Compost is decomposed organic matter used to enrich soil, providing nutrients and beneficial microorganisms to the garden. This guide will walk you through the practical steps, from selecting the right material to calculating the necessary volume, ensuring a successful purchase for your project.
Understanding Different Compost Types
The source materials used in composting directly influence the final product’s nutrient content and texture, making the selection specific to the project. Yard waste or municipal compost is often the most cost-effective option, typically made from grass clippings and leaves. This type is generally lower in nutrients but excellent for improving soil structure, especially in heavy clay soils, due to its high organic matter content.
Manure-based compost, derived from animal waste, offers a higher concentration of nitrogen and other nutrients, reducing the reliance on supplemental fertilizers. It has a heavier texture and should be used with caution, as it carries a higher risk of containing residual persistent herbicides like aminopyralid or clopyralid. These chemicals can pass through an animal’s digestive system and remain active in the finished compost, potentially damaging sensitive broadleaf crops like tomatoes or beans.
Worm castings, also known as vermicompost, are the most nutrient-dense option, produced by earthworms feeding on organic material. Vermicompost is typically used as a concentrated soil amendment or a top-dressing rather than for bulk application across large areas. Mushroom compost, the spent substrate from mushroom farming, offers a good balance of nutrients and organic matter, often containing gypsum and straw. Fresh mushroom compost usually has a near-neutral pH (averaging around 6.6), making it a good soil conditioner, especially for balancing out overly acidic soils.
Identifying High Quality Compost
The most straightforward way to determine quality is to perform a physical inspection, regardless of whether the compost is bagged or sold in bulk. High-quality, mature compost should have a dark brown or black color and a uniform, crumbly texture, resembling rich soil. The material should pass the “smell test,” exhibiting a pleasant, earthy aroma, indicating a complete and aerobic decomposition process.
A sour, rancid, or ammonia-like smell suggests the compost is either too wet or has an excessive nitrogen content and is still actively decomposing, meaning it is immature and can harm plants. Visually, a finished product should contain no large, recognizable pieces of the original feedstock, such as wood chips or recognizable leaves, which is another sign of incomplete processing.
To mitigate the risk of contamination from persistent herbicides, especially when buying manure or hay-based compost, always ask the supplier about their feedstock sourcing and testing procedures. These herbicides are odorless and colorless, surviving the high temperatures of the composting process and remaining active for months or even years. Some commercial suppliers voluntarily test their product and may participate in programs like the US Composting Council’s Seal of Testing Assurance (STA), which indicates the product has undergone laboratory analysis for various quality parameters.
Bagged Versus Bulk and Volume Calculation
Deciding whether to purchase bagged or bulk compost depends on the size of your project, cost, and logistical considerations. Bagged compost, typically sold in one cubic foot bags, is convenient, easily transported, and suitable for small-scale projects like potting or amending a few garden beds. However, the unit cost is generally much higher compared to bulk material.
Bulk compost is sold by the cubic yard and is the most cost-effective choice for large gardens, raised beds, or extensive landscaping projects. Purchasing in bulk requires arranging delivery or having a truck or trailer capable of hauling a large volume of loose material. A single cubic yard is a substantial amount, covering 324 square feet at a depth of one inch.
To calculate the volume needed, first determine the square footage of the area you plan to cover by multiplying the length by the width. Next, select the desired application depth; a standard recommendation is to incorporate one to two inches of compost into a garden bed’s top six to eight inches of soil. For a rough estimate of the cubic yards required, multiply the square footage by the depth in inches, and then divide that total by 324.