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Environmental Horticulture (Gardening)

Understanding Fertilizers

Fertilizers are not plant food! Although it is common practice to call them plant foods, this is a misnomer. Plants produce their own food using water, carbon dioxide, and energy from the sun.

Plant nutrients consist of 17 elements essential to plant growth. Nitrogen, phosphorous, and potassium are considered fertilizer macronutrients because plants require them in larger quantity for maximum growth.

Fertilizer Analysis:

All fertilizers are labeled with three numbers. These three numbers give the percentage by weight of nitrogen (N), phosphate (P2O5), and potash (K2O). Nitrogen is important for leaf and stem growth and provides the rich green color in a plant. Phosphorous (derived by the plant from phosphate) provides for root and flower growth. Potassium (derived by the plant from potash) helps build plant tissue and aids the production of chlorophyll.

A fertilizer is said to be complete when it contains nitrogen, phosphorus, and potassium. Examples of commonly used fertilizers are 10-10-10, 16-16-16, 20-10-5. An incomplete fertilizer will be missing one of the major components.

Slow-release fertilizers release nutrients (make them available to the plant) over an extended period. Caution is needed when slow release fertilizers are applied around trees or shrubs, as the later nutrient release may keep the plants growing into the fall when they should be hardening off for the winter.

Cottonseed meal, blood meal, bone meal, hoof and horn meal, fish emulsion and all manures are examples of organic fertilizers. Organic fertilizers usually contain relatively low concentrations of actual nutrients, but they perform other important functions which the synthetic formulations do not. These functions include: increasing organic content of the soil, improving physical structure of the soil, and increasing bacterial and fungal activity.

Effects of Over-Fertilizing:

Fertilizers are salts, much like our familiar table salt except that they contain various plant nutrients. If tender plant roots are close to the fertilizer granules, water is drawn from these roots. Plant cells in these roots begin to dehydrate and collapse, and the plant roots are burned or dried out to a point where they cannot recover.

It is important to apply fertilizer according to instructions at the proper time and rate to prevent water quality problems. Avoid getting fertilizer on sidewalks and driveways where it can easily wash into storm drains and, eventually, into creeks, streams, and rivers. Nutrients, particularly nitrogen, become a water quality problem through leaching or run-off. Leaching is the effect of nutrients being washed through the lower soil layers and into the groundwater supply. Leaching and run-off not only rob your soil of nutrients, but also lead to erosion. Provide your soil with holding power by planting groundcovers in bare spots.

Additional information on fertilizers and fertilizing.