Fertilizer Basics

Fertilizer is any material that once added to the soil hastens the growth of plants. Fertilizers are plant nutrients.

Fertilizer is a term commonly used to refer to manufactured products sold in bags and boxes or in liquid form. That is most often the case but not always; natural organic materials such as compost or animal manure contain plant nutrients. It is always the case that fertilizers contain nutrients.

Fertilizers fall into one of two categories: synthetic fertilizers and natural or organic fertilizers.

Natural and organic fertilizers are derived from rock or living things such as animals and plants; they added to garden soil in a more or less natural form.
Synthetic fertilizers are chemical fertilizers synthesized from coal, natural, gas, or petroleum products or derived from minerals treated with chemicals to make them more soluble.

Synthetic fertilizers may be dry—in granular, powder, or pellet form—or liquid. Synthetic fertilizers are fast-acting and often concentrated. They are potent, but they also last in the soil for just a few weeks. If applied too heavily, the concentration of nutrients in synthetic fertilizers can harm and even kill plants. Synthetic fertilizers are often blended, meaning they may contain a multitude of nutrients—both essential and trace nutrients.

Natural or organic fertilizers also come in dry or liquid form. The nutrients in natural fertilizers are less concentrated than those in synthetic fertilizers. Natural fertilizers can be blended like synthetic fertilizers but sometimes a natural fertilizer will contain just one nutrient, for example, bone meal, made of animal bones, contains almost wholly phosphorus. Natural fertilizers are slower acting than synthetic fertilizers; they break down in the soil like other organic materials over an extended period of time. Natural fertilizers release nutrients over months, not weeks; it is difficult to overfertilize with natural fertilizers.

Table of Contents

Nutrients in fertilizers

Macronutrients

Most fertilizers whether synthetic or natural contain the three basic plant nutrients, nitrogen, phosphorus, and potassium in varying amounts. Here’s what those nutrients do:

Nitrogen aids green, leafy growth.
Phosphorus aids root development, flowering, seed formation, and fruiting.
Potassium aids general growth and disease resistance.

These three nutrients are listed on a fertilizer label in the order above by their chemical abbreviation, NPK (N=nitrogen, P=phosphorus, K=potassium). They are listed by the ratio of the nutrient to the whole; for example, a package that contains 10-10-10 fertilizer has 10 percent nitrogen, 10 percent phosphorus, and 10 percent potassium in the package; the remaining 70 percent of the material in the package is an inert carrying agent to help spread the nutrients.

Nitrogen, phosphorus, and potassium are called macronutrients; these are all drawn from the soil in a mineral form mixed with soil moisture. Other macronutrients plants need are carbon, oxygen, and hydrogen; these are found in air and water.

Secondary and micro-nutrients

Plants need additional nutrients beyond the macronutrients—these are called secondary and micronutrients.

Secondary nutrients are Calcium (Ca) which is important for plant cell formation and growth, especially roots, Magnesium (Mg) which helps form the core of chlorophyll molecules in the cells of greens leaves, and Sulfur (S) which acts with nitrogen to manufacture protoplasm for plant cells. Secondary nutrients are usually readily available in most soils, unlike the macronutrients. However, many fertilizer manufacturers add secondary nutrients to fertilizer blends. You will see them listed on the package.

Micronutrients are nutrients that plants need in small amounts. Micronutrients are also called trace elements. Micronutrients include Zinc (Zn) and Manganese (Mn) which function in plants as catalysts for the utilization of other nutrients, Iron (Fe) which is essential for chlorophyll formation, and Boron (B). Trace elements are often found in blended fertilizers and they also can be purchased alone.

Bagged and boxed organic and synthetic fertilizers

Fertilizer terminology

Buying fertilizer can be daunting at times; there are many manufacturers, many forms of delivery, and many nutrient blends. Here are some terms to know:

Balanced: This is a fertilizer with equal or nearly equal proportions of the major nutrients—nitrogen (N), phosphorus (P), and potassium (K). Balanced fertilizers are also called “all-purpose” fertilizers. 10-10-10 is a balanced fertilizer, so is 5-10-5.
Complete: A complete fertilizer contains all three of the primary plant nutrients, N, P, K.
Dry: These are powder, granule, or pellet fertilizers; they can be spread on the ground and scratched or dug into the soil. They can also be spikes or tablets; use a mallet to pound a spike into the soil; dig holes for tablets.
Foliar: This is a liquid fertilizer that is sprayed on the plant; the nutrients are absorbed through the leaves. Be sure to follow directions on the label.
Granular: This is a pelleted fertilizer; the pellets can be as small as a grain of sand. Granular fertilizers can be applied with a measuring cup or a spreader.
Inorganic (chemical): This is a fertilizer made from synthetic substances with precisely formulated amounts of specific nutrients; these can be fast or slow release.
Liquid: This can be a liquid concentrate fertilizer or a water-soluble powder, crystals, or granules. They can be applied with a spray bottle or hose-end applicator. Follow directions on the label. Liquid fertilizer spread in the soil immediately as opposed to granular fertilizers which must dissolve in water. Fish emulsion is an organic liquid fertilizer.
Organic (Natural): These are fertilizers made from the remains or by-products of living or once-living organisms. Manure, fish emulsion, bonemeal, cottonseed meal, and kelp meal are all organic fertilizers. They can be used in combination to produce a complete fertilizer. Most organic fertilizers are slow-release they also usually have lower proportions of nutrients than inorganic fertilizers.
Simple: This is a fertilizer that contains just one macronutrient; ammonium sulfate (21-0-0) is a simple fertilizer; superphosphate (0-20-0) is a simple fertilizer.
Slow-release: This is a fertilizer that releases its nutrients slowly over a few months; it can be organic such as bonemeal or blood meal or synthetic.
Soluble: This is a fertilizer that dissolved in water; it can also mean a fertilizer that is activated when it comes in contact with water.
Specialty Fertilizers: These are fertilizers that are formulated for specific plants or specific needs. African violet or rose fertilizers are specially formulated. Bulb food is a specialty fertilizer; it is high in phosphorus which is needed by bulbs to root.
Spike or Tablets: These are methods of delivering fertilizer to plants; spikes can be pressed or driven in the soil; tablets can be worked into the soil. Slow-release fertilizers are often delivered via spikes or tablets.
Time-Release: This is a fertilizer that is formulated to release nutrients over a period of time, not all at once. Time-release fertilizers can match a plant’s growing cycle.