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GARDENING : Fertilizer: Health Food for Plants


Before you shake granules around your garden or mix in manure, it's important to understand what fertilizers can do.

They are not miracle workers capable of resurrecting half-dead plants. But they can provide already healthy plants with nutrients that enable them to reach their full potential.

There is a wide variety of fertilizers on the market, and although the selection appears confusing, rest assured that most of them contain three main ingredients: nitrogen, phosphorus and potassium.

"Nitrogen is essential to a plant's growth. The plant uses it to create protein and can't get larger without it," said Gary Matsuoka of Laguna Hills Nursery.

"Another important nutrient is phosphorus, which is used in the formation of chromosomes and the transfer of energy," he said. "And potassium serves a lot of functions and contributes to the plant's overall health."

"Our main need in Southern California is for nitrogen fertilizers," said Forrest Fullmer, an agronomist for the Soil and Plant Laboratory in Orange. "This nutrient is water-soluble and leaches out of the soil easily, so it must be reapplied on a regular basis."

To spot nitrogen deficiency, look for a plant that won't grow or grows slowly, Matsuoka said.

"Nitrogen-poor plants are usually pale green or yellow," he said. "The foliage and stems may also be stunted, and the plant is likely to dispose of old leaves quickly as it moves what little nitrogen it has to new leaves for growth."

Although nitrogen deficiency is fairly common, Matsuoka said phosphorus and potassium deficiencies are rare in Southern California's clay soil, which holds tight to these nutrients. But sometimes reapplication is necessary.

Phosphorus deficiencies don't show, although plant growth may be poor. The same holds true with potassium deficiencies, although at times lack of this nutrient can cause a mottled look around the edge of the leaves or burning of older leaves.

Fertilizers often also contain small amounts of minor nutrients needed by plants such as iron, zinc, calcium, sulfur, manganese, copper and magnesium.

These trace minerals are usually not lacking in heavy clay soil, either.

"If you have snails or pill bugs, then your soil probably doesn't lack calcium," Matsuoka said, adding that their bodies contain calcium. "There is also usually plenty of sulfur in the soil."

If you do have a trace mineral deficiency, it may alter the look of your plant but probably won't affect its productivity.

"Iron deficiency can cause chlorotic markings on the foliage, but that's about the extent of the damage," Matsuoka said.

Determining what your plants lack is not easy, said Chris Amrhein, assistant professor of soil chemistry at UC Riverside.

"There are many reasons why plants yellow," he said. "Yellowing could be due to nitrogen deficiency or even a lack of zinc. It's often difficult to determine a problem just by looking at a plant, because plants all react differently to deficiencies and toxicities."

For this reason, if you have a problem with your plants, you may want to consider soil testing or plant analysis.

"It's nice to get your soil tested to find out what is lacking," Matsuoka said. "The test costs from $20 to $50, depending on if you bring the soil in to a lab or they come to you."

Soil testing is best for shallow-rooted plants such as grass, ground cover and vegetable gardens, said Fullmer, adding that testing for trees is often a waste of money.

"If we get a call from someone who wants a soil test on an established tree that is doing poorly, we will advise against it because a tree that has been growing for 15 or 20 years wouldn't have grown if there was a deficiency," he says. "More than likely the tree has a disease or other problem."

In addition to soil testing there is plant analysis, which may be even more helpful at pinpointing deficiencies, Amrhein said.

A soil and plant lab technician will "take a portion of your plant and do a tissue analysis, which tells you if the plant is deficient in something," he said.

"For the home gardener, I wouldn't suggest this as a first resort, though. Instead, take a portion of your sick plant to a nursery. Nursery people are likely to know the symptoms of deficiencies in certain plants for your area."

When fertilizing, stick to a high-nitrogen fertilizer regimen, unless you know that you have a deficiency in another nutrient.

"If you need to add phosphorus and/or potassium to your soil, choose a complete fertilizer with numbers such as 16-6-8," Fullmer said. "This refers to 16% nitrogen, 6% phosphate and 8% potassium by weight of the product. The fertilizer will also contain some minor elements that you may need."

After you've used this fertilizer once or twice, go back to a complete nitrogen program; otherwise the potassium and phosphorus will build up.

Nitrogen comes in many forms.

"In a chemical fertilizer, it can be found as ammonium sulfate, ammonium nitrate, urea and calcium nitrate," Fullmer said.

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