Compost improves soil in many ways. By adding loose, crumbly organic matter, it helps sandy soils retain water and clay soils drain. It loosens heavy, tightly compacted earth and creates a biologically active soil that contributes to overall plant health. However, most gardeners think of compost simply as a form of inexpensive fertilizer. Compost improves fertility, but it doesn't add nutrients such as nitrogen and phosphorus in the concentration of commercial fertilizers.
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Using compost as a fertilizer is entirely different from using a soluble chemical fertilizer. Soluble fertilizers could be thought of as "vitamins" for your garden. They're designed to be absorbed quickly by your plants, making up for any deficiencies in the soil and preparing plants for optimum production. Compost is more analogous to a long-term balanced diet. It enriches your soil steadily over years but won't provide a quick hit of nutrients such as nitrogen and phosphorus to help plants immediately (see Reference 1, pages 24 and 25).
Phosphorus is one of the three major nutrients, or macronutrients, used by plants. On a bag of commercial fertilizers, it's usually listed after nitrogen and before potassium in the standard "N-P-K" form (see Reference 1, pages 23 and 24). Ordinary home compost made from yard wastes and food scraps tends to be low in phosphorus (see Reference 1, pages 24 and 25). This isn't necessarily bad, because surplus phosphorus is a serious pollutant in wastewater. However, if your soil needs phosphorus, you can add well-composted manure to your pile or a small quantity of ground rock phosphate (see Reference 1, page 25). Either one increases phosphorus content in your compost.
Compost piles are constructed by mixing nitrogen-rich "green" feedstocks such as food scraps and grass clippings with carbon-rich "brown" feedstocks such as dead leaves, straw and sawdust (see Reference 2). A portion of the nitrogen is used in the decomposition process or is temporarily bound in compounds your plants can't use. However, once the compost is fully matured, the nitrogen is again available to your plants. Nitrogen releases slowly from compost, so it usually takes several years of composting to enrich soil. Vegetables are especially heavy feeders and might require more nitrogen than ordinary home compost can provide (see Reference 1, page 25).
Although compost improves your garden's fertility in the long term, it can have a negative impact on short-term fertility. If you blend compost that hasn't fully matured into your garden, the compost continues to decompose in your garden's soil. Just as it does in your composter, that temporarily binds up a portion of the soil's nitrogen in forms your plants can't use, costing them nutrients. Over time, as the organic matter in the compost decomposes completely, the nitrogen again becomes available to nourish plants. However, in the short term, it's important to only use well-matured compost (see Reference 1, page 25).
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