Soil Calculator and Raised Garden Bed Basics

For gardeners, the single most important ingredient for a thriving garden is good, quality soil. It is imperative that soil contains vitamins, nutrients, and organic matter that can feed the plants, as well as give the roots plenty of room to expand. As more Americans have limited access to large lots of land with loose, rich soil, raised beds and container gardens are becoming popular option.

Raised Beds and Containers

One obvious benefit to growing plants in pots or raised beds, the region’s natural soil types do not have to limit what types of plants can grow.

Raised beds and garden containers are also typically easier to maintain. Weeds are not as pervasive and large tools such as tillers are not generally required. They can be custom-made to fit any space and filled with fertilized soil ready for planting.

How Much Soil or Mulch Do I Need?

Most plants need soil that is loosened 12 inches to 18 inches deep. For a raised bed or pot that will be placed on a patio or other hard surface, make sure that the container is deep enough to accommodate root development.

To use a raised bed over native soil, be sure to turn the ground with a hoe or garden tiller first.

For instance, if a raised bed is 12 inches deep, the native soil will only need to be turned about 6 inches deep to give the roots plenty of room. However, if the raised bed is only 5 inches deep, the ground under it will need to be turned at least 12 inches deep.

Key Factors to Consider Prior to Calculating Soil Volume

The amount of soil necessary will depend on the size of the raised bed.

First, consider what the bed will be used for. Make sure that there is plenty of room to accommodate the number of plants in the raised bed.

Different plants require different spacing, so do some research and calculate how much room the raised bed will need.

Also, consider what depth the raised garden should be. Some root vegetables like carrots and potatoes need deep soil to mature underground

When determining how much soil to add to a raised bed, first, calculate the square footage by multiplying the lengths of the sides together. For instance, a raised bed that is 6 feet by 6 feet has a square footage of 36 feet (6 x 6 = 36). To calculate a square yard, multiply the area by 9.

Use our handy soil calculator above to determine the volume of soil needed for specific garden bed dimensions.

Example: How Much Soil Do I Need for a 3×6 Bed?
Bed Dimensions: 3×6
Bed Side Height: 10″
Blended Soil Volume: 15 cubic feet
Blend Components:
Topsoil: 9 cubic feet (9 20-quart bags)
Compost: 4.5 cubic feet (4.5 20-quart bags)
Soilless Potting Mix: 1.5 cubic feet
Organic Granular Fertilizer: 1.5 cups

What is Mulch and Why Do I Need Any?

Mulch is a covering layer over soil, and can consist of various materials. Mulch is often made of some organic material, most typically bark, either shredded or chipped. Alternative types of mulch can be grass clippings, leaves, wood chips, straws, or even cardboard.

Mulch is useful not just to give garden beds a nicer look, but more importantly, to protect the soil, keep it moist, and suppress weeds and other kinds of contamination.

Organic mulch is also helping to improve soil quality and richness as they decompose.

How Much Mulch Do I Need?

As always, it depends, but the standard is to have at least 2 inches of organic mulch to cover garden beds and the soil. The volume can be difficult to guess so please use our soil and mulch volume calculator above!

Soil Type Basics

Garden Soil Type Basics

After calculating the correct volume of soil to use for a raised bed, the next step is deciding what types of soil most garden beds need.

Generally, the ingredients for quality soil are:

  • 60 percent topsoil
  • 30 percent compost
  • 10 percent potting mix

Most Common Types of Soils


Topsoil, as its name implies, is the top layer of soil. The topsoil consists of the top five to ten inches of soil and is rich with organic matter and microorganisms. Topsoil can be purchased in bags or in bulk from nurseries or the lawn and garden department of big box retailers.

Potting Mix

Potting mix is soil that is designed to work best in pots or containers. The potting mix consists of topsoil enriched with forestry products (wood chips, bark, etc.), peat moss, and other nutrients. It holds in water and moisture while still allowing airflow. Because the potting mix is designed to hold in necessary moisture and is loose enough to allow for good root expansion, it is important to incorporate a potting mix into container gardens.


Compost is organic matter that has been decomposed into the soil, enriching it with nutrients. Compost is the best organic fertilizer and it is important to work compost into garden soil, especially in container gardens and raised beds. Find out more about composting and how worms can help create excellent compost.

Sandy Soil

Sandy soil has lots of tiny sand particles in the dirt. As water passes through the dirt, the sand dries out the water and quickly replaces it with air. Sandy soil is much lighter and looser than clay and does not hold moisture very well. Carrots, parsnips, and potatoes grow well in sandy soil because the loose particles allow them to expand nicely.

Clay Soil

Clay soil is dense and often muddy, as it retains water very well. Clay soil packs in moisture, but also packs in nutrients. Mulching plants in clay soil can help absorb heavy rains so that the soil does not get too dense. Plants with shallow root systems grow well in clay soil such as broccoli, lettuce, chard, green beans, cabbage, and sweet corn.

Peaty Soil

Peaty soil consists of partially decomposed vegetation and organic matter. It is native to wetlands and grows moss, shrubs, and sedges.

Loamy Soil

Loamy soil consists of equal parts sandy, clay, and silty soil. It is designed to merge the unique advantages of different soil types while counteracting the disadvantages. Loamy soil is considered the best type of growing soil and can accommodate almost any type of plant.

Can I Reuse Potting Soil?

YES. It is perfectly fine to reuse potting soil from previous seasons. Simply remove any old plants from the container, turn the soil by hand or with a small cultivating tool, and re-plant. If the same soil has been reused for several seasons or has developed a white crust, mix it with some fresh potting soil or add fertilizer to spruce it up.

Raised Garden Bed Basics

There are numerous advantages to building a raised garden bed. First of all, plant selection is not limited by the region’s natural soil. If a gardener wants to grow beautiful, bright orange carrots but has dense, clay soil in the yard, a raised bed filled with a good mix of loose, sandy soil will do the trick.

Second, compact spaces can be utilized to yield a much larger harvest. Every square inch can be used and the space is more manageable than traditional garden rows. Companion planting is much easier to plan and maintain, also.

When planning the raised bed, consider the manner in which specific plants grow. Watermelon and pumpkin grow along thick, sprawling vines that spread horizontally. Pole green beans and cucumbers climb vertically along stakes or trellises. Tomatoes and some beans are wide and bushy. Research how far apart seeds or seedlings should be spaced and fit as much as possible into the bed.

Choosing the Perfect Location

Most plants require a minimum of 8 hours of sunlight each day. Be sure to build raised garden beds in sunny sites where the shade will not hinder their growth. Avoid low-lying areas that stay damp and soggy. The proper soil will be well-drained, but the garden bed should still not be in a low, wet area.

Even though quality soil composed of topsoil, compost, and potting mix will not dry out as fast as regular soil, sometimes the garden will require extra watering. Consider where the outdoor faucets are located and whether or not a garden hose can reach the plants to ensure they get a good soaking every so often.

How Big Should A Raised Garden Bed Be?

Size is really a matter of preference, but there are a few factors to take into consideration. The length of the raised bed does not matter, but it is a good idea to keep the width less than four feet. It is important that the middle of the garden be accessible for harvesting and weeding.

Generally, a good starting size is 4 long by 4 feet wide. Of course, it can be as long as it needs to be, but since the lumbar is usually cut in increments of four feet, it is easier if the length is divisible by four.

Be sure that the depth of the raised bed can accommodate root expansion and plant growth. As a rule of thumb, the soil should be well-loosened and 12 to 18 inches deep. Six inches is the bare minimum, even for plants with shallow root systems.

Before building the frame for the garden bed, turn the native soil underneath and remove any weeds or grass in the area. How deep the natural soil should be turned will depend on how deep the bed is. If the bed is 12 inches deep, the ground will only need to be turned about 6 inches. If the bed is 6 inches, then the ground needs to be turned at least 12 inches.

Building a DIY Raised Garden Bed

Raised Beds DIY Steps

Anyone can build a well raised garden bed with just a few materials and a free afternoon. First, gather the right materials:

  • Wooden stakes
  • Galvanized nails or screws
  • Wooden boards (not railroad ties)
  • Level
  • Hammer or drill
  • Shovel
  • Angle-square
  • Tape measure

After a sunny location has been selected and the site has been tilled and cleared of weeds, follow these steps to construct a raised bed:

  • Rake the ground so that the wall will set flush on the ground.
  • Stake the four corners of the garden bed with one foot 4 by 4 wooden posts.
  • Attach the boards to the corner stakes to build the garden bed frame. Use a level to make sure that the boards are straight.
  • Once the boards are attached, measure the bed diagonally to make sure the bed frame is square.
  • Fill the garden bed with dirt consisting of 60 percent topsoil, 30 percent compost, and 10 percent potting soil.

While acquiring the materials for a DIY raised garden bed is usually simple and cost-effective, there are some kits available for purchase online and at most major retailers. These kits come with everything needed for assembling the garden bed frame and instructions. While the kits may cost a little more, for DIYers that are short on time, the convenience may be worth purchasing a kit.

Topsoil, compost, and potting mix can all be purchased in pre-packaged bags. Composting at home is an easy process that takes just a few months, as well. Composting bins are also available on the market that converts waste into organic fertilizer at a quicker rate.

When To Plant

Just as with a traditional garden, do not plant any seeds or seedlings until the final threat of frost has passed. The average final frost date varies from region to region, but it is generally within the months of March through May. To find the average final frost date for a specific zone, refer to the USDA zone map.

Although temperatures are a good indicator regarding planting season, also consider soil temperatures. Typically, seeds that are planted in soil that is still cool, around 50 to 55 degrees Fahrenheit, will germinate within 10 days. Soil that is already 70 degrees, Fahrenheit or warmer may germinate in less than 7 days. To determine the soil’s temperature, use a soil thermometer or even a meat thermometer.

Once the seeds have been planted, give them a good drink of water. This is why it is important to consider how far a garden hose can reach when selecting the right location for a garden bed. Some seedlings can benefit from a good, water-soluble fertilizer after planting, and then once a month thereafter.

We have discussed what to plant in the fall, bulbs or vegetables. There are some good tips as well for succession planting to increase harvest in an easy way.


Raised garden beds are an excellent alternative to traditional row gardens. They can maximize productivity by allowing more plants to thrive in compact spaces.

It is easy to maintain and weed-raised garden beds and they can be filled with only the best soil for nourishing and housing plants. Gardeners do not have to be limited by lack of space or depleted soil, simply fill a garden bed with rich soil, arrange the seeds or seedlings, and enjoy the harvest!