Like all living things, plants need some basic nutrients to live. There are about 20 nutrients that are essential to plant growth. Most of these are readily available in the soil, so we don’t need to provide them. Four of the essential macro-nutrients are carbon (C), nitrogen (N), phosphorus (P), and potassium (K). Here is a scientific explanation of why plants need these nutrients.
At the cellular level, the dry weight of a plant cell is about 50% carbon. Plants process carbon dioxide (CO2) from the air via photosynthesis, and combine it with the hydrogen in water (H20) to create carbohydrates like sugar, starch, and cellulose. This is also called carbon sequestration – the plant is trapping the carbon in its cells for a time. (The carbon is released again when the plant dies and decomposes.) We never worry that a plant doesn’t have enough carbon since carbon is readily available in the atmosphere.
A plant’s cells also have a significant portion of nitrogen (N). Nitrogen is part of the nucleic acids DNA and RNA – the genetic information contained in the cells. Nitrogen is also necessary to produce amino acids. Our atmosphere is some 78% nitrogen as diatomic nitrogen (N2), but most plants can not process nitrogen as a gas. (Legumes like beans and peas are an exception.) The nitrogen must be “fixated” first – that is combined with other compounds to become usable. In nature, soil bacteria fixate the atmospheric nitrogen creating either ammonium (NH4+) or nitrate (NO3). Without sufficient nitrogen, the cells cannot divide and reproduce so the plants’ growth is stunted. Some of the chlorophyll cells may also die without nitrogen, seen as the leaves yellowing. Soil-dwelling bacteria are fundamental to providing nitrogen to a plant’s roots.
Phosphorus (P) is an essential nutrient since the DNA and RNA inside the cell are held together with phosphorus bonds. Phosphorus is also key to changing glucose to energy. Since producing seeds requires more energy than growing leaves, a plant will translocate phosphorus to be near the flowers. When there is insufficient phosphorus, the plant will conserve its energy by flowering less.
The fourth essential nutrient is potassium (K). Potassium is an activator for enzymes which function for protein synthesis, sugar transport, and metabolism. As a salt mineral, potassium also helps regulate osmosis across cell walls, keeping plant cells hydrated and moving water throughout the plant. A potassium deficiency often looks like a nitrogen deficiency – stunted growth and yellowing leaves. This is because without enough potassium, the fixated soil nitrogen isn’t transported to the plant’s leaves.
For more information about choosing fertilizer, see this previous blog post: Fertilizer for Annual Container Gardens
For a really in depth look at soil nutrients, refer to this CSU Master Gardener resource:
