Back to the Wild.
Imagine a forest. Leaves fall and settle on the growing pile of leaf litter. The debris at the bottom is already decomposing and gradually becoming part of the topsoil layer. As this matter breaks down into smaller pieces, valuable nutrients start to become available to plants and microorganisms within the soil.
This is a fundamental process that occurs in almost every ecosystem around the world. The recycling of organic matter back into the soil completes the plant’s life cycle and supplements the soil with nutrients and carbon compounds. Mulching replicates this process and has significant benefits for both the soil, and the plants.
Very rarely in the ‘wild’ do you see bare soil; even in an area crammed with plants, if you look closely between their stalks, the soil is matted in a thick layer of thatched plant debris, twigs and leaves. This organic covering acts as a barrier for the soil, a kind of skin or plaster, which protects and shelters the soil from any fluctuating environmental conditions.
Similarly when mulch is added, it alters the structure of the soil and acts a physical barrier. A thick layer of mulch acts a bit like a cushion, dampening the effects of rain; without which, rain would hammer away on the surface, squashing air out and causing severe ground compaction. But, just like a sponge, mulch can also help to trap moisture in, preventing evaporation and drought-like conditions. Sticky high clay soils can become more aerated upon the addition of mulch, and the water holding capacity of sandy soils can be dramatically increased.
In a way, mulch acts as a buffer, safeguarding the soil against dramatic climatic changes.
The same applies with fluxes in temperature. Mulch adds greater thermal mass to the soil, and this means that the soil is better able to absorb and retain heat. It helps to slow the rate at which the sun heats the soil and the rate at which the soil loses heat at night. Without this thermal mass, heat from the sun would simply just re-radiate back out, cooking the soil in the sun and becoming overly cold without it – not ideal for your plants or all of the wonderful organisms that live there!
If you take a closer look at the surface of a forest floor, you’ll soon start to notice hundreds of tiny bugs crawling all over the leaves and twigs. Likely these are springtails, small invertebrates commonly found on leaf litter. However, there are thousands of other different species that live in this matter, each helping to speed up the process of decomposition. Microbes shred up this organic matter and the larger surface area of all these little pieces promotes bacterial growth. As the mulch slowly breaks down, it encourages the soil particles to cling together – this is known as granulation. As the microbes feast on the organic material, they secrete a sticky substance that binds the soil together, creating lots of air pockets and aggregates. Mulch encourages these microorganisms by creating a cushy, hospitable environment; without the biology, soil would be lifeless.
Okay. So now it’s time to get into the extra sciencey stuff!
Mulch affects the soil on a chemical level. Adding it to the soil increases the soil organic matter or SOM for short. SOM consists of an assortment of components ranging from labile compounds that mineralize rapidly to more stable humic substances which tend to be more resistant to decomposition.
However, it’s the weak acid tendency of the stable humic compounds that gives soil its unique chemical properties. Humic acid is able to store cations (positively charged molecules or nutrients such as sodium) and protect them from leaching out of the soil. But it’s also able to easily release them for plants to use; this is known as the Cation Exchange Capacity (CEC). The higher the CEC the more nutrients are potentially bioavailable for exchange.
The capacity for organic matter to retain cations is due to the negative charges created as hydrogen (H) is removed from weak acids during neutralization. Some of these acids are produced as a result of organic matter decomposition by microorganisms, secretion by roots, or oxidation of inorganic substances (Bot & Benites, 2005). Hydrogen ions are released and exchanged with the cation nutrients, which are then able to be taken up by the surrounding plant root hairs.
Mimicking ecosystems by adding mulch to the soil seems to produce, over time, the same beautiful, crumbly and friable soil akin to that of a mature woodland soil. Not only does it bring about a plethora of benefits (well documented in scientific journals), it can also drastically increase the carbon content of the soil, helping to sequester it from the atmosphere.
Addressing the health of soil, through regenerating soil biodynamics and increasing SOM, you can significantly increase productivity whilst maintaining a sustainable carrying capacity – a perfect solution!
The importance of soilorganic matter. Key to drought-resistant soil and sustained food production. Food and Agriculture Organization of the United Nations, 2005. ISBN 92-5-105366-9/ ISSN 0253-2050.
By Alexandra Bot (FAO Consultant) and José Benites (FAO Land and Plant Nutrition Management Service)