Definition - What does Insoluble mean?
The term insoluble refers to a substance that is unable to be dissolved in a solvent. A substance can either be soluble or insoluble, and its solubility capabilities is often based on the solvent in question (water vs. oils, for example).
An example of an insoluble substance is chalk. Chalk cannot be dissolved in water; it just forms small particles or chunks in the water and never truly liquefies. Sand is another substance that is considered insoluble.
Some substances such as certain vitamins are insoluble in water, but are soluble in fats. Vitamins A, D, and E are examples of insoluble vitamins that do not dissolve in water but become soluble in fat.
MaximumYield explains Insoluble
Insoluble substances often exist in garden fertilizers. Gardeners have to take into consideration the solubility of certain fertilizers before applying them. Many fertilizer companies state the solubility of the various fertilizers on the label of the fertilizer.
Oftentimes, mixing fertilizers, such as potassium nitrate together with potassium sulfate, decreases their solubility and renders them insoluble. Calcium, magnesium, and sulfate are all minerals used to fertilize plants, but it takes exact ratios of each to render them soluble. If the ratio is not exact, the minerals are insoluble. Mixing fertilizers together can also make insoluble salt formations, which can be harmful to plants.
Fertilizers that contain calcium will create insoluble salt when mixed with phosphate or sulfate.
Nutrients that are considered insoluble when mixed are kept separate until it is time to feed them to plants. This is why hydroponic formulas in particular are often sold in two parts: A and B formulas, and Grow and Bloom formulas.