MisterBlah
Well-Known Member
FYI this has also been posted on my website, which as it turns out, is more like a blog. You will also find a few charts there which are helpful. It's not really practical for me to recreate them in a forum post.
Applying fertilizers to irrigation water (fertigation), hydroponic systems, or foliar sprays is essential to grow big and healthy plants. It is important to be familiar with the facts regarding fertilizer solubility.
Creating a Stock Solution
A stock solution is a concentrated solution of the fertilizers you wish to use. Often times concentrated at 500X or higher. That is, it may be added to irrigation water at a concentration of 2mL/L. The concentrations you wish to make your stock solution in are entirely up to you, but are also limited by fertilizer solubility. If you are irrigating a grow or garden at home and using smaller reservoirs, it’s okay to make your stock solutions in smaller containers and lower concentrations, 1 L to 20 L tanks at 200-400x concentration, before creating your final nutrient solution that you use for irrigation, hydroponics, or foliar spraying. If you are working on the commercial scale, large fertigation tanks of 1000 gallons or more are common. Higher concentrations, 500x-2000x or higher, are recommended.
Dissolving a Fertilizer
The solubility of a fertilizer in water is the maximum amount that can be dissolved in a given volume of distilled water at a given temperature. Often times, manufacturers will provide you with solubility data for their fertilizers, but usually only upon request. It is often not provided in data sheets or labels.
Dissolving a fertilizer is as simple as putting a bunch of it in a bucket of water, right? Sort of. When you add a fertilizer to water, you should not exceed its solubility. Otherwise a precipitate will form and clog your irrigation system hoses, foliar sprayers, or hydroponic lines. Furthermore, you are are wasting fertilizers as those precipitates will often never find their way beyond any irrigation lines to plants roots or leaves.
Mixing Fertilizers
Most fertilizer stock solutions contain multiple fertilizers. Therefore, we need an understanding of what can occur when mixing different fertilizers. If you are mixing fertilizers that contain a common element, like potassium nitrate and monopotassium phosphate, the solubility of each fertilizer will drop. The same will occur if your source water is already rich in minerals. In any case, calculations become more complex as more fertilizers are mixed together. In addition, there are some fertilizers that should not be mixed together in a stock solution. Insoluble precipitates(salts) can form very quickly. A good example of this is mixing fertilizers containing calcium with those that contain phosphate or sulfate. A double replacement reaction will occur and the calcium will precipitate out of solution as a calcium phosphate or a calcium sulfate. Have a look above at the Fertilizer Solubility table and you will notice that Calcium Sulfate and DiCalcium Phosphate have incredibly low solubility when compared to other fertilizers.
As such, all these possible cases are not calculated in the field. Trial and error becomes the best method for determining if a mixture will be suitable. Enter the Jar Test.
The Jar Test
The Jar Test is a simple test. Take a small container, usually 1 L, and mix your fertilizers in the concentrations you are planning on using for your stock solution. You need to be careful to be accurate with this mixture as significant deviations from it can produce different results. I recommend being accurate to 3 significant digits when doing your measurements, and for a 1L solution, that can mean needing milligram precision. The Jar Test is simply a test for compatibility of the fertilizers you are using. Mix everything and wait for all compounds to dissolve. If a precipitate forms, that is, you either see a milky white tint to your mixture or there are salts on the bottom of the container, then you need to change your recipe. What I usually do once I see a precipitate is heat the solution up to 100°F or a little higher to see if the precipitates dissolve. If they do, it means I have too much of one or more fertilizers in solution. If the precipitates do not dissolve, then I have to change the recipe entirely.
Applying fertilizers to irrigation water (fertigation), hydroponic systems, or foliar sprays is essential to grow big and healthy plants. It is important to be familiar with the facts regarding fertilizer solubility.
Creating a Stock Solution
A stock solution is a concentrated solution of the fertilizers you wish to use. Often times concentrated at 500X or higher. That is, it may be added to irrigation water at a concentration of 2mL/L. The concentrations you wish to make your stock solution in are entirely up to you, but are also limited by fertilizer solubility. If you are irrigating a grow or garden at home and using smaller reservoirs, it’s okay to make your stock solutions in smaller containers and lower concentrations, 1 L to 20 L tanks at 200-400x concentration, before creating your final nutrient solution that you use for irrigation, hydroponics, or foliar spraying. If you are working on the commercial scale, large fertigation tanks of 1000 gallons or more are common. Higher concentrations, 500x-2000x or higher, are recommended.
Dissolving a Fertilizer
The solubility of a fertilizer in water is the maximum amount that can be dissolved in a given volume of distilled water at a given temperature. Often times, manufacturers will provide you with solubility data for their fertilizers, but usually only upon request. It is often not provided in data sheets or labels.
Dissolving a fertilizer is as simple as putting a bunch of it in a bucket of water, right? Sort of. When you add a fertilizer to water, you should not exceed its solubility. Otherwise a precipitate will form and clog your irrigation system hoses, foliar sprayers, or hydroponic lines. Furthermore, you are are wasting fertilizers as those precipitates will often never find their way beyond any irrigation lines to plants roots or leaves.
Mixing Fertilizers
Most fertilizer stock solutions contain multiple fertilizers. Therefore, we need an understanding of what can occur when mixing different fertilizers. If you are mixing fertilizers that contain a common element, like potassium nitrate and monopotassium phosphate, the solubility of each fertilizer will drop. The same will occur if your source water is already rich in minerals. In any case, calculations become more complex as more fertilizers are mixed together. In addition, there are some fertilizers that should not be mixed together in a stock solution. Insoluble precipitates(salts) can form very quickly. A good example of this is mixing fertilizers containing calcium with those that contain phosphate or sulfate. A double replacement reaction will occur and the calcium will precipitate out of solution as a calcium phosphate or a calcium sulfate. Have a look above at the Fertilizer Solubility table and you will notice that Calcium Sulfate and DiCalcium Phosphate have incredibly low solubility when compared to other fertilizers.
As such, all these possible cases are not calculated in the field. Trial and error becomes the best method for determining if a mixture will be suitable. Enter the Jar Test.
The Jar Test
The Jar Test is a simple test. Take a small container, usually 1 L, and mix your fertilizers in the concentrations you are planning on using for your stock solution. You need to be careful to be accurate with this mixture as significant deviations from it can produce different results. I recommend being accurate to 3 significant digits when doing your measurements, and for a 1L solution, that can mean needing milligram precision. The Jar Test is simply a test for compatibility of the fertilizers you are using. Mix everything and wait for all compounds to dissolve. If a precipitate forms, that is, you either see a milky white tint to your mixture or there are salts on the bottom of the container, then you need to change your recipe. What I usually do once I see a precipitate is heat the solution up to 100°F or a little higher to see if the precipitates dissolve. If they do, it means I have too much of one or more fertilizers in solution. If the precipitates do not dissolve, then I have to change the recipe entirely.
