Compost contains a variety of the basic nutrients that plants require for healthy growth, including micronutrients such as iron, manganese, copper, and zinc. They are only needed in small doses, like vitamins in our diet, but without them plants have difficulty extracting nutrients from other foods. Micronutrients are often absent from commercial fertilizers, so compost is an essential dietary supplement in any soil.
Compost also contains small amounts of the macronutrients that plants need in larger doses, including nitrogen, phosphorous, and potassium (the three numbers listed on fertilizer bags), and calcium and magnesium. Although compost generally contains small amounts of macronutrients, they are typically present in forms that may not be immediately available to plants.
Due to the variable content and slow release of major nutrients in compost, it is often considered a supplement to fertilizers that contain more readily available nutrient sources. However, when applied in 3- to 5-inch layers over a period of years, compost may provide significant amounts of these nutrients and additional fertilizers may not be necessary.
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