Why Mulch and Which Mulch
Mulch is a very important factor in gardening in Australia. It helps to reduce evaporation which is essential in times of drought. It also reduces the number of weeds in your garden which is always a bonus. The main purposes of mulching are:
- to adjust soil temperature by helping soil retain more heat in spring and autumn, and by keeping soil cool and evening out temperature swings during hot and variable summer conditions
- to control weeds by blocking the sunlight necessary for germination
- to retain water by slowing evaporation
- to add organic matter and nutrients to the soil through the gradual breakdown of the mulch material
- to repel insects
- to incrementally improve growing conditions by reflecting sunlight upwards to the plants, and by providing a clean, dry surface for ground-lying fruit such as squash and melons
- for erosion control - protects soil from rain and preserves moisture
- for sediment control - slows runoff velocity
I guess I never really put that much thought into mulch but obviously some people have. Yates actually did some research on the differences between chunky and fine mulch. The findings are reported here but I’ve quoted the most important points below.
The main objected of the tests was to determine which kind of mulch tended to soak up water rather than allowing it to travel through to the soil and plant roots.
Chunky mulch rated the best for reducing evaporation rate, and for allowing water to travel through. The blend of Mostly Chunky also rated well, but had a tendency to soak up twice the amount of water as the Chunky blend.
Mostly Fine mulch did not perform well. It did the poorest job of reducing evaporation and, worst of all, it actually robbed the soil of moisture. This is because water moves into fine mulch by capillary action, so that the water loss can be higher than from bare soil.
The fine mulches also soaked up over four times more water than chunky mulches. This means that only water from heavy rain or irrigation will actually reach the soil under a thick layer of fine mulch.
So what does all this mean? Well, here are some recommendations for home gardeners using mulch:
If using mulch with fine particles that are 2mm or smaller, mix it with chunkier mulch with a particle size greater than 5mm.
Don’t apply a thick layer of fine mulch, as it will hold a lot of water that plant roots won’t ever benefit from.
The chunkier the mulch, the thicker it can be applied. Chunky mulches can be used up to 600mm in depth without concern for blocking rain and irrigation.
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