Twenty Facts about Mulches - Kevin Handreck SA

[This article is taken from the Garden Design study Group Newsletter No. 58, May 2007, and was previously published in ‘Gardening Australia’, page 74, November 2005]

In nature, virtually all soils have a mulch on their surface. The soils of forests have litter and leaf mould; those of grasslands have a layer of decaying grass and mosses; many desert soils have a stony surface; the sand of sandy deserts is an excellent mulch.

In our gardens we use mulch as a substitute for these vital natural soil covers.

1. Look at the organic litter on a forest soil. It grades from very fine highly decomposed humus at the soil surface to the coarseness of recently fallen leaves and twigs on top. In our gardens the ideal mulch will be like that.
   
   
2. The most important property of a garden mulch is that it should reduce the rate of water evaporation from the soil below.
   
   
3. A mulch is like a blanket on the soil. The best mulches allow rain or irrigation water to move into the soil below, but they minimise the loss of water by evaporation. They reduce evaporation partly by providing a break between soil water and air. Water is not then simply ‘sucked’ out of the soil by the sun and wind. It has to pass as a vapour through the still layer of air within the mulch and this is much slower than direct evaporation.
   
   
4. Many of the organic mulches available in retail packs are too fine to be of top effectiveness. A thick layer (eg 50 mm) of fine mulch will hold most of the light rain that falls on it; only with heavy rain will the water actually reach the soil. There is no break between water and air so water wicks up through fine mulches. The rate of water loss from fine mulches can in fact be higher than that from bare soil.
   
   
5. In good mulches most of the particles will be larger than about 5mm. Only a small proportion will be smaller than 2 mm. The high the proportion of ‘fines’, the less effective will be the mulch in reducing water loss.
   
   
6. Loss of rain or sprinkler water from fine mulches can therefore be faster than that of bare soil. But if most of the water is applied below them (by drippers or soaker), they will reduce evaporation rate.
   
   
7. Fine mulches are excellent seed beds for weed seeds that are blown into the garden.
   
   
8. The large particles of coarse mulches will gradually decompose, so that after several years of additions, the earlier additions will have decomposed to be like fine humus of the bottom layer forest litter. The later additions will be doing the work of reducing evaporation. To speed up this process on a new bed, you could first apply a thin (eg 10 mm) layer of fine mulch and then a thicker layer (eg 40 mm) of coarse mulch that has few fines.
   
   
9. In summary the best benefit is obtained from water by applying irrigation beneath a coarse organic mulch.
   
   
10. Organic mulches are living mulches. They are gradually decomposed by small soil animals, fungi and bacteria. In the process nutrients are released for use by the plants.
    
    
11. But in addition plants actually help themselves to the nutrients in the mulch. Most plant roots have beneficial fungi (Mycorrhiza) growing on them. These fungi send out hyphae into the lower layers of leaf mould where they secrete acids and enzymes that dissolve nutrients such as phosphorous and take them back to the plant.
    
    
12. Proteaceous plants do the same thing by producing clusters of fine roots in the humus layer. If you repeatedly remove leaf litter from a garden bed, you are robbing you plants. Poorer health and growth are inevitable.
    
    
13. Another benefit of mulches is that they protect the soil from the pounding of rain and irrigation water. On sloping ground soil erosion is minimised.
    
    
14. Mulches also shade the soil below. In summer the lower temperature under an organic mulch allows roots to continue to grow into the topsoil.
    
    
15. But in winter mulched soil will tend to be cooler than bare soil and plant growth may be slightly reduced and the effects of frost severe.
    
    
16. Any problems? There can be, but they are minor compared with the benefits.
    
    
17. Uncomposted ‘waste’ material may contain weed seeds, may be temporarily toxic to plant roots and can reduce oxygen supply to plants for some months. Composting kills weed seeds and eliminates toxicity. Some extra nitrogenous fertilizer should be applied to woody mulches.
    
    
18. If you find that repeated heavy mulching produces water repellence in you soil, overcome this in the short term with a wetting agent; reduce applications.
    
    
19. Organic mulches are the best, but what about non-organic mulches (stones, crushed rock etc). These can give interesting decorative effects, but they are difficult to maintain in good condition. Fallen leaves have to be removed, so the benefits of the leaves to the plants is lost.
    
    
20. Plastic sheeting must not be used a s mulch. Unless it has holes punched into it, neither water nor oxygen can move into the soil below. Plants will be harmed. If you do want to use palstic, use woven products such as weed mat, whose holes allow water and oxygen to pass.

Kevin Handreck is the author of the book "Gardening Down-Under" (CSIRO).

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