A balanced nutrient approach is critical to maintain watermelon plant growth and maximize flower production. Nitrogen is a key element in boosting leaf growth. It is important that nitrogen supply is not limiting prior to flowering, or plant vigor and marketable yield will be adversely affected. However, too much nitrogen can restrict flowering, and therefore fruit set and yield. Up to one third of all female flowers can be lost due to over application of nitrogen at flowering, leading to significant reductions in fruit set.
Nitrogen form is particularly important during vining, flowering, and fruit set. Trials confirm reductions in leaf mass and yield losses from use of nitrogen fertilizers with too high a proportion of ammonium compared to nitrate (Figures 1 and 2). Crop quality, using Brix as indicator, also deteriorates (Figure 1).
Figure 1- Muskmelon yield and brix is maximized with an abundance nitrate (average of four varieties)
Figure 2- Nitrogen source and leaf mass- Cv Sugar Baby- watermelon, USA
Another important reminder when it comes to nitrogen fertilizer source is that ammonium nitrogen can be antagonistic to the uptake of calcium, magnesium, and potassium while nitrate nitrogen promotes the uptake of those nutrients. This cationic competition, as a result of over-use of ammonium as the main source of nitrogen, can increase fruit nutrient disorders such as Blossom End Rot (BER).
Nitrate nitrogen in YaraLiva promotes uptake of critical nutrients such as calcium, magnesium and potassium.
As a result of early BER issues, young fruit may be lost. BER is often caused by lack of calcium in the flower-end of the fruit and it is worse when lack of moisture or use of saline irrigation water restricts calcium uptake.
Calcium availability is critical during fruit set and early fruit development in order to ensure a good supply of calcium will reach the flesh and rind of the fruit in later stages. Trials show that calcium accumulation in the fruit is almost complete a month after fruit set (Figure 3). This is confirmed with another research which suggests that 80% of the calcium present in ripe melon is already in the fruit 20 days after flowering. The reason for this is that the calcium enters the fruit with water via the xylem, a route that is shut down as the fruit matures.
Figure 3 – Calcium and Potassium Accumulation - cv Maestro - Cantaloupe
In addition, calcium deposits in the cell walls where it will reduce the incidence of pathogen infections. It is important to be proactive and maintain good calcium nutrition throughout the early and mid-season of watermelon production. Maintaining a consistent supply of calcium in the plant will help support the goals of maximizing yields, melon quality and storability.