Part 8: Adding Soil
Adding soil
Just how complex soils are was the most surprising thing to me when I started gardening. I thought it would be pretty linear: call the garden supply company and have them deliver dirt. Or I thought I would go to the home improvement store and buy some dirt. Done! Nope, not even close. I am still perplexed by the different types of soil, nutrition, organic matter versus inorganic matter, and the fact that lots of “soils” are “soilless”.
Soil is made up of lots of different parts but we need to focus on these:
A big chunk of the dirt in your yard is made up of crushed minerals (rocks). This is the inorganic material. If you get topsoil delivered from the garden supply company or buy a bag of top soil at a home improvement center it probably will be mostly made up of inorganic material.
The other part of the soil that is important to us is the organic material. This is the decaying matter (leaves, grasses etc.) in the soil. Farmers have greatly differing opinions on how much of your soil should be “organic matter.” But we’ll talk more about it later.
There also are compounds--think of them as vitamins and minerals--in the soil that the plant can access for its nutrition. The classic nitrogen, potassium, phosphorus is what is listed on most bags of fertilizer (the N-P-K ratio). But there are lots more out there and people are getting smart about looking at soil as an entire system. (Just as your diet is an entire system, not just a few key vitamins and minerals. The carrot is good for you because it’s a carrot, not just because it has beta carotene in it)
Soil can be alkaline or acidic and we’ll fix this easily enough.
You have two choices: buy bags of soil from a home improvement/garden center or have soil delivered to your driveway. There are pluses and minuses to both that we’ll discuss.
Bulk delivery: In Northern Virginia I have a garden center deliver top soil and then I have a compost company deliver compost to my driveway; and then I mix the two at a 50-50 ratio and fill the garden beds. You can use any soil or compost delivery company you desire. But you have to do some work and research in this area with these things in mind:
Ask the landscape supply company where they get their topsoil from and see if they have testing results they can share with you.
Ask your compost supply company what they make their compost from. Some compost and pre-made garden soil is made with sewage byproducts. Do your research and figure out what you are comfortable with.
You are going to see some pre-blended options out there. A landscape supply company might sell something in bulk that is “raised bed soil” for example. Just ask the same questions as above: where is the topsoil from and where is the compost from?
Compost is different than mulch. The I-66 transfer station gives away free mulch, but mulch is ground up wood and grass and isn’t composted; mulch will use nutrients as it decays and is not what you want in your garden beds.
Bags of soil from the home improvement store vary widely and I have no real good specific recommendations for you. I’ve grown in garden beds with the “raised garden bed” soil mixes sold by a few different well-known companies and they seem to produce well. I would stick with something that specifically says for raised garden beds; don’t use a potting mix. Here are a few things to keep in mind with bagged soils:
Look at the ingredients: some may be made with the byproduct from human wastewater treatment plants. Not necessarily bad, just do your research and decide.
Try to find one that has “OMRI” listed on it; this isn’t foolproof but it is at least a sign that it’s approved for use on organic farms.
Make sure they do not have chemical fertilizers in them; or any chemicals for that matter.
The amount of soil you need depends on the number and size of your beds but here are some general rules of thumb I use:
An 8 by 4 bed that is 10.5-11 inches tall needs one cubic yard (27 sq ft) of dirt/compost (half this for a 4 foot by 4 foot bed of the same height).
An 8 by 4 bed that is 5.5-7 inches tall needs about ½ cubic yard (13.5 sq ft) of dirt/compost (half this for a 4 foot by 4 foot bed of the same height).
You can find online soil calculators out there that can provide rough calculations for you.
Soil that you buy at the big box stores usually comes in bags that are 1.5 cubic feet. So for one 11-inch tall bed that is 8x4 you would need to buy 18 bags of soil at the store. You will not fit that in your car in one trip by the way (unless you have a truck). And if you are building two beds the number of trips really adds up. Also the money adds up; as of May 2020 the price for one bag of raised bed garden soil was about 8 dollars. So for one of your beds you might spend 144 dollars.
Soil in bulk is usually cheaper but the delivery fee is where things get tricky. This delivery fee is usually fixed (ranges from 50 to 100 dollars) regardless of the amount you buy. So to fill one garden bed it might be cheaper to buy bags of soil but once you get to filling 2 or 3 or more garden beds then delivery is probably going to be a more economical option.
Remember that moving all the soil is going to take some physical work. If you are starting off building 6 raised beds that are all 8x4 and 11 inches deep you are going to be moving 6 yards of dirt/compost. Go find a yardstick in your house and then use it as a guide to imagine a cube where each side is one yard; now times that by six… that is how much dirt you are going to have to haul with a wheelbarrow. I’ve done it, but it takes some time.