Just like animals, plants need a number of nutrients to survive in thrive. So far, researchers have identified sixteen nutrients that are essential for plant life (some sources have identified seventeen, but I'm sticking with the more conservative basics). These nutrients are broadly divided into two groups: nine macronutrients and seven micronutrients. Macronutrients are needed in much greater quantities than micronutrients, which are often needed in minuscule amounts (although they are still necessary).
The macronutrients include:
- Carbon (C)
- Hydrogen (H)
- Oxygen (O)
- Nitrogen (N)
- Phosphorus (P)
- Potassium (K)
- Calcium (Ca)
- Magnesium (Mg)
- Sulfur (S)
The first three—carbon, hydrogen, and oxygen—are known as structural elements and are present in the atmosphere and growing environment.
The next three are sometimes called the "fertilizer elements" since they are the familiar N-P-K identified on fertilizer labels. The NPK rating of a fertilizer identifies how much of the fertilizer by volume is comprised of these three elements. For example, a 10-10-10 balanced fertilizer would contain 10% each by volume of nitrogen, phosphorous, and potassium. Although each of these fertilizer elements has multiple roles, in general nitrogen encourages strong leaf growth, phosphorus encourages flowering and budding, and potassium encourages stronger root growth.
The next two macronutrients, calcium, and magnesium are crucial for many cellular functions in plants, as well as fruit and flower formation. Be aware they might not always be present in your fertilizer mix or soil (although dolomite lime is a good way to deliver both). If you're growing houseplants, make sure your fertilizer is a complete one, with both calcium and magnesium present.
The final macronutrient, sulfur, is often not included on the lists of "critical" major elements because it's so common. In most fertilizers, the other nutrients are delivered in the form of sulfate salts, which automatically includes sulfur. As a result, sulfur deficiency is extremely uncommon, and there is some debate over whether or not many plants even have upper tolerances for sulfur (within reasonable limits, obviously).
So...what's the final word? If you're growing houseplants, you'll want to feed your plants and make sure you're supplying each of the five critical elements, either in your balanced fertilizer or through a combination of your fertilizer and soil amendments.