The Matter of Mulching

Many enthusiastic gardeners and farmers use mulch but still have little understanding of the functions of it, how to apply it, and what to avoid. Firstly, what is mulch? The mulch mentioned hence forth will be of the organic variety, or only from a previously living organism. Some sources mention inorganic mulches, but these don’t provide all the pros of organic mulch and result in more cons such as increased costs and toxic leachates and pollution. Mulch can thus be best described as a protective layer of organic material to cover the soil.

The concept of mulching comes from observing nature. Simply walk into a forest and one can clearly see that nature prefers carpeted floors. The same can be said for all other natural biomes but unfortunately humans have removed many of the means for mulching to occur naturally. The examples are numerous, from removing large herds of herbivores from the environment or changing their behaviour, to bad applications of burning and poor grazing practices of livestock.

Mulch is used as a regulatory “blanket” for the soil. It keeps the soil cooler in summer and warmer in winter, which is very beneficial to both plants and soil biota. Correct applications of mulch will result in large quantities of water being saved, with figures ranging from 50 to 90 percent. This is a result of a wide range of factors. Firstly, mulch shades the soil from sunlight and insulates it from heat and wind. This prevents moisture from evaporating easily and therefore more moisture remains in the soil underneath the mulch.

As the surface temperature is cooler than un-mulched soil, the radiation of heat from the soil is less and so heat stress on the plants is minimised. The plants thus require less water and photosynthesize more efficiently. This is one of the reasons why transplants and seedlings do so much better in mulched areas. Another factor which is very seldom noted, is that when mulch decomposes, the detritivores release water as a by-product, reducing additional water use further. Mulching also protects the soil from cold weather. Root crops and bulbs can be protected from frost damage by having a layer of mulch to insulate them.

A common mistake in the irrigating of mulched areas, is that one doesn’t water for long enough at a time. It is true that mulch reduces water use, but the amount of water needed to wet the mulch and infiltrate deep enough into the soil is greater than when irrigating un-mulched soil. One would then water once a week for several hours at a time, rather than for a few hours multiple times a week.

A good way to change a lawn into garden beds and successfully kill the grass, is to place a thick layer of newspaper, then compost, wood chips and manure. One can now plant directly into this mix, but it is important to make a hole through the newspaper under any new plants.

The mulch itself is a protective barrier for the soil. It prevents mud splashing up onto plants and erosion from occurring. It is also beneficial when harvesting as it provides a clean surface for produce. Many don’t realize that it is easy to “thatch” the mulch, so that water runs off away from the plants. Mulch such as hay or straw, should be placed along the contour as to stop water and let it infiltrate. Hence it is often recommended to chip or shred mulch into irregular shapes and sizes to prevent “thatching” from occurring.

Mulch has the benefit of providing nutrients to the soil biota. It provides a habitat and food for many beneficial invertebrates such as insects and earthworms. Microbes also break down mulch into compounds which can be exchanged with plants for sugars. A positive sign of a good mulch is that there will be a weave of fungal hyphae and plant feeder roots interacting between the soil and the mulch.

Different mulches will have varied effects on the soil when they break down. Pine needles for example, will make the soil more acidic. This can be beneficial or detrimental depending on the original soil pH and what the species of plants to be cultivated in the future are. Grass mulch can particularly add silica and potassium. High carbon mulches such as wood chips often take longer to break down and can result in nitrogen deficiencies while in the process of being broken down. A way to combat this phenomenon is to add high nitrogen sources to the mulch such as chicken litter.

Mulch should be as thick as possible without becoming anaerobic and becoming a compost pile. This way it will serve its purpose best for as long as possible. The type of mulch used, and the environmental conditions will affect the thickness required and the speed of the decomposition of the mulch. Soil should never be visible, and reapplication of mulch is necessary when original layers have noticeably thinned. Recommendations as to the thickness of mulch generally range from 10 to 15 centimetres as the most effective.

Care should be taken to keep mulch, especially green mulch, from getting too thick, and from pressing up against the stems of living plants. This is because it can form a site for infections, rot and disease to affect the plants. Inversely, mulch is often useful in reducing disease. Studies show that having rots or infections from plants introduced to the mulch, results in the mulch being a repository for the microbes that attack the disease. It is therefore possible to say that having diseased material in one’s mulch is beneficial, as it will act as an inoculation against future disease.

Hay and straw make an excellent mulch. Notice the absence of weeds.

Another crucial reason for mulching is to allow the soil organisms to add carbon to the soil. Organic matter in the form of humic compounds is crucial for soil life. Humic compounds have a high cation exchange capacity, facilitating the transfer of nutrients. Another advantage of adding carbon to the soil is that for every 1% of organic carbon, the water holding capacity increases by approximately 57 litres per cubic metre of soil. However, most of the organic carbon in the deeper parts of the soil profile, come from root exudates also known as the liquid carbon pathway.

Adding organic matter as a mulch, facilitates the formation of good soil structure close to the surface of the soil. Having a diverse soil food web facilitates the formation of aggregates and reduces compaction. This in turn allows proper oxygenation of the soil and infiltration of moisture. The upside of being able to place carbon in the soil is that it becomes an easy way to sequestrate carbon from the atmosphere. This means one could combat climate change by simply mulching!

Lastly, but not least, mulching reduces competition from weeds. This occurs as a result of the seeds being out of the reach of sunlight and other factors that aid germination and growth. This results in saving one from the time and effort required for weeding. However, placing large amounts of weeds that have reached maturity as mulch, can result in an even bigger weed problem.

A good way of using mulch is to wait for the weeds to germinate and then smother them with a thick mulch. This is very effective in reducing weed numbers as it kills the weeds before they produce seed. It also provides a very nutritious layer under the mulch for decomposition via the soil organisms. Mulch is thus very good at keeping areas under trees weed or grass free. This means that one does not damage the tree or equipment when mowing, as it isn’t necessary.

It is now easy to see why so many people use mulching as an integral part of their farming or gardening toolkit. Mulching has become a recommended practice in orchards, vineyards and parks for its many benefits to ease management. It has long been practiced in regenerative farming and gardening circles for the nutritional and soil altering properties. As long as one follows the basic rules of mulching, the results will be unimaginable compared to how easy the practice is. Happy mulching!

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