Setting the Record Straight: Primary vs. Secondary Plant Nutrients

Essential nutrients, primary nutrients, secondary nutrients, micronutrients, macronutrients, trace elements, etc; it’s easy to see how new growers may be confused about what’s what. Learning the terminology will help “decode” what is being discussed and help you make an informed decision about what nutrients your plants need.

Essential Plant Nutrients

Any nutrient that is needed for plant growth or functioning is an “essential nutrient”. There are 17 recognized essential nutrients for plants. Some charts and articles will refer to only 16 essential nutrients. The element nickel (Ni) is necessary for plant health, but in such a minute quantity, that there is some disagreement among botanists and horticultural professionals as to whether it should be on the list or not, or just be considered a “trace element”. To further muddy the waters, some include cobalt (Co) as an essential nutrient, bringing the total number of essential nutrients by some accounts to be 18.

Macro Nutrients

A macro nutrient (sometimes expressed as macro-nutrient or macronutrient) is any element needed by plants in high quantities. There are nine macro nutrients. Three of them come from the atmosphere and are not generally deficient: Carbon (C), Hydrogen (H) and Oxygen (O) are supplied to plants by air and water. The other six are further subdivided into primary and secondary nutrients depending again on the volume necessary for proper plant health and function.

The primary nutrients are: nitrogen (N), phosphorus (P) and potassium (K). These are the “big three” of fertilizers. On any bag or container of fertilizer, there are usually three (sometimes more) numbers listed. These represent the percentage of each respective element in the order N-P-K. Nitrogen is the element that plants need more than any other and it is highly mobile. Knowing what the level of nitrogen in your soil is now, will not help you later in the season. Most professionals therefore suggest a default rate of nitrogen of one pound (.45 kg) of nitrogen per 1,000 square feet (93 square meters) of growing area in the absence of specific analysis suggesting more or less.

The secondary nutrients are: calcium (Ca), magnesium (Mg) and sulfur (S). These are not needed in quantities as large as the primary nutrients, but are needed in higher quantities than any of the micro nutrients.

Micro Nutrients

Micro nutrients (micro-nutrients or micronutrients) are the other seven to nine (depending on who you ask), nutrients that are essential to proper plant growth, development and function. The undisputed ones include: boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), and zinc (Z). Nickel and cobalt are two other possible micro nutrients.

“Trace elements” can be micronutrients, or any other nutrient not found in a large or sufficient quantity. This is a sampling measure and does not mean that the plant does not need the elements in higher quantities. It may also refer to elements that are present in small amounts but that shouldn’t be there at all such as with heavy metals.

Roles of essential nutrients

All of the 17 plus essential nutrients serve one or many roles. Many of them assist plants in proper usage of some of the other nutrients. Some plant functions require multiple nutrients. Some of the complex interconnections between these elements and the roles they play in the matrix of plant functions are still unknown. The following list is an oversimplification, but highlights the main purposes of each of the essential elements. Each likely does a whole lot more to help keep our plants healthy:

Hydrogen with C and O, this provides the building blocks for plants to produce carbohydrates.

Carbon along with H and O, carbon helps provide the structure of the cellular walls, which in turn strengthen leaves, stems and roots.

Oxygen with H and C, provides a source of energy for the plant to perform all of its other functions.

Nitrogen supports vigorous leaf, stem and trunk growth and development.

Phosphorus supports root development, along with flower and seed development.

Potassium supports overall health of the plant, giving it the ability to adapt to environmental stressors.

Calcium strengthens cell walls and new shoots and roots.

Magnesium part of chlorophyll and a metabolizer of other nutrients.

Sulfur helps with dark green color in the production of chlorophyll.

Boron helps with cell development and to regulate a plant’s metabolism.

Chlorine helps with photosynthesis.

Copper helps plants better use nitrogen.

Iron assists in the production of chlorophyll and aids with other plant processes.

Manganese aids in the production of chlorophyll.

Molybdenum assists plants to make use of nitrogen.

Zinc aids in the development of enzymes and hormones.

Nickel helps with iron metabolism in the plant.

Cobalt possibly helps with ethylene production, nitrogen fixation and plant metabolism.