Reading the Fertilizer Bag

By Melinda Myers, Contributing Editor

All fertilizers contain the same basic information. Once you know what you're looking for, the task become less overwhelming.

The front of every bag contains three numbers, such as 10-10-10 or 10-15-10 or 6-3-0. These represent the percentage of nitrogen, or N (first number), phosphorous in the form of phosphate, or P (middle number), and potassium in the form of potash, or K (last number). Plants need all three elements in relatively large amounts. But you don't want to overdo it.

Breaking Down the Numbers

Too much nitrogen can result in lush plants with no flowers, brown or "burned" leaves, or even death. Years of using complete fertilizers such as 10-10-10 or 12-12-12 also results in high levels of phosphate and potash. You can't remove these elements the soil, but you can decrease the problem by adding only the nutrients your soil and plants need.

A 100-pound bag of 5-10-10 contains 5% (5 pounds) of nitrogen; 10% (10 pounds) of phosphate; and 10% (10 pounds) of potash. The other 75 pounds is filler that dilutes the fertilizer, making it easier to apply. It also reduces danger of fertilizer burn.

Many fertilizers will be labeled as fast-or slow-release, indicating how quickly the nutrients are available to the plant.

Fast-release fertilizers dissolve in water and are readily available. They're fast-acting and less expensive, but pose a greater risk of fertilizer burn and groundwater pollution if misapplied.

Slow-release fertilizers release small amounts of nutrients for plant use over time. They have a lower burn potential and require fewer applications, but usually cost more.

The release of the nutrients in slow-release fertilizer is controlled by the nutrient formulation, by microorganisms acting on the fertilizer, or by a physical coating on the granule. Weather, temperature and moisture can all affect their release.

The Right Mix

To get the results you need, you may want to adjust your application or use a combination of materials. For example, I use mostly compost and organic fertilizers in my yard. A few years ago, we had a cool spring and the nutrients were not available to the plants. I supplemented with a little quick-release fertilizer to give the plants a boost until the soils warmed and the slow-release nutrients became available to them.

Signs of Nutrient Deficiencies

This information, from the North Central Regional Extension Publication No. 356, explains deficient nutrients in soil and tells you what to look for.

• Nitrogen. Pale green or yellowish lower leaves; slow growth.
• Potassium. Older leaf edges are yellow or brown. May be chlorotic, curl or have spots.
• Phosphorus. Stunted extremely dark green leaves. Purplish to reddish veins, leaves or stems. Flowers and fruits late.
• Calcium. Deformed or failed terminal buds and root tips. Results in blossom end rot in tomatoes and peppers.
• Magnesium. Yellow in areas between veins and may show mottling of older leaves.
• Sulphur. Entire plant is light green; chlorotic younger leaves.
• Iron. Upper leaves are yellow between veins and eventually look bleached. New leaves may be yellowish white.
• Manganese. Yellow leaves between veins followed by spots that occur on middle leaves first.