Question
What are the advantage and disadvantages of slow release fertilizers?
Thanks,
Jim  

Answer
Advantages of slow release fertilizer is that the nutrients are available gradually over an extended period of time. This means that the gardener can fertilize less often, and the nutrients are provided gradually (at lower rate but consistently). This is how most plants prefer to be fed (like us who eat a little every day compared to just one (bigger) meal per week). Most slow release fertilizers (organic and synthetic) release at specific (warm) soil temperatures. The benefit of this is that plant roots generally are most active in warm soil and therefore the slow release fertilizer will start to make fertilizer available as soon as the plants actually needs them. As the plant roots become more active (in warmer soil) more fertilizer will automatically be released.

The draw back is when certain plants prefer to take up fertilizer nutrients from cold soil (some slow release fertilzers can release nutrients in cold soil, e.g. IBDU coated fertilizers for example, but most can not)

Fast release fertilizer is often cheaper than slow release fertilizers. They can burn plant roots if provided in too big quantities, and even when they are dosed correctly, they will have to be reapplied more frequently as they dissolve quickly and disappear from the root zone sooner. They can be excellent for plants which require higher amounts of nutrients, and/or for plants which require a quick supply of nutrients (e.g. if a nutrient deficiency is noted). They are also favorable for plants which take up nutrients at specific times.


Example: cool season grass (rye, fescue, kentucky blue) benefit tremendously from a fertilizer application in late fall after topgrowth has ceased but while the roots are still growing actively (before the roots are forced into dormancy from a heavy freeze). This window is rather short (from 7-20 days) and a quick release fertilizer, such as ammonium nitrate applied in late fall can be very useful. In spring, for example, the same grass plants refer to take up nutrients only very moderately, and forcing growth with too much fertilizer can be a disadvantage. Therefore, quick release fertilizers would for these same plants not be useful in spring, but a slow release fertilizer (which release according to soil temperatures) would be ideal.

In the above example, you can see how the use of both quick release fertilizers and slow release fertilizers can be useful for the same plants depending on the specific circumstances.

In other instances, some plants do better with just one or other type. Tulips, which are typically only active in cold soil and grows actively only during a short period of time will typically require the use of fast release fertilizers. Most garden perennials which consume fertilizer nutrients slowly in cool soil, but very fast in warm soil, benefit from a slow release fertilizer. A quick release fertilizer during the wrong period of the year could set the plant back.

I hope this helped.