By Jim Chatfield
Fertilizers are key products for plant health and for garden center sales. But what are the basics about these products you should know and be able to teach your customers?

Using fertilizers properly makes your customers better gardeners and they will share this success with you, their green-industry partners. Don't just sell products -- sell success. Here are 10 things your customers should know about fertilizers.

1. Fertilizers are not plant food.

First, it's important to know what fertilizer is not. The wonderful world of plants is one in which energy from the sun, in the chlorophyll of plant leaves, combines with oxygen and carbon dioxide to produce sugars that are the basic food that energizes the plant. We are beneficiaries of this process. As we cannot make our own energy from these raw ingredients, we eat plants and animals that eat plants to get our food.

2. Fertilizers provide needed minerals.

So, what are fertilizers if they're not plant food? Fertilizers provide elements that are essential to plant growth and development. Though plants produce the sugars, fertilizers provide needed elements for the development of proteins, the structure of plant cells and many other processes.

3. The minerals in most fertilizers are NPK.

The main ingredients of most fertilizers are the three essential elements of nitrogen (N), phosphorous (P) and potassium (K). Some fertilizers contain other minerals, such as iron (Fe), sulfur (S) manganese (Mn) magnesium (Mg) and others.

4. Fertilizer numbers refer to NPK percentage.

The three numbers on most fertilizer products are the percentages of NPK. Thus a 20-5-10 fertilizer contains 20 percent nitrogen. How much of this fertilizer should be applied to a lawn area if the recommendation is for 1 pound of nitrogen per 1,000 square feet? For each 1,000 square feet (say a 40-by-25-foot area), 5 pounds of the 20-5-10 fertilizer (.20 times 5 equals 1) should be applied. If it was a 10-4-6 fertilizer with the same 1 pound of nitrogen per 1,000 square feet recommendation, then the amount would be 10 pounds of the fertilizer.

5. Knowledge is power.

There are several ways to know if and when fertilizers should be applied. First, don't guess, soil test! Soil tests can tell you the levels of minerals such as phosphorous and potassium and, if they are deficient, how much to add.

Soil tests can also tell you about soil pH. If soil pH is basic (above 7), then acid-loving plants like rhododendrons, pin oaks, pachysandras and river birches will not thrive. Sulfur and chelated iron fertilizers can help alleviate this, but you may also want to steer customers toward alkaline-tolerant plants or encourage them to buy good topsoil to replace the high-pH soil in a planting bed.

You also should know how to diagnose mineral deficiencies on plants. Abnormally pale leaf coloration, poor annual twig growth and small leaf size can signal nitrogen deficiency. Yellowing between leaf veins can be a symptom of iron deficiency that occurs on acid-loving plants growing in alkaline soil. Diagnosis is never easy and you can be fooled, but learn to read what the plants are telling you. This is a great accompaniment to reading soil test results.

6. Know where to apply fertilizers.

Though some fertilizers are formulated to be applied directly to leaves or to be injected into stems, most are for root uptake. The key for placement is to get the fertilizer to the feeder roots of the plant. In the past, some tree fertilizers were injected to a soil depth of several feet, but this was below where the feeder root system thrived and thus was ineffective. Also remember that the feeder roots of mature trees and shrubs extend at least to the drip line, so fertilizer should not be just applied near the trunk.

7. The amount of fertilizer is important.

The amount of fertilizer applied is also critical. Houseplants, for example, typically need less fertilizer in winter when light levels are lower, since growth is slower during this time. Overfertilization, though far less common then many think, does occur when too much available fertilizer is applied at once. In such cases, high levels of fertilizer salts may actually result in movement of water out of plant roots.

Schedules for fertilization have also changed over the years based on new knowledge of how plants use fertilizer. Years ago, spring turf fertilizations were much higher in nitrogen. We learned that this resulted in plenty of leaf growth (as if we needed more) and poor root establishment. Nitrogen fertilizer is still critical for quality turf in spring but the recommended amount is now lower. The key is to encourage your customers to follow the directions on the fertilizer labels of the products you sell.

8. Different fertilizers fit different situations.

Many wonder why there are so many different fertilizers. First, different plants need different combinations of minerals and different combinations are needed at different stages of plant growth. Nitrogen is arguably the most important fertilizer mineral. It plays a critical role in growth and can be lost in many ways from plants' roots due to leaching and other processes. Because of this, many fertilizers have nitrogen as their highest percentage, often in ratios two to four times higher than phosphorus and potassium. It is common for turf and ornamental fertilizers to have ratios of 3-1-2 or 4-1-2 for this reason.

However, there are times when fertilizers high in other minerals are best. Bulbs need phosphorous to develop and phosphorous does not move readily downward in the soil. So when you plant tulips, the addition of superphosphate fertilizer with just phosphorous at the bottom of the planting hole is recommended.

Readily available nitrogen pushes vegetative growth of leaves and stems on tomato plants. When the flowers and fruits are developing they need other minerals for this reproductive growth, so side dressings comparatively high in nitrogen are not recommended at that time. Instead, 1-1-1 and 1-2-1 ratios such as 10-10-10 and 5-10-5 should be used.

9. All fertilizers are not created equal.

So, is the amount of NPK all she wrote when it comes to fertilizers? No on several counts.

One variation is whether the percentage of nitrogen is in a slow-release or fast-release form. Some of the costlier fertilizers are ones that have slow-release forms of nitrogen that are more expensive to produce. This can be a big advantage if steady plant growth is needed between applications. During some greenhouse production, when fertilizers are applied in small amounts at each watering, this may not be necessary. But on landscape plants, when infrequent applications are the only practical way to fertilize, then slow-release products are quite helpful.

Another difference is between organic and inorganic fertilizers. Plants take up nitrogen the same way from both types of fertilizers, but that doesn't mean all fertilizers can be simply boiled down to their NPK amounts. This is because some organic products contain an added bonus -- needed organic matter to improve soil structure.

Fertilizers contain many variations of minerals. Some are only phosphorus, some are only potassium, some may be NPK with added micronutrients and so on. One size does not fit all.

10. Fertilizers are important ingredients in plant health management.

Whether it is for maximizing plant growth in nurseries and gardens or maintaining plant health in soils deficient in needed nutrients, fertilizers are important pieces of the plant health puzzle.

Teach your customers about how fertilizers fit with the overall practices of proper plant selection, proper planting practices, good maintenance and pest control management. And don't forget to keep up to date yourself.

Learn from your green industry peers and frequently consult trade publications, books and the Internet. Here's just one Web source with thousands of horticultural fact sheets, including hundreds on plant fertilization alone: http://plantfacts.osu.edu.

© 2001 Branch-Smith Publishing