Blossom end rot is a watering fault, not a calcium shortage

The sunken, leathery black patch at the base of a tomato or capsicum fruit is blossom end rot. It is caused by calcium failing to reach the growing tip of the fruit. That sounds like a calcium shortage — and so growers buy calcium nitrate, spray calcium foliar feeds, and keep losing fruit.

Calcium travels with water

Calcium moves through the plant almost entirely in the water stream, from the roots up to where water is being lost. Leaves lose a lot of water and pull calcium strongly; fruit loses very little. So when water supply is interrupted — a dry spell, then a heavy soak — the fruit is the first part of the plant to go short, even in soil that holds plenty of calcium.

That is why blossom end rot follows irregular watering far more often than genuinely calcium-poor soil. It also explains why foliar calcium sprays rarely help: calcium sprayed on leaves hardly moves into the fruit.

What causes irregular watering

• Watering by the clock rather than by the soil — too much on cool days, too little on hot ones
• Long gaps between irrigations, then one long flood to “catch up”
• Two zones open at once on a small main, so the far end of the block runs starved
• Hot, dry, windy afternoons when the plant loses water faster than roots can supply it
• Salty root zone (high EC), which makes water harder for roots to take up

The fix: little and often, driven by the root zone

Steady moisture fixes it. In practice that means several short pulses a day rather than one long irrigation, each zone watered when its own probe says the root zone has dried to a set point — and never two zones on one small main at the same time.

When calcium really is short

Some soils genuinely lack calcium, and very acid soils can too. A soil test or pH and nutrient map settles it. If calcium is low, correct it in the soil (often with lime where pH is also low). But check the watering first — it is the cause far more often.