Soils 101: Topping off Raised Beds

When we talk about adding soil to your school garden beds, it may come as a surprise that the physical work of shoveling and hauling the soil may be the easiest part. The purchase and delivery process may involve numerous people and can take significant time. Every school system has their own process for purchasing supplies and when you add the delivery aspect of bulk soil, where you are generally dealing with different people in a different department, the task becomes more challenging as well as time consuming. Being prepared ahead of time can prevent last minute stress and a smooth purchasing and delivery process. A little homework, outlined here, will have your garden beds looking good and ready to go.

When should beds be topped off?

Top-off beds prior to spring and fall planting seasons. There may not be enough time between the harvest of spring crops and planting sweet potato slips to add soil, so it’s important to do a good job of topping-off before planting the spring crop. Plan to order soil in December (yes, December!) to make sure you can have it on-site and in beds for February planting. Some businesses may even offer a winter discount since it may be a slower season for them. For the fall season, ordering in early August should be sufficient for September planting.

How much soil do I need?

If possible, and grade-level appropriate, include students in this process of calculating the quantity (volume) and perhaps the price (or a price comparison) of the soil needed. 

Step 1: Length of bed (ft) x width of bed (ft) x depth of soil needed (ft) = Volume of soil needed (cubic ft)

Hint: You will need to convert depth of soil needed from inches to feet in order to have the correct unit of measure.

Step 2 – Bagged Soil: Volume of soil needed (cubic feet) / 2 cubic feet per bag = number of bags needed

Bagged soil is commonly sold as 2 cubic feet per bag.

Step 2 – Bulk Soil: Cubic feet needed (from Step 1) / 27 cubic feet per cubic yard = number of cubic yards of soil needed

Bulk soil is sold by cubic yards.

What kind of soil should I get?

Soil mixes vary. A primary consideration to choosing a soil is the quantity you are adding. If the depth of soil you need to add to a bed is 2 inches or less you may consider topping-off with compost. If the depth of soil needed is more than 2 inches, consider adding a soil mix that has an adequate amount of aged compost – 1/3 to ½ by volume.



Ultimately the goal is to have beds ready on time with a good soil mixture that will support a successful growing season. Twice yearly topping-off will limit the need for big loads of soil to be brought in. Many students enjoy the physical process of filling a bed with soil, but remember to also involve them in the mental process of determining how much soil is needed.

Work Smarter, Not Harder
Do you have garden beds in a courtyard that require interior access? Adding soil can be a potentially laborious and messy task. A teacher we work with offered up a great solution: ask the custodians to borrow their 55-gallon trash drums on rolling dollies, fill the cans outside and roll them to the courtyard eliminating the risk of hallway spills.