Fertilizers
By: Mike Bush
20-20-20, 16-4-8, 10-10-10 --- These numbers are listed on every bag of fertilizer. Do you know what they mean?
Let's start with the first number, which represents nitrogen, one of the most important nutrients for a plant's good health. Nitrogen is responsible for the production of chlorophyll. Without it, plants can't produce their own food. If they don't have enough nitrogen, their foliage will take on an overall yellow tint.
The second number indicates phosphorus. Another significant nutrient, phosphorus creates abundant roots, flowers and fruits. A plant with a phosphorus deficiency will turn a slightly purplish color.
The last number stands for potassium, a nutrient that determines the quality of a plant’s flowers and fruits and how well it withstands heat, cold, insect infestation and disease. Without potassium, the edges of a plant’s leaves will begin to turn brown.
The elements should be applied in the following order: nitrogen, phosphorus and potassium. These three elements, along with calcium, sulfur and magnesium, are known as macronutrients. Plants with little calcium develop young leaves with hooked ends. If they're lacking in magnesium, the leaves turn yellow with green veins. These macronutrients are needed in greater quantities than micronutrients or trace elements: iron, copper, manganese, boron, zinc, molybdenum and chlorine.
How do you know which nutrients your plants need? Nothing can beat a soil test for accuracy. You can pick up a kit at your local Co-Operative Extension Service. It will explain how to properly dig the sample and how to package it for mailing. In four to six weeks, you will receive a very accurate analysis of your soil.
Here's what you can expect to see: Many soil tests will recommend liming. That's because most Piedmont clay soil has a pH between 5.0 and 6.0, while a pH of about 6.5 is more ideal for general growing conditions. Fertilizers work best at this higher pH level. The soil test may also recommend adding nitrogen to your soil, because it's used quickly and almost always in short supply. Other nutrient deficiencies will be listed, along with recommendations for correcting them.
When it comes time to buy a fertilizer or soil amendment, there are many possible choices. Fertilizers like cottonseed meal and Osmocote break down slowly and provide a continuous source of nutrients. They don't need to be applied as often, and there's less risk of burning tender plant roots. Other fertilizers act more quickly, breaking down within weeks. These have to be applied more frequently. The following are just a few types of fertilizers to consider:
Ammonium Sulfate - rapidly adds nitrogen
Blood Meal - slowly adds nitrogen
Bone Meal - slowly adds phosphorus and calcium
Calcium Nitrate - rapidly adds nitrogen
Compost - slowly adds nitrogen, phosphorus and potassium
Cottonseed Meal - slowly adds nitrogen, phosphorus and potassium
Dolomitic Limestone - slowly raises pH, adds calcium and magnesium
Epsom Salt - rapidly adds magnesium and sulfur with no effect on pH
Greensand - slowly adds potassium and trace elements
Gypsum - slowly adds calcium and sulfur with no effect on pH
Osmocote - slowly adds nitrogen, phosphorus and potassium, may contain trace elements
Potassium Sulfate - rapidly adds potassium
Urea - rapidly adds nitrogen
Ureaformaldehyde - slowly adds nitrogen, phosphorus and potassium, may contain trace elements
Check the label of any fertilizer you buy. It will most likely contain a combination of some of the above ingredients. Chlorine may also be used and is harmful to plants in large quantities. It is found mainly in inexpensive fertilizers. Look for less than two percent chlorine.
Finally, keep in mind that the best time to fertilize your woody trees, shrubs and vines is in early spring before new growth begins. That would be mid-February through March here in the Piedmont. You may fertilize throughout the growing season, but be sure to stop in late summer, so your plants have a chance to slow down and enter dormancy before the weather turns cold.
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