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Make most of compost, other fertilizers in garden

By Darbie Granberry
University of Georgia

If you want to make the most of your garden, you'll need to make the best use of fertilizers, including compost and other organic products.

Fertilizers fall mainly into two classes: organic or inorganic.

Inorganic fertilizers, sometimes called synthetic or mineral fertilizers, are store-bought mixtures of inorganic nutrients such as nitrates, phosphates and potassium.

They have much higher concentrations of plant nutrients than organic fertilizers. The nutrients in them are released into the soil soon after you apply them, too.

Organic fertilizers come from plants or animals. The plant nutrients in them have been taken up by living plants or animals and chemically bound in their tissues and by-products.

Slow release

Because they're integrated into complex organic molecules, these nutrients are in relatively low concentrations. They're not water soluble. As a result, plants can't take them up as soon as you apply them.

Organisms in the soil have to break down the organic matter before the bound nutrients are released for plants' roots to take them up.

Besides the slow-release nutrients, organic fertilizers typically have generous amounts of helpful organic matter.

Compost is an excellent organic fertilizer for your garden.

Simply put, compost is decayed organic matter. Through the composting process, plant and animal materials are broken down into smaller particles. The final product is 35 percent to 45 percent organic matter. It resembles potting media.

What's in it?

The nutrient content varies, depending on what material was used and exactly how it was composted.

Generally, though, it's 1 percent to 2 percent nitrogen, 0.2 percent to 1 percent phosphorus, 0.5 percent to 1.5 percent potassium and 0.05 percent to 2 percent calcium. Besides these nutrients, compost also contains small amounts of micronutrients such as boron, copper, manganese and zinc.

If they aren't composted, organic fertilizers from animal manure can harbor weed seed and pathogenic organisms.

Because of the heat generated during composting, though, compost is free of viable weed seeds. This helps keep troublesome weeds from getting a start in your garden.

Safety factor

Perhaps even more important, the high heat phase of composting destroys plant and human pathogens. Getting rid of these critters helps safeguard the health and well-being of you and your garden veggies.

Nutrients in inorganic fertilizers are released into the soil quickly. But they also start leaching from the soil as soon as you apply them.

Heavy rains or watering early in the season can move nutrients deep down, beyond the reach of plant roots. Unless you apply more fertilizer, plants become stunted. This lowers the quality and yields of your garden crops.

Compost releases its nutrients slowly, over many months. This keeps them from leaching and makes sure they're available to plants all season long.

Other benefits

Compost has a lot of organic matter, which improves garden soils and helps your garden vegetables grow better. Besides providing its nutrients over a long time, organic matter:

  • Improves soil structure and reduces soil compaction.
  • Increases soil aeration.
  • Helps soil to hold moisture and nutrients.
  • Supports beneficial soil organisms.
Don't rule out the use of inorganic fertilizer.

You usually have to apply organic matter many times over several years to build up the soil's organic content and nutrient reserves. Until then, most gardens will do better if you apply both organic and inorganic fertilizer.

Moderate amounts of inorganic fertilizer can be especially helpful in getting garden vegetables off to a good start. As always, know the nutrient content of the product you use. And base your application rates on your plants' needs.

(Darbie Granberry is a horticulturist with the University of Georgia College of Agricultural and Environmental Sciences.)

(Darbie Granberry is a Cooperative Extension horticulturist with the University of Georgia College of Agricultural and Environmental Sciences)

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