Should I Plant In Soil Or Compost?

Do you grow vegetables in soil or compost? This is a question asked by many gardeners, and unfortunately, the answer is not a simple yes or no.

Whether your plant in soil or compost really depends on the quality of your soil, and what kind of compost you have. Ideally, you should plant your vegetables in soil, and healthy soil will contain lots of decomposing organic matter (compost). However, soil without decaying matter will not have enough nutrients and have poor aeration and water retention, while compost on its own might be too rich and not have enough structure.

This is a difficult question to answer straight up, and the unfortunate answer is both, either, or neither. To answer this question, let us look at what soil and compost are.

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Soil: The Structure Of The Garden

At its foundation, the soil is composed of particles of rock that are the remnants of large boulders that have slowly been eroded. A rock particle is a rock that is less than 2mm in diameter. What type of soil you have is determined by the size of these particles. Clay particles are smaller than 0.02mm, silt particles are 0.02mm to 0.06mm, and sand is made of particles that are 0.06mm to 2mm. These particles mix together to form the particular soil makeup of your area.

Healthy Soil For Growing Vegetables

However, there is more to soil than just rock particles. Healthy soil is a composition of these rock particles mixed with air, water, and organic matter. This organic matter is the lifeblood of the soil, and nothing would grow without it. Ironically, it is made from everything that has died. In nature, when a plant (or animal) dies, it falls to the ground and decomposes. Bacteria and fungi that live in the soil convert this organic “waste” into nutrients that can be used by plants to grow. The more decomposing matter in the soil, the more soil organisms, earthworms, and other creatures will abound, and the more food there will be for plants.

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Image by Carmen Edenhofer
Image by Carmen Edenhofer

Healthy soil such as this is the ideal place to plant seeds and grow vegetables. However, the soil can be stripped of its nutrients and not be healthy enough to grow anything. Every plant that grows takes nutrients out of the soil. When we continually harvest the plants, we slowly deplete the soil over time. When European settlers first arrived in North America, they were amazed at the rich topsoil that was over 1 meter (3.3ft) deep. After centuries of continuously harvesting food without replenishing the nutrients, the topsoil has become so depleted that in many places it is no more than 15cm deep, and sometimes even less. We need to continually feed the soil with decomposing organic matter.

Well-rotted organic matter is called humus. Humus mixes with the soil’s rock particles to aid water retention, improve aeration and create a nice soil structure. When humus mixes with clay, for example, it will open up the clay and improve drainage and aeration. On the flip side, humus will bind the sand together and keep water from washing away the nutrients.

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Unhealthy Soil

Our farm used to be a wooded forest in the foothills in the rocky mountains. Around the 1950s, most of the fields were clear-cut and used to grow crops. For the next few decades, every hay or grain crop stripped the Gray Wooded soil of nutrients. When we started farming it, the soil at the top of the hills had turned to sand, and the soil at the bottom of the hills had become so compacted that you had to chip it with a pickaxe. There was no humus left to give structure to the soil, let alone feed the crops. This type of soil is not suited to planting vegetables.

When we plant in the garden, we are sowing seeds in the outermost skin of the Earth’s crust called the topsoil. As we saw above, this soil is a vibrant ecosystem ideal for growing. This is different from bags of topsoil that you purchase at a store, which are usually sterilized inert “dirt.” This type of topsoil does not have the essential nutrients to grow healthy plants, and it will quickly compact if used in a plant pot because it has no structure other than the “dirt”. While you can plant vegetables in bags of topsoil and probably produce a good harvest, this is not the ideal growing medium for your vegetable garden.

Image by Carmen Edenhofer
Image by Carmen Edenhofer

Compost: Much More Than Plant Food

Compost is man-made humus. Instead of plants dying in the field where they are naturally absorbed by the soil, we put the dead plants in a pile and decompose them into humus that can be used in the garden. When this organic matter has been completely decomposed, it will be dark, sweet-smelling humus that can be applied right on top of the garden. My wife’s Opa in Germany has a natural garden on the border of a forest that is a small piece of vegetable-bearing wonder. In his compost bin, he produces rich, fertile humus called komposterde, or “compost soil.”

Compost like this is perfect for growing, and you can plant your vegetables right in this medium.

Growing Vegetables In Raw Compost

When you pile all of your garden waste and kitchen scraps in the compost bin, it will start to decompose. You can plant vegetables right into your partially decomposed compost, but this will not always produce the desired results and should be done with caution. Here are a few things to consider.

First off, the partially decomposed material is usually fairly coarse and might not be suitable for many small-seeded vegetables. Also, the structure of the compost might be not strong enough to support the roots of larger plants. It is best to wait until your pile has already started the decomposition process.

Secondly, compost can get very hot when it decomposes, so hot that it can almost burn you. If your compost bin heats up, it will quickly burn any seeds or roots that you have growing in it. Make sure the compost has already started to cool before you plant. Third, your compost might actually be too rich for your plants, especially some light-feeders. Fourth, some root crops might rot if the compost is not fully decomposed and there is too much wet organic matter still decomposing.

However, if you manage your compost heap, you can often grow vegetables right in the compost bin. Squash is a popular choice for growing right in compost, as are corn and tomatoes. In another article, we talk about potatoes in the compost bin as a great way to save space.

Growing In Store-Bought Compost

Can you grow vegetables right in compost that you buy from a store? Technically yes, but these bags are meant more as soil amendment than as a growing medium. They are often concentrated strength and will be too rich for vegetables on their own. These bags are mostly marketed as a specialized mix of components to boost plant growth. This is different from the rich komposterde that builds soil and naturally grows healthy plants.

Turning the compost Image by Carmen Edenhofer
Turning the compost Image by Carmen Edenhofer

Growing Vegetables In Composted Manure?

Whenever an animal poops in the wild, it is feeding plants. The great plains that stretch across much of western Canada were fed by the migrating herds of bison, and the feces they left behind. On our farm, all of our animals’ poop goes into the compost and eventually makes its way into the garden.

Manure is a wonderful addition to the garden, and we should use as much of it as we can. However, manure should be treated carefully when used as compost. Manure can contain dangerous pathogens (depending on where you source it from) and can be very dangerous if ingested. Take great care growing vegetables directly in your compost if you use animal manure.

Unfortunately, most of the manure available to gardeners is not beneficial, and in many cases is actually detrimental. This is not the fault of the manure itself, but of the unsavoury agricultural practices used to produce it. Recent years have seen the introduction of aminopyralid, a hormone-based herbicide that is used on pastures and hayfields to kill unwanted plants. This herbicide will not only survive after passing through an animal’s body, but it will survive through composting and is the cause of numerous issues with plant growth in the garden.

So what is best: soil or compost? Ideally, we want to build healthy soil in our garden by adding as much compost as we can. Improving our soil should be the ultimate goal of our self-sufficient garden, and this is done with both soil and compost. Soil and compost can both make a good growing medium, but nothing compares to the two of them working together.

In the end, it is best to forget about these kinds of questions and start planting. Or we can start trench composting, and we can ask instead: “am I growing on compost, or am I growing in soil?”