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Tampa Bay Estuary Program


Lawn Fertilizer and Tampa Bay  »  Time it Right: Use Slow-Release Fertilizer in Spring and Fall

Q.  What is Slow-Release Fertilizer and How Do I Use It?

A:   Slow-release fertilizers release their nutrients gradually, over time, reducing the chance of "overdosing" your lawn, and our precious waterways.

Plants fed a timed-release fertilizer benefit from a consistent supply of the elements they need throughout their growth phase. Unlike quick-release or fast-acting fertilizers, slow-release fertilizers aren't water soluble. This quality, plus the fact that nutrients are released in useful amounts, means they aren't washed away in stormwater runoff.

Slow-release fertilizers are usually more expensive than fast-acting products. However, they require less application so the small increase in upfront investment can be cost-effective. Additionally, they are less likely to "burn" your yard, they help establish sustainable growth and they are an important way that homeowners can reduce water pollution.

Look for products with 50% slow-release (or water insoluble) nitrogen, and then apply this product at the recommended rate twice a year, in the spring and fall.

Q:  How can you tell if a product has 50% of the nitrogen in slow release form?

A:   Fertilizer labels always list nitrogen first among the three numbers on the bag. They also are required to list how much of the nitrogen is in water-insoluble, or slow release, form.

Q:  To calculate the percent of slow-release nitrogen, divide the total nitrogen listed on the bag by the percent water-insoluble nitrogen, and multiply by 100.

A:   Example A

If you have a bag of 10-10-10 fertilizer, with the 3 numbers representing Nitrogen, Phosphorous and Potassium in order, and 5% of the nitrogen is in water-insoluble form, here is what you get:

5 ÷ 10 = .5 x 100 = 50

This bag contains 50% slow release nitrogen.

Example B

If you have a bag of 16-2-6 fertilizer, with 4% in water-insoluble form, here is what you get:

4 ÷ 16 = .25 x 100= 25

This bag contains 25% slow release nitrogen.

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