Organic Raised Garden Bed Soil
One of the most confusing things for a first-time gardener is getting the right combination of soil. There are many options out there, all of which claim to be the best organic raised garden bed soils. Some bagged mixes are so good, they're often a good starting point for new gardeners.
But soil science isn't just about opening a bag and getting amazing, healthy plants out of it. Your garden soil is a living mixture of various components. In the natural environment, soil consists of many layers, some inorganic, such as crushed rock, and some organic, such as rotting leaves. Microorganisms, fungi, etc. also live in it. Together, they form the basis on which we grow our plants.
When you take soil out of its natural environment and place it in a raised bed, different goals should be met. Raised garden bed soils heat up faster in the spring, generally require better drainage than other forms of soil, and decrease in height as the material decays and turns to plant nutrients. So let's review loft bed mixes, what they are, how they work, and what you can do to keep them active and healthy. Healthy soil makes for healthy plants, so getting started right is critical!
What exactly is soil?
The term "soil" is often misused, but in essence it should refer to what we often call dirt. The average garden soil in a yard usually contains at least 45% minerals, about 25% air and water, and about 5% organic matter such as rotting leaves. These numbers vary by region and soil quality, but this is an average for what is considered good soil. While mineral content varies by type, soil types are generally organized by their particle size. The largest particle size is what we call sand. The best is clay. Between the two is silt.
Each type of soil has different properties. Clay tends to stick together and become hard when baked in the sun. Sand is loose, but its large particle size means it can't hold as much water. Silt is slippery and prone to erosion.
A mixture of these three types is called loam, but if it has the predominant amount of any type of soil, add it to its name. For example, a sandy loam soil will be a relatively uniform mix of mineral sizes, sloping slightly towards the sandy side. The loamy sand will be dominated by sand, but there will also be some loam mixed in. These terms help us determine the average particle size of soil, as well as some of its most common properties.
What is the difference between soil and mixture?
Things sold as "potting soil" are almost always mixed. In fact, it may or may not contain any soil at all! A better term is "potting mix," which some sellers use instead of including the word soil. For most raised beds, what we want is usually a soil mix. Soil mixes include mineral-rich dirt as well as other organic components, adding to the rich organic matter that plants need to thrive.
Do you always need to mix?
Demand is a pretty strong word here, but technically you don't need to run out and buy any kind of mix. You can make one at home using your own compost, aged leaves from your yard, or any other inputs. But to start a new raised bed, it's best to use the mix as at least part of the initial organic soil mix. Depending on the size of your loft bed, you can buy bagged mix to fill it. Alternatively, many landscape supply companies sell their own mixes in bulk. Both options are great for getting you started.
Is soil always necessary?
For container growing, soil is not always required. Many plants are perfectly happy in a potting mix optimized for their specific needs. But generally, sand is used as an additive to mix bags because of its large particle size, which provides better drainage, so even if you don't think you do, you may actually have a small amount of soil. As a general rule, a raised bed uses at least some soil to ensure good drainage, but also fill it up. Since garden beds can require a lot of fill material, using some soil and other components can significantly reduce the cost of filling a raised bed.
What does raised garden bed soil contain?
The various bagged mixes out there have proprietary blends of different ingredients. To provide an example of what we really like, organic mixes for raised gardens bed contain the following ingredients on their labels: 55%-65% aged forest products, sphagnum peat moss, perlite, limestone, alfalfa meal, kelp meal, Feather meal, worm castings and yucca extract, and some mycorrhizal life. It's technically a soilless garden medium for large containers or raised gardens beds. It works well if you want the option to premix straight from the bag.
But what are all these components and what do they do? Let's discuss it in more detail.
Aeration and Moisture Related Additives
It stands to reason that moisture is necessary for your plants, but not too much. Drainage needs vary by plant type, but most common garden plants prefer excess water in the soil to drain freely. Adding aeration is also necessary if you start with garden soil that is usually compacted year after year. Just like plants need water for their roots, they also need air!
Perlite is a common additive used for both. This expanded volcanic rock is very lightweight and easy to use, and it also reduces soil compaction. Because it has a much larger particle size, it allows airflow through the soil, but also drainage. Many types of additives are used for both, but some are specific to moisture or drainage. Below is a short summary of the most common additives to improve aeration or drainage and retain/absorb moisture.
Perlite: Improves drainage, is lightweight and captures small amounts of water.
Rice Husk: Drainage for final decomposition. Extremely lightweight.
Lava cinders: These are tiny pieces of volcanic rock that improve aeration and drainage while capturing small amounts of water. Unlike perlite, it will not eventually be crushed by the weight of the soil.
Aged forest products: These not only improve drainage, but also absorb water for later use. Older adults are already partially decomposed, which means they don't steal nutrients from raised garden beds.
Coir: This lightweight material absorbs moisture for later plant use, but excess moisture is easily expelled. It will also loosen the soil, which will eventually decompose.
Sphagnum peat moss: While it is difficult to wet it initially because dry peat can be hydrophobic, once wetted, it retains moisture for a long time. Keep it moist, it's a great addition.
Worm Castings: Worm castings are capable of absorbing nearly 10 times their weight in moisture, making them ideal for retaining moisture.
Vermiculite: This mineral absorbs water like a sponge. Over time, it releases moisture into the soil.
Coarse straw/hay: More commonly used as mulch. The large size makes it useful for aeration purposes and will absorb some water. It breaks down quickly. For best results, use a healthy straw!
While the various ingredients mentioned above are great for regulating moisture and improving airflow through the soil, only a few of them provide any nutritional value. Soil needs not only the fertility of plants, but also the microorganisms that live in it. A variety of materials can be added to enhance the fertility of a raised bed garden. Let's review those lists too.
Plant-Based Composting: No matter how you compost, you'll find that you always need more compost. It enriches the soil and improves texture and tillage as it decomposes.
Mushroom compost: This waste from mushroom cultivation is great at improving your raised bed.
Cow Dung: Once composted, this is a great amendment and should be used often! All manure is also food for soil microorganisms.
Horse manure: This is also an option if it is well composted and aged.
Poultry manure: Again, if used around vegetables, it should be composted first, or you can compost directly on the bed during the off-season.
Worm Casting: This incredible resource also increases soil fertility, albeit less so than other fertilizers.
Foliage or Leaf Mold: Foliage is a fantastic amendment to all gardens, providing your soil with many valuable materials. It is also an excellent top covering.
Alfalfa meal/bone meal/blood meal/kelp meal/other meals: Every meal listed, along with other meals, is a source of natural fertilizer. Alfalfa is rich in nitrogen, kelp powder is rich in potassium, etc.
Vermiculite: This provides some trace minerals that can help plants grow.
Organic fertilizers: If you don't add your own mixed meal as a fertilizer source, this may be a good option for you. If possible, choose a product that contains micronutrients like calcium, sulfur, and iron.
Azolite: This and other powdered minerals can provide additional micronutrients to kickstart your garden.
There are a few other supplements that can help with professional things. Products like agricultural lime or calcium carbonate can help neutralize the pH of acidic soils, but be careful not to add too much. Excessive amounts of these materials can slow down the ability of plants to absorb nutrients from the soil.
Mycorrhizae, or beneficial bacteria, are contained in many bagged mixes and organic fertilizers. These can be added to your raised bed, but your soil also already has naturally occurring mycorrhizae and bacteria. Still, it doesn't hurt to increase the population occasionally.
Build a good mix
We've discussed tips for filling loft beds in the past, but it's important to know what you're filling. Even bagged mixes can use a little extra fertility. So, for that, I recommend that if you choose bagged material, use some good quality compost or compost manure in addition to starting with what's in it. Not only does it lower the price, but it improves the overall quality.
If you are building from scratch, a good basic recipe is 30% good quality topsoil in your yard (sieved to remove large rocks), 40% compost material, 20% aeration and drainage improver, and 10% other ingredients (fertilizer, Additional moisture retention, pH neutralizer if necessary, even 10% worm casting). You can also build a mix commonly known as a "mel mix," which is equal parts compost, peat, and vermiculite. Many people swear by this concoction and it works amazingly well! Sphagnum moss is usually inexpensive at big box stores like Home Depot or your local garden center. If necessary, use aged and composted manure in place of plant-based compost in the bed. Make sure to mix things together very thoroughly so that the mixture is fully incorporated before adding it to the bed.
Once you've filled your raised garden bed and put it to work, it looks as if you're done. However, the organic soil mix gradually decreases over time as the material in the bed breaks down. The mineral content will stay, but the compost and other organic matter should break down and disappear over time. Adding more compost should be your first step in rejuvenating the mix. An annual application of a good, thick layer of compost to the soil surface will cover the raised beds and ensure they remain fertile. It also reduces the number of weeds that sprout. Really, compost is very helpful and should be applied regularly if you are doing organic gardening!
In addition to compost, you can occasionally add another mixed product if you want to supplement with this. Try not to add too much extra sand to the bed unless drainage requires it, and try to avoid clumping clay-type soils. But regular application of vermi compost or manure compost is always good. You can even stack the top of the bed with fall leaves and let them break down over time. By spring, most of the leaves will compost on their own!