Many people confuse plant nutrition with plant fertilization, but they are not the same, says LSU AgCenter horticulturist Dr. Tom Koske.
"Plant nutrition refers to the needs and uses of the basic chemical elements in the plant," Koske explains. "Fertilization is the term used when these materials are supplied to the environment around the plant. But chemical change may occur before a plant nutrient supplied in a fertilizer can be taken up and used by the plant."
To achieve adequate plant nutrition, an appropriate balance of nutrients in the soil is necessary, Koske says, explaining that plants need 16 elements for normal growth. Three are found in air and water. They are carbon, hydrogen and oxygen. Six are found in soil - nitrogen, potassium, magnesium, calcium, phosphorus and sulfur. They are used in relatively large amounts by plants and are called macronutrients.
Seven other elements, known as micronutrients or trace elements, are used in much smaller amounts. These essential micronutrients are found in soil and include iron, zinc, molybdenum, boron, copper, manganese and chlorine.
"All 16 elements, both macronutrients and micronutrients, are essential for plant growth," Koske says, adding, "Other minor elements associated with some plant growth include cobalt, selenium and silicon."
Most of the nutrients a plant needs are dissolved in water and then absorbed by the roots, Koske says.
"Ninety-eight percent of these plant nutrients are absorbed from the soil solution, and only about 2 percent are actively extracted from soil by the roots," he says.
Most of the nutrient elements are absorbed as charged ions, the smallest particle of a substance that can exist and still retain the characteristics of the substance. For example, nitrogen may be absorbed as nitrate, an anion with one negative charge. The potassium ion is a cation with one positive charge.
"The balance of ions in the soil is very important," Koske says. "Just as ions of opposite charges attract each other, ions of similar charges compete for chemical interactions and reactions in the environment."
Some ions are more active than others and compete better, he explains. "For example, both calcium and magnesium are cations with two charges, but magnesium, with a smaller radius, is more active and can compete better," Koske says, adding, "If both are in competition to be absorbed, the magnesium will be absorbed."
In such cases, although a soil test may indicate sufficient calcium in the soil, the plant may still exhibit a calcium deficiency because of an excess of the more active magnesium.
"Therefore, we look for a 10- to 1-ratio in soil tests," Koske says, explaining, "What may be expressed as a deficiency in one micronutrient may actually be caused by an excess of another." Testing your soil can help you ensure macronutrient and micronutrient balances, as well as monitor and adjust soil acidity, Koske says.
Such testing is available for a minimal charge through the LSU AgCenter. Contact a county agent in your parish LSU AgCenter Extension office for instructions on gathering a soil sample and for help performing tests.