|MadSci Network: Agricultural Sciences|
You’re right, they are all highly soluble. The difference is really the form of the fertilizers, which essentially just slows the dissolution. Fertilizers are often coated to slow the dissolution rate. Time-release fertilizers represent the far ends of this scale. Those, meant to be spread by mechanical means, are often pelletized and thinly coated to keep clumps from forming and to allow free flowing through the spreader. Fertilizers marketed as water-soluble left in their crystalline form and sometimes powdered to enhance their dissolution rather than their solubility. They really should be labeled as "Quick Dissolving Fertilizers".
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