Those three numbers on every fertilizer bag — like 10-10-10 — aren't marketing. They're the percentage of nitrogen, phosphorus, and potassium inside, and getting the ratio right for the right crop at the right time is the difference between a thin harvest and a heavy one.
Nitrogen, phosphorus, and potassium Nitrogen builds leaves and greens up growth fast — so fast that too much of it late in the season trades fruit for foliage on tomatoes and squash. Because nitrogen moves freely inside the plant, a shortage shows up first as yellowing on the oldest, lowest leaves. Phosphorus does the opposite job: it drives root development, flowering, and fruit set. It barely moves through soil, so work it in before you plant rather than side-dressing later. Potassium regulates water movement and sugar transport, and demand for it spikes hard once fruit starts ripening. Deficiency looks like yellowing at the leaf edges that turns into a scorched, burnt look on older leaves.
The quieter players: calcium, magnesium, and sulfur Calcium gets blamed for blossom-end rot on tomatoes and peppers, but the soil usually has plenty of it already. The real trigger is often uneven watering, which disrupts calcium's path into the fruit — so a consistent watering schedule does as much for blossom-end rot as any calcium spray. Magnesium sits at the center of chlorophyll, and its deficiency shows as yellowing between the veins of older leaves. Sulfur deficiency looks similar but shows up on younger leaves first.
Trace nutrients still matter Boron, zinc, iron, manganese, and their trace-nutrient cousins are needed in tiny amounts, but a shortage still caps yield. Boron is essential for pollen tube growth, so a shortage can mean poor fruit set. Zinc deficiency is considered the most common micronutrient problem in crop production worldwide.
Why soil pH runs the whole show Nutrients can be sitting right there in the soil and still be unavailable if the pH is off. Most vegetables and fruit want soil in the 6.0-6.8 range. Push pH above 7 and iron, manganese, zinc, and boron become harder to absorb; drop it below 6 and nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur all become less available instead.
Get a soil test before you guess A soil test from your local cooperative extension office is the only real way to know whether you have a nutrient problem, a pH problem, or both. Guessing wastes money and can wash excess nutrients into nearby waterways.