Mulch is a layer of material applied to the surface of soil to conserve moisture, moderate soil temperature, and suppress weed growth. As home recycling has increased, gardeners are seeking ways to repurpose common household waste, including shredded paper generated from home offices and junk mail. Utilizing this readily available material as a garden mulch offers a sustainable way to manage waste while providing traditional mulching benefits. This analysis evaluates the suitability of shredded paper, examining its physical characteristics, chemical interactions, and best practices for safe application.
Physical Performance: Moisture Retention and Weed Suppression
Shredded paper is effective at blocking sunlight, which is the primary mechanism for suppressing weed seed germination. A layer of two to four inches of paper will deprive emerging weeds of the light they need to grow, reducing the competition for water and nutrients with desired plants. This physical barrier also helps to retain moisture in the underlying soil by reducing the rate of surface evaporation.
However, the shredded nature of the material presents several practical challenges. Paper is extremely light when dry, making it highly susceptible to being blown away by wind before it can be settled and wetted. If a layer of paper mulch is allowed to dry out completely, it can coalesce and form a dense, hard crust. This matted layer can become nearly impervious to water, causing subsequent irrigation or rainfall to simply run off the surface rather than penetrate the soil below. This crusting effect also limits the natural exchange of air and water vapor.
Managing the Nitrogen Trade-Off
The primary technical hurdle when using shredded paper involves its chemical composition, specifically its high Carbon-to-Nitrogen (C:N) ratio. Paper is composed mainly of cellulose, a high-carbon material, giving it a C:N ratio that can be around 170:1. For comparison, the ideal ratio for rapid microbial decomposition is approximately 30:1.
When this high-carbon material is introduced to the soil environment, soil microbes begin the decomposition process. To efficiently break down the large amount of carbon, these microbes require a source of nitrogen. They draw free nitrogen from the surrounding soil, a process known as nitrogen immobilization or “nitrogen tie-up.”
This temporary depletion of available nitrogen in the topsoil layer can result in a nutrient deficiency for nearby plants, particularly young seedlings that require significant nitrogen for initial growth. This effect is minimized when the paper is used only as a surface mulch, as most decomposition happens slowly and away from the plant roots.
Mitigation Strategies
A simple mitigation strategy is to mix the shredded paper with high-nitrogen materials, such as fresh grass clippings or compost, before application. Applying a balanced fertilizer beneath the paper layer can also ensure that sufficient nitrogen remains available for plant uptake.
Safe Application and Material Selection
Proper application is necessary to overcome the physical and chemical drawbacks of shredded paper mulch. A layer approximately two to four inches deep is effective for weed suppression and moisture retention. The paper must be thoroughly wetted immediately after application, which helps to bind the shreds together, preventing them from blowing away and reducing the risk of crust formation.
It is also important to maintain a small buffer zone, keeping the mulch several inches away from the base of plant stems. Direct contact can trap excessive moisture, which may encourage rot and create a hospitable environment for pests. Safe application requires careful selection of the paper material itself.
Material Selection
Only plain, non-glossy papers should be used, such as office paper, newsprint, and brown corrugated cardboard. Many modern inks, especially black and white, are soy- or vegetable-based and pose little environmental risk. Materials to avoid include:
- Glossy magazine pages
- Brightly colored advertisements
- Thermal receipts
- Any paper with heavy dyes or adhesives
These materials may contain potentially harmful chemicals or heavy metals that could leach into the soil.