I think you probably misheard, as “fast acting” and “slow release” are opposites, and a fertilizer will generally be one or the other. I guess it’s possible that a product will contain a mixture of fast acting and slow release fertilizers, and thus provide the benefits of both?
In any case, whether it’s fertilizer or drugs, the answer is kind of the same: how fast does it dissolve?
Both fertilizer and drugs only get taken up and used when they’re dissolved into a fluid, either the groundwater or your stomach acid. And both of them use a lot of different strategies to change how fast they get dissolved.
One strategy both use is a coating, either on the pill or the grains/tablets of fertilizer. This coating may be mixed with the active ingredient in layers or in a sort of microscopic honeycomb pattern, so that the coating has to dissolve before a bit of the active ingredient gets dissolved. The coating could also be porous, allowing some of the active ingredient to seep through it at a slow, controlled rate.
There are many more strategies when it comes to drugs, and I won’t go into them all here; most essentially boil down to “make the active ingredient dissolve into stomach acid more slowly.” And that’s just oral preparations – there are things like patches which deliver a slow and steady dosage, but that’s another can of worms.
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