Gardening for Geeks. Christy Wilhelmi
so that the top end of the stake sits about 1 to 2 inches (2.5 to 5 cm) below the top edge of the raised bed. You will not be able to see the stakes once the bed is filled with soil.
6.Secure each stake to the wood frame with four 1½-inch (3.75-cm) wood screws as shown (see “Metal stake placement inside front view” on the diagram). If you plan to attach a sitting rail or cap to the top edge of your raised bed, this is the time to do it.
7.Congratulations! Now you are ready to fill your bed with soil.
8.If you don’t have enough existing soil and compost to fill your raised beds, you will need to start off with bagged nursery planting mix, bulk organic vegetable garden soil, or a combination of ingredients. Once the beds are established, regular composting will keep you supplied with soil amendments.
A Formula for New Raised-Bed Soil
Before we get started, let’s get something straight: there’s no actual soil in potting soil. It’s a misnomer. Potting soil is usually made up of decomposed organic matter, like compost, wood chips, lumberyard waste, and peat moss. It also has perlite—that white puffy stuff that looks and sounds like Styrofoam when you crush it. Perlite is puffed volcanic glass that allows water to flow better through the soil medium. It doesn’t hold nutrients or add nourishment to the plants. It’s simply there as a placeholder for air and water.
Peat moss is widely used in potting soils to hold moisture and improve soil texture. It is acidic and can help adjust alkaline pH when needed. The trouble is that peat moss is a natural resource that is being depleted around the world. It takes about 3,000 years for nature to make a peat bog, and we’re using it up faster than it can reproduce. As an alternative, some forward-thinking soil companies are starting to incorporate a peat moss substitute into their potting soils instead. That alternative is called coir. It’s mispronounced by most, but the correct pronunciation (in English, anyway) is “coy-yer.” Coir works like peat to hold moisture and improve soil texture, but it’s made from a waste product, so it’s renewable.
Coir is made from the outside hull of coconuts—the part that is thrown away. The raw product has a high salinity, so it must be thoroughly rinsed before use. Thankfully, most manufacturers producing coir for the hydroponics industry are meticulous in eliminating salts before packaging the product. Investigate your provider before using a new material and inquire with your nursery professional about carrying coir-based products for your neighborhood.
That bag of potting soil from the store will probably say “organic,” but know this: there is no regulation about the use of the word organic in soil amendments. When we go to the grocery store and buy organic produce, we know it has been grown without pesticides and chemical fertilizers according to United States Department of Agriculture (USDA) organic standards. In the world of soil amendments, organic simply means “of, relating to, or derived from living matter.” Keep this in mind as you read about fertilizers as well.
So now that you have an idea of what’s in those bags, let’s talk about how to make your own raised-bed planting mix. Here’s the formula: start with 50 percent compost, 40 percent coir, and 10 percent perlite or vermiculite, depending on your existing soil conditions. Vermiculite does the opposite of perlite. It is a puffed mineral that is exploded like popcorn. It holds water like a sponge, which helps sandy soils retain moisture. It doesn’t break down, so you need to apply it only once. Vermiculite has a checkered past: at one point, a major mining source discovered that their vermiculite was contaminated with asbestos, but OMRI (Organic Materials Review Institute) put regulations into practice to ensure that each batch is now certified asbestos-free before sale.
You can also throw in coffee grounds and organic fertilizer if you like. This is just a starting point, however. If you mix in some of your existing soil, you will want to adjust the percentages a bit. If you have clay soil, add more compost to break up tight particles. If you have sandy soil, use more coir to hold moisture better. Play around with this. It’s all part of the experiment of gardening.
Now, to calculate your soil needs. If you have a raised bed that is 12 inches tall, it will be easy to figure out how much planting medium you will need. It’s simply length x width x 1 = cubic feet (because your bed is 1 foot tall). If you have a shorter raised bed, you will need to do a little more math: length x width x height, but this time the height will be a fraction of 1 foot. For example, let’s say you have a bed that is 4 feet wide, 6 feet long, and 8 inches tall. First, multiply 4 feet x 6 feet to get 24 feet. Then divide 12 inches into 8 inches to get 0.667 (this is the fraction of 1 foot). Multiply 24 feet by 0.667 to get 16 cubic feet of soil. If your bed is 10 inches tall, use 0.834 for the height; if it is 6 inches tall, use 0.5 for the height.
Volume = 4 feet x 6 feet x (8 inches/12 inches) = 16 cubic feet (or 4 feet x 6 feet x 0.667 = 16 cubic feet)
Once you have figured out the cubic footage, multiply that total by 50 percent to find out how much compost you need. Then multiply the cubic footage total by 40 percent for how much coir, and then by 10 percent for perlite. For example:
Our bed needs 16 cubic feet of soil.
50 percent of 16 is 8 cubic feet of compost.
40 percent of 16 is 6.4 cubic feet of coir.
10 percent of 16 is 1.6 cubic feet of perlite.
Most bags of nursery potting soil and compost are sold in quantities of either 2 cubic feet or 1.5 cubic feet. Divide your totals for each material by the bag size you plan to buy, and that will tell you how many bags to load into your car.
Geeky Gardening TiP:
Old-School String Lines
When setting up mounded raised beds, place stakes at each corner of your target bed area and run string along the sides to map out clear lines for your beds. Whether you are building stacked mounds with layers of organic material, or just hilling up soil and compost, these lines will ensure that your beds are straight.
Berm Beds
Often called unconstructed beds or mounds, berm beds are the right choice for those who want to build a garden without actually building anything—not even stacks of organic matter. These beds also allow for more creativity. They can be formed into any shape, length, and size to accommodate your grand garden vision. Do you want a snaking berm with sunflowers along the pathway? A circle mound for your potato patch? Start digging.
There are several ways to build an unconstructed bed. A common practice is to mark out the bed areas (see the Old-School String Lines tip above) but instead of digging inside the bed area, dig soil out of the pathways and dump it onto the bed areas. This leaves the pathways lower than soil level and the beds higher than where they started. This technique was used by indigenous cultures to irrigate their crops and is often used in commercial agriculture today. The lower pathways fill with water, and the water is absorbed at the base of the mounds. Plant roots reach deeper for water, making them more resilient and drought tolerant. In permaculture, a variation of the pathway-turned-trench is called a swale and is often used to divert water away from flood-prone areas toward plant root zones (we’ll talk more about this in Chapter 7). Turn to Appendix D for other ways to build mounded beds, including “no-dig” beds.
Mounded raised beds are usually between 4 and 8 inches (10 and 20 cm) tall, though some adventurous gardeners have been known to build them waist-high. Pathway soil is mixed with compost to create loamy, well-drained soil that will hold together in mounds. The sides are sloped, and the surface of the mound either is flat across the top or has a gentle convex curve.
The theory behind this kind of mounded bed is that it creates more surface area. Sloped sides plus a flat top equals more soil exposed to the sun, which means that the soil thaws and warms more quickly in spring. Some gardeners take advantage of the extra surface area by planting crops on the slopes of each bed. You gain up to another foot of growing space if your beds are 6 inches (15 cm) tall. Be aware that slopes can erode during watering, so keep the