Straw mulches such as hay, lucerne, pea straw and sugar cane mulch are popular for use in vegetable gardens because they’re easy to apply, and they work well to conserve water and effectively suppress weeds.
These non-woody mulches breaks down very quickly, in around six months, adding carbon to the soil very quickly. They’re classified as labile soil carbon materials, the carbon they contain enters the soil quickly, but leaves the soil relatively quickly back to the atmosphere. They do create a wonderful humus to enrich the soil when they break down, but it’s not a very stable form of humus. This isn’t a problem as the mulch is replenished at the beginning of the warm and cool seasons in vegetable gardens.
How to Calculate the Amount of Mulch Needed to Cover a Garden Bed
When applying any mulch to a garden bed, there are two things we need to know to determine the total amount of materials required.
- The area of the garden bed
- The thickness of the mulch required
To calculate the area of a rectangular or square garden bed, we simply multiply the length by the width. If we measure the length and width in metres, our final area calculation will be in square metres.
When applying mulches, the minimum thickness is usually around 5cm (2”), with around 7.5cm (3”) being preferable. For heavy mulching, a mulch layer up to 10cm (4”) can be used, but at that thickness, a lot more mulching material is needed.
Mulches are sold by volume, either in bags by the litre, or in bulk by the cubic metre, so once we know the area we’re mulching and the thickness of mulch we want to apply, we can calculate the volume of mulch required.
- First convert all dimensions to centimetres. There are 100cm in1m, so multiply any measurements in metres by 100 to convert them to centimetres.
- Multiply length x width x height, then divide by 1000 to convert the result from cubic cm to cubic litres.
- If we need to convert the result from cubic litres to cubic metres, divide by 1000.
For example, if we had a 10m long by 1m wide garden bed, and wanted to mulch it very heavily with a 10cm thick layer of mulch:
The area of the garden bed (in square metres) is simply length (in metres) x width (in metres) = 10m x 1m = 10 square metres (10m2)
- Length = 10m x100 (to convert to cm) = 1000cm
- Width = 1m x 100 (to convert to cm) = 100cm
- Height = 10cm
Volume of mulch = length x width x height /1000 = 1000 x 100 x 10 /1000 = 1,000,000 / 1000 = 1000 cubic litres
To covert cubic litres to cubic metres, divide by 1000, so 1000L / 1000 = 1 cubic metre (1m3).
Therefore, 1 cubic metre of mulch will be required to cover 10 square metres of garden bed with a 10 cm layer of mulch.
This formula works great for uncompressed mulches, because their volume doesn’t change, but what about compressed mulches?
How Much Straw is in a Bale? How to Calculate Straw Bale Will Coverage
Straw bales are compressed, so their true volume is much greater than their compressed volume.
How do we work the mulch coverage that a straw bale will provide? We have to calculate is uncompressed volume.
The problem here is that the baling machines can be set to increase the bale compression. At maximum compression, a bale can contain one third more straw. To be able to work out the average compression ratio of various types of straw bales,
I looked at a range of labelled plastic wrapped bale products worldwide, and calculated the compression based on the dimensions of the bales and their specified coverage.
A standard bale has fixed height of 35cm (14”) x width of 45cm (18”) but can vary in length, depending on the setting of the baler machine, but is usually around 90cm (36”), 100cm (40”) or 120cm (48”).
Bales are generally compressed around 3 to 4 times their original volume*, so if we calculate the volume of a compressed bale, and multiply it by 3 or 4, we would have an approximate volume of uncompressed mulch material that it will provide.
To calculate the volume of a compressed bale:
- Measure the length, width and height in centimetres.
- Multiply length x width x height, then divide by 1000 to convert the result from cubic cm to cubic litres.
To calculate the uncompressed volume of bale:
- Multiply the compressed volume by 3 for a conservative estimate of the uncompressed bale volume, in litres.
- Convert the result from cubic litres to cubic metres, divide by 1000.
To calculate bale coverage:
Determine the mulch thickness (in centimetres) and divide by 100 to convert it to metres.
Divide the uncompressed bale volume (in cubic metres) by the mulch thickness (in metres) to give the square metre coverage.
A Practical Example of Straw Bale Coverage Calculation
Using a 35x45x90cm (14x18x36”) bale, and a mulch thickness of 5cm (2”) lets do the calculations:
Step 1 – To calculate the volume of a compressed bale, we multiply the length x width x height, then divide by 1000 to convert the result from cubic cm to cubic litres.
- 35x45x90cm / 1000 = 141,750 / 1000 = 141.75 L
Step 2 – To calculate the uncompressed volume of bale, we multiply the compressed volume by 3, then divide by 1000 to convert from cubic litres to cubic metres.
- 141.75 L x 3 / 1000 = 425.25 L/ 1000 = 0.425 cubic metres, a bit less than half a cubic metre of uncompressed straw mulch.
Step 3 – Divide the uncompressed bale volume (in cubic metres) by the mulch thickness (in metres) to give the square metre coverage.
- mulch thickness of 5cm = 5/100 = 0.05m
- uncompressed straw mulch volume = 0.425 cubic metres
- 0.425 / 0.05 = 8.5 square metres of coverage at 5cm mulch thickness
- Divide area by 0.09290304 to covert to square feet.
- 8.5 / 0.09290304 = 91.5 square feet at 2” mulch thickness
Knowing our true volume of uncompressed mulch, and deciding what mulch thickness we want, we can calculate how much coverage we’ll get across a garden bed.
Estimating the exact compression ratio of baled straw products is challenging because coarser mulches will ‘fluff up’ and decompress better than finely cut straw mulches, and the type of straw mulch will determine how high it sits when in an uncompressed state and laid out on the soil surface.
Most packaged mulches will usually specify an estimated coverage of square feet or square metres for a particular thickness of mulch in inches or centimetres.
Using this method, we can calculate approximate estimates of uncompressed volume for standard, string-bound, unwrapped bales.
*Additional Information – Calculating Straw Bale Compression
For anyone interested in how I calculated the compression ratio of a straw bale, I’ve included the information below, but this is just additional information and not really necessary:
If we look at a bale from Lowes in the US, the manufacturer specifies the coverage of 1 bale as 80 sq. ft. at 3-in depth.
The bale dimensions are 36” x 18” x 14” and the compressed volume for the bail is listed as 20 cubic ft.
Converting the imperial dimensions to metric for an international audience…
coverage area (conversion: 1 square ft = 0.09290304 square m)
- 80 sq ft = 7.4322432 sq m
mulch depth (conversion: 1” = 2.54cm)
- 3” = 7.62cm
bale dimensions (conversion: 1” = 2.54cm)
- 36” = 91.44cm
- 18” = 45.72cm
- 14” = 35.56cm
compressed bale volume (conversion: 1 cubic foot = 28.31685 litres)
- 5.25 cu ft 148.66 L
uncompressed bale volume (conversion: 1 cubic foot = 28.31685 litres)
- 20cu ft 566.34 L
The Volume Compression Ratio = uncompressed bale volume / compressed bale volume = 566.34 / 148.66 = 3.81