Most cucurbit crops and tomatoes do not convert nitrogen to fruit very well. Their response to excess levels of nitrogen is to grow more plant, which potentially leads to increased disease problems and later production. Increased diseases can occur because the increased leaf canopy inhibits air movement leading to increased humidity. Late fruit production can happen because the plants are busy producing stems and leaves before they settle down to making fruit.
Photo by Mindy Tape, MSU
One of the benefits of fertilizing through drip irrigation (fertigation) is being able to have higher control over the fertilization program. If you notice plants are growing too much, reduce the rate or do not fertilize that week. It is easy to tell if the rate is too high by looking at the leaf number, size and color. Many big, dark green leaves and little to no flowering means the nitrogen rate is too high. In watermelon, there will be an excessive amount of up rights (ends of the vine) pointing up.
Most plants will respond quickly to nitrogen adjustments when they are fertigated since the nitrogen is in liquid form and readily available. Once plants begin to set, fruit nitrogen rates can be increased some to encourage continued plant growth and additional fruit set and development.