Injecting Slow Release Arbor Fertilizers

Why Do Slow-Release Fertilizers Have Problems Dissolving in Water?

This is a common issue encountered by landscapers who apply arbor fertilizers by soil injection. Fertilizers that have at least 50% slow-release nitrogen are recommended by various experts, including the American National Standards Institute (ANSI). However, these slow-release fertilizers typically have poor water-solubility, so that undissolved particles may settle in the tank, or worse yet, can clog screens or mechanical parts. Why can’t anybody make a slow-release fertilizer that fully dissolves in water?

The answer is all too simple. But before explaining the matter, it is important to realize this fundamental fact: Plants can only absorb nutrients that are liquefied by becoming dissolved in water.

Quick-Release and Slow-Release

Nutrients that are not dissolved in water are unavailable to the plant, because roots cannot absorb them. They only become available if and when they are changed to a liquid (dissolved) form. So very simply put, a fertilizer is released only when it becomes dissolved in water. Fertilizers are quick-release if they dissolve quickly. Fertilizers are slow-release only if they dissolve very slowly, over several weeks or months.

Different Types of Slow-Release

Some slow-release fertilizers need to biodegrade to be released. That is, soil bacteria and other microbes use enzymes to slowly digest the fertilizer. As enzymatic digestion takes place, the fertilizer gradually becomes water-soluble so the microbes can absorb them. Plant roots and other nearby life forms can absorb them too.

Some fertilizer manufacturers use coatings to convert a soluble mineral to a slow-release mineral. The coating keeps water out for an extended time to prevent the mineral from dissolving immediately. As the coating wears away, water gradually gets in to dissolve the fertilizer, thereby releasing it gradually through leaks in the coating. The longer the coating lasts, the slower is the mineral release. In addition to biodegradation and coatings, there are other strategies to make a fertilizer slow-release. But in all cases, the strategy involves extending the time period before the mineral becomes completely dissolved in water. The moment a fertilizer becomes dissolved, it has been released.

Dissolved = Released

So, after reading the explanation given above regarding the meaning of slow-release, you should have come to this inescapable conclusion: Dissolved = Released. Therefore: It is impossible to make a slow-release fertilizer that completely dissolves in your tank.

Although It Can’t Be Dissolved, We Can Make it Suspend in the Water

The best we can do is to make the insoluble ingredients very tiny to keep them lightweight so they can remain suspended in the water. Typically, they are ground down into a fine powder. Although these powders still won’t dissolve, at least they take longer to settle in the tank, and they won’t settle if you have good agitation. Also, tiny particles are less likely to clog screens.

Currently, PHC offers 3 arbor fertilizers for soil injection. These are:

Of these, the first two contain slow-release nitrogen in the form of ureaformaldehyde. This material is not water-soluble, so it has been ground down to a powder. To prevent the powder from piling up on the bottom, you have to do two things:

  1. Get the powder suspended in the tank.
  2. Keep the suspended powder from settling.

Step 1 involves the way that you add the product to the tank. The tank must already contain some water, preferably more than half-full. If you dump the whole bag of product into the tank carelessly, then chances are good that a lot of the insoluble powder will end up at the bottom. You need to slowly add the product into the tank while the water is being agitated. This way, you will ensure that the particles become suspended in the swirling water.

Step 2 involves maintaining agitation. If the agitation is turned off for too long, then the powder will gradually settle, and it may take some effort to get it to re-suspend. The powder will remain suspended in the tank only if you have adequate agitation, and you keep that agitation going. Paddle agitation is the best. Jet agitation can work if it is a strong jet system. If you don’t have a good agitation system, then you cannot inject slow-release fertilizers. PHC for Trees 11-22-22 (without the SRN designation) is fully soluble in water, and will not settle. However, it is not slow-release at all.


  • Plants can only absorb nutrients that are liquefied by being dissolved in water.
  • Dissolved = Released.
  • It is impossible to make a slow-release fertilizer that will completely dissolve in the tank.
  • Slow-release fertilizers must be introduced into the tank in a manner that will properly suspend the insolubles in your tank of water.
  • Slow-release fertilizers require a good agitation system in your tank to maintain suspension.
  • If you don’t have a good agitation system, then you cannot inject slow-release fertilizers.