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A gardener should avoid fertilizing seedlings as if they were full-grown plants. Seedlings are young plants and need to be treated with care. Too much fertilizer early on can actually do more harm than good. If the seedling must be fertilized, the fertilizer should be diluted and only a small amount applied. Ideally, the fertilizer will contain nitrogen, phosphorous and potassium.
Once the plants have the first set of true leaves, it is okay to begin fertilizing seedlings. True leaves are the leaves the plant will grow as it matures, rather than the cotyledons, which are the first two leaves to appear on the seedling. Cotyledons are actually part of the seed and are what feed the plant initially. If a gardener applies fertilizer when only the cotyledons are visible, she risks burning the plant and its roots or otherwise damaging it.
Initially, a diluted fertilizer should be used to fertilize seedlings. Ideally, a 25 percent solution of fertilizer should be applied. A gardener can produce the 25 percent solution by mixing one part fertilizer with three parts water.
Before fertilizing seedlings, the gardener should water the plants. A gentle spray should be used so that the roots are not disturbed. Once the soil is damp, she should give each seedling no more than a tablespoon (15 ml) of fertilizer.
When a gardener first begins fertilizing seedlings, she should not use the fertilizer too often. It should not be applied everyday. She may want to try fertilizing the plants every week or two. If the plants begin to grow tall and lanky-looking, that suggests she is using too much fertilizer and should cut back. Short, dwarf plants may require more fertilizer.
As the seedlings continue to grow and develop into mature plants, a gardener can begin to increase the concentration of fertilizer. Two weeks after she begins fertilizing seedlings, she can try increasing the concentration to 33 percent, then to 50 percent after another two weeks. She may need to reduce the concentration if the plants seem over-fertilized, though.
Plant and seedling fertilizer typically contains the three primary macro-nutrients needed, nitrogen, phosphorus and potassium. Most fertilizers are labeled NPK for each respective nutrient along with a number next to each letter. For example, N10, P5, K5 means the fertilizer contains twice as much nitrogen as phosphorus and potassium. Plants need nitrogen to grow lush foliage, phosphorus for healthy roots and potassium to resist disease and retain moisture.