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Organic gardeners work to build nutrient-rich soil by continually adding organic matter (particularly compost) to the soil rather than feeding plants directly. Mixing well-rotted farm animal manures (especially cow, horse and chicken manure) into the soil provides an abundant supply of nitrogen to your plants. Nitrogen is the key nutrient plants use for growing leaves. If you can add an inch or so of compost to your garden each year, you probably don't need any additional fertilizers.
You'll see a lot of different packaged organic fertilizers in garden centers and mail-order catalogs. Many derive their nitrogen boost from fish or feather meal in both cases, the ingredients typically are food processing waste. The best of these fertilizers are "slow-release" foods that nourish plants incrementally as they decompose in the soil. Though fertilizer made from municipal sludge (sometimes called "biosolids") is often sold as "organic," we discourage gardeners from using it because research has found that this sludge contains heavy metals that can build up in your soil.
To give your plants a more immediate boost or for feeding those growing in soil that is more difficult to amend (like plants in containers), you can use liquid fertilizers that are commonly made from fish processing waste combined with mineral-rich seaweed (often kelp). You'll find these liquid fertilizers as concentrates that you mix with water and apply directly to plants' leaves.
