Fertilizing

N-P-K: the three numbers on every fertilizer label

The three numbers (e.g., 10-10-10) represent the percentage of nitrogen (N), phosphorus (P), and potassium (K) by weight. Nitrogen drives leafy, vegetative growth; phosphorus supports root development, flowering, and fruiting; potassium boosts overall plant health, disease resistance, and fruit quality.

Secondary nutrients and micronutrients matter too

Calcium, magnesium, and sulfur are needed in moderate amounts. Trace elements like iron, manganese, zinc, and boron are needed in tiny quantities but are critical — deficiencies cause yellowing, stunted growth, and poor fruiting. A full soil test reveals any gaps.

Signs of the most common deficiencies

Yellow leaves overall usually signal nitrogen deficiency. Yellow leaves with green veins (chlorosis) suggest iron or magnesium deficiency. Blossom end rot on tomatoes and peppers indicates calcium uptake problems, often caused by irregular watering rather than a lack of calcium in the soil.

Organic fertilizers feed the soil, synthetic fertilizers feed the plant

Synthetic fertilizers deliver nutrients directly and immediately, but do nothing to improve soil biology or structure over time. Organic fertilizers (compost, fish emulsion, kelp, bone meal) break down slowly, feed soil microorganisms, and build long-term fertility. Most experienced gardeners use a combination.

Slow-release granular fertilizers are low-maintenance

Granular fertilizers (organic or synthetic) are worked into the soil at planting and release nutrients gradually over weeks or months. They're ideal for in-ground beds where you don't want to fertilize repeatedly through the season.

Liquid fertilizers act fast for quick corrections

Fish emulsion, liquid seaweed, and water-soluble synthetic fertilizers are absorbed quickly through roots and leaves. Use them to correct deficiencies mid-season or to give transplants and seedlings an immediate boost. They require more frequent application than slow-release granulars.

More fertilizer is not better

Over-fertilizing with nitrogen produces lush, soft foliage that attracts aphids and resists flowering. Excess phosphorus locks out micronutrients. Always follow label rates. If in doubt, err on the low side — deficiencies are easier to correct than toxicities.

Fertilize at the right growth stage

Heavy feeders like tomatoes, corn, and squash benefit from a nitrogen boost early in the season for vegetative growth, then a switch to lower-nitrogen, higher-phosphorus feeding once flowers appear. Too much nitrogen late in the season produces leaves at the expense of fruit.

Side-dressing mid-season keeps heavy feeders producing

Side-dressing means applying granular fertilizer along the row or around the drip line of a plant mid-season and watering it in. This is particularly valuable for sweet corn (when it's knee-high), tomatoes (after first fruit sets), and brassicas (after transplanting).

Foliar feeding for fast uptake

Plants can absorb nutrients directly through their leaves. Diluted liquid seaweed or fish emulsion applied as a foliar spray (early morning, so it dries before midday) is a fast-acting supplement — especially useful for correcting micronutrient deficiencies or giving stressed transplants a boost.