Plants need 17 essential nutrients to survive. Three of these come from air and water: carbon, hydrogen, and oxygen. The remaining 14 are minerals absorbed through the roots from soil. Each plays a distinct role, and a shortage of even one can stunt growth, discolor leaves, or kill the plant entirely.
The Big Three From Air and Water
Carbon, hydrogen, and oxygen make up roughly 95% of a plant’s dry weight, and none of them come from fertilizer. Plants pull carbon dioxide from the atmosphere through tiny pores in their leaves, then use sunlight to combine it with water in a process called photosynthesis. This produces the sugars and starches that form the plant’s physical structure and fuel its growth. Water, absorbed by the roots, supplies both hydrogen and oxygen.
Because these three nutrients are freely available from the environment, gardeners rarely think about them. But carbon dioxide availability does matter. Aquatic plants, for instance, can struggle with carbon uptake because CO₂ diffuses through water about 1,000 times slower than through air. For land plants in a typical garden, though, the limiting factors are almost always the mineral nutrients below.
Primary Macronutrients: N, P, and K
Nitrogen, phosphorus, and potassium are the nutrients plants consume in the largest quantities, which is why they appear as the three numbers on every fertilizer bag. A label reading “18-4-10” means 18% nitrogen, 4% phosphorus (expressed as phosphate), and 10% potassium (expressed as potash) by weight.
Nitrogen is the engine of leafy growth. It’s a core building block of chlorophyll (the molecule that captures sunlight) and of every protein in the plant. When nitrogen runs low, older leaves turn uniformly yellow because the plant cannibalizes them, shipping stored nitrogen up to newer growth. Some species also develop red or purple pigmentation on those older leaves.
Phosphorus drives energy transfer inside every cell. It’s central to the molecules that store and move energy (ATP), and it plays a key role in root development, flowering, and seed formation. Deficient plants often show purple or reddish discoloration on lower leaves, followed by yellowing and tissue death.
Potassium regulates water movement in and out of cells, controls the opening and closing of leaf pores, and activates dozens of enzymes. Without enough potassium, leaf edges and tips on older leaves brown and die, sometimes with spotting across the leaf surface. Plants also become less resilient to drought and disease.
Secondary Macronutrients: Ca, Mg, and S
Calcium, magnesium, and sulfur are needed in smaller amounts than N-P-K but are no less critical.
Calcium strengthens cell walls and helps cells communicate with each other. Because calcium barely moves once it’s deposited in a leaf, deficiencies show up in new growth first. You’ll see distorted young leaves, blossom end rot in tomatoes, or tip burn on lettuce.
Magnesium sits at the center of every chlorophyll molecule, so without it, photosynthesis falters. Deficient plants develop yellow patches along the edges of older leaves while the veins stay green, a pattern called interveinal chlorosis.
Sulfur is a structural part of several amino acids, vitamins, and proteins essential for photosynthesis. A sulfur shortage reduces the plant’s ability to build its main carbon-fixing enzyme, which slows sugar production and causes yellowing in young leaves. It also disrupts the balance between nitrogen and sulfur inside the plant, leading to a buildup of unused nitrogen compounds. The result is stunted growth and significant yield loss in crops.
The Eight Micronutrients
Plants need tiny amounts of eight trace elements: iron, manganese, zinc, copper, boron, molybdenum, chlorine, and nickel. “Tiny” doesn’t mean optional. Each one fills a specific catalytic role that no other element can replace.
Iron, manganese, copper, nickel, and molybdenum all facilitate chemical reactions involving electron transfer, the kind of reactions that power photosynthesis and cellular respiration. Zinc acts as a helper for enzymes that need a chemical “spark plug” to get reactions going. Boron is involved in cell wall formation and reproductive development. A boron imbalance can cause crinkling in young leaves, where patches of cells simply fail to develop in the leaf blade. Chlorine helps regulate water pressure in cells and plays a role in photosynthesis.
Because these nutrients are needed in such small quantities, deficiencies are less common in healthy garden soil. But they do occur, particularly in soils with the wrong pH.
How Soil pH Controls Nutrient Access
A nutrient can be physically present in the soil yet chemically unavailable to roots. The main factor controlling this is soil pH. Most plants grow best in a pH range of 6.0 to 7.2, where all essential nutrients remain soluble enough for roots to absorb.
When pH climbs above 8.0, iron, zinc, manganese, and phosphorus become locked into insoluble compounds that roots can’t take up. This is why plants in alkaline soils often show iron deficiency (yellowing between the veins of new leaves) even when a soil test shows adequate iron levels. On the acidic side, a pH below 6.0 can make certain nutrients too available, reaching toxic concentrations, while others become scarce. Adding lime raises pH in acidic soils; sulfur or acidifying fertilizers lower it in alkaline ones.
Mobile vs. Immobile Nutrients
One of the most useful things you can learn about plant nutrition is which nutrients move inside the plant and which don’t. This single concept lets you diagnose problems just by looking at where symptoms appear.
Mobile nutrients, including nitrogen, phosphorus, potassium, and magnesium, can be pulled out of older leaves and redistributed to younger, actively growing tissue. So when these run low, the oldest leaves show damage first. Yellow lower leaves, for example, are a classic sign of nitrogen deficiency.
Immobile nutrients, including calcium, sulfur, iron, manganese, copper, and boron, stay put once a leaf incorporates them. The plant can’t recycle them to new growth, so deficiencies appear at the top of the plant or on the youngest leaves. If your newest leaves are pale, distorted, or dying while the rest of the plant looks healthy, suspect one of the immobile nutrients.
If a deficiency continues long enough, symptoms spread throughout the entire plant regardless of mobility. But catching the early pattern, old leaves versus new leaves, helps you narrow the culprit quickly.
Beneficial but Not Essential Elements
A handful of elements aren’t required by all plants but can improve performance in certain species or stressful conditions. Silicon, cobalt, sodium, selenium, and aluminum fall into this category. Silicon, for example, strengthens cell walls in grasses and can help plants resist fungal diseases and insect feeding. Sodium can partially substitute for potassium in some salt-tolerant species. These beneficial elements have also been shown to boost resistance to drought, salinity, and nutrient toxicity.
You won’t find these on a standard fertilizer label, and most home gardeners don’t need to think about them. But if you’re growing rice, sugarcane, or other grasses in challenging conditions, silicon amendments can make a noticeable difference.
Putting It Into Practice
A soil test is the single most useful step you can take before adding any fertilizer. It tells you which nutrients are actually low, what your pH is, and how much to apply. Without one, you’re guessing, and adding a nutrient that’s already abundant can be just as harmful as a deficiency. Excess phosphorus, for instance, blocks the uptake of iron and zinc.
For most garden and landscape plants, maintaining a pH between 6.0 and 7.2, adding compost regularly, and using a balanced fertilizer when soil tests indicate a need will keep all 14 mineral nutrients in a healthy range. If you spot symptoms, check the youngest and oldest leaves first, match the pattern to mobile or immobile nutrients, and confirm with a soil test before making corrections.