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Fertilizer blending- Things to know before you take up fertigation?

Fertigation, the application of fertilizers with the irrigation water, allows applying small amounts of fertilizer in each fertigation event which contributes to high flexibility in fertigation frequency.

When mixing fertilizers, it is important to check fertilizer compatibility before application. If incompatible fertilizers are mixed, they form insoluble precipitations that can clog drip emitters and damage sprayers used to apply foliar fertilizers. This article discusses a few scenarios for which special attention should be paid to the solubility of mixed fertilizers.


Scenario 1. A grower is using 20-20-20, a complete fertilizer to fertigate tomatoes. To prevent blossom end rot, he decided to add calcium nitrate in his fertigation program. However, problems may be caused by the application of these two fertilizers. The reason is that calcium from calcium nitrate and phosphate from ammonium phosphate in the complete fertilizer may form calcium phosphate, which is insoluble in water.


Scenario 2. A soil test indicates that tomato plants are low in magnesium. Epson salt (magnesium sulfate) is recommended to correct magnesium deficiency. The grower should avoid applying Epson salt together with complete fertilizers because magnesium from Epson salt forms an insoluble compound when it meets phosphate in the complete fertilizer. In both cases, calcium nitrate and magnesium sulfate should be applied separately with the complete fertilizers.


Scenario 3. The grower wants to apply both calcium nitrate and magnesium sulfate through drip to add calcium and magnesium during the season. These two fertilizer should not be applied together because calcium nitrate and magnesium sulfate form calcium sulfate, which has a low solubility.


Scenario 4. A grower wants to apply foliar spray of high phosphorus fertilizer at blooming stage to increase flowering and help fruit set. He also wants to apply a foliar micronutrient fertilizer solution because plant tissue test indicates plants are low in Boron and Zinc. However, several cations (such as Boron, Zinc, Copper, Iron etc.) in the foliar fertilizer solution are insoluble with phosphate, especially if they are not in a chelated form. The high phosphorus fertilizer should not be applied together with the foliar nutrient solution.

It is impossible to list all the incompatible fertilizers. The best approach is to conduct a jar test before mixing fertilizers. When doing a jar test, the fertilizers should be mixed in the same concentration as intended to be used. If you notice the mixed solution has a milky appearance, the fertilizers should not be mixed and applied together.

Improper mixing and storage of fertilizers can result in large nutrient losses. Some important aspects to consider in fertilizer mixing and storage include the following:

· Urea should not be mixed with ammonium calcium nitrate (CAN), Potassium chloride, and Single super phosphate.

· Urea can be mixed with most other fertilizers but fertilizer mixtures containing urea should be applied immediately after mixing. Do not store fertilizer mixtures containing urea.

· Ammonium phosphates and super phosphates should not be mixed with lime, slag, rock phosphate or CAN.

· Potassium chloride and sulfate of potash can be mixed with most fertilizers, but mixtures of these fertilizers with urea and calcium ammonium nitrate should not be stored.

· CAN should not be mixed with basic slag but can be mixed with urea, single superphosphate, and ammonium phosphates immediately prior to application. Do not store fertilizers in damp or dirty places. Make sure that bags of fertilizer in the store do not absorb moisture from leaky roofs or water seepage through walls and floors.

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