Reasons for using potassium nitrate in a foliar spray

Potassium nitrate can be used for one or more of the following reasons:

• To prevent the occurrence of nutrient deficiency before the first deficiency symptoms appear. This is especially the case when foliar analysis shows lower nutritional levels than the desired optimum levels.
• To correct nutrient deficiency.
• To increase pest and disease resistance.
• To promote well-balanced fruit and/or plant growth and development, especially in orchards with physiological disorders.

When to use potassium nitrate in a foliar spray

Potassium nitrate should be used only where there is a recognized need.

Situations that make foliar plant nutrition with KNO₃ very beneficial

One or more of the following situations make foliar plant nutrition with KNO₃ very beneficial:

• When there is a limited nutrient uptake via the root system, caused by:
  • Damaged roots: diseases, mechanical damage.
  • Unfavourable soil conditions:
    • Too wet (water logging, low oxygen content), too cold, too dry.
    • Nutrient deficiency or excess in the soil.
    • Fixation (K-fixing clay soil).
    • Salinization (cation competition).
  • When peak demand exceeds the nutrient supply via the roots.
• In situations when localized within-plant demand may exceed the capacity of within-plant nutrient redistribution
  • As in the vicinity of large fruit and nut clusters with high localized demand of many nutrients in the fruit.
• In the case of limited nutrient mobility, when flowering precedes leaf expansion and thereby limits xylem nutrient transport.
• For faster recovery of the leaves after stress caused by night frost.

Recommendations and guidelines for foliar sprayings

• Add potassium nitrate when the tank is half filled with water and stir until all potassium nitrate is dissolved.
• Do not exceed the maximum dissolution rates of potassium nitrate in water.
• The temperature of the dissolution water drops when potassium nitrate is dissolved.
• The dissolution time of potassium nitrate can be shortened when the temperature of the dissolution water is increased.
• Apply early in the morning or late in the afternoon, when the air humidity is sufficiently high in order to expand the leaf wet period and promote nutrient absorption by the leaf.
• Make a jar test to check the compatibility with pesticides.
• Make a test with different concentrations of potassium nitrate to assess when leaf burning takes place.
• Nutrient uptake is enhanced when the pH of the spray tank solution is slightly acidic (pH 4-5). Therefore, the use of an acidic potassium nitrate for foliar spray is recommended.
• pH levels of the final tank mix solution below 4 may provoke scorching.

Table 1 shows crop-specific recommendations for foliar KNO₃ sprayings in vegetables, flowers and field crops.

Table 2 shows crop-specific recommendations for foliar KNO₃ sprayings in fruits and nut tree crops.