Growing Medium for the Grow Box

Soilless Growing Medium? What is it? If you are confused by this term, it simply means NOT dirt or field soil. It is the material that you will fill the newly constructed grow box with. Understanding the functions of growing media is important since it will help you decide which is best suited for your grow box garden.

Growing medium has several functions:-

1) It supports the plant by giving the plant roots anchorage.
2) It supplies the roots with water, nutrients, and air.

What material is best suited for the Grow Box?

 The material that you choose should:
1) allow the plant roots to grow freely in between the particles of the material.
2) allow air and water to travel through the pore spaces.
3) be able to hold enough water and nutrients to keep your plant healthy.
4) allow excess water to drain away easily and prevent waterlogging. If excess water does not drain away and fills up the pore spaces in the growing medium, then plant roots cannot get fresh air and they will die.
5) be free from weeds, pests, and diseases.

Here are some soilless materials that are well-suited for your grow box

(Only one of these will be required for your growing medium mixture)

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

Other materials that you will require:

(All of these will be required for your growing medium mixture)

• Sharp sand - The sharp sand, because of its larger particle size, is added to allow for excess water to drain away.

• Pre-plant Fertilizers (before planting).
• Limestone 

How much material will be required?

 Depending on the size of the grow box you have constructed you will need to determine how much material you will require for filling the grow box.
This is easily done by calculating the volume of the grow box.

e.g. If the size of your grow box is 3 feet (90 cm)wide by 10 feet (300 cm) long then

Calculating the volume of the grow box

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

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

NOW

 Use the same method to calculate the volume of your wheelbarrow

THEN
Divide the volume of the grow box by the volume of your wheelbarrow. This will tell you how many barrows of material you will require to fill the grow box.

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RATIO OF MATERIAL FOR THE GROWING MEDIUM

A 'good' growing medium is usually a 3:1 Ratio and should be made up of the following ratios of materials.

For every 3 barrows of Soilless Material (eg. pro mix) use:

 

 

3 Barrows Promix  +  1 Barrow Sharp Sand = 4 Barrows of material

How to mix the growing medium for the grow box

METHOD

• Measure ALL the materials you will require for the mixture.
• Spread the soilless material on a clean concrete area, as close to the grow box as possible. 
• Spread the sharp sand on top of the soilless material.
• Sprinkle the other materials as evenly as possible over the soilless material and sharp sand.
• Mix the materials thoroughly using a garden shovel.

Limestone and pre-plant fertilizers are some of the materials (nutrients) required for the growing medium. They are required in small quantities and are therefore measured using a measuring scale.

The Limestone and pre-plant fertilizers are sprinkled evenly over the promix and sharp sand before mixing.

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

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Filling the grow box

After thoroughly mixing the growing medium materials, they can now be filled into the grow box.  This can be done using the wheelbarrow or if the mixing was done close to the grow box, the mixture can be shovelled in.

 

Spread the growing medium evenly throughout the grow box during the process of filling to ensure that there are no air pockets.

Fill in the holes (cores) in the concrete blocks as you will also be planting inside them. Level the growing medium using a piece of wood. Do not stand on the material to compress it. 

Wet the growing medium thoroughly to allow it to settle. After wetting, you may need to top up the grow box with a bit more growing medium. You will know that the growing medium is adequately watered when you see water trickling through the spaces in the base of the concrete blocks.

Your grow box is now ready for planting seedlings.