Blossom end rot in tomatoes and peppers. Bitter pit in apples. Tipburn in lettuce. Internal browning in brassicas. These are all calcium disorders, and they routinely appear on soils that a laboratory would report as having ample calcium.
That apparent contradiction is the whole subject. Understanding it changes what you do about it — and explains why applying more calcium often fails to solve the problem.
How calcium moves, and why it matters
Calcium travels in the xylem, carried passively by the transpiration stream. It moves upward with water pulled through the plant by evaporation from the leaves.
Two consequences follow, and everything else in this article comes from them.
1. Calcium goes where the water goes
Tissues that transpire heavily receive plenty of calcium. Leaves, being large, thin and exposed, transpire a great deal. Fruit does not — it has a waxy cuticle, relatively few stomata, and a small surface area for its volume. Fruit therefore receives far less calcium than leaves do, even though both are on the same plant drawing from the same soil.
2. Calcium is not redistributed
Once calcium is incorporated into cell walls it stays there. It does not move in the phloem, so the plant cannot withdraw it from an old leaf to supply a developing fruit. A tissue that formed without adequate calcium cannot be repaired afterwards.
Which is why blossom end rot appears on fruit while the leaves above it are perfectly healthy. The plant is not short of calcium. That particular fruit was short of calcium during the two or three weeks it was forming — and nothing you do later will change what happened then.
What actually causes calcium disorders
Irregular water supply
The most common cause, and the least often addressed. Any interruption in transpiration interrupts calcium delivery. A hot afternoon where the plant closes its stomata to conserve water, a missed irrigation, a period of water stress followed by heavy watering — each produces a window in which the fruit gets no calcium.
This is why blossom end rot often appears in a distinct band on a crop, corresponding to fruit that was developing during a particular week. The soil calcium level did not change. The water supply did.
High humidity
Under protection this is a significant factor. Very high humidity reduces transpiration, which reduces calcium movement even when soil moisture is perfect. Growers sometimes see calcium disorders worsen after improving humidity control in the wrong direction.
Nutrient competition
Calcium, potassium, magnesium and ammonium are all cations and compete for uptake. A programme heavy in potassium — common during fruit fill — can suppress calcium uptake at exactly the moment fruit needs it most.
Ammonium is particularly antagonistic to calcium. Where nitrogen is supplied largely as ammonium or urea, calcium uptake suffers. This is one reason nitrate-based nitrogen sources such as calcium nitrate and potassium nitrate are preferred in intensive horticulture.
Salinity
High EC in the root zone reduces water uptake, which reduces calcium transport. Saline conditions make calcium disorders more likely regardless of how much calcium is present.
Rapid growth
Fast-growing fruit dilutes the calcium it receives. Excessive nitrogen driving vigorous growth can precipitate calcium disorders through dilution alone.
Root damage
Poor root health limits water uptake and therefore calcium delivery. As with most nutritional problems, if treatment is not working, look at the roots.
What to do about it
Manage irrigation first
If you address one thing, address this. Steady, frequent irrigation that avoids swings in soil moisture does more for calcium disorders than any product. Little and often beats large infrequent applications.
Under protection, watch the interaction between irrigation and humidity. On very humid days the plant transpires less regardless of water availability, and calcium delivery falls.
Supply calcium continuously
Because calcium is not remobilised, a single corrective application achieves very little. It needs to be present in the root zone throughout fruit development.
Calcium nitrate is the standard source for fertigation: highly soluble, supplies nitrate nitrogen, and readily available. Include it in the base programme from fruit set onward rather than adding it when symptoms appear — by then the affected fruit is already lost.
Balance the cation ratios
Watch the potassium-to-calcium relationship. Pushing potassium hard for fruit quality while calcium stays flat is a common way to induce blossom end rot. If you increase potassium, review calcium at the same time. The fertigation guide covers building a balanced solution.
Keep nitrogen in nitrate form
Minimise ammonium during fruit development. Nitrate-based sources support calcium uptake; ammonium competes with it.
Be realistic about foliar calcium
Foliar calcium has limited value for fruit disorders, and it is worth being direct about why. Calcium applied to a leaf largely stays in that leaf — it does not move to the fruit. Sprays directed at the fruit surface itself can help in some crops, notably apples for bitter pit, where repeated direct applications during fruit development are a recognised practice.
Spraying the canopy and expecting it to prevent blossom end rot is unlikely to work. The mechanism does not support it.
Crop-specific notes
Tomato and pepper — blossom end rot
Dark, sunken, leathery lesion at the blossom end. Appears on fruit that developed during a period of stress. Steady irrigation and continuous calcium in the fertigation programme are the effective measures. Avoid pushing potassium and nitrogen hard while calcium is static.
Apple — bitter pit
Small sunken brown spots, often appearing in storage rather than at harvest. Large fruit on vigorous trees with low crop load is most susceptible, because rapid growth dilutes calcium. Managing vigour and crop load matters as much as nutrition. Direct fruit sprays through the season are standard practice.
Lettuce and leafy greens — tipburn
Necrosis on leaf margins of young inner leaves, which transpire very little because they are enclosed. High humidity and rapid growth increase risk. Air movement over the crop, moderating growth rate and steady root-zone conditions are the practical levers.
Brassicas — internal browning
Similar mechanism to tipburn, affecting internal tissue. Consistent water supply and avoiding excessive nitrogen are the main controls.
Diagnosis: is it really calcium?
Before restructuring the programme, confirm the problem:
- Check the pattern. Calcium disorders appear on fruit and enclosed young tissue, not on exposed mature leaves.
- Check the timing. Symptoms in a distinct band of fruit point to a specific stress period.
- Analyse fruit, not leaves. Leaf calcium is almost always adequate and tells you nothing. Analyse the affected tissue.
- Review irrigation records. Look for gaps, hot spells and equipment failures around the time the affected tissue was forming.
- Check EC and cation ratios in the root zone and drainage.
Summary
- Calcium disorders are almost always transport problems, not supply problems
- Calcium moves with the transpiration stream and is never redistributed once deposited
- Irregular irrigation and high humidity restrict delivery more than low soil calcium does
- Supply calcium continuously through fruit development; a corrective dose after symptoms appear is too late for that fruit
- Watch potassium and ammonium, which both compete with calcium for uptake
- Foliar canopy sprays have limited effect on fruit disorders — direct fruit application is the exception
If you are dealing with a recurring calcium disorder, send us your fertigation programme and water analysis. We will look at where calcium sits in the schedule and what is competing with it. See also calcium nitrate and how to keep calcium out of your phosphate tank.
Need this product, or advice on which grade fits?
Send us the crop, your water analysis and the quantity you need. We will reply with the options that actually apply to your situation.
