Compost and Phosphorus

Compost is Mother Nature's recycling program and, in nature, works to release nutrients contained in dead plant and animal matter and return them to the soil, where new plants can use them. Microbes found in the soil break down organic material, freeing the nutrients it contains. As one of the primary nutrients used by plants -- and therefore present in significant quantities in decaying plant matter -- compost provides one source of phosphorus.

Plants need three mineral nutrients, called primary macronutrients, in large quantities: nitrogen, phosphorus and potassium. They extract these nutrients from the soil with their roots. Phosphorus comes from the weathering of rocks and plays a role in photosynthesis, respiration and other metabolic functions, as well as helping plants to use water efficiently. Phosphorus encourages the growth of young plants and promotes the development of seeds and fruits. Because it comes from the slow erosion of rocks, phosphorus tends to be one of the more difficult nutrients for plants to obtain.

Compost contains the nutrients present in the plants used to form it and, therefore, provides small amounts of all of the primary macronutrients, including phosphorus. Because different ingredients with different levels of phosphorus can all end up as compost, the amounts of phosphorus present in compost varies. "Rodale's Ultimate Encyclopedia of Organic Gardening" indicates that commercially available compost tends to include 1 percent phosphorus. Homemade compost may contain between 0.5 and 4 percent phosphorus, depending on the ingredients used to produce the compost.

Compost itself, applied to a garden over time, makes it easier for plants to extract phosphorus from the soil. Because phosphorus tends to stick to clay particles, increasing the amount of organic matter in the soil leaves fewer clay particles to grab up the available phosphorus and allows plants to take it up instead.

Ants and earthworms in your compost also help to free up more phosphorus for your plants. These organisms break up soil particles, releasing more phosphorus into the soil. Earthworm castings -- those small mounds of dark soil you see near earthworm burrows in the garden -- free up phosphorus for plants to use.

When you consider that superphosphates -- commonly used to boost phosphorus levels in the soil -- contain between 20 and 46 percent phosphorus, the maximum 4 percent found in compost seems rather paltry. If a soil test reveals that your garden needs a phosphorus fertilizer, several organic options can supplement the compost you provide. Bone meal provides about 11 percent phosphorus, and rock phosphate provides about 3 percent. Rabbit, chicken and duck manure, added to compost, will increase phosphorus levels better than any other livestock manures.