How to Use Urea Rationally and Why Urea Works Well as a Foliar Top-Dressing

Urea contains 46% nitrogen, the highest nitrogen content among solid nitrogen fertilizers.It is also a physically neutral fertilizer and is currently the most commonly used high-quality nitrogen fertilizer in production. So how should urea be used rationally? Why does urea work well as a foliar top-dressing?

The nitrogen form in urea differs from that in ammonium bicarbonate, ammonium sulfate and the like. After it is applied to the soil, although a portion can be directly absorbed by crops, most of it is used by crops only after being hydrolyzed into ammonium carbonate or ammonium bicarbonate through the action of urease. Its hydrolysis rate is related to temperature. When soil temperature is high, conversion is fast and the fertilizer effect is shorter; when soil temperature is low, conversion is slow, but the fertilizer effect lasts longer. In addition to nitrogen nutrients, urea also contains carbon dioxide, which helps crop photosynthesis. In recent years, fluidized bed technology has been used domestically to produce large-granule urea with a diameter of 1.8-4.5 mm, and even larger granules can be produced according to agricultural production needs. It has good pressure resistance and moisture resistance, and also has slow-release properties, with a longer fertilizer effect, making it a good raw material for bulk blended fertilizer (BB fertilizer).

When urea is used as a base fertilizer, it is best to apply it deeply. If used as a base fertilizer in paddy fields, it should be applied deeply before irrigation, because urea is not easily adsorbed by soil and is prone to loss with water. Urea used as a top-dressing should also be applied deeply. Urea has a high nitrogen content, so the amount used each time should not be too large; too high a concentration will instead affect crop growth. Urea should not be used directly as a seed fertilizer, because high-concentration urea in direct contact with seeds can often penetrate into the protein of seed cells, causing protein denaturation and affecting seed germination.

Appendix: Why does urea work well as a foliar top-dressing?

Urea used for foliar spraying has a better effect than other nitrogen fertilizers. This is because:

(1) Urea is a neutral organic-form nitrogen with a low degree of ionization and a small molecular volume, so it easily enters the leaf and is not likely to damage crop stems and leaves;

(2) Its diffusion rate is faster than that of ammonium ions and nitrate ions, and after entering the plant body it can quickly participate in the nitrogen metabolism of the crop;

(3) Urea has a certain hygroscopicity. When the water in the spray solution evaporates and urea remains on the leaf surface, it can still reabsorb moisture and dissolve, so its utilization rate is relatively high;

(4) Urea aqueous solution is neutral and stable in nature, and can be mixed with many pesticides for spraying, both providing nutrients and controlling pests and diseases, improving work efficiency. The concentration of urea used as a foliar top-dressing is 0.2%-1.0%. If the concentration is too high, it will damage the leaves and may even lead to crop death.

Suitable spraying concentrations for various crops: 1% for rice and wheat, 0.5%-1% for vegetables and fruit trees, and 0.2%-0.3% for strawberries, flowers, and cucumbers, tomatoes, eggplants and others in protected cultivation. Usually spray once every 7 days, for 2-3 consecutive sprays, using 50-75 kg each time per 667 sq m, with the leaves sprayed wet as the standard.