Northeast Wisconsin
  • Northeast Wisconsin
  • April 2015
Written by 

Compost: The soil amender! — Say what?

Compost isn’t considered soil; it is a soil amendment. How exactly does compost “amend” soil? Why does soil need to be amended? How does that dynamic work?

The first step is to understand what soil and compost are.

An inch of topsoil can take up to 500 years to create, so you can see why you want to understand your soil. It’s not simply “dirt.” Soil has a unique blend of 45 percent minerals, 25 percent water, 25 percent air and 5 percent organic matter. The minerals in soil consist of nitrogen, phosphorus and potassium. The organic matter comes from compost. Compost is easier to understand. It is a blend of carbon and nitrogen material that breaks down and turns into organic matter.

Nitrogen allows plants to use carbohydrates to create energy for growth and strength. Phosphorous allows plants to breathe. It also allows energy to be transferred and stored, as well as helps plants use water efficiently. Potassium helps a plant metabolize its food to create energy. Potassium also helps control hydration and biochemical reactions inside the plant.

You want your soil to have a healthy balance as mentioned above. Plants cannot grow well without the proper soil, similar to how humans need a healthy diet. If your soil lacks essential minerals you can compensate by mixing it with compost to supply nitrogen, bone meal for phosphorus, and wood ash for potassium.

How can compost amend your soil?

If you live with heavy clay soil you know it can take forever for water to drain and you’ve seen how the soil clumps up into clay balls. It is best to work a large amount of compost into clay soil when the moisture level is normal — not too wet and not too dry. After working compost into clay soil you will see improved drainage because the compost combines with the tiny clay particulate to form larger chunks. The larger compost-clay particles also improve air penetration into the soil. Increased air in the soil limits compaction, which allows roots to penetrate the soil better. Lastly, compost binds to the clay, making the soil more workable and less sticky.

You can apply compost to sandy soil at any time without the additional challenges of working with clay. Those with mostly sandy soil may be frustrated by the speed with which water or fertilizers seep right through it. Compost mixed with sandy soil increases water retention anywhere from 10 to 1,000 times. The compost also keeps all the important nutrients in the root zone rather than letting them just wash away.

Once you have figured out what type of soil you have, you need to think about what types of vegetables, flowers, trees or shrubs you want to grow. Different plants have different soil requirements. To simplify things, you can’t go wrong mixing compost in with the top 2 inches of your soil to make sure you have a healthy balance of nutrients. Remember to reapply compost as a topdressing in the garden all season long. Compost not only amends soil and gives plants a boost right away but also slowly releases nutrients so plants can thrive for months.

References: Biospheric Sciences Research. NASA.


“Soil Structure Benefits with Compost.” The Compost Gardner.

“Vegetable Garden Soil: Soil Requirements for Vegetable Plants.” Heather Rhoades. Gardening Know How.

Sarah Everson

Sarah Everson is the business manager for Compost Joe’s Premium Soils and Organics, a private composting facility located between Fond du Lac and Oshkosh. Sarah also offers seminars and private classes on composting. For more information, visit or call her at 920-921-6223.

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