A large proportion of watermelon acres in Indiana are not currently using drip irrigation. Farmers may choose not to use drip irrigation because precipitation is usually abundant in most years for watermelon production. There is a desire to reduce costs and/or fear that over-irrigation may increase the risk of mature watermelon vine decline, a disease often appears after heavy rains in the field, and management recommendations include judicious irrigation.
Watermelons generally accumulate 145 to 160 lbs nitrogen (N)/acre in the vegetation and fruit. In the non-irrigated system, farmers apply most N fertilizers preplant. Urea is often used as the primary nitrogen source.
In 2021, we experimented to understand how the different amounts of preplant nitrogen as urea affect watermelon growth and yield. The experiment was conducted in a loamy fine sand soil with 0.7% organic matter. The treatments include 75 (Trt1), 100 (Trt2), 150 (Trt3), 200 (Trt4), and 250 (Trt5) lb N/acre preplant as urea. Trt 1 received an additional 75 lb N/acre as urea seven weeks after transplanting.
Yield
Trt2 that received 100 lb N/acre had the lowest yield among the treatments. Trt1 that received split N application had the highest yield numerically but did not differ statistically from yields of Trt3-5, which received equal or higher rates of N preplant.
Did Trt2 have the lowest yield because of insufficient N?
Probably. The soil total N was the lowest in Trt2 at ten weeks after transplanting. Trt2 leaf tissue also had the lowest N rate (3.87 % N) compared to other treatments ranging from 4.28 to 4.54% N.
Why did increasing the preplant N rate not increase watermelon yield in Trt 4 and 5?
The first consideration is whether the watermelons have reached the maximal yield in the study. This experiment was conducted in small plots (20’ W by 28’ L), including two rows and six plants per row. We converted the small plot yield to yield/acre; it ranged from 36280 lbs/acre (Trt 2) to 53925 lbs/acre (Trt1). Even though yield in the small plot experiment may not reflect yield in a large field, we do not think the lack of yield response was because the plants have reached maximal yield.
The second consideration is whether the extra N applied to Trt 4 and 5 has lost during the season. This was not likely because the total N in the soil at ten weeks after transplanting was the highest in Trt5, followed by Trt 4, then Trt 3; Trt1 and Trt2 were lower than Trt 3-5. These results indicated that the extra nitrogen applied to Trt 4 and Trt 5 was still in the soil toward the end of the season.
Why did watermelons not take the extra N and achieve a higher yield? Plant tissue tests provided us with a clue. Leaf potassium (K) was in the range of 1.9 to 2.19% at the end of the season, which was considered low or deficient. Trt5 had lower K compared to Trt1. Leaf magnesium (Mg) and Calcium (Ca) were low or deficient at the fruit-expanding stages. These results indicate that a K, Mg, and Ca deficiency may be the yield-limiting factors in Trt3-5.
All the treatments received 100 lb K/acre and 30 lb Mg/acre, which should be adequate for watermelon production. Then why did plants not take up the K and Mg?
We found soil ammonium-N (NH4-N) to nitrate-N (NO3-N) ratio in the early season was greater as preplant N rate increased. Trt1 and Trt2 had relatively lower NH4‑N:NO3-N ratios (0.2 to 0.5) than Trt4 and Trt5. The NH4-N:NO3-N ratio was 1.4 at two weeks after transplanting and decreased to 0.5 at six weeks after transplanting in Trt 5. The high NH4-N:NO3-N ratio in the early season will likely inhibit plant uptake of cations including K, Mg and Ca. A study on melons found that leaf composition of K, Ca, and Mg was significantly lower under NH4-N:NO3-N ratio at 0.5 compared to NH4-N:NO3-N ratio at 0.1. The soil NH4-N:NO3-N ratio in the current experiment was higher than 0.1, especially in the treatments with a higher preplant N rate.
When urea is applied to soil, it converts to NH4-N through urea hydrolysis; NH4-N then converts to NO3-N through nitrification, a microbial process that requires moisture and air. We noticed soil water content at the top 6” soil were frequently above 50% water depletion in the first 6 weeks of the 2022 season. The dry soil may have suppressed the nitrification process.
Take home message: in the non-irrigated system and in a dry year, we think the current N management practice may result in high NH4-N:NO3-N ratios that inhibit plant update of K, Mg, and Ca, which prevent watermelon from reaching maximal yield.
We are planning to conduct additional studies in the 2022 season to improve watermelon fertilization management recommendations for the region.