Fertilizing Appropriately

Many trees and landscape plants require little or no fertilizer once they are established and mature. In fact, fertilizers can be hazardous to the health of your yard and the environment when they are misused.

When over-applied, fertilizers may aggravate insect and disease problems and force excessive growth which must be mowed or pruned. Excess fertilizers can run off yards into waterways or seep into aquifers, polluting drinking water.

The decision to fertilize should be based upon the health of the plant, the desired rate of growth, and a soil analysis. A soil analysis will tell you the soil pH and the amounts of nutrients in the soil that are available for plant growth. Soil pH affects the availability of nutrients in the soil. When you choose the fertilizer to use, it should have an analysis which provides the nutrients that are lacking in the soil.

How much fertilizer should be applied?

It's spring, and Bob and Linda want to fertilize their lawn. Here are the steps they take to determine how much fertilizer to buy:

  • They measure their lawn area and calculate square feet:
    Backyard: 60 X 50 feet = 3,000 square feet
    Frontyard: 50 X 40 feet = 2,000 square feet
    Total lawn area = 5,000 square feet
  • Based on their soil analysis, they determine that they need a fertilizer with low phosphorus and a moderate amount of potassium. They find a slow release fertilizer with the analysis, 16-4-8. This fertilizer contains 16% nitrogen, 4% phosphorus, and 8% potassium.
  • They use this simple formula to determine the application rate of each fertilizer:
    100 divided by the % nitrogen = the amount of fertilizer to spread over 1,000 square feet.
  • Therefore, 100 ÷ 16 = 6.25 pounds of fertilizer per 1,000 square feet. 6.25/1000 x their 5,000 square foot lawn = 31.25 pounds of fertilizer. A 40 pound bag of fertilizer will be all that they need to buy to fertilize their lawn.
  • Before spreading the fertilizer, they calibrate the fertilizer spreader to apply the correct amount. (Hint: this information is available from the spreader's manufacturer or HGIC fact sheet 1654, Applying Fertilizers.)

The Fertilizer Tag:

South Carolina law requires that fertilizer manufacturers supply a tag with every bag of fertilizer. There's a wealth of information on the tag once you understand how to interpret it. Use fertilizers in which 30% or more of the nitrogen is in a slow- or controlled-release form.

Slow-release Fertilizers

When fertilizer nutrients are in "slow release" forms, they are available to plants over a longer period of time and less nutrients are wasted or lost as pollutants. Look for these terms on the product or fertilizer tag:

  • Timed-release, slow-release or controlled-release.
  • Water insoluble nitrogen, Activated sludge, Sulfur-coated urea (SCU), IBDU, Ureaform (UF), Nitroform, or Polymer-/Plastic-/Resin-coated urea.

Fertilizer Facts:

Fertilizer is not plant food.

Food to a plant is the sugars it makes through photosynthesis. Fertilizer nutrients are used in this process, but a lawn or plant growing poorly in too much shade will not grow better if fertilized.

The truth about "100% Organic."

The "100% Organic" claim often refers only to the nitrogen in the bag. Furthermore, the nitrogen can be derived from natural products such as manure or it can be from synthetic chemicals such as urea. Read the label to determine where the "organic" nitrogen is coming from.

Buy nutrients, not fertilizer.

Many fertilizers contain a number of plant nutrients even though only one or two may sometimes be needed. Submitting a soil sample for analysis every two years will allow you to wisely choose exactly which nutrients you need. Soil analysis can be done through your county Extension office or through private labs.

 

Resources for fertilizing appropriately:

Additional Resources: