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):
|Fertilizer / Temperature (C˚)||5||10||20||25||30||40|
|MAP (Mono Ammonium Phosphate)||250||295||374||410||464||567|
|MKP (Mono Potassium Phosphate)||110||180||230||250||300||340|
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.
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.
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. calcium, magnesium or sulphate.
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:
Click on image to enlarge it
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.
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.