Plants need nutrients the way you need a diet: a few things in large amounts, a few in traces. Nutrients needed in large amounts are macronutrients; those needed in traces are micronutrients. The following paragraphs, and the Plant Nutrient Table later in this book, list what matters. Amounts are typical ranges in dried plant tissue, to give you a sense of scale — not a recipe to pour on the bed. Even the macronutrients are only a small percentage of a dried plant; almost all the rest of that dry weight is carbon. A living plant is mostly water.

Plants do not eat pure elements. Nitrogen, for example, makes up 78% of the air as dinitrogen gas (N₂), which plants cannot use. They need nitrate, ammonium, or related forms made by soil life, by lightning, and in the roots of legumes. Industrial nitrogen fertilizer is made with large amounts of fossil fuel; it is not used in organic production. Organic nitrogen comes from manures, compost, and meals (blood, feather, alfalfa).

Macronutrients

Nitrogen (N), 2–5% of dry weight — leaves, growth, green color. Phosphorus (P), 0.2–0.5% — roots, flowers, fruit, energy transfer. Potassium (K), 1–4% — water regulation, cold and disease resilience. Calcium (Ca), 0.5–1.5% — cell walls, growing tips. Magnesium (Mg), 0.2–0.4% — chlorophyll. Sulfur (S), 0.2–0.5% — proteins. These atoms are bonded to each other to make the molecules plants are built from — they are not optional seasoning.

Micronutrients

Iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl). Needed in tiny amounts. A soil test plus compost usually covers them. Dumping “more micronutrients” can be as harmful as a deficiency.

Read the table above as a map, not a shopping list. Compost and a diverse rotation supply most of this. Targeted organic fertilizers fill gaps a test reveals.