If you have spent painstaking months creating the best possible compost, lovingly turning and aerating your compost pile, and you’ve now got some of the famous gardener’s “black gold,” you might be wondering what the next step is.
Perhaps you’re wondering, can you plant in just compost?
You can theoretically plant in just compost, but it may not be the best idea – so let’s explore why.
If you’re thinking of planting your new plants in pure compost, you are going to hit problems, and the biggest of these is water retention. Plants are thirsty things and they need access to lots of water.
They get the water from the soil, which traps and holds pretty large quantities sometimes – giving plants something to tap into when they need a drink. The structure of the soil holds onto the water and keeps it available.
However, compost has very poor water retention rates.
It doesn’t trap the moisture in the same way that soil does, but lets most of it drain through and disappear deeper into the ground, beyond the reach of plants – especially new plants you may just have put into the ground, that don’t have an established root network.
This is particularly important to pay attention to in summer, but it is relevant all year round.
Your compost will not hold enough moisture for your plants; you need to mix it with topsoil (just your garden soil should be fine) to create a good texture that will ensure your plants always have enough to drink.
A plant planted in pure compost might survive if you water it very often, always topping up the moisture content, but it will likely struggle for other reasons too – so avoid planting in pure compost, even if you can overcome the moisture issue.
Wrong Balance Of Nutrients
Compost is full of nutrients, but they are not necessarily well-balanced for a plant to grow from.
Remember that plants have not evolved to be planted into compost, but into soil. Boosting soil and making it rich is great, but compost is just that: a boost, an amendment, not a replacement.
Some of the things in your compost are only wanted by plants in very small quantities. You might even be lacking in other nutrients that are only found in topsoil and the deeper parts of the earth your plant will reach down to.
Planting in pure compost can lead to excessive salinity and ammonia toxicity. It will often leave plants surprisingly weak, or even dead, because they are in an unnatural balance of earth, and cannot grow properly.
So, how do you use compost?
Adding compost to the soil is a crucial way of keeping it rich and revitalized, especially if you are growing nutrient-hungry plants, such as in a vegetable garden.
This will deplete the soil of certain nutrients over the years and you need to add them back – especially if you grow things in the same place year on year.
Mixing your compost with your soil, or spreading a thin layer on top that can be gradually mixed in by nature, is a better solution.
If you have compost at the surface, worms will pull it down and rain will wash it down, making it available to your plants in much better quantities, and ensuring the soil has a balance of nutrients to tap into.
Compost also lacks stability. Remember when we said it doesn’t have the right structure to retain water? That’s because it’s so soft – the water just seeps through and straight down.
Equally, that softness means it will wash around and sink into the topsoil over time.
Compost does not hang around once it is exposed to rain and sun and insects. It will keep breaking down and apparently disappearing (going into the soil), and that isn’t great news for your plants, especially any large ones.
Plants depend on the soil as an anchor, as well as a source of water and nutrients. If they are not fixed into the soil, they’re at risk of being blown over and damaged by the wind and rain. They can easily be washed away.
Large plants are particularly at risk of this, but it’s true even of seedlings and other plants. They can’t take strong root in compost; it is too fine and will simply wash away. It’s a little like planting in nutrient-rich sand – not a good idea!
Compost is also quick to compact for the same reasons. It forms a dense layer that has little to no oxygen in it – and plants need oxygen around their roots.
You want good, aerated soil beneath a plant, and generally, your garden’s top layer is perfect. It is what plants have evolved to root into, after all.
Give your topsoil a boost by providing a thin compost topping, but don’t plant your new plants directly into compost, or they may soon start to slump and slip because there’s nothing solid for them to hold onto.
This is more important for seedlings than for other plants, and only applies if you’re using homemade compost, but it’s worth mentioning.
Seedlings will enjoy the nutrient-rich compost (though as mentioned, it may not be well-balanced), but they may also encounter aliens in your compost that hurt them.
Homemade compost is not sterile, and this can make it a challenge for seeds to grow healthy and strong. If you buy your compost from a garden center, it should be sterile, but it may still not be the best thing to plant in.
Mix it with some topsoil, and your seedlings will grow better.
You should also be aware of any weed seeds in your compost if you are using home compost. These can affect both adult plants and your seedlings.
If you throw weeds into your compost bin and it doesn’t reach temperatures that are sufficient to kill the seeds, you may be spreading them around your garden when you use the compost – with a good amount of nutrients and soft soil to help them grow, too.
Be careful when adding weeds to the compost bin, and make sure it is getting hot enough to kill them. Remember that the seeds can be tiny and you may not see them, and many seeds can stay dormant for years before they start to grow.
If you’re concerned, it is better to burn weeds (particularly highly invasive ones) and add the ash to your composter where possible, particularly if your compost bin is not operating at high temperatures.
How Much Compost Is Good In The Garden?
So, with all of that in mind, how much compost should you be using? It does depend to some degree on the plant. You need to find out if your plant likes rich soil, or prefers poor soil.
There are many that can’t cope with large quantities of nutrients being available.
Many food crops will like a rich dressing of compost, and a lot of standard garden plants will benefit from it too.
You can use up to three inches of soil to top your garden off, particularly where you want high levels of nutrients, such as in a vegetable garden. Do not use more than this, or you will compromise your plants’ growth.
You can do this once or twice a year. For ornamental areas of the garden, add less, between half an inch and two inches, to give them a nutrient boost.
You shouldn’t need to mix it in; the worms, rain, sun, and wind will do the job of pulling it down into the soil over a period of time.
If you want it to be available to your plants quickly or you have very heavy or very sandy soil, you can speed the process up a little bit by mixing it in, but remember that you are disturbing the natural life in the soil, and this can have a negative impact.
The mycorrhizal fungi help plants to tap into the nutrients deep below the soil, and these don’t like being disrupted. Regularly stirring and shifting your soil will stop them from doing their job, and your plant may suffer.
However, in some circumstances, it is worth it to get compost into your soil.
How Much Compost Is Good In A Pot
If you are mixing compost for a pot, you might also be wondering about quantities. This is harder, because it will depend heavily on the plant and how much drainage it needs; drainage issues are likely to be more significant for a potted plant.
A very basic mix will include: one scoop of compost, one scoop of topsoil, one scoop of perlite, and a scoop of coconut coir or peat or something similar. Look into your plant’s specific requirements and increase or decrease ratios to meet its needs.
It is often tempting to plant straight into compost because it looks so rich and soft and lovely. How could your plants possibly not like it? If you have made your own, this is even more likely to be the case.
However, you need to approach your gardening sensibly and not overload your plants with an unnatural balance of nutrients. Use your compost to dress your existing soil, and don’t plant directly into it.