Growing Medium for the Raised Bed and Grow Box

What Is a Soilless Growing Medium?

A soilless growing medium is any material used for planting that does not contain field soil or dirt. It’s what you’ll use to fill your raised bed or grow box. Understanding how growing media work helps you choose the best option for healthy, productive plants.

Why Growing Media Matters.

A good growing medium performs two essential functions:

• Anchorage: It supports the plant by holding the roots firmly in place.
• Root Health: It supplies roots with water, nutrients, and air, all of which are critical for growth.

What Makes a Good Growing Medium for Raised Beds or Grow Boxes?

The material you choose should:

1. Allow roots to grow freely between particles.
2. Provide pore spaces for air and water movement.
3. Hold enough moisture and nutrients to keep plants healthy.
4. Drain excess water easily to prevent waterlogging.
5. Be free from weeds, pests, and diseases.

Recommended Soilless Materials

(Choose one of the following as your base material)

• Pro-Mix or other commercial potting soil
• Sawdust
• Bagasse
• Rice hulls (husks)
• Coconut coir

Additional Materials Needed

(All of these are required for your growing medium mixture)

• Sharp sand - Improves drainage due to its larger particle size.

• Pre-plant Fertilizers: Supplies essential nutrients before planting.
• Limestone: Helps balance pH depending on the material used.

How to Calculate How Much Growing Medium You Need

  To determine how much material is required, calculate the volume of your raised bed or grow box.

Example

Raised bed size: 3 ft (90 cm) wide × 10 ft (300 cm) long × 8 in (20 cm) high

Using Feet

VOLUME   = length   X  width    X   height

                    = 10 feet  X  3 feet   X  8 inches ( 0.66 feet) (i.e. the height of the concrete block)

                    =  19.8 cubic feet

Using Centimeters

VOLUME   = length   X   width    X   height

                    = 300cm  X   90cm     X   20cm (i.e. the height of the concrete block)

                     =  540000 cm³  or  0.54 m³

Next steps

1. Calculate the volume of your wheelbarrow.
2. Divide the raised bed volume by the wheelbarrow volume.
   This tells you how many barrows of material you’ll need.

Ratio For Mixing a Good Growing Medium

A standard mixture uses a 3:1 ratio:

For every 3 barrows of Soilless Material

Add 1 barrow of sharp sand
Total = 4 barrows of growing medium

How to Mix the Growing Medium 

1. Measure all materials before mixing.

2. Spread the soilless material on a clean concrete surface near the raised bed.

3. Spread the sharp sand evenly over it.

4. Sprinkle limestone and pre‑plant fertilizers evenly across the mixture.

5. Mix thoroughly using a garden shovel until uniform.

Note: Limestone and fertilizers are added in small, measured amounts using a scale.

Limestone and pre-plant fertilizers are required. They are added in small quantities and are measured using a measuring scale.

All of the materials are thoroughly mixed using a garden shovel.

Filling the Raised Bed or Grow Box

1. Transfer the mixed growing medium into the raised bed using a wheelbarrow or shovel. 

  

2. Spread the mixture evenly to avoid air pockets. 

3. Fill the holes (cores) of the concrete blocks — these can also be planted. 

4. Level the surface with a piece of wood.

* Do not step on the medium to compress it.

5. Water thoroughly to help the mixture settle.

         - Add more medium if the level drops.

        - You’ll know it’s fully watered when you see water trickling out from the base of the blocks.

Your Raised Bed or Grow Box Is Ready for Planting
Once the growing medium is mixed, leveled, and watered, you can begin transplanting your seedlings.

Last Updated: December 20, 2025