Mulch and compost — what is the difference?

There is some confusion about the terms ‘mulch’ and ‘compost’ which are often used interchangeably even though they are quite different products with different uses and potential benefits.

Mulch can be defined as material, organic or inorganic, that is used for placement on the soil surface as a protective cover. In vineyards, mulch is generally applied beneath the vines and includes materials such as cereal straw, wood chippings, cover crop slashings and plastic film. Organic materials are generally coarse in nature and commercially supplied products such as wood chippings may have been pasteurised to eliminate pathogenic organisms and weed seeds. The use of mulch offers a number of potential benefits including improved soil moisture retention, weed suppression and reduced topsoil temperature variation and erosion. Mulch is also likely to enhance the population of soil saprophytic fungi as they are the primary organisms

responsible for the breakdown of high lignin material. Nutrients are imparted to the soil, albeit very gradually, as this material is broken down.

Compost is a generic term used for a product generated from the composting process. It can be defined as organic material that has undergone controlled biological and chemical decomposition (including pasteurisation), resulting in a stable substance that is resistant to further decay. In vineyards, compost is applied either to the topsoil or incorporated into the subsoil as a conditioner, depending on the objectives of the vineyard.

The majority of composts contain nutrients and satisfactory numbers of bacteria which help to improve soil nutrient levels and biological health and result in positive vine growth responses. Well-managed ‘quality’ composts have undergone a degree of humification and therefore have the added benefit of imparting colloidal material (humus) and a large diversity of organisms, including protozoa and nematodes, to the soil. These additions to the soil help reduce the risk of pathogen growth and may also increase vine resistance to diseases.

In summary, you can use compost as mulch but you cannot use mulch as compost. This distinction might seem trivial, but it is important to understand in order to make an informed decision on the suitability of a particular product for a particular task.