concentrations of NO3-N in the soils of watermelons grown under the fertigation treatment were significantly lower than those of the other treatments at 2, 4, 6, and 10 WAT (Fig. 4). At the end of the season (14 WAT), the soil NO3-N level in the fertigation treatment was similar to that of the high irrigation treatment, and both of them were significantly lower than that in the low irrigation and no irrigation treatments.
Average levels of nitrate-nitrogen (A) and ammonium-nitrogen (B) in soils of experimental plots under high irrigation, low irrigation, no irrigation, and fertigation treatments at the Southwest Purdue Agriculture Center, Vincennes, IN, USA, in 2023. WAT = weeks after transplanting. Error bars represent ± SD.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05483-24
At 4 WAT, the high irrigation treatment had a significantly higher soil NO3-N level than that of the low irrigation and no irrigation treatments. At 6 WAT, the no irrigation treatment had the highest NO3-N concentration compared with that of the other treatments.
A significant difference in the NH4-N concentration in soils among the different treatments was detected at 6 WAT, with the no irrigation treatment having a significantly higher NH4-N level compared with that of the other treatments.
Plant nutrient status
All fertilizers were applied before planting for the high, low, and no irrigation treatments, whereas the fertigation treatment received both preplant fertilizers and liquid fertilizers that were applied with irrigation during the season. The total N applied to the fertigation treatment was less than that applied to the other treatments. Although the total amount of K applied to the fertigation treatment was the same as that applied to the other treatments, K was exclusively supplied by liquid fertilizers during the 2023 season.
Significant differences in leaf N contents were detected at 7 WAT among treatments in both years (Fig. 5). The leaf N contents of watermelons grown under fertigation and high irrigation treatments were significantly lower compared with that of the no irrigation treatment in 2022. In 2023, the fertigation treatment had a significantly lower leaf N content than that of all the other treatments at 7 WAT. No significant differences were detected on the other sampling dates. There was also no significant difference among treatments for the other nutrients (data not shown).
Average leaf nitrogen content (%) of watermelons grown under the high irrigation, low irrigation, no irrigation, and fertigation treatments in 2022 and 2023 at the Southwest Purdue Agriculture Center, Vincennes, IN, USA, in 2023. WAT = weeks after transplanting. Error bars represent ± SD.
Citation: HortTechnology 34, 5; 10.21273/HORTTECH05483-24
Discussion
The different water applications caused great variations in the soil water content among treatments and influenced watermelon vegetative growth in both years. However, the treatment effects on plant vegetative growth diminished after significant rainfall occurred. Despite the differences in early-season vine growth, the treatments had little effect on yield. This lack of irrigation impact on watermelon yield has also been noted by other studies conducted in humid regions. For instance, a study in Florida found that reducing irrigation to approximately 30% of crop evapotranspiration had minimal effect on watermelon yield and quality compared with doubling or tripling the irrigation amounts (Clark et al. 1996). Similarly, a study in Delaware observed little yield difference among various irrigation treatments, including no irrigation (McCann et al. 2007). These findings suggest that in humid regions, enough water is often stored in the soil to prevent yield-reducing stress in watermelons during periods without rain. This stored water may accumulate before the growing season or after heavy rains.
When rainfall was low, the soil water content at a depth of 1 ft at the base of the plants significantly decreased in the no irrigation treatment. In such cases, depending on the root structures, the plants may use water stored in deeper soils or access water over a larger area. Greenwood et al. (1982) observed that watermelons possess a distinctive root structure; instead of a deep tap root, they develop a relatively shallow taproot with numerous robust roots that spread laterally across the upper soil layers as far as the length of the vines. We have observed watermelon roots extended 4 ft away from the base of the plant at 4 weeks after transplanting (data not shown), thus supporting the previously documented root growth patterns.
In 2022, without irrigation, when soil water at a depth of 2 ft in the bed was at 15% water depletion, the depletion level at the depth of 1 ft reached approximately 75%. In 2023, the soil water content at the depth of 2 ft in the bed in the no irrigation treatment declined slightly during periods of little rain, even as water was depleted at the depth of 1 ft. Although we did not monitor the soil water content beyond the bed area, these observations suggest that watermelon roots likely grow horizontally to explore water in a larger soil area before seeking water in deeper soil, supporting previously observed root growth patterns.
The relatively stable water content at the depth of 2 ft in 2023 during the dry period suggested that the silty clay loam soil in the experiment site retained enough water in the first 1 ft of the soil to supply the needs of the watermelons during periods of little rain. In contrast, in 2022, the sandy loam soils held less water, prompting the plants to explore deeper soil layers for water.
It is important to note that after 7.95 inches of rain on 25 and 26 Jul 2022, the soil water content at the depth of 2 ft in both high irrigation and low irrigation treatments exceeded field capacity for an extended period. This saturation could create anaerobic conditions at this depth, potentially negatively impacting watermelon plants if their roots extended into the deeper soils. This risk may be lower for the frequently irrigated plants because they can easily access water in the top 1 ft of soil, making them less likely to extend roots into deeper soils. In contrast, the water content at the depth of 2 ft in the no irrigation treatment declined more rapidly after the excessive rainfall. Thus, even if some roots grew into deep soil, the likelihood of them being damaged by the anaerobic conditions was low. Those observations may explain why the low irrigation treatment had the lowest yield, although the difference was not significant compared with that of the no irrigation treatment in 2022.
The fertigation treatment resulted in higher early yields, and its total yield was generally higher than that of the low irrigation and no irrigation treatments. Unlike the other treatments, the fertigation treatment received split N applications with a total N rate lower than that of the other treatments.
Throughout most of the season, the soil NO3-N content under the fertigation treatment was the lowest among all treatments. By the end of the 2023 season, the soil NO3-N content was approximately 50 ppm in the low irrigation and no irrigation treatments, suggesting that a significant amount of N was not used by the plants in those treatments. Excessive N before blooming can delay flower initiation in other cucurbit crops and, consequently, delay harvest (Swiader et al. 1994). The highest early yield in the fertigation treatment indicated that the soil N levels might have been excessive during the vegetative growth stage for the treatments that received 150 lb/acre of N preplant.
This hypothesis is further supported by tissue analyses. Across all treatments, leaf N contents were more than 5.5% at 3 WAT and 5 WAT during the vegetative growth stage, which is considered above the sufficient range (4.0%–5.5%) for these growth stages (Bryson et al. 2014). The fertigation treatment had the lowest leaf N content compared with that of the other treatments at 7 WAT.
Conclusions
This study examined the effects of drip irrigation and fertilization methods commonly used by Indiana watermelon farmers on watermelon yield over two growing seasons. During both years, June experienced minimal rainfall, resulting in dry periods overlapping with the watermelon flowering and fruit set stages. Consequently, treatments without irrigation or with limited irrigation yielded fewer fruits by count. However, the total yield by weight for these treatments was not significantly different from the total yield by weight for the treatment with frequent irrigation. The minimal impact of irrigation on watermelon yield suggested that in humid regions, sufficient water is often stored in the soil to prevent yield-reducing stress during dry periods. Similar results were observed during watermelon irrigation experiments conducted in Florida and Delaware (Clark et al. 1996; McCann et al. 2007). In Indiana, it is a common practice to apply required fertilizers before planting, with most being incorporated into the beds while laying plastic mulch. This practice can result in excessive N levels around root zones during watermelon vegetative growth, potentially delaying harvest.
This study provides the following insights into optimizing irrigation and fertilization management for watermelon production in southern Indiana. Similar to other vegetable crops, watermelons use between 1 and 1.5 inches of water per week during peak season, based on evapotranspiration rates. However, simply ensuring that this amount is supplied each week from rainfall plus irrigation does not account for the water reservoir stored in the soil that watermelon roots can access. Therefore, if farmers irrigate weekly or more frequently, then they may benefit from reducing irrigation amounts and taking advantage of stored soil moisture instead.
Monitoring soil water content can help make informed decisions about when to irrigate. Because watermelons develop extensive root systems that can access water beyond the bed area, it may be beneficial to monitor soil water content not only under the plastic mulch-covered beds but also in the row middles with bare soil.
In Indiana, it is common for watermelons to be irrigated once per week or every few days to meet the crop’s water needs of the entire period during one irrigation event. Applying a large volume of water during a single irrigation event can cause water to penetrate deeply into the soil, particularly with drip irrigation in light-textured soils. This can readily occur in sandy Indiana soils when an acre-inch of water is applied through drip irrigation on one day. Early in the season, this practice may lead to nutrient leaching beyond the root zone. Later in the season, when plant roots have reached deeper soil, prolonged irrigation followed by heavy rainfall can cause deep soil saturation. This is particularly problematic if there is a hard pan or heavier subsoil that limits water infiltration. Such deep soil saturation can create anaerobic conditions that negatively affect plant growth, which is a risk that is becoming increasingly common with frequent excessive rainfalls.
For nutrient management (if possible), N fertilizer applications should be split throughout the season to avoid early excessively high N levels. Fertigation through drip tapes or sprinkler irrigation as well as side-dressing are known to be effective methods for applying N incrementally. Additional studies are needed to further optimize N application using these different approaches. The optimized N application rate and method should consider plant growth stages, soil types, and irrigation practices and should be adjustable based on environmental conditions.
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