Groundwater Clogging Problems

Mixing Fertilizers Containing Calcium and Sulfate

Mixing a fertilizer that contains calcium with one that contains sulfate can cause calcium sulfate (gypsum) to precipitate. One example of this occurs when you mix calcium nitrate with potassium sulfate. While both of these fertilizers are water soluble, mixing them together in irrigation water causes the formation of gypsum, which is much less soluble and will precipitate out of the water.

Testing a Mixture

The potential for precipitation from injecting fertilizers can be evaluated by analyzing the water for calcium, magnesium, bicarbonate, and sulfate content. Concentrations of any of these constituents in exceed of 2.5 meq/l suggest a potential for the water to form precipitates when mixed with certain fertilizers.

You can also conduct a jar test. Mix the fertilizer in the water at the concentration that will occur during fertigation. Cover the mixture and let it stand overnight. If the mixture turns cloudy or white, or if a precipitate collects on the bottom of the container, precipitation will probably occur in the drip lines. This testing procedure may have to be repeated frequently, particularly when phosphate fertilizers are used, since characteristics may differ by manufacturer. Use extreme caution when mixing fertilizers for a jar test since some mixtures may generate heat. Check with the manufacturer before you mix a product.

The best policy is to not mix fertilizers because of the many unknowns regarding these chemical reactions.

References

Burt, C., K. O’Conner, and T. Ruehr. 1995. Fertigation. San Luis Obispo: California Polytechnic State University Irrigation Training and Research Center.
Rolston, D. E., R. J. Miller, and H. Schulbach. 1986. Management principles. In F. S. Nakayama and D. A. Bucks, eds., Trickle irrigation for crop production. Amsterdam: Elsevier.
UNOCAL. 1996. N-pHURIC reference manual.