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Selecting the Right Water-Soluble Fertilizer

To balance your growing media pH, select a water-soluble fertilizer that works with your irrigation water.

By Bill Argo, Ph.D., Blackmore Co.

There are two primary goals to keep in mind when selecting a water-soluble fertilizer for your facility:

1. Maintaining the growing media pH within an acceptable range, and

2. Supplying your plants with a sufficient amount of essential nutrients for good growth and flowering.

The “best” fertilizer to use on your plants is the one that not only supplies nutrients, but also complements the alkalinity and nutrient content of your irrigation water.

Water alkalinity influences the fertilizer’s pH reaction

When discussing how water-soluble fertilizer affects growing media pH, it is important to understand that water-soluble fertilizer cannot be applied without irrigation water. The best guide when selecting an appropriate water-soluble fertilizer is to balance the proportion of nitrogen in the ammoniacal form (acid) against the irrigation water alkalinity (base). Although other factors affect growing media pH, research has shown that it is the balance between the ammoniacal nitrogen in the fertilizer and water alkalinity that has the greatest effect on growing media pH of long-term crops.

Optimize the health of your crops by selecting a water-soluble fertilizer that best matches your irrigation water’s alkalinity.

Fertilizers with Iron-EDDHA are shown to be the most effective at supplying iron in growing media,
resulting in more robust crops.

To understand how the alkalinity concentration in the water and the percentage of ammoniacal nitrogen in the fertilizer interact, picture a balance with water alkalinity on one side pushing the pH up, and on the other side, with the ammoniacal nitrogen pushing the pH down.

If either of these factors is out of balance, the growing media pH will be affected. For example, using a fertilizer very high in ammoniacal nitrogen (like 21-7-7 acid) with low alkalinity water (like RO or rain water) is very effective at driving the growing media pH down, because there is nothing to neutralize all the acidic hydrogens (H+) being produced through nitrification or plant uptake.

Proven Winners® Professional Fertilizers

"Let us take the guesswork out of finding the right fertilizer for your greenhouse. With a simple water sample, our Director of Growing will evaluate and select the right fertilizer for your water quality. Each of the six specialized Proven Winners Professional Fertilizer blends include 100% Iron EDDHA, the most readily available form of iron.

  • Low alkalinity — 14-3-14 + Minors
  • Moderate alkalinity — 16-4-16 + Minors
  • Moderately high alkalinity — 18-5-18 + Minors
  • High alkalinity — 17-4-17 + Minors
  • Cold weather formula — 12-3-12 + Minors
  • Greening/pH reducing formula — 16-4-8 + Minors

To request the water sample form, please call us at (734) 654-6420 or email:

Another example would be using a fertilizer low in ammoniacal nitrogen (like 14-0-14) with a high alkalinity water source (like well water commonly found in the U.S. Midwest). In this case, there would be little if any acidic hydrogens (H+) produced to neutralize the liming effect of the water alkalinity, plus the large amount of nitrate nitrogen uptake would also add to the liming effect.

Remember — the two things that affect growing media pH the most (water alkalinity and ammoniacal nitrogen) cannot be directly measured with a pH meter. Water alkalinity must be measured with an alkalinity test. The percentage of ammoniacal nitrogen in the fertilizer needs to be calculated based on the information supplied on the fertilizer bag or from your fertilizer supplier.

Iron requirements are key

The form of iron is very important with iron nutrition. The three common chelated forms (iron-EDDHA, DTPA, and EDTA) differ in their ability to hold onto the iron (and therefore keep iron soluble and available to plants) as the growing media pH increases.

Between a growing media pH of 4.0 to 5.5, any form of iron will work (including iron sulfate) at supplying iron to the plant. However, as the growing media pH increases above 7.0, only the iron from Iron-EDDHA has high solubility.

Research has shown that the ranking of iron forms from most effective to least effective at supplying iron at high growing media pH is: Iron-EDDHA > Iron-DTPA > Iron-EDTA > Iron sulfate. If iron is applied in a form that is not soluble because of high growing media-pH, most of the nutrient will not be available to plants until the growing media pH is lowered.

Choose the fertilizer that fits your needs

Understanding how to fertilize your crop requires more than just selecting a fertilizer formulation off the shelf. Start with a recent water test that is less than a year old so you know what is in your water. Select fertilizers that complement your water in both pH reaction and desired nutrient levels. Finally, if you constantly have high-pH related problems, choose an iron form that is more soluble in the high pH range.

For more information, contact Bill Argo at 800-874-8660 or email:

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