Mulching and its impact on the environment

The term “mulch” is derived from the German word “molsh”, which means soft. It reminds the soft layer found in forest ecosystems, although not all mulches are soft.

Mulches are defined as materials that are applied to the soil surface. On the contrary, materials that are incorporated into the soil, in order to improve its texture or fertility, are called amendments.

Therefore, any material laid or grown over the soil surface can be considered a mulch. If bare soil were to be exposed to heat, wind, and compacting forces, it would lose water through evaporation. Consequently, it would be less able to absorb rainfall or irrigation as it becomes increasingly compressed.

Mulches can increase soil water by increasing percolation and retention, and reducing weeds, which can decrease soil moisture by 25% in a summer day. In other words, mulching may reduce evapotranspiration.

There is a wide variety of mulching materials, which influence water movement. Some permeable materials do not limit soil water infiltration and do not inhibit water movement between the soil and above-ground environment. For instance, black plastic generally may influence surface permeability, limiting recharge. In fact, they will create soils that are unnaturally dry over the long term.

In particular, organic and inorganic mulches are better conservers than synthetic. Mulches with demonstrated ability to retain water include a vast array of plant materials, such as grass clipping, leaves, slowly decomposing timber residues including sawdust, and barks. In other words, these materials can reduce needs for supplemental irrigation, and can also protect trees and shrubs from environmental stresses, such as cold injury. As it is known, Council Regulation (EC) n.° 834/2007, setting out the principles about organic production, gives greater emphasis on environmental protection. And the main aim of the Regulation is to ensure and develop an overall system of farm management that combines best environmental practices and a high level of biodiversity.  Rules on organic farming policy of the EU are strictly related to mulching practices, considering that organic  production respects natural systems, and this means that internal resources are preferred to open cycles based on external resources.   

Furthermore, a layer of straw mulch may reduce soil erosion, consequently it is important to leave fallen vegetation on forest sites. Of course, mulches cannot e used to “stabilize” a part of the side of a hill and its difference in level. Slope stabilization requires an engineering solution, not a horticultural one. Mulches are also useful in urban landscape, where rainfall will compact unprotected soils.

It is better to apply mulch, such as bark and jute, before compaction occurs rather than after the fact. In this perspective, mulches applied after compaction could not reverse bulk density changes even after two years. In 2002 some experts demonstrated that common urban contaminants, such as lead and cadmium, can be removed from the soil solution by mulched leaves of eucalyptus, pine, poplar, and arborvitae.

In addition, mulches protect soils from extreme temperature, which will kill fine roots, which can be killed by freezing. If extreme temperature were to kill pine roots, the plant would spend energy to generate new pine roots. On the contrary, hot soil can ill new transplants that have not had time to generate a large root mass. Of course, soils can be kept cooler in hot conditions and warmer in cold conditions.

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