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How to Fertilize Your Garden for Healthy Plant Growth

Actively growing plants need a steady supply of nutrients.

Remember the old adage, "You are what you eat?" Well that applies to your plants, too. Healthy plants need a healthy diet and a good fertilizer is a key component.

Actively growing plants need a steady supply of nutrients—many nutrients are present in soil, water, and air, but gardeners need to provide others. The most likely plants to need feeding are vegetables, flowers, lawns, container plants, and fruit trees, but not all need to be fed the same. Finding the right product packed with the right nutrients will help your plants grow big and strong this season.

Read on to learn more about what nutrients plants need, how to best provide them, and how to tell when your plants need your help.

What Are Plant Nutrients?

The first step in successful fertilization is understanding the nutrients plants need and in what amounts. Plant nutrients fall into three categories: macronutrients, secondary nutrients, and micronutrients.

Macronutrients

These elements—nitrogen, potassium, and phosphorus—are needed by plants in large amounts.

  • Nitrogen (N) helps synthesize proteins, chlorophyll, and enzymes. Nitrogen is the nutrient most likely to be inadequate in garden soils. Too much can make plants too leafy (often at the expense of flowers and fruit) and prone to attack by sucking insects. Applied too late, it promotes new growth that's vulnerable to frost damage.
  • Phosphorus (P) promotes flowering and fruiting, strong root growth, and the transfer of energy within the plant. Phosphorus deficiency is rare, and an overdose can interfere with a plant's absorption of other essential elements.
  • Potassium (K) helps regulate the synthesis of proteins and starches that make sturdy plants. It also helps increase resistance to diseases, heat, and cold. Too much potassium interferes with the absorption of calcium and magnesium, making plants grow poorly.

Secondary Nutrients

Plants need these in about the same amounts as they do the macronutrients. But they're less likely to be deficient in most soils.

  • Calcium (Ca) plays a fundamental role in cell formation and growth, and most roots require some calcium right at the growing tips.
  • Magnesium (Mg) forms the core of the chlorophyll molecules in the cells of green leaves.
  • Sulfur (S) acts with nitrogen in the manufacture of protoplasm for plant cells.

Micronutrients

Also known as trace elements, micronutrients are required in very small quantities (excess amounts can be toxic). Among them are zinc (Zn) and manganese (Mn)—both thought to function as catalysts for utilizing other nutrients— and iron (Fe), essential for chlorophyll formation. Some plants, particularly vegetables such as Swiss chard, need boron (B), an element often lacking in alkaline soils.

Understanding the Types of Fertilizers

The right fertilizer can make all the difference in the health of your plants. Below are the common types of fertilizer available at most nurseries and garden centers. While you can use the info below to help you choose the best fertilizer for your needs, you should also talk with a knowledgeable nursery employee about what type of plants and soil you have and what soil amendments you're already using so they can steer you in the right direction.

  • Simple vs. Complete: Complete fertilizers contain nitrogen (N), phosphorus (P), and potassium (K). Some may also include secondary and/or micronutrients. Simple fertilizers supply just one macronutrient. Most familiar are the nitrogen-only types—ammonium sulfate (21-0-0)—and phosphorus-only superphosphate (0-20-0). Incomplete fertilizers fall between simple and complete; an example is 0-10-10, providing phosphorus and potassium but no nitrogen.
  • General Purpose: Fertilizers labeled "general purpose" or "all purpose" contain equal or nearly equal amounts of the macronutrients N, P, and K (a 10-10-10 formula, for example). They are intended to meet most plants' needs throughout the growing season.
  • Special Purpose: These formulas are designed to meet specific needs. High-nitrogen blends (such as 29-3-4), for instance, help keep lawns green and growing quickly. Higher-phosphorus mixes (6-10-4, for example) are intended to promote flowering and fruiting. Other packaged fertilizers are formulated for particular types of plants. Those designed for acid lovers, such as camellias and rhododendrons, are especially useful, as are fertilizers for citrus.
  • Inorganic (Chemical): Fertilizers made from synthetic substances with precisely formulated amounts of specific nutrients, primarily nitrogen, phosphorus, and potassium are considered inorganic or chemical fertilizers. These can be formulated for fast or slow release.
  • Organic (Natural): Organic fertilizers are made from the remains or by-products of living or once-living organisms. Manure, fish emulsion, bone meal, cottonseed meal, and kelp meal are all examples of organic fertilizers that can be used alone or in various combinations to produce complete fertilizers. Most release their nutrients more slowly than inorganics (but some, like blood meal, release quickly). Organics also usually have slightly lower proportions of nutrients than most inorganic fertilizers. They make up for this by adding organic matter to the soil.

Signs of Nutrient Deficiency

Your local nursery can often recommend a foliar spray to solve the immediate problem and a fertilizer supplement for long-term care.

Common Symptom Deficiency
Leaves are yellow and smaller than normal. On some plants they may turn red or purple. Overall growth is stunted or dwarfed Nitrogen
Small leaves, with edges scorched, purplish, or blue-green in color. May fall early. Overall growth is reduced and weakened. Flower and fruit production diminished. Rare. Phosphorus
Leaf tips and edges become yellow and scorched looking, with brownish purple spotting underneath. Potassium
Leaves turn dark from the base outward and die. Tomatoes and related vegetables may rot on the blossom end. Calcium
Leaves yellow between the veins, which remain green or slightly yellow. Growth is weak. Usually occurs in alkaline soils. Iron
Leaf centers turn reddish or yellow. Dead spots appear between veins. Usually occurs in very acid soils. Magnesium
Upper leaves yellow in center, between veins, with no sign of red. New fronds on palms curl and yellow. Usually occurs in alkaline soils. Manganese
Veins grow lighter in color than the leaf tissue in between. Usually occurs in soils with low organic matter Sulfur

How to Read Fertilizer Info Labels

Every fertilizer label states the percentage by weight of the three macronutrients used in mineral form, in alphabetical order: nitrogen (N), phosphorus (P), and potassium (K). So a fertilizer labeled 10-8-6 contains 10 percent nitrogen, 8 percent phosphorus, and 6 percent potassium. The label also tells you the source of each nutrient and lists any micronutrients present.

  • Nitrogen may be included in a fertilizer in nitrate form. If so, it will be water soluble and fast acting, especially in cool soils, but is easily leached (requiring frequent replenishment) and can pollute surface and groundwater if used to excess. Nitrogen in the form of ammonium is from organic sources (such as blood meal) and IBDU (isobutylidene diurea, a synthetic organic fertilizer). These are released more slowly and last longer in the soil.
  • Phosphorus is expressed on product labels as phosphate, P2 05 and listed as "available phosphoric acid."
  • Potassium is expressed as potash, K2 0, and may be described in various ways, including "water-soluble potash." Phosphorus and potassium do not move readily through the soil in solution and must be applied near plant roots to do the most good.

How to Apply Fertilizer

Spread dry fertilizers (powders, granules, or pellets) on the ground or dig them into the soil. Controlled-release types may be beadlike granules, spikes, or tablets. Dig granules into soil at planting time or scratch them into the soil surface. Use a mallet to pound spikes into the ground; dig holes for tablets.

Liquid fertilizers are sold as crystals, granules, or liquid concentrates that you mix with water and apply with a hose or spray bottle. The spray types, known as "foliar feeds," deliver instant supplies of specific nutrients. To avoid burning leaves, follow label directions.

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