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The physics of mulches: how they work

We all know that mulches benefit plant growth. The beneficial effects are more obvious in dry and hot climates. Mulches can sometimes provide nutrients, but most of the benefit is due to the physical processes at the ground surface. These can be summarised under the following three headings.

Water infiltration

A coarse open mulch will allow rapid intake of rainfall and irrigation and reduce runoff. This results in a higher effective rainfall. The effect is greater on degraded soils that are compacted or crusted, on sloping sites and in climates with intense stormy rainfall. A dry compacted soil on a slope in Sydney may absorb only 200 mm of the annual 1000 mm of rainfall. It is effectively a desert soil! By laying a mulch you can increase this to 700 mm or more.

To be effective at trapping rainfall, a mulch should be coarse (less than 30% below 16 mm). Fine mulches can actually intercept light rain and simply evaporate it again, keeping the soil dry.

Water evaporation

A surface mulch holds little water itself and dries out rapidly after rainfall. Instead, it traps an insulating layer of still air, which greatly decreases evaporation from wind and heat at the soil surface. This is a very profound effect in dry climates and ensures that the water is retained in the surface soil where roots can draw on it, and is not lost from the soil surface by evaporation. A deep turf or grass layer has the same effect as a mulch, reducing wind velocity at the ground surface and thus reducing direct evaporation.

Aside: Farmers “mulch” soil to store water by ploughing the surface so that it becomes dry. This dry surface layer acts just like a mulch in preventing evaporation. By doing this they can store water over the off-season so that, by planting time, enough water is stored in the soil to grow the crop even if it doesn’t rain again after planting. However, this important dryland farming technique carries the risk of leaving a bare ploughed soil prone to erosion if heavy rain falls.

Temperature damping

The surface of a bare soil on a hot summer’s day can exceed 50 °C. This heat travels slowly down the soil profile in a “wave” over the next few days. A layer of warm soil will drive off water, where it will condense in a cooler layer such as the atmosphere. This results in rapid drying of the soil surface by the formation of dew at night.

A mulch will greatly reduce the surface soil temperature by acting as an insulator. The four essential properties of an insulator are that it:

• traps a layer of still air
• has a low specific heat capacity (i.e. takes little heat to raise its temperature)
• is deep enough to be effective
• is reflective (i.e. light in colour).

A blond medium-coarse wood chip at least 50 mm thick is a very effective thermal mulch.

Aside: Farmers can manipulate soil temperatures with mulches. To keep soils cooler through summer for summer crops, they mulch in later winter. To keep soils warmer into winter, they mulch in late summer. To warm soils up, for example for early strawberry crops, they can lay a black plastic sheet, which absorbs heat.

Chemical benefits

The physical benefits of mulch alone will aid plant growth, but will also have indirect benefits on soil chemistry. Where the soil surface is moister and has a steadier temperature, roots grow better, taking up more nutrients and improving plant nutrition.

In addition, some mulches have a nutritional effect in their own right. Lucerne, for example, has up to 4% nitrogen, 0.5% phosphorus and 3% potassium. This is higher than animal manure, making lucerne an excellent mulch for both its physical and nutritional benefits in vegetable, rose and bedding gardens.

Suitable mulches

Commonly available, good-quality mulches for urban areas include:

• straw, hay and sugarcane bagasse
• lucerne for high-value garden areas
• pine bark
• wood mulches, e.g. pine flakes, eucalypt chips
• composted and screened green waste.

Whether you have a small garden, a commercial forest or a local government area to manage, you can make your own very valuable mulch by collecting all plant trash, especially sticks and branches, and using a wood chipper – either a small one hired locally or your own commercial-scale one. Compost the chips with water and some added urea to cause the pile to heat up (which kills weed seeds and disease organisms) before use.

Aside: Farmers can use crop stubble and other residues as mulch. In orchards and vineyards the inter-row space can be used to grow green manures that are then mown and thrown over the row areas as a mulch in summer.

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