“Macronutrients,” “micronutrients,” “organic” and “inorganic” fertilizers – what does it all mean? There are lots of fertilizers on the market and very little clarifying information to help gardeners choose what’s best for their plant needs. Although the terms used to describe fertilizers may sound intimidating, they’re really not that difficult to decipher as long as you stick to the basic essentials.

Various fertilizer options

The array of fertilizer choices can be dizzying. Always read labels carefully and make sure you’re using the right type of fertilizer for your specific plants.

Photo Credit: Jessie Keith

Adding granular fertilizer into soil

It’s best to mix granular fertilizer directly into the soil before planting (if possible).

Photo Credit: Jessie Keith

Fertilizer numbers

The front of the fertilizer bag will generally tell you the product’s N-P-K ratio.

Photo Credit: Jessie Keith

Macronutrients – primary and secondary – are the most crucial to the plant-nutrient puzzle. Plants need these nutrients in large (or “macro”) quantities. The three most well-known are the “primary” macronutrients: nitrogen (N), phosphorus (P) and potassium (K). “Secondary” macronutrients are calcium, magnesium and sulfur. Each of these six nutrients plays a different role in plant growth: Nitrogen is needed in the highest quantities and encourages leaf and stem growth; phosphorus encourages blooming and rooting; potassium helps stem and fruit development; and calcium, sulfur and magnesium contribute to aspects of cellular health. Together, they promote essential growth and function, from germination to fruit-set. Macronutrient deficiencies in your plants cause poor overall health – even death.

Every plant fertilizer on the market lists primary macronutrients in “NPK” ratios – from blood meal (NPK 13-2-0) and Nitrozime® (NPK 0-4-4) to greensand (NPK 0-0-3). These are given as percentages of each nutrient per 100 pounds. So, if a package of Scotts Miracle-Gro® Tomato Plant Food lists that it has an NPK of 18-18-21, it has 18 pounds of nitrogen, 18 pounds of phosphorus and 21 pounds of potassium for every 100 pounds of fertilizer.

Different NPK ratios achieve different ends. Those higher in each nutrient contribute more or less toward foliar growth (N), rooting (P) and shoot/fruit growth (K). To encourage rooting and blooming, fertilizers high in phosphorus and potassium (NPK 1-18-18) are needed. Those highest in potassium, like tomato food (NPK 3-4-6), encourage fruiting. Nitrogen-rich plant foods, like standard lawn fertilizer (NPK 4-1-2), encourage foliar growth. All have greater or lesser amounts of NPK components to maintain overall plant health.

Micronutrients are those trace nutrients needed by plants in smaller (or “micro”) quantities. There are six key micronutrients, each needed by different plants in different amounts. The most basic of these (in order of importance) are chlorine, iron, boron, manganese, zinc and copper. All help plants in many ways: Chlorine aids with photosynthesis, iron helps energy transfer and protein development, boron increases sugar balance and movement, manganese encourages chlorophyll and enzyme production, zinc aids chlorophyll and carbohydrate production, and copper helps photosynthesis and respiration (among other functions).

Some fertilizers are organic, meaning they derive from living organisms. Others are mined or chemically-created inorganic minerals. Not all sources are the same in terms of release rates or environmental safety. Some are broken down and released into the soil quickly, while others are slow. The release rate dictates nutrient availability to plants and the rate it’s released into the environment.

Fertilizers with slow-release rates tend to be better for the environment because they don’t flush into waterways and cause pollution. These fertilizers are also better for areas where soils are highly porous and don’t hold nutrients. Fast-release fertilizers can be useful where soils are deep and high in nutrient-binding organic matter and clay. Still, they should be used away from waterways where surface runoff of fertilizer may occur.

Deficiencies, or the physical lack of needed nutrients, manifest themselves differently in different plants (see the table below). If your plants or their fruits look ill, and you suspect a nutrient deficiency, then refer to the table of deficiency symptoms and feed your plants accordingly with one of our recommended fertilizers. (If you’re not sure what ails your plant, contact Learn2Grow or your local Cooperative Extension office).

Nutrient Deficiency Symptoms and Sustainable Plant Nutrient Sources

Macronutrient Plant Deficiency Symptoms Organic or Inorganic Sustainable Sources Release Rate
Nitrogen (N) Overall pale green or yellow-green foliage. The oldest leaves are the lightest and sickliest. Organic Blood meal (13-2-0) Fast
Organic Fish meal (9-3-0) Fast
Organic Soybean meal (7-2-1) Medium
Organic Worm castings (3.2-1.1-1.5) Medium
Inorganic Sulfur coated urea (30-0-0) Slow
Potassium (P) Basal or older leaves look wilted or burnt and have interveinal chlorosis (yellowing) at the leaf bases. Organic Kelp meal (1-0-2) Medium
Organic Liquid seaweed (varies) Medium
Calcium (Ca) Leaves at growing tips look distorted. Fruits – particularly tomatoes, eggplant and peppers – have black ends (“blossom end rot” disorder). Inorganic Calcium Nitrate (15.5-0-0 + 19% Ca) Fast
Organic Bonemeal (4-21-0) Medium
Inorganic Gypsum (Calcium Sulphate) Slow
Magnesium (Mg) Basal or older leaves develop yellowish edges and have green lance shapes in their centers. Inorganic Epsom salts Fast
Inorganic Magnesium Oxide Slow
Phosphorus (K) Leaf tips look scorched. Basal or older leaves are dark green or purple-red. Organic Nitrozime Marine Algae Extract (0-4-4) Fast
Organic SuperSwell Guano (0-7-0) Fast
Organic Bonemeal (4-21-0) Medium
Inorganic Rock Phosphate (0-3-0) Slow
Sulfur (S) New leaves at the growing tips turn yellow, sometimes followed by older or basal leaves. Inorganic Epsom salts Fast
Inorganic Calcium Sulfate Slow
Micronutrient Plant Deficiency Symptoms Organic or Inorganic Sustainable Sources Release Rates
Chlorine (Cl) Leaf edges roll upward and become somewhat cupped. Young leaves develop yellowing between the veins, and plants wilt. Inorganic Potassium Chloride
(This is a very rare disorder because rainwater contains chlorine.)
Fast
Iron (Fe) New leaves at the growing tips have yellowing between the leaf veins, a disorder called “chlorosis.” Inorganic Iron DPTA Chelate Medium
Inorganic Lutz Iron Pellets Slow
Boron (B) Growing tips die and multiple stems regrow, forming “witches’ brooms.” Inorganic Solubor® Fast
Manganese (Mn) Plants are stunted and new leaves develop light yellowing between leaf veins. Inorganic Lutz Manganese Spikes Slow
Zinc (Zn) Leaf tips become dense and form odd rosettes. New leaves develop yellowing between leaf veins. Inorganic Zinc EDTA Chelate Fast
Copper (Cu) Plants are stunted and leaves turn unusually dark green. Inorganic Copper Sulfate Pentahydrate Fast

When choosing an all-purpose plant food for your garden, be sure to use one formulated for your growing needs. There are lots of general formulations for vegetables, flowers, orchids, trees and so on. Be sure to apply the recommended fertilizer doses on product packages. Overfeeding can kill plants quickly!

Lastly, gardeners should learn what their soils need before fertilizing. In addition to fertility and drainage considerations, pH is essential to know. Highly alkaline or acid soils can bind certain nutrients, making them unavailable to plants no matter how much feeding is done. In these cases, one can choose plants adapted to pH extremes or amend soil to achieve a more neutral pH. Iron sulfate helps acidify soil, and garden lime increases alkalinity. Cooperative Extension Services also help gardeners test soil for pH and other qualities for a fee. (At-home soil-testing kits are also available at garden centers.)

Now go forth and feed plants wisely with your newfound understanding of basic fertilizer essentials!