Organic Gardening Guide Series 2021: How to Measure Your Raised Bed & What to Put in It!

February 8, 2021

Now that we have selected our seeds (and started our inside/outside sowing), it is time to focus on our soil! Most gardeners know that this is a crucial step in achieving a successful growing season. This year I have become particularly focused on the magic recipe in the various formulas that are used to amend and fill raised planting beds. My Organic Veggie Garden will contain three different sizes and styles of raised beds. Although slightly different, each one must contain the magic recipe so that my garden will yield the best crop of veggies!


Most new planters have a tag listing the dimensions of the box, but it takes a bit of math to figure out how much soil is needed to fill it. If the dimensions aren’t listed, take a measuring tape and measure the length, width and depth of the box, in inches. Multiply these numbers together to determine the volume of the box in cubic inches. For example, if a planter box measures 20 inches long by 12 inches wide by 6 inches deep, it has a volume of 1,440 cubic inches. But bagged potting soil is sold in cubic feet, so you’ll need to know the box’s volume in cubic feet. Divide your box measurements by 1,728, which is the number of inches in a cubic foot, to determine the cubic foot volume of your box. In this case, the planter box would be 0.83 cubic feet, or just over three-quarters of a cubic foot.

How Much Soil Do I Need?

Calculate the amount of soil you need for your raised bed or planter by using the formula below.


1. Multiply: Length in. x Width in. x Height in = the total area in inches

2. Divide:total in inches by 1,728 (which equals number of inches in a cubic foot) = total in cubic feet

3. Divide:total in #2 by 27 (which is the number of cubic feet in a cubic yard) = total of cubic yards….some stores sell the soil in cubic yards!

Each bag of soil at the store/nursery equals 2 cubic feet of dirt each.

Determine the volume of your planter box by multiplying the width by the length by the depth. For example, if you have a length of 5 feet, a width of 4 feet and a depth of 2 feet, the volume would be 40 cubic feet (5 x 4 x 2 = 40). This is the amount of dirt you will need to fill your raised planter


How many bags of soil do I need for a 4×8 raised bed? 4 bags

For a 4×8-foot raised bed: 4 bags (2 cubic feet each) topsoil (Note: Avoid using topsoil from your yard, as it may contain weeds and pests.) 2 pails (3 cubic feet each) coconut coir (to improve drainage) 2 bags (2–3 cubic feet each) compost or composted cow manure.

75 cubic feet

A 40-lb bag of topsoil is about . 75 cubic feet (or ft3).Jun 9, › questions


Use an All Purpose ORGANIC fertilizer in your vegetable raised bed:

5-10-10 OR 12-12-12

Use 2 pounds of fertilizer for every 10 square feet of bed space.


If you’ll be filling more than one raised bed, you may want to buy your soil in bulk — by the cubic foot or cubic yard. Use the Soil Calculator to figure out the total amount of soil you’ll need for each bed. For most situations, I recommend these proportions:

  • 60 percent topsoil
  • 30 percent compost
  • 10 percent soilless growing mix—also known as potting mix.

(other combination: 1/3 peat moss, 1/3 vermiculite, 1/3 compost)
Use 2 pounds of ORGANIC fertilizer for every 10 square feet of bed space.


Pre-mixed potting soils are formulated to retain moisture yet allow excess water to pass through freely, and are typically a blend of peat moss, compost, and vermiculite or perlite, but they may also contain other materials including coconut fiber, milled tree bark and sand.


1. Fill the planter with equal parts compost, perlite and sterile soil. Mix these ingredients together until they are fully combined.

2. Add 1/2 tablespoon of 14-14-14 slow-release fertilizer OR GENERAL ORGANIC FERTILIZER for each gallon of soil mix. Mix the fertilizer into the soil until it’s thoroughly incorporated into the potting soil.

3. Water the potting soil before you plant the vegetables in the planter/raised bed. Add water until the excess begins to drain from the bottom. Allow the planter/raised to drain for an additional 30 minutes, then water a second time if the soil still feels dry. Allow the soil to drain a second time before planting.

4. Amend the soil with a soluble fertilizer OR GENERAL ORGANIC FERTILIZER every two weeks beginning six weeks after planting the vegetables in the planter/raised bed. For example, mix 1 tablespoon of 24-8-16 fertilizer with 1 gallon of water in a watering can. Water the vegetables with this solution at 14-day intervals.


Planters/raised beds should be filled to within an inch of the lip of the planter, which allows the box to trap moisture and prevent soil from spilling over the edges when the box gets watered. Because many plants don’t grow much deeper than 18 inches, the bottom third of very deep containers can be lined with foam or other lightweight, rot-resistant filler to reduce the overall amount of soil needed to fill the box/raised bed.

Note: Remember that certain root vegetables and climbers will require a deeper planting soil (not filler material) since their roots may reach from 18” deep onwards.

The following is my suggested

LAYER 1: Upper most layer or top—3-inch layer of Organic Mulch to prevent weeds and to keep moisture in the soil. LAYER 2:60 % Topsoil

LAYER 3: 30 % Compost

LAYER 4: 10 % Soilless Growing Mix or Potting Mix

LAYER 5: Last layer= Sand and Rocks for drainage


Mulches serve multiple purposes in a planter box. Mulch inhibits weed growth and slows water loss from evaporation. Mulch can also insulate soil and protect plants from pests in the soil. Organic mulches, like straw and wood chips, primarily insulate and stop moisture loss. Plastic mulch slows moisture loss, insulates, warms the soil and protects plants from soilborne pests and pathogens. A drip irrigation system is often necessary when using plastic mulch because overhead watering can’t penetrate to the soil.

Organic Mulch

Mulch shields the soil surface from the sun, which prevents evaporation and helps retain soil moisture. Organic mulches provide an attractive and useful option. Shredded bark and pine straw is often used in ornamental beds, while seed-free straw is best for vegetable beds. A 2- to 3-inch mulch layer helps keep soil moist, and it also works to improve the soil as it breaks down so it retains moisture better. Mulched beds still require irrigation to replenish the moisture in the soil as plants use it.

Inorganic Mulch:

Plastic and landscape fabric are examples of inorganic mulch. Fabric is primarily used to prevent weeds, while plastic prevents weeds and retains soil moisture. Almost no moisture is lost through evaporation with plastic mulch, but rainfall and overhead irrigation can’t penetrate it. Drip irrigation lines beneath the plastic supply the necessary moisture, while the plants grow through holes cut in the plastic. Plastic mulched beds often require less irrigation to stay moist since little moisture is lost.


Gophers and other digging pests can dig under the garden box and disturb or eat the plant roots and root vegetables if the box has no bottom and sits on soil. Lining the box’s interior with chicken wire or hardware cloth prevents these pests from getting into the box from beneath the structure. If squirrels or other animals dig inside the box from its top, then covering the box’s soil with chicken wire cut to fit around the plants can help.


Most flowering plants and vegetable plants need 1 to 2 inches of water per week from irrigation or rain. The moisture in a garden box drains out the box’s bottom or evaporates from the top of the soil quickly because the bulk of the box sits above ground. Prevent wilting and encourage healthy roots by watering the box’s soil when it begins to dry; determine whether or not the soil is beginning to dry by feeling it every two or three days. Keeping the soil evenly moist but not soggy results in healthier roots. A mulch — plastic, wood chips or straw — prevents soil from drying too quickly.


Best suggested system is a DRIP IRRIGATION SYSTEM! The following system is the one that I plan to use in my veggie garden this season:

Although retaining existing moisture helps keep soil moist, irrigation is the key to supplying a plant’s water needs. Most garden plants require 1 to 2 inches of water weekly, from either rainfall or irrigation. Generally, soil needs water when the top 1 inch dries, and it needs enough water to moisten the top 6 inches of soil. Water the plants early in the day so the moisture can penetrate the soil before it evaporates. Using drip irrigation, soaker hoses or other methods that deliver the moisture to the soil further helps water penetration. Overhead watering often results in dry soil and wet foliage, since minimal moisture penetrates to the ground.


The University of California Cooperative Extension Napa County recommends rejuvenating the soil in your raised bed every three years by removing the existing soil, mixing it with organic amendments and returning it to the bed. During the middle of the summer, solarize the soil by covering it with plastic sheeting for several weeks to complete the rejuvenation process.

What are you planting this season? What types of planting beds or boxes are you using? Please share in the comments. Thank you for sharing!

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