Creating the Right Soil Mix for Square Foot Gardening

Planting a watermelon plant in a raised bed garden.
What You'll Need
Peat moss
Coarse vermiculite
Large Tarp
Raised garden boxes
Seeds and starts

Square foot gardening is a method for intensively growing plants and vegetables in raised beds. Some square foot gardeners use a soiless mixture of planting material that are lighter than dirt. Soil and even soil mixed with compost can be quite heavy, full of clay or slow draining. Creating the right mix of soil is a simple matter of knowing the proportions, measuring the volume of your growing area, and mixing the ingredients.

TIP: Our expert gardening advisor, Kathy Bosin adds, "Soilless mixes are used in popular vertical garden schemes, as well as rooftop gardens. If you're using a soilless mix, be sure to research the plants you want to grow to make sure you find a mix recipe that will be appropriate."

Step 1 - Measure Your Growing Area Volume

Volume of a space is equal to width times the length times the height. So, for a raised garden box with dimensions of 4 by 4 feet and 1 foot deep, that equals 16 cubic feet of volume. Calculate the volume for each of your raised boxes and then add the volumes together to get total volume.

Step 2 - Purchase the Ingredients

The lighter-than-dirt mixture used for square foot gardening is the following: 1/3 peat moss, 1/3 vermiculite and 1/3 compost. For a 3.5 cubic foot bag of vermiculite, on average you will spend $18. A 3.8 cubic foot compressed amount of peat moss is roughly $9. You should acquire different types of compost. You can usually find several different mixtures of compost in bulk for around $30 a truckload. You can also buy it pre-bagged. A 1 cubic foot bag usually costs around $2.

Step 3 - Mix the Ingredients

If you only have one raised box to work with, you can probably mix the ingredients in several batches in a wheelbarrow. Otherwise, lay out a large tarp and put down the measure of compost followed by the peat moss and vermiculite. For example, if you have three 4x4 beds that are each 1 foot deep, you need 48 cubic feet of mix. That equals 16 cubic feet of each of the ingredients. According to the above measures, you need approximately 5 bags of vermiculite, 5 measures of peat moss, and 16 bags or one truckload of compost.

Once on the tarp, mix thoroughly with a pitchfork or shovel. You want the ingredients well blended before you shovel them into the garden boxes.

Step 4 - Fill the Boxes

Once mixed, each box should get roughly the same amount of the blend. Now you can plan out what will be planted where. You can use both seeds and starts to plant your garden. The light mixture allows the seeds to germinate easily and the roots to expand. Another great aspect of this mixture is that it is almost impossible to over water. The mix is so light and well draining that the water never pools up.