An average-yielding watermelon crop in an acre accumulates approximately 150 pounds of both nitrogen and potassium in the vegetative tissue and fruit. These nutrients are supplied by the soil’s existing nutrient pool and through supplemental fertilizer applications. Soil testing is a valuable tool for assessing what nutrients are present in soils and the amount of supplemental fertilizers needed. However, estimating nitrogen availability in the soil is more complex, and standard soil tests do not directly measure plant-available nitrogen. Instead, soil organic matter is used as an indicator of a soil’s nitrogen-supplying potential. Since most soils used to grow watermelons in Indiana have less than 2% organic matter, it is generally assumed that soil organic matter contributes minimally to the crop’s nitrogen needs. As a result, a recommendation of 150 pounds of nitrogen per acre has become a standard guideline in watermelon production.
Despite this general recommendation, there is no standardized approach to nitrogen fertilizer application among Indiana watermelon growers. Instead, a variety of methods are used—each considered “best” by different growers based on their experience. To better understand the impacts of these differing methods and provide research-based recommendations—or advise against certain practices—we conducted a series of field trials over the past few years. I want to share what we have learned from this process.
Does band nitrogen fertilizer application perform better than broadcast application?
Some believe applying granular fertilizers only under plastic mulch-covered beds is more effective than broadcasting fertilizers across the entire field. The reasoning behind this is to improve fertilizer use efficiency as plastic mulch prevents nutrient leaching.
To better understand fertilizer distribution, we measured nitrogen content in soils under plastic mulch and bare soil between rows in a field where fertilizers were broadcast applied. We found that, at the beginning of the season, nitrogen content in the beds was about three times higher than in the areas between the beds. This suggests that most of the fertilizer had been moved into the beds during the bedding process, even though fertilizers were broadcast applied. In this case, we used 4-foot-wide plastic mulch on beds approximately 3 inches tall, with 8-foot center-to-center bed spacing. Clearly, factors like different bed widths, heights, and spacing would influence the fertilizer distribution.
When using banded fertilizer application, an important question arises: how much fertilizer should be applied? Unfortunately, there are no clear-cut recommendations. One thing we do know is that applying all the needed fertilizers in a banded application increases the risk of salt accumulation in the young plants’ root zone. This can potentially delay plant growth or even damage the plants. We’ll discuss this issue in more detail below.
Does fertigation work better than applying all fertilizers preplant in watermelon production?
We explored this question through three separate field trials to compare fertigation with applying all fertilizers preplant. In two of the trials, the fertigation treatments received 20% of total nitrogen before planting, followed by weekly in-season fertigation using a complete liquid fertilizer. In the third trial, at preplant, the fertigation treatment also received 20% of total nitrogen, in addition to all other nutrients, followed by weekly fertigation using liquid nitrogen.
We did not find a total yield difference among the treatments in the three trials. However, early yields were consistently higher in the fertigation treatments, with statistically significant differences observed in one of the three trials. Soil nitrogen levels under the beds remained lower in the fertigation treatments throughout the season. Leaf nitrogen concentrations were above the sufficiency range early in the season across all treatments, but the excess was less pronounced with fertigation. These findings suggest that applying all nitrogen fertilizers preplant may delay early plant growth and flower initiation, ultimately leading to a delayed harvest.
Notably, in our trials, fertigation treatments received 20–40% less total nitrogen compared to the other treatment because fertigation was skipped in some weeks when the soil was wet. Despite the lower total nitrogen input, yields were similar. This result indicates that fertigation improved nitrogen use efficiency in the production system. The excessive nitrogen might be lost through leaching, runoff, denitrification, or left unutilized in the beds, depending on irrigation and natural rainfalls of the season.
Can granular nitrogen fertilizers be applied in the season?
It has become clear that applying all required fertilizers at once is not the most effective approach. While fertigation has shown promising results, it is not feasible for all situations, as a significant portion of watermelon acreage in Indiana is either not irrigated or irrigated using center pivot systems. In these situations, questions about the effectiveness of in-season granular fertilizer application have arisen.
To explore this, we tested in-season urea application over two years. In both years, leaf burn was consistently observed following application. In one year, plant tissue analysis near the end of the season indicated a more balanced nutrient profile in the split nitrogen treatment compared to the preplant-only treatment, even though both received the same amounts of nitrogen and other nutrients. The split nitrogen treatment produced numerically higher yields, but the difference was not statistically significant. In the second year, however, the opposite trend was observed—plants that received split nitrogen yielded less. The application occurred later in the season, followed by heavy rainfall, which likely caused fertilizer runoff.
In-season application of urea is not ideal, as it can cause leaf burn, and may not reliably increase soil nitrogen availability or plant uptake when not incorporated into the soil. Its effectiveness is highly dependent on weather conditions following application and carries a significant risk of nitrogen loss to the environment. When other methods of fertilizer application are not feasible and additional nitrogen is needed, a better approach is to apply split nitrogen at the watermelon layby. Incorporating the fertilizer into the soil through cultivation at that time helps improve nutrient uptake and reduce environmental loss.
Can foliar-applied liquid nitrogen fertilizer benefit watermelons?
The question about foliar fertilizers has come up frequently across different watermelon production systems. I agree that plants are more efficient at taking up nutrients through their roots than through their leaves. If complete nutrients are already supplied through fertigation, I don’t believe plants gain additional benefits from foliar fertilizer applications.
However, I have observed improved crop performance in fields where liquid fertilizers that contain nitrogen were applied through overhead irrigation or spray tanks. These effects were most noticeable under dry conditions. In such conditions, watermelon plants develop extensive root systems that spread far beyond the plastic-covered beds. When nutrients reach the soil, they can be taken up efficiently along with water.
Although I observed these positive effects in growers’ fields, our research trials did not show differences when different foliar fertilizers were used. Looking back, I suspect our trials did not replicate the same level of drought stress seen in grower fields. It’s also possible that the lack of effect was related to the total water volume we used to apply the fertilizers.
Additional thoughts
After exploring various fertilizers and application methods, I’m more convinced than ever that improving soil health should be our top priority. The key message I share with growers is this: no fertilizer or application technique can substitute for the benefits of fertile, healthy soil. Moving forward, our research will focus on developing practical, science-based recommendations to help watermelon farmers build and restore soil fertility for sustained, productive yields.