What To Do With Your Old Compost From Pots

When you have raised beds, potted indoor and outdoor plants, as well as a lawn and garden to maintain, you may have a lot of soil turnover throughout the year.

As you move plants around from pot to pot and spot to spot, you don’t have to throw away the old compost or soil. As long as it’s free of diseases and pests, you can fortify and keep the material on-site, reducing the amount you need to get rid of and purchase each year.

Can You Reuse Compost From Pots?

The compost you use in your seed germination and potted plants can be reused, pretty much ongoing, until it depletes. As long as it doesn’t carry any pests or disease, compost can be mixed back into a composting pile or refreshed with new compost to replenish the bulk and lost nutrients from its most recent plant resident.

Compost is decomposed organic material, the nutrient-rich byproduct of digestion by worms, bacteria, and fungi. Compost can be used to germinate seeds, and it is an important soil additive to provide growing and mature plants with a source of nutrients. As the organic material continues to break down and release nutrients to the plants, it loses mass as well as nutrients.

As long as there is compost, however, it has some nutrients and can be broken down further. This means you can still use the leftovers, either by adding it to a compost heap, using it in your gardening as a soil amendment or as a mulch topdressing for vegetables and grass seeds. Compost can also be mixed with new compost and/or soil to use it as a potting mix.

Is There A Difference Between Compost And Soil?

Compost and soil provide similar support to plants, but they aren’t necessarily the same thing. They differ in several ways:

  • The materials they’re made of
  • How long they last
  • Which plants live in them

The Materials They’re Made Of

Compost is the result of breakdown and decay by worms, other insects, bacteria, and fungi, and it’s made intentionally by gardeners who collect organic materials like fruits, vegetables, grass clippings, and yard waste and direct the decomposition process.

Soil, on the other hand, is made of decomposed organic material, like compost, but a large proportion of soil is composed of minerals and stone (sand, clay, silt, and/or gravel), as well as living organisms like microbes, insects, and fungi. Soil also contains air and water.

How Long They Last

Since compost is made of organic materials like humus, solid carbon-rich materials, bacteria, and fungi, it breaks down much more quickly than soil. Depending on the materials added, compost takes anywhere from several weeks to several months to decompose completely into the brown-to-black nutrient-dense material that gives soil its life-supporting properties.

In contrast to compost, soil’s high content of inorganic material gives it a longer lifetime, although the nutrients become depleted as the organic material is broken down and absorbed by the plants. This “spent” soil needs to be amended, whether with store-bought fertilizer or homemade compost.

Which Plants Live In Them

Compost holds a lot of moisture and nutrients, and too much can be overwhelming for certain plants. Seeds can be germinated in compost, and some plants like potatoes, pumpkins, fruit trees, and annuals do well in it, too. Grass, perennial plants, trees, and shrubs, however, need to be in soil to grow.

Seeds and seedlings aren’t able to store much nutrients or moisture, as well as bigger plants with large root systems. Since those smaller plants have thin, fibrous roots, an abundance of nutrients and moisture help the plants grow and mature quickly. Soil-based plants need the air and drainage that soil provides for their roots to dry out between waterings.

How To Refresh Old Compost

Plant in Pot With Compost

Old compost can be refreshed by adding new compost to it or by adding the old to an ongoing compost heap. When it comes to compost, mass matters, and the more, the better. Compost is host to bacteria and fungi that decompose and breaks it down into usable nutrients for plants in your yard and garden: it’s a food source for them, and the more food, the more good bacteria.

As old compost is mixed with new or into a heap-in-progress, it will be aerated and moistened, improving the growing environment for the decomposers to live. The more bacteria there is, the better and faster the material will break down. A fine end-product provides the easiest nutrients for the plants when you add it to their soil.

If you have a lot of old compost that can become a new heap on its own, adding an inoculant or a dormant form of the good bacteria and fungal spores that aid in breakdown, will help revive the compost micro-ecosystem. This can be a compost starter, worm castings, manure, or samples from fresh compost to get the process going. Some gardeners use a mixture of sugar-based soda (not diet), beer, and water to provide energy and sugars to the bacteria already present in a compost heap.

What Do You Do With Old Soil After Repotting?

Similar to refreshing compost, you can add spent soil to new soil, or vice versa, to revitalize and reuse the growing medium. Soil should be measured for its nutrient content and pH level before amending it. Unlike compost, which neutralizes as it breaks down, the inorganic materials in the soil can change the pH level, which affects the availability of certain nutrients, as can over-amendment of certain materials.

Limestone, for example, lowers the acidity of the soil, but too-alkaline soil can tie up iron and make it unavailable to your plants. Soil should be balanced for its destination plant’s nutrient preferences based on the condition of the old soil. Some may still be high in one or another nutrient. Adding compost to soil can help balance those nutrients with slow-release nutrients that won’t overwhelm plants.