The labels of all commercial fertilizers contain, three numbers separated by hyphens, such as 2-1-1 or 14-14-14 or 4-8-3. These areÂ “N-P-K” indicator for Nitrogen, Phosphorus and Potassium, represented by the letter K. These are the elements most required in quantity for plant growth and health.
The numbers represent the percentage of each element the fertilizer makes available to plants. For example, a fertilizer labeled 5-5-5 contains 5% available nitrogen, which helps plants create rapid growth, 5% available* phosphorus, which stimulates flowering and fruiting, and 5% available potassium, which is needed for root and stem growth. (*The term available indicates that the element is in a form that a plant can readily absorb.)
Nitrogen: Key To Healthy Plant Growth
The first number is for the amount of Nitrogen, which is the basic building block of plant proteins and a major ingredient of chlorophyll, the chemical that helps plants capture energy from the sun and makes them green.
Â Too much nitrogen fertilizer can result in weak, leggy plants with few flowers and fruit but lots of lush, tender foliage, which makes them susceptible to pests and diseases. That's why it is especially important to apply nitrogen fertilizers according to the rates on the package.
Phosphorus: The Plant Energizer
Phosphorus, the second number on the label, helps store and transfer the sun’s energy during photosynthesis so it
• stimulates early plant and root growth
• improves a plant’s ability to absorb water and other nutrients
• stimulates blooming and the development of fruits and seeds
• strengthens plants against environmental stresses.
Â Most annual plants and many vegetables require more phosphorus, particularly in the beginning of their growing season. Older, more established plants need less phosphorus. There are several good "bloom fertilizers" that are formulated with extra high levels of phosphorus.
Potassium: The Immune
The third number is for Potassium (K) which is often called the “regulator,” because it is involved with more than 60 different plant enzymes that control all aspects of plant growth. Potassium
• helps slow down plant diseases
• helps plants build the cellulose (plant fibers) needed for stalk and stem strength
• aids in photosynthesis
• increases root growth
• increases the size and quality of flowers, fruits, grains, and vegetable
• improves drought resistance by reducing water loss from leaves.
New Mexico soils are naturally high in potassium so plants grown in the ground here rarely need very much potassium. However, plants grown in containers filled with potting soil need potassium (as well as nitrogen and phosphorus), because it is not naturally present in commercial potting mixes.
Â Plants Also Need Micronutrients
Â N-P-K are by no means the only nutrients plants need for optimum growth and health. Iron, zinc, copper, magnesium, manganese and many others are needed, too, in small but essential quantities. Such nutrients are called trace minerals or micronutrients, and without them, plants can develop trace-mineral-deficiency diseases. Check fertilizer labels for the presence of micronutrients (the best brands will contain them).
Â A Word About Iron
Northern New Mexico's clay soils can be naturally high in iron, but they're also highly alkaline, with a pH greater than 8.0. At such high alkalinity levels, iron molecules are bound up in the soil and cannot be absorbed by the roots of most of the ornamental and edible plants gardeners favor. For this reason, extra supplements of iron may be necessary to keep plants from developing iron-deficiency symptoms, such as chlorosis (yellowing of the leaves).
Iron sulphate is probably the most popular iron supplement product, containing iron in a form plants can readily absorb. But be sure to follow the directions on its label exactly. Too iron sulphate can burn plant roots