How to Use Compost in Your Garden

Mature compost

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Compost is often referred to as black gold, the stuff makes gardens thrive, and if you make your own compost, it’s a type of gold that does not cost anything. But before you start spreading it in your yard, you need to make sure that you follow a few basic rules because using compost too early, too much of it, or at the wrong time can also be harmful to plants. 

How to Know When Compost Is Ready

It’s easy to tell when compost is ready to be used. Mature compost has a rich dark brown, almost black color. The original materials are not recognizable any longer; everything has turned into a crumbly, soil-like substance with an earthy smell. Still recognizable original materials and the compost releasing heat indicate that the compost is not ready yet and that you need to give it a few more weeks. The presence of hard materials that take a long time to break down, such as fruit pits and twigs, is no hindrance to use the compost, simply screen them out. 

Shoveling compost

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When to Use Finished Compost

The best time to use compost depends on the maturity of the compost. Fully matured compost can be used any time in the spring, summer, or fall. Compost that is not fully matured or just about to reach that final stage, on the other hand, should be used in the fall. The reason for this is that the microorganisms in immature compost require nitrogen to do their decomposition job, and that nitrogen is taken away from the plants. If you spread the compost in the fall when plants go dormant, microorganism activity can take place without adverse effects. 

4 Ways to Use Compost in Your Garden

For Soil Improvement

All types of soil benefits from amending the soil with organic matter, and compost is an excellent way to do that. Organic matter is key for soil productivity, it improves the soil texture, microorganism activity, and nutrient uptake. Even rich, good-quality loamy soil gets compacted over time and adding organic matter loosens up the soil. Sandy soil with its large soil particles does not hold water and nutrients very well and amending it with organic matter helps the soil retain moisture better so that it does not drain faster than the plants can absorb water and nutrients. Clay soil with is dense soil particles sticks together, which leads to poor drainage and insufficient oxygen. Adding organic matter to heavy clay soil loosens it up and improves aeration.

Adding organic matter in the form of compost to soil also improves the soil’s ability to hold key nutrients such as calcium, magnesium, potassium. The acids in organic matter help plant roots with the uptake of water and nutrients and makes soil minerals available to plant roots by dissolving them. By adding compost to soil you furthermore keep the soil pH stable because a pH that is off kilter also affects the plants’ nutrient availability and uptake.

Only 3 to 6 percent of soil consists of organic matter so as important as compost is, it is used in moderate amounts but applied repeatedly over time, about once a year. Spread a layer 2 to 3 inches thick over the soil and incorporate it with a tiller, shovel, or a garden fork.

For Potted Plants

Just like in garden soil, mixing mature compost into your usual potting mix for container improves its water-holding capacity. Keep in mind though, that in the confined space of a container, too much compost can lead to root burn from the excess phosphorus in the soil. You can either spread a layer of 2 to 3 inches thick on top of the potting mix, or combine up to 30 percent compost and 70 percent potting mix to fill the container.

Using potting mix and and compost for potted plants

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As a Fertilizer

Compost adds some nutrients to the soil but with few exceptions, it is not sufficient as the sole fertilizer. Especially plants with very specific nutrient needs, such as lawns (nitrogen) and vegetables and other crops (nitrogen, phosphorus, potassium, plus a range of micronutrients) need to be fed with a commercial fertilizer according to an established fertilization schedule.

The nutrient content of compost, unlike commercial fertilizer, organic or other, is unpredictable. Spreading a thin layer of mature compost over your lawn as a top-dressing or in your vegetable garden is always a good idea, as long as it is not the only source of nutrients. 

For Mulching

Compost is sometimes recommended as a mulch around plants but that only works to a limited extent. While compost usually does a good job retaining moisture in the soil, it does not have the same ability as other mulches to suppress weed growth. Compost is a good choice if you are mainly after enriching the soil and keeping moisture in the soil, and if you don’t mind applying compost repeatedly. If you also want the mulch to cut down on weeds, bark mulches and other woody materials work better, and they won’t have to be applied quite as often as they take a long time to decompose. Ideally, you’d use both, a bottom layer of mulch, followed by a top layer of bark mulch or wood chips. 

Storing Compost

Unlike a compost pile with lots of produce scraps that can attract rodents, fully matured compost is less attractive to unwanted critters. However, your compost should still be covered. An impermeable cover protects it from the elements, so it won’t leach all its goodness (think compost tea). It also prevents weeds from growing on the compost, which should be pulled promptly because if you let them go into seed, your valuable compost will be contaminated with weed seeds. Only leave your compost uncovered if you use it for growing crops, which thrive on a compost pile.

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  1. Soil Organic Matter. Cornell University Cooperative Extension.

  2. Organic Soil Amendments. University of Florida.