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Flowers, Fruits, and Frass

Local and statewide information on a variety of current topics for home gardeners and market growers.

Fertilizers 101

Fertilizers demystified

Why do I need fertilizer? When? What kind? How do Master Gardeners fertilize plants? The answer to this question is easy to answer: It all depends on the plant.

Now that you have been left even further mystified about fertilizers, learning some basic principles about plant nutrition can help you make a decision like a pro.

Plants require nutrients in varying quantities for proper growth, flowering, fruiting, disease resistance, winter hardiness and root development that are derived from the soil. Some nutrients are required in large amounts: Nitrogen, Phosphorus and Potassium and are represented by the three bold numbers on a fertilizer package. Fertilizer may also contain micronutrients like sulfur, calcium and iron needed in smaller amounts. Nutrients are available to the plant according to the pH (acidity or alkalinity) of the soil. This is the logic behind getting a soil test: to determine pH and nutrient availability. In other words, you can apply a fertilizer to a plant but if the soil pH is incorrect it will not be available and a waste of time and resources.

However most plants grown in Illinois can benefit from the addition of a fertilizers with the nutrients: Nitrogen, Phosphorus and Potassium because they are required in large amounts. A general rule of thumb for Master Gardeners is to fertilize newly planted annuals, perennials and vegetables with a liquid fertilizer treatment about two weeks after planting to give it a boost. Annuals will then need to be fertilized with a liquid feed every 2-3 weeks or with a controlled release fertilizer that is dry and encapsulated where nutrients are released slowly over a period of time. Each vegetable you grow may have different nutrition needs and then should be fertilized accordingly. Trees in Illinois seldom need fertilizers unless nutrient deficiencies are evident (wrong color or poor growth). Lawns are generally fertilized in spring and fall and when first seeded. It is important to know that lawns must not be fertilized with phosphorus unless newly planted, a soil test determines a need or you are a professional. Most landscape perennials and shrubs do not need but one fertilizer treatment a year with a liquid feed or a controlled release fertilizer. Always follow directions on the back of the fertilizer package.

Inorganic fertilizers benefit the plant because they are readily available and quick but do little for the soil. Organic fertilizers release slower improves the soil structure but can be more expensive. Some organic fertilizers that can be considered are composted manure, fish meal and emulsion, kelp meal and earthworm castings. Organic fertilizers generally have single digit bold numbers like 7-4-2 rather than the 20-20-20 of a synthetic fertilizer.

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