The most confusing thing for first-time gardeners is finding the right soil mix. There are so many options out there that they all claim to be the best organic raised bed flooring. Some packaged mixes are quite good and usually a good starting point for new gardeners.
But soil science is a little more than opening a bag and wondrous, healthy plants out of it. Your garden soil is a living and breathing mixture of different components. In a natural setting, the soil is made up of many layers, some inorganic like powdered rock and some organic like rotting leaves. Microbes, fungi and more also live in it. Together they form a basis for what we grow plants on.
Taking the soil out of its natural environment and placing it in a raised bed should achieve several goals. Elevated soil warms up faster in spring, often needs better drainage than other types of soil, and can lose height as the material breaks down and turns into plant nutrients.
So let's go over the raised bed mixes, what they are, how they work, and what you should be doing to keep them alive and healthy. Healthy soil makes for healthy plants, so it's important to start right!
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What is soil anyway?
The term “soil” is often misused, but it should essentially refer to what we often refer to as dirt. The average garden soil in your yard is usually made up of at least 45% mineral content, around 25% each of air and water, and around 5% organic matter like decayed leaves. These numbers vary somewhat depending on the region and soil quality, but that's an average for what is considered good soil.
While mineral content varies in species, soil types are most often organized according to their particle size. The largest particle size is what we call sand. The finest is clay. There is silt between the two.
Each type of soil has different properties. Clay tends to stick together and can get rock hard when baked in the sun. Sand is loose, but because of its large particle size, it doesn't hold as much water. Silt is slippery and can be prone to erosion.
A mixture of these three types is called clay. However, if it has a dominant amount of a soil type that is added to its name. For example, a sandy loam would be a relatively even mixture of mineral sizes with a slight slope towards the sandy side. Loamy sand would be sand dominant, but also with a little loam mixture. These terms help us determine the average particle size of soil, but also some of its most common attributes.
What is the difference between soil and mixes?
The stuff sold as “potting soil” is almost always a mix. In fact, it may or may not contain soil at all! A better term for this would be "potting soil," and some sellers use this term instead of including the word soil.
What we want for most raised beds is usually a soil mix. A soil mix contains the mineral-rich dirt along with other organic ingredients, thus promoting the rich organic matter plants need to thrive.
Do you always need mixes?
Need is a pretty strong word here, but technically you don't have to go out and buy a mix of any sort. You can make one at home with your own compost, aged leaves from the yard, or any number of other inputs. To start a new raised bed, however, it's a good idea to use a mixture as part of your original organic soil mix.
Depending on the size of your raised bed, you may be able to purchase bag mixes to fill it up. Alternatively, many landscape suppliers sell their own mixes in bulk. Both options are good to get started with.
Is floor always necessary?
Soil is not always required for container cultivation. Many plants are very happy to have potting soil that is tailored to their specific needs. However, sand is often used as an additive to mix bags because its large particle size gives it better drainage, so you may have a small amount of soil even if you don't believe it.
Raised beds usually use at least some soil to ensure good drainage, but also to fill them. Since garden beds can hold a surprising amount of filler material, using some soil along with other components can significantly reduce the cost of filling your raised bed.
What goes into the raised bed floor?
The various packaged mixes contain proprietary mixes of various ingredients. To provide an example that we really like, Espoma's organic mixture for raised bed gardens contains the following ingredients on the label: 55% -65% old forest products, sphagnum peat moss, perlite, limestone, alfalfa flour, seaweed flour , Feather meal, worm castings and yucca extract as well as some mycorrhizal life. Technically, it is a soilless garden medium for large containers or raised beds. It works well if you want to opt for a pre-mix straight from sachets.
But what are all these components and what do they do? Let's talk about this in more detail.
Ventilation and moisture related additives
Obviously, moisture is necessary for your plants, but not too much moisture. Drainage needs vary between plant types, but most common garden plants like excess water in the soil to drain freely. If you start out with garden soil, which typically thickens year after year, additional ventilation is also required. Just as plants need water at the root level, they also need air!
Perlite is a common additive that is used for both aspects. This expanded volcanic rock is extremely light and easy to work with, and it also reduces soil compaction a little. Since it is much larger, it allows air to flow through the floor, but also allows drainage.
Many types of additives are used for both, but there are some that are specific to either moisture or drainage. Here is a brief summary of the most common additives used to improve ventilation or drainage, and to hold / absorb moisture.
- Perlite: Improves drainage and is light, catches small amounts of water.
- Rice husks: for drainage that will eventually decompose. Extremely light.
- Lava slag: These are fine pieces of volcanic rock that improve ventilation and drainage while collecting small amounts of water. In contrast to perlite, this is ultimately not crushed by the weight of the soil.
- Aged forest products: These not only improve drainage, but also absorb water for later. Elderly people are already partially decomposed, which means that they do not deprive the raised beds of food.
- Coconut coconut: This lightweight material absorbs water for later use in plants, but excess material can easily drain away. It also loosens the soil and will eventually decompose.
- Sphagnum peat moss: While it can be difficult to get it wet initially, 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 casting: Worm castings can hold almost ten times their weight in water and 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 than mulch. The large size makes it useful for ventilation purposes and absorbs some water. It decomposes quickly.
Fertility supplements
While the various above components are great for regulating water and improving airflow through the soil, few of them provide any nutritional value. And the soil not only needs fertility for the plants, but also for the microbiology that lives in it.
There are a variety of materials that can be added to improve the fertility of your raised bed garden. Let's go over a list of these as well.
- Vegetable compost: No matter how you compost, you will find that you always need more compost. It enriches the soil as it is broken down and improves texture and slope.
- Mushroom compost: This by-product from mushroom growing will do a great job of improving your raised beds.
- Cow dung: Once composted, this is a fantastic change to use on a regular basis! All fertilizers also serve as nourishment for the microbiology of your soil.
- Horse manure: If it's well composted and aged, this is an option too.
- Poultry manure: Again, it should be composted first if you are going to use it around your vegetables, or you can compost it right in bed in the off-season.
- Worm casting: This incredible resource also improves the fertility of your soil, although less so than other fertilizers.
- Leaves or leaf mold: Leaves are a fantastic addition to any garden and are a great source of valuable material for your soil. It also makes an excellent top mulch.
- Alfalfa meal / bone meal / blood meal / seaweed meal / other meals: Any of the listed types of meals as well as others serve as a natural source of fertilizer. Alfalfa is high in nitrogen, seaweed flour is high in potassium, etc.
- Vermiculite: This provides some trace elements that can help with plant growth.
- Organic fertilizers: If you don't add your own meal mix as a source of fertilizer, this might be an excellent option for you. Choose one that contains micronutrients like calcium, sulfur, and iron whenever possible.
- Azomite: This and other powdered minerals can provide additional trace elements to get your garden going.
Other additives
There are a few other supplements that can help with specific things. Products like agricultural lime or calcium carbonate can help neutralize the pH of the acidic soil, but be careful not to add too much. An excess of these materials can slow down a plant's ability to absorb food from the soil.
Mycorrhizae or beneficial bacteria are found in a number of bag mixes and organic fertilizers. These can be fine added to your raised beds, but you also have naturally occurring mycorrhizae and bacteria in your soil. Still, it can't hurt to occasionally strengthen the population.
Build a good mix
If you're in a hurry, using bag mixes as a way to get started can be a good choice. However, it depends on how big the bed you want to fill. If you're using one of the galvanized steel 6-in-1 beds from the Epic Gardening store, you may need a little more than a cubic meter of earth (or 30 cubic feet) and that is some bags to buy. The price for all of these bags can add up quickly! A multi-bed garden may require bulk delivery from your local landscaping company.
While we've discussed tips for filling a tall raised bed in the past, it's important to know what you're filling with as well. Even packaged mixes can use up a little more fertility. To this end, I recommend that if you choose to go with pre-packaged material, in addition to what's included initially, you also use good quality compost or composted manure. This not only lowers the price but also improves the overall quality.
If you're building from scratch, a good basic recipe is 30% high quality topsoil from your garden (sifted to remove large stones), 40% composted materials, 20% ventilation and drainage improvers, and 10% other components (fertilizers, additional moisture retention, pH neutralizers if needed, or even 10% worm cast).
You can also make a mix, often called "Mel's Mix," which is equal parts compost, peat, and vermiculite. Lots of people swear by this mixture, and it works really well! Peat moss is often cheap at a large store like Home Depot or the local garden center. Aged and composted manure can be used in the beds instead of plant-based compost if necessary.
Make sure you mix things together very thoroughly so that they are fully incorporated before adding your mixture to the beds.
Maintain your soil
Once you have your raised beds filled and operational, it will seem like you are done. However, organic soil mixes gradually decrease over time as the material in the beds breaks down. Mineral components are retained, but compost and other organic matter are supposed to decompose and disappear over time.
Adding more compost should be your first step in rejuvenating your mix. A good, thick layer of compost that is applied to the soil surface once a year rounds off the raised beds and ensures that they remain fertile. It can also reduce the amount of weeds that sprout. Compost is really very helpful and should be used regularly if you are organic gardening!
In addition to compost, you can occasionally add another mixed product if you want to top it off. Try not to add a lot of extra sand to the bed unless it is necessary for drainage and avoid lumpy clay floors. However, regular applications of vermicompost or manure compost are always good. You can even stack the top of the bed with fall leaves and gradually let them degrade over time. By spring most of the leaves will have turned into compost by themselves!
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