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FERTILIZER SOLUBILITY

When applying fertilizers through the irrigation water (fertigation), it is essential to be familiar with some important facts regarding fertilizers solubility.

 

Solubility of a fertilizer - The solubility of a fertilizer is defined as the maximal amount of the fertilizer that can be completely dissolved in a given amount of distilled water at a given temperature.

 

Manufacturers may provide you with solubility data of their fertilizers upon request. Here is an example of solubility data of various fertilizers (expressed in  g/liter):

 
  Solubility g/l
Potassium nitrate 133 170 209 316 370 458
Ammonium nitrate 1183 1510 1920 . . .
Ammonium sulfate 710 730 750 . . .
Calcium nitrate  1020 1130 1290 . . .
Magnesium Nitrate 680 690 710 720 . .
MAP (Mono Ammonium Phosphate) 250 295 374 410 464 567
MKP (Mono Potassium Phosphate) 110 180 230 250 300 340
Potassium chloride 229 238 255 264 275 .
Potassium sulfate 80 90 111 120 . .
Urea 780 850 1060 1200 . .

 

Different manufacturers may provide slightly different solubility data for the same fertilizer. This is because they use different additives in their products. There are also some fertilizers that may contain insoluble residues.

 


DISSOLVING A FERTILIZER

When you dissolve a fertilizer, you should not exceed its solubility. Otherwise, a precipitate may form and might clog the irrigation system. Moreover, the nutrients you intend to provide through the solution may not be fully available.

 

For example, according to the data in the table above, the solubility of Potassium Nitrate in 20oC is 209 g/l and the fertilizer contains 38% Potassium. If you attempt to dissolve 300 g/l in the fertilizer stock tank, you will not get 114 g/l of Potassium (38% of 300g), but only 80g. The remaining 34g will precipitate and will not be available.
 

 

MIXING FERTILIZERS

When mixing fertilizers that contain a common element (for example potassium nitrate together with potassium sulphate) the solubility of the fertilizers is decreased. In such case, we cannot refer to the fertilizer solubility data alone. The same happens when the water used for dissolution is highly rich with minerals, e.g. calciummagnesium or sulphate.

 

In such cases, additional chemical reactions come into play, and calculations become more complex. Usually, these are not calculated in the field and instead, trial-and-error practices are common.

 

 

MIXING FERTILIZERS - FERTILIZER COMPATIBILITY

Some fertilizers should not be mixed together in one stock tank because an insoluble salt might form very quickly. An example for such incompatibility is mixing fertilizers that contain calcium with those that contain phosphate or sulphate.

 

Use this fertilizers compatibility chart:

 

Fertilizer-compatibility.jpg 

                      Click on image to enlarge it

 

THE JAR TEST

We've established that when mixing fertilizers, one must be familiar with the solubility data of the fertilizers used, as well as with the chemical reactions that may take place. In order to avoid unwanted precipitates, a common recommendation is to perform a "jar test". In this test, the fertilizers are initially mixed in a jar containing the same water used for irrigation.

 

jar-test.jpg

  Complete dissolution              Formation of a precipitate

The fertilizers should be mixed exactly in the same concentration as intended to be used in the stock tanks. If a precipitate forms or if the solution has a "milky" appearance, the test should be repeated with lower concentrations of the fertilizers. 

 


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