While wandering around my garden just recently, a friend noticed my compost bin (actually, I have two) and asked me “Can I plant directly into compost”? As I pondered the answer to this question, I thought that my readers might also be asking this and decided I would write this article.
I’ve written a lot of articles about composting, including how to make the perfect compost, because it’s a subject I’m quite passionate about. Composting is also something that I’ve done for a good number of years and my plants and vegetable gardens have greatly benefitted from this.
I also like the fact that I’m helping the environment by repurposing green waste into healthy food for my plants and the soil. But, can you plant directly into just compost? My answer to this would be no for a variety of reasons which I’ll explain a little further.
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Let me clarify just a little further. While you can plant into compost that you’ve topdressed your garden beds with, pure compost will not support plant growth on its own. It just doesn’t have the right structure and lacks the minerals that are found in soil.
But, before I go into this, here is how I commonly use compost in my garden.
How I Use Compost In My Garden
The main thing I do with my homemade compost is use it to top up my vegetable-growing beds. I have a few shallow raised beds that need to be topped up every year because the soil seems to settle.
So, before I plant my summer crops in spring, I’ll usually just gather as much compost as I can from my compost bins and then, just spread this over my garden beds.
Generally, I don’t really dig it in but just use a small rake to mix it lightly with the existing soil. As a sustainable gardener, I generally don’t like to disturb the topsoil too much because there is a lot of microbial activity going on that we can’t see.
Plus, the earthworms that are in my garden will do a good job of pulling the compost deeper into the soil for me.
Sometimes, I also use compost to mix with potting mix in order to enhance the mix and make it go further. This is a good practice for when I have quite a few pots that I want to fill.
At other times, I’ve also mixed some compost with coconut coir (instead of peat moss) to make my own seed-raising mix as the coco coir is fairly good at retaining moisture once it’s wet.
So, although I use compost fairly regularly in my garden as I find it’s the best way to improve my soil, I never just plant solely into it. Compost just doesn’t have the right structure on its own to support plant life continuously.
Compost Does Not Have Good Water Retention
When you look closely, you’ll notice that compost has a fairly loose and fine structure. This means that it doesn’t have the same capacity to hold moisture like regular soils do. You’ll find that when you add water, it will just drain through fairly quickly.
While this is good when you want to ensure that you have good drainage, your soil does need to hold a certain amount of water in order to keep your new plants hydrated.
Therefore, when you top up your garden beds with straight compost and rake it into the existing soil a little, it will encourage the roots of your plants to go deeper to where the moisture is being held.
This works well on all types of soils including sandy soil and even all clay soils.
This is also one of the reasons that I like to cover my growing beds with a layer of straw mulch. It helps to conserve the moisture a little and prevents it from evaporating too quickly on hot, sunny days.
Compost Does Not Provide The Widest Range Of Nutrients
While your compost will definitely add some vital nutrients to your soil, after all, that’s one of the major benefits, it may not contain a balanced range. It really all depends on what you’ve added to the compost pile.
For example, my compost gets a fair amount of coffee grounds added to it on a daily basis. These are great because they’re quite high in nitrogen which is essential for healthy plant growth.
However, if I’m growing fruiting vegetables such as tomatoes and cucumbers, I really don’t want an oversupply of nitrogen. This will only promote lots of green growth and will not encourage flowering and fruiting.
For fruiting plants, I also need potassium plus some phosphorus to promote healthy root growth. Plus, there are plenty of micronutrients in ground soil such as iron, manganese and calcium. These may not be present in compost.
This is precisely why compost should only be used as a soil additive rather than a complete growing medium.
Compost Does Not Provide A Stable Structure For Strong Root Growth
The structure of compost is vastly different from garden soil or even potting mix. It is very soft and can easily wash away when it rains or when it’s watered.
That’s why it’s perfectly fine to spread compost on top of your garden. It will eventually be washed down into the soil. However, this fine and loose structure will not support strong root growth.
Your plants need the coarser structure offered by the soil in your garden or the potting mixes that you buy. This allows the plant roots to grow freely. It also supports them so that they can, in turn, support the plant.
Compost Can Harbor Bacteria And Weed Seeds
While there is a certain level of bacteria in most soils, these are usually not concentrated enough to harm young plants. However, when you make your own compost, it generally won’t get hot enough to kill all bacteria.
While this is generally not a problem for established plants, it can be harmful to young seedlings.
This is also one of the reasons that I always suggest that you use gloves when you’re gardening. In fact, even commercially produced compost and potting mixes can sometimes harbor things that you don’t want to get on your skin.
Weed seeds can also be a problem if you toss your weeds into the compost. Once again, backyard compost bins generally don’t get hot enough to kill weed seeds. That’s why it’s often not a good idea to throw weeds in the compost.
It means that your young plants may need to compete with any weeds that crop up for both water and nutrients.
How Does Soil Differ From Compost?
By now you understand that soil and compost are different. That’s why you should not plant solely into compost because it doesn’t support healthy plant growth. But, how do these two differ?
What Is Compost
Essentially, compost is decomposed organic material that has been created from the breaking down of plant materials such as kitchen scraps, food scraps, dead leaves and grass clippings. It is sometimes referred to as black gold and is designed to be used for soil amendment and nutrient enhancement.
Think for a moment of a forest floor. If you take a closer look, you’ll notice a layer of organic matter like leaves and twigs. Under this, will be a layer of rich compost that has resulted from the breakdown of the top layer of organic matter.
Further below the surface, you’ll see the actual soil that will be rich in organic matter and can hold moisture. This is what supports the trees and other plants that grow in the forest.
What Is Soil?
Soil is a cocktail of organic matter, minerals derived from rocks, water, air and tiny living organisms. All of these work together to support the growth of healthy plants.
All the soils that cover the surface of our planet formed millions of years ago from the weathering of rocks and a blending of decomposing organic matter. And, they are continuously being formed in order to sustain healthy plant life.
Comparing Compost To Soil
While compost has lots of lovely nutrients that your plants will love, it does not have the same structure as soil. And, it also lacks the minerals and living organisms that are found in garden soils.
All of these are essential for healthy plant growth. This is why I only use compost as a soil additive to ensure that my plants get the absolute best and never use only compost to support healthy plant growth.
Frequently Asked Questions
While this is possible, it’s not a good idea and something I wouldn’t do. Most seeds need to be kept moist and this is difficult with just compost.
Ideally, the best way to use compost for growing seeds is to mix it with some other materials such as coco coir, perlite and even sand. This will improve the structure and the water-holding capacity of the compost.
Compost adds numerous benefits to the soil. It adds valuable nutrients such as nitrogen, it can allow sandy soils to hold moisture better and it can reduce the pH in highly alkaline soils. It’s the best thing that you can do for your garden and the health of your plants.
It’s no secret that I’m passionate about compost. It is one of the most valuable resources that you can use in your garden. Plus, it’s so easy to make. And, once you’ve got your compost bin or composting set up, it’s free fertilizer for your plants.
While I love using compost in my garden, I would never use it solely for growing plants. Compost is very different to soil. It doesn’t contain things like minerals and other living organisms and doesn’t have quite the same structure.
Therefore, I do recommend that you use plenty of compost in your garden. But, only apply it as a top dressing or mix it with other ingredients if you want to make potting mix.