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How to mix Powder fertilizers which have reached soluble saturation levels

March 08, 2018

In an earlier article we discussed how to prepare Water Soluble (Powder) Fertilizers for use with fertigate automatic fertilizers and provided a formula to assist you to mix powder fertilizers in appropriate ratios for use in an automatic fertilizing system.

After reading the blog, Andrea an avid gardener, contacted us and rightly pointed out that some powder fertilizers reach saturation when mixing and can be difficult or impossible to totally dilute. In response to Andrea’s query we discuss below several methods that can be used to assist in the mixing of powder fertilizers which have reached soluble saturation levels.

Some powder fertilizers can be comparatively easy to mix in summer, yet nearly impossible to totally dilute it in Autumn or Winter, depending on your local conditions at the time.

Summer water temperatures are warmer (particularly on a hot day) than say Autumn or Winter, and warm water makes it easier to dilute fertilizer powder.

The earlier discussion paper included an example using an excellent powder fertilizer available in Australia called "Miracle-Gro® MaxFeed All Purpose Plant Food". In the example, 200 scoops of powdered fertilizer were mixed with 4L of water to create a concentrated liquid fertilizer. This concentrated liquid fertilizer was then to be used in conjunction with a fertigate automatic fertilizer system.

Whilst the above powdered fertilizer can be mixed at the nominated concentration, it can be difficult to achieve complete dilution if cool water is used. We have noticed that water temperatures below say 15°C can be troublesome to totally dilute the powdered fertilizers.

Not all powder fertilizers prove troublesome to mix at low water temperatures, but if you come across this problem there are several solutions you may like to consider.

In the “Miracle Gro” example we assumed fertilizer concentrate would be delivered at a rate of 50ml/10L and then calculated the quantity of powdered fertilizer scoops required. This lead to mixing 200 scoops of powdered fertilizer with 4L of water. As stated above, problems diluting this concentration may be experienced if the water is cool.

Whilst there are most likely numerous methods to solve the dilution problem, some of the ones we know are:

1. Change the dilution concentration of the fertilizer concentrate.

Perhaps the easiest solution is to decrease the fertilizer concentration. This will mean that a larger volume of fertilizer concentrate would be required for the automatic fertilizer system.

By way of example, instead of 50ml / 10L fertilizer concentrate ratio, assume 75ml / 10L of fertilizer concentrate will be consumed by the automatic fertilizer unit.

Therefore, our fertilizer concentrate would be comprised of 133 scoops of powder fertilizer being added to 4L of water as follows:

Fertilizer powder dilution calculation

If you have already added 200 scoops to 4L of water and found a significant amount of powder undiluted and precipitating at the bottom of your mixing container all is not lost !!!  You can still dilute the fertilizer in a similar manner as above by simply adding water so that the proportion of fertilizer concentration is equivalent to 75ml / 10L

To create a concentration of 75ml / 10L we need to add more water to that you have already added 200 scoops of powder fertilizer.

In total we would require (75ml/50ml) x 4L = 6L and therefore, add a further 2L of water.

Store the mixed fertilizer concentrate in an airtight container clearly labelled as to the contents and that it should be used with a fertigate automatic fertilizer at a rate of 75ml/10L (in this example).


2. Increase the water temperature.

    If your water temperature is low eg 10°C or lower and an amount of powder remains undissolved, you may consider increasing the mixture / water temperature to say 25 or 30°C. This can be easily achieved by placing your mixing container in a laundry tub filled with hot water at that approximate temperature. [As a handy hint, don’t fill the laundry tub water level higher than the fertilizer level in your container or it may tip over].

    When your fertilizer mixture has warmed, stir and completely dissolve the fertilizer concentrate.

    Store the mixed fertilizer concentrate in an airtight container clearly labelled as to the contents and that it should be used with a fertigate automatic fertilizer at a rate of 50ml/10L (in this example).

    It is probable that your fertilizer concentrate will remain mixed. However, it is good practice to stir it before use. In an extreme rare instance (eg cold climate) you may need to heat the fertilizer concentrate back to circa 20°C prior to use to ensure all precipitate is fully dissolved.

    You may also need to stir the concentrated fertilizer while it is being consumed.

    Whilst there will undoubtedly be some powder fertilizers which cannot be made into a liquid concentrate for use in an automatic fertilizer, these will hopefully be few and far between.

    We trust the above is of assistance if you find a powder fertilizer difficult to dilute and thank Andrea again for raising this topic.

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