Get to know your fertilizers

Part 1 of 2

Have you ever wondered what comprises fertilizer?

Perhaps you should be curious about what you sprinkle liberally on your lawn and planting beds. After all, a little knowledge about plant nutrition could go a long way towards keeping your landscape healthy.

Keep in mind that plants can become too dependent upon supplemental fertilizers. You are best served by using fertilizer only when needed, which varies by plant species. If you fertilize too often, the affected plants will always require this level of attention. You may never be able to “wean” them away from supplemental nutrition.

What nutrients are included the typical package of fertilizer? Nitrogen, phosphorus and potassium are considered fertilizer “macronutrients” because plants require them in larger quantities for maximum development.

The secondary macronutrients of calcium, magnesium and sulfur are usually present in the soil in sufficient quantities, but they may be added to some fertilizers or landscape products. Plants require other “micronutrients” in smaller amounts; they are seldom packaged in fertilizers. However, if plants are lacking in any of the 16 essential elements, they generally exhibit signs of nutrient deficiency.

Fertilizers are designed to provide missing nutrients. Most products are actually salts that contain plant nutrients, which is one reason why applying too much fertilizer can dehydrate and harm plants, especially during hot weather. Also, excess nitrogen can result in excessive, lanky plant growth and cause plants to need a lot of water.

Labels on fertilizer packages generally provide an analysis as to how much of each nutrient is present in the product. Fertilizers are labeled with three numbers that give the percentage by weight of nitrogen (N), phosphorus (P) and potassium (K). For example, a 100-pound bag of fertilizer may be labeled 10-10-10, which means there are 10 pounds of nitrogen included, etc. The remaining 70 pounds of product is simply inert “filler,” which aides in distribution.

A fertilizer is called “complete” if it contains nitrogen, phosphorus and potassium. However, the specific fertilizer ratio needed in your landscape depends on the soil nutrient level. It may also depend upon the type of plants you are feeding, such as azaleas that require acidic plant foods. Be aware that some fertilizers packaged for specific plants or purposes may not have valid scientific data behind their formulation. For instance, product labels that claim to boost blooms usually have high amounts of phosphorous, but research shows high phosphorous is no guarantee of increased blooms.

Scientists now recommend that you have your soil tested before fertilizing and that you use fertilizers that have only nitrogen (1-0-0) or nitrogen with potassium. Extensive research indicates that phosphorus has accumulated in excess in Texas soils. This poses a threat to the environment, particularly the water supply. Also, high phosphorous levels can cause other nutrients to be less available to plants.

Fertilizers are either classified as quick or slow release based on how fast the nutrients dissolve into the soil and become available for plant use.

Resources: AgriLife Extension, Doug Welsh’s Texas Garden Almanac

For horticulture answers, call the Texas AgriLife Extension, Hood County at 817-579-3280 or visit the website at