Watermelon Fertilization Standards and Detailed Explanation of Fertilization Techniques After Fruit Set

The general principle of watermelon fertilization is to apply heavy organic fertilizer, a moderate amount of nitrogen fertilizer, steady phosphorus fertilizer, increased potassium fertilizer, combined application of the three elements nitrogen, phosphorus and potassium, and a moderate supplementary application of medium and trace element fertilizers. So what are the watermelon fertilization standards? How should fertilization be done after fruit set?

The fertilizer amount is determined by the target yield. For example, under medium fertility conditions in North China, when the target yield is 2500~3500 kg/mu, 20~30 kg of nitrogen, 8~12 kg of phosphorus and 30~40 kg of potassium are required.

Fertilization should refer to the fertilizer requirement patterns of watermelon at each growth stage. As a crop that likes fertilizer and warmth and is not tolerant of waterlogging, watermelon has a preference for potassium and boron, and its demand ratio for nitrogen, phosphorus and potassium is roughly 3:1:4.

Overall, nutrient uptake by watermelon is low in the early stage, high in the middle stage, and low in the late stage. The total uptake of nitrogen, phosphorus and potassium at the seedling stage accounts for about 0.18%~0.25% of the entire growth period; during the vine extension stage, stems and leaves grow rapidly, and the uptake of nitrogen, phosphorus and potassium increases accordingly, accounting for 20%~30% of the whole growth period; after fruit set, the uptake of nitrogen, phosphorus and potassium during the fruit enlargement stage reaches the peak of the entire growth period, accounting for 65%~75% of the whole growth period; the total nutrient uptake at the maturity stage accounts for 5%~10% of the whole growth period.

Throughout the whole growth period, watermelon absorbs the most potassium, followed by nitrogen, and the least phosphorus. Before fruit set, nitrogen fertilizer is dominant; after fruit set, potassium uptake increases sharply; at the mature stage, nitrogen and potassium uptake decreases significantly, while phosphorus uptake relatively increases; at the dehairing stage, nitrogen and potassium uptake is basically equal. The approximate absorption ratios of nitrogen, phosphorus and potassium at different growth stages of watermelon are respectively 3.8:1:2.8 at the seedling stage, 3.6:1:1.7 at the vine extension stage, and 3.5:1:4.6 at the peak fruit growth stage.

Therefore, sufficient nitrogen fertilizer is needed in the early growth stage to ensure and promote vegetative growth. At the seedling stage, apply nitrogen fertilizer as a topdressing; at the middle stage, attention should be paid to the moderate application of phosphorus fertilizer; after entering the enlargement stage, potassium fertilizer should be mainly applied as a topdressing to promote the improvement of fruit yield and quality. The critical period for nitrogen and phosphorus nutrition in watermelon is at the seedling stage, the critical period for potassium nutrition is at the vine extension stage, and the period of maximum nutrient efficiency for watermelon is at the fruiting stage.

In addition, in the current watermelon cultivation process, there are problems such as excessive nitrogen and phosphorus fertilizers, too little potassium fertilizer and organic fertilizer, and a lack of trace elements, which watermelon growers should pay attention to avoiding.

Base fertilizer

Base fertilizer is the foundation of the nutritional supply throughout the life of watermelon. It not only supplies the nutrients needed for growth, but also has the effect of improving the soil and enhancing soil fertility. It accounts for 1/2~2/3 of the fertilizer used during the entire growth period.

First, organic fertilizer. Generally apply 1~2 cubic meters/mu of fully decomposed organic fertilizer such as chicken manure or cow manure, or 200~400 kg/mu of commercial organic fertilizer, and 80~100 kg/mu of decomposed cake fertilizer (bean cake is best for cake fertilizer).

Second, chemical fertilizer. Apply 25~35 kg/mu of compound fertilizer (15-15-15) or other proportions of compound fertilizer and 6~15 kg/mu of diammonium phosphate.

Topdressing

Principle: apply seedling fertilizer lightly, apply vine extension fertilizer skillfully, and apply fruit enlargement fertilizer heavily.

1. Seedling stage

Timing: when there are 2~4 true leaves.

Effect: applying a small amount of quick-acting fertilizer to watermelon at the seedling stage can accelerate seedling growth.

Method: first, open an arc-shaped furrow 15 cm away from the seedling and apply; second, make a hole 10 cm away from the seedling and apply, scatter the chemical fertilizer into the hole, then cover with soil and level the ground, then water at the spot, watering 2~3 kg per plant.

Fertilizer choice: mainly nitrogen fertilizer, generally applying 8~15 kg/mu of urea.

2. Vine extension stage

Timing: watermelon enters the vine extension stage after 5~6 leaves (when the vine is about 15 cm long).

Effect: after watermelon vines extend, the growth rate accelerates and the nutrient requirement increases. Topdressing fertilizer at this time can promote the rapid elongation of vines and expand the leaf area.

Method: open a topdressing furrow about 10 cm deep, 10 cm wide and about 40 cm long between two watermelon seedlings, and tread it firm after fertilization. For chemical fertilizer, the furrow can be smaller, about 5~6 cm deep, 7~8 cm wide and about 30 cm long. After fertilization, water once to promote the absorption of fertilizer. When applying cake fertilizer, water one more time.

Fertilizer choice: combined application of organic fertilizer and chemical fertilizer. Apply 40~100 kg/mu of decomposed cake fertilizer or 100~150 kg/mu of biogas fertilizer. Add 15~20 kg of urea and 15~20 kg of potassium sulfate. For seedling transplanting, the topdressing amount can be appropriately larger, applying 20~25 kg/mu of ammonium sulfate or urea.

3. Flowering and fruit setting stage

Generally no topdressing and no watering. However, if poor plant growth, small ovary, and difficulty in fruit setting are found, measures should be taken as early as possible. When female flowers begin to open, observe the growth of the plants. When topdressing is needed, foliar topdressing can be used as a remedy. Prepare a 0.3% potassium dihydrogen phosphate solution and spray it on the leaves, spraying once about every 5 days, preferably in the evening for the best effect, and strictly avoid spraying around noon. Spray fertilizer 2~3 times. If very weak growth is found, fertilizer can also be applied once at flowering and watered once. The fertilizer is mainly nitrogen fertilizer, and the amount should be small, applying about 5~6 kg/mu of urea.

4. Fruit enlargement stage

Timing: when the female flower at the normal fruit-setting position has set fruit and the young fruit grows to the size of an egg, it enters the fruit enlargement stage.

Effect: promote fruit expansion and prevent premature aging.

Method: open a furrow on one side of the plant 30~40 cm from the root, and water in combination with fertilization.

Fertilizer choice: mainly phosphorus and potassium fertilizer, combined with nitrogen fertilizer. If nitrogen fertilizer is applied excessively, the quality will decline. Fruit enlargement fertilizer is generally applied in two topdressings. The first is when the young fruit grows to the size of an egg; at this period, nitrogen and potassium fertilizer are dominant. Apply 15~20 kg/mu of diammonium phosphate and 10~15 kg of potassium sulfate. The second is when the fruit grows to the size of a bowl mouth. Apply 5~7 kg/mu of urea and 5~10 kg of potassium sulfate, which can be applied with water or broadcast and then watered immediately.

Skillful application of foliar fertilizer and water flushing fertilizer

Spray 0.1%~0.2% borax solution twice consecutively before watermelon flowering and when enlargement begins. This can not only effectively prevent overly thick rind and hollow fruit, but also has obvious effects of increasing yield, promoting thin rind, and making the flesh sweeter.

After applying fruit enlargement fertilizer until harvest, generally no more topdressing is done, to prevent delaying the maturity period. If there is a fertilizer deficiency phenomenon (such as withered yellow and premature aging of vines), 0.2%~0.3% urea and 0.2%~0.3% potassium dihydrogen phosphate can be used for foliar spraying. After the watermelon vines close the rows, water-soluble fertilizers containing humic acid, amino acids, etc. can also be applied by flushing to enhance root activity and improve yield and quality.