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Compost in garden soil multiplies rewards

By Darbie Granberry
University of Georgia

Compost is a hot topic. If you've been reading the latest gardening articles or watching your favorite TV gardening guru, you've probably seen its use enthusiastically touted.

Most gardeners agree that compost is good for the garden. But why is it good? And how should it be applied?

Compost is what's left of organic matter after microbes have thoroughly decomposed it. Simply put, it's decayed organic matter.

Through the composting process, plant and animal materials are broken down into smaller particles. The final product has an organic-matter content around 35 percent to 45 percent and resembles potting media.

Organic fertilizer

Because it's high in organic matter and doesn't contain "synthetic" chemical fertilizers, compost is a good source of organic fertilizer.

Generally speaking, organic fertilizers come from plants or animals that took up these nutrients, or fertilizer elements, and chemically bound them in their tissues and by-products.

Because they're integrated into complex organic molecules, the plant nutrients in organic fertilizers are in relatively low concentrations. They're not water soluble. And they're not readily available. They have to be broken down by organisms in the soil before the bound nutrients are released for plant roots to take up.

This keeps the nutrients from being washed out of the soil by heavy rains. It results in its slow release over many weeks or even months.

Nutrient content

The nutrient content of compost varies with the materials composted and the specific composting process. Generally, though, it falls within these ranges: nitrogen, 1 percent to 2 percent; phosphorus, 0.2 percent to 1 percent; potassium, 0.5 percent to 1.5 percent; and calcium, 0.05 percent to 2 percent.

Besides these major nutrients, compost also contains small amounts of micronutrients such as boron, copper, manganese and zinc.

Remember, organic fertilizer is slowly released. So, it usually works best as a supplement to conventional fertilizer, not as a replacement.

Compost, though, does more for the garden than just provide organic fertilizer. It also helps:

  • Increase the soil's capacity to hold water and nutrients.

  • Reduce soil compaction, allowing more air and water to move among soil particles.

  • Improve the soil's tilth, or structure, making it easier for roots to grow and thrive there.

How much, when to apply

Eight to 10 weeks before you plant, broadcast compost over the garden. Any amount is helpful. But for best results, initially apply 20 to 30 pounds of compost per 100 square feet of garden soil. Scatter it uniformly over all of the garden.

And immediately after you spread it out, for best results, till the compost into the top 8 to 10 inches of soil. To keep a good thing going, follow up the initial application each year with 10 to 15 pounds of added compost per 100 square feet.

Compost will help give you your best garden ever. It will help you have a richer, more rewarding gardening experience.

(Darbie Granberry is an extension 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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