For first-time gardeners, one of the most confusing things to deal with is getting the right soil mix. There are so many options out there, all of them claiming to be the best organic raised bed soil. Some bagged mixes are quite good, and they’re usually a great place to start for new gardeners.
But soil science is a bit more than opening a bag and having miraculous, healthy plants from it. Your garden soil is a living and breathing blend of various component parts. In a natural setting, the soil is composed of many layers, some inorganic, such as pulverized rock, and some organic, such as decaying leaves. Microbes, fungi, and more live within it as well. Together, they form a basis for what we grow plants in.
When you take the soil from its natural setting and put it in a raised bed, there are different goals that should be met. Raised bed soil warms up more rapidly in the spring, often needs better drainage than other forms of soils, and can reduce in height as material decays and turns to plant nutrients.
So let’s go over raised bed mixes, what they are, how they work, and what you should do to keep them alive and healthy. Healthy soil makes for healthy plants, so it’s crucial to start right!
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What Is Soil, Anyway?
The term “soil” is misused often, but in essence, it should refer to what we often call dirt. The average garden soil in your yard is usually at least 45% mineral content, roughly 25% each of air and water, and about 5% organic material like decayed leaves. These numbers vary a bit depending on region and soil quality, but that’s an average for what is considered good soil.
While the mineral content varies in type, soil types are organized most often by their particulate size. The largest particle size is what we refer to as sand. The finest is clay. Between the two is silt.
Each type of soil has different properties. Clay tends to stick together and can become rock-hard when baked in the sun. Sand is loose, but its large particle size means it doesn’t hold as much water. Silt is slippery and can be prone to erosion.
A blend of these three types is referred to as a loam, but if it has a dominant amount of any type of soil, that is added to its name. For instance, a sandy loam would be a relatively even blend of mineral sizes with a slight lean towards the sandy side. Loamy sand would be sand-dominant but with some loam blend as well. These terms help us to determine the average particle size of the soil but also some of its most common attributes.
What’s The Difference Between Soil And Mixes?
The stuff sold as “potting soil” is almost always a mix. It may or may not actually contain any soil at all, in fact! A better term for it would be “potting mix”, and some sellers use that term instead of including the word soil.
What we want for most raised beds is usually a soil blend. A soil blend includes the mineral-rich dirt along with other organic ingredients, thus boosting the rich organic matter that plants need to thrive.
Do You Always Need Mixes?
Need is a fairly strong word to use here, but technically speaking, you don’t need to run out and buy a mix of any sort. You can make one at home using your own compost, aged leaves from the yard, or any number of other inputs. But to start a new raised bed, it’s a good idea to use a mix as at least part of your initial organic soil blend.
Depending on the size of your raised bed, you may be able to purchase bagged mixes to fill it. Alternately, many landscape supply companies sell their own mixes in bulk. Both options work well to get you started.
Is Soil Always Necessary?
For container growing, soil is not always required. Many plants are perfectly happy in a potting mix blend that is optimized to their specific needs. But often, sand is used as an additive to bags of mix as it provides better drainage due to its large particle size, so you may actually have a small amount of soil even when you don’t think you do.
As a general rule, raised beds use at least some soil to ensure good drainage capability, but also to fill it. As garden beds can take a surprising amount of fill material, using some soil along with other components can reduce the cost to fill your raised bed significantly.
What Goes Into Raised Bed Soil?
The various bagged mixes out there have proprietary blends of different ingredients. To provide an example that we really like, Espoma’s organic mix for raised bed gardens contains the following ingredients on its label: 55%-65% aged forest products, sphagnum peat moss, perlite, limestone, alfalfa meal, kelp meal, feather meal, worm castings, and yucca extract, as well as some mycorrhizal life. Technically speaking, it is a soilless garden medium for large containers or raised beds. It works well if you want to opt for a straight from bags pre-mix.
But what are all of those components, and what do they do? Let’s talk about that in more detail.
Aeration And Moisture-Related Additives
It stands to reason that moisture is necessary for your plants, but not too much moisture. Drainage needs vary between plant types, but the majority of common garden plants like any excess water in the soil to drain away freely. If you’re starting with garden soil that normally compacts down year after year, adding aeration to it is also necessary. Just like plants need water at the root level, they also need air!
Perlite is a common additive used for both aspects. This expanded volcanic rock is extremely lightweight and easy to work with, and it reduces soil compaction a bit as well. Because it is much larger in particle size, it allows for airflow through the soil, but also for drainage.
Many types of additives are used for both, but there are some which are specific to either moisture or drainage. Here’s a short summary of the most common additions for improving aeration or drainage, and for holding/absorbing moisture.
- Perlite: improves drainage and is lightweight, catches small amounts of water.
- Rice hulls: for drainage that will eventually decompose. Extremely lightweight.
- Lava cinders: These are fine pieces of volcanic rock that improve aeration and drainage while catching small amounts of water. Unlike perlite, this will not eventually get crushed by soil weight.
- Aged forest products: these not only improve drainage, but they absorb water for later. Aged ones are partially-decomposed already, which means they won’t rob nutrition from the raised garden beds.
- Coconut coir: This lightweight material absorbs water for later plant use, but excess drains easily away. It also loosens the soil and will eventually decompose.
- Sphagnum peat moss: While it can be hard to initially get it wet as dry peat can be hydrophobic, once moistened it will hold moisture for long periods of time. Keep it moist and it’s a fantastic addition.
- Worm castings: Able to absorb nearly 10 times their weight in water, worm castings are great for moisture retention.
- Vermiculite: This mineral absorbs water like a sponge. Over time it releases moisture to the soil.
- Coarse Straw/Hay: More often used as a mulch. The large size makes it useful for aeration purposes, plus it absorbs some water. It decomposes rapidly.
While the various components mentioned above are fantastic for regulating water and improving airflow through the soil, only a few of them provide any nutrient value. And soil needs not only fertility for the plants, but for the microbiology that lives within it.
There’s a wide array of materials that can be added to improve the fertility of your raised bed garden. Let’s go over a list of those as well.
- Plant-based compost: No matter how you’re composting, you’ll find that you always have a need for more compost. It enriches the soil as it breaks down and improves the texture and tilth.
- Mushroom compost: This waste product from mushroom farming is extremely good at improving your raised beds.
- Cow manure: Once composted, this is a fantastic amendment and one which should be used regularly! All manures also act as food for your soil’s microbiology.
- Horse manure: If it’s well-composted and aged, this is also an option.
- Poultry manure: Again, it should be composted first if using around your veggies, or you can compost this directly in the bed during the off-seasons.
- Worm castings: This incredible resource also improves the fertility of your soil, although less than other manures do.
- Leaves or leaf mold: A fantastic amendment for all gardens, leaves provide lots of valuable material for your soil. It also makes an excellent top mulch.
- Alfalfa meal/bone meal/blood meal/kelp meal/other meals: Each type of meal listed, plus others, acts as a natural fertilizer source. Alfalfa is nitrogen-rich, kelp meal is potassium-rich, etcetera.
- Vermiculite: This provides some trace minerals that can help in plant growth.
- Organic fertilizers: If you’re not adding your own blend of meals as a fertilizer source, this may be an excellent option for you. Choose one that includes micronutrients like calcium, sulfur, and iron if possible.
- Azomite: This and other powdered minerals can provide additional trace nutrients to kickstart your garden.
There are a few other additions that can help with specialized things. Products like agricultural lime or calcium carbonate can help neutralize acidic soil pH levels, although be careful not to add too much. An excess of those materials can slow down the ability of a plant to absorb nutrition from the soil.
Mycorrhizae or beneficial bacteria are included in a number of bagged mixes and organic fertilizers. These are fine to add to your raised beds, but you also have naturally-occurring mycorrhizae and bacteria in your soil already. Still, it can’t hurt to boost the population occasionally.
Building A Good Blend
If you’re in a hurry, using bagged mixes as a way to get started can be a good choice. But it depends on how large the bed you’re trying to fill is. If you’re using one of the 6-in-1 tall galvanized steel beds from the Epic Gardening shop, you may need as much as just over a cubic yard of soil (or 30 cubic feet), and that’s quite a few bags to buy. The price for all these bags can add up quickly! A multiple bed garden may require a bulk delivery from your local landscape supply company.
While we’ve discussed tips on filling a tall raised bed in the past, it’s important to know what you’re filling with as well. Even bagged mixes can use a little extra fertility. So to that end, I recommend that if you’re opting for bagged material, work in some good quality compost or composted manure in addition to what’s in there to start with. Not only does it drop the price, but it improves the overall quality.
If you’re building from scratch, a good basic recipe to work with would be 30% high-quality topsoil from your yard (sifted to remove large rocks), 40% composted materials, 20% aeration and drainage improvers, and 10% other components (fertilizers, extra moisture retention, pH neutralizers if necessary, or even 10% worm castings).
You can also build a mixture often referred to as “Mel’s Mix”, which is equal parts compost, peat, and vermiculite. Many people swear by that blend, and it works really well! Peat moss is often inexpensive at a big box store like Home Depot or at the local garden center. Aged and composted manure can be used in lieu of plant-based compost in the beds if necessary.
Make sure to blend things together very thoroughly so it’s fully incorporated before adding your blend to the beds.
Maintaining Your Soil
Once you’ve got your raised garden beds filled and in use, it might seem as though you’re done. However, organic soil blends will gradually reduce in time as the material in the beds breaks down. Mineral components will stay, but compost and other organic matter is supposed to decompose and vanish over time.
Adding more compost should be your first step to rejuvenate your mix. A good, thick layer of compost applied on the soil surface once a year will top off the raised beds and ensure that they continue to remain fertile. It also can reduce the amount of weeds that sprout. Really, compost is very helpful and should be applied regularly if you’re doing organic gardening!
Beyond compost, you can occasionally add another mix product if you would like to top things off with that. Try not to add much extra sand to the bed unless it’s necessary for drainage, and try to avoid clay type soil that clumps. But regular applications of vermicompost or manure compost are always good. You can even pile the top of the bed with fall leaves and let them break gradually down over time. By the spring, most of the leaves will have turned to compost on their own!
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