Pine shavings are an inexpensive byproduct of lumber milling, often kiln-dried or aged, and are widely used in animal bedding. Many gardeners consider using these small, thin pieces of pine wood in their landscape. The use of wood-based products in the garden is debated, requiring an understanding of their biological and chemical properties and how they interact with the soil environment.
The Mechanism of Nitrogen Depletion
Pine wood products, including shavings, possess a high carbon-to-nitrogen (C:N) ratio, meaning they contain a large amount of carbon structure but very little nitrogen. This ratio (often 200:1 to 500:1) is far beyond the ideal 25:1 to 30:1 needed for efficient decomposition. Microorganisms require nitrogen to break down the high-carbon material.
When pine shavings are mixed directly into the soil, microbes begin decomposition. To acquire the necessary nitrogen for growth, these organisms draw soluble nitrogen (nitrates and ammonium) directly from the surrounding soil. This process is known as nitrogen immobilization or nitrogen tie-up.
This temporary depletion of available nitrogen can starve nearby plants, leading to yellowing leaves and stunted growth, especially in annuals. Mixing high C:N materials directly into the root zone creates the primary biological hazard for plant health. The nitrogen is not permanently lost, but it is locked up in microbial biomass until the microbes die and release it, which takes time.
Optimal Application as Surface Mulch
Nitrogen depletion is largely avoided when pine shavings are used exclusively as a surface mulch. When kept on the surface, microbial activity and nitrogen immobilization occur only in the thin layer between the mulch and the soil. This boundary layer is outside the main feeding zone of plant roots.
Pine shavings function well as a physical barrier, suppressing weed growth by blocking sunlight and moderating soil temperatures. They are also effective at retaining soil moisture, reducing the need for frequent watering during hot or dry periods. For best results, maintain the mulch layer at a depth of two to three inches, which is sufficient for weed suppression.
The most practical application is using pine shavings in permanent areas, such as pathways, between raised beds, or around established, nitrogen-frugal plants like fruit trees and shrubs. In these locations, the shavings provide a clean surface and a durable, slow-to-decompose ground cover. For annual vegetable gardens, apply a nitrogen-rich material like compost or a high-nitrogen fertilizer before laying down the shavings to act as a buffer.
Acidity and Resin Considerations
Pine shavings are often associated with making soil too acidic, though this concern is overstated. While fresh pine wood has a mildly acidic pH (typically 3.0 to 5.0), this acidity does not significantly impact the underlying soil’s pH over time. The soil has a natural buffering capacity that resists rapid changes in acidity from surface-applied materials.
Any slight acidification from decomposing pine shavings is usually confined to the top few millimeters of the soil surface. This small change is generally negligible in a healthy garden, and it can be beneficial for acid-loving plants like blueberries and rhododendrons. Regular soil testing remains the most reliable method for monitoring pH levels.
A separate consideration is the presence of resins and terpenes in pine wood, which can be phytotoxic (harmful to plants), particularly when shavings are fresh or “green.” Kiln-dried or aged pine shavings have a lower concentration of these volatile compounds. Using well-aged or processed shavings is recommended near sensitive seedlings or young vegetable plants to mitigate potential chemical interference with growth.