Calcium in Plants: What It Does and When Your Crop Needs It

So calcium matters most wherever new tissue forms fast: root tips, young leaves, the growing point and every fruit. A tomato, a pepper or a melon short of calcium as it swells gets a rotten, sunken spot at its blossom end, and a cabbage head gets brown, burnt inner leaves.

A building block that stays where it lands

Calcium is a building block. Once it is laid into a cell wall it stays there for good. The plant cannot pull it back out to feed a new leaf, the way it does with nitrogen or potassium.

Calcium also travels only with the water the plant draws up. So anything that slows the flow of water, like dry soil, very humid air or a fruit that loses little water, keeps calcium from reaching the parts that need it, even in a soil full of it.

Where calcium shows first

Because calcium does not move inside the plant, a shortage starts on the newest growth. The young leaves come out hooked, wrinkled or burnt at the tips, the growing point can die, and fruit rot at the blossom end. The old leaves stay normal.

Most calcium problems in the field are about water and timing, not an empty soil. See calcium deficiency for how it shows in each crop.

When calcium in plants is most critical

Calcium matters most while a crop forms its fruit or its head, and in peanut while the pods fill underground. Rice and corn need little of it and most soils give enough.

DAT means days after transplanting, DAS days after sowing in the field, and DAP days after planting. The days are the usual ones for a typical variety, so a long or short variety moves them a little. Timing calcium in plants to these windows gets more harvest from the same bags. Our guide to fertilizer timing by growth stage puts them together for a whole season.

Here is when each crop needs it most, from the anee.io stage book:

Rice, corn and other grains

Vegetables and legumes

Root crops, sugarcane and banana

When calcium comes too late

• Fruit vegetables (tomato, eggplant, pepper): Calcium given after blossom end rot shows cannot save the spoiled fruit.

Keep the soil evenly moist through fruit set, because a dry spell followed by heavy watering brings blossom end rot even when the soil holds plenty of calcium.

What keeps calcium from the crop

Strongly acid soil, sandy soil and heavily leached upland soil can run low on calcium. More often, though, the soil has enough and the plant cannot move it: dry soil, uneven watering, roots damaged by too much fertilizer, or heavy ammonium and potassium that crowd calcium out at the roots.

On sodic soil the trouble is the reverse: too much sodium pushes calcium off the soil and the ground turns hard and sticky. Gypsum puts the calcium back and lets the sodium wash away.

Where calcium comes from

Lime gives calcium and raises the pH of acid soil at the same time. Gypsum gives calcium without changing the pH, so it suits sodic and neutral soil. For fruit crops, calcium nitrate on the soil or as a spray reaches the fruit fastest. See fertilizer for tomatoes for a fruit crop schedule.

• Agricultural lime and dolomite: calcium for acid soil, dolomite with magnesium too.
• Gypsum (calcium sulfate): calcium and sulfur without raising the pH.
• Calcium nitrate: quick calcium with nitrogen, on the soil or the leaves.

Calcium questions