top of page
Debunking The Myth of Calcium

John Kempf • The American Fruit Grower

Published on April 26, 2017 

Nutrient management and crop quality clearly go hand in hand. But not always in the ways you might think.

In my nutrition management work, I have focused on understanding how to manage nutrition at different stages of fruit development to affect fruit quality. I have learned that it is relatively easy to substantially impact fruit size, firmness, sugar and solids content, shelf life, and storability with nutrition management. Many of these improvements in fruit quality are in some way associated with calcium metabolism in the fruit.

By closely measuring calcium flow in the trees of many different orchards, I have observed that bitter pit on ‘Honeycrisp’ apples and other calcium-related physiological disorders often are not the direct result of calcium shortage. In fact, many growers have been applying substantial calcium, with very little effect on bitter pit.

Bitter pit and other calcium-related disorders result when there is unregulated absorption of potassium, leading to potassium concentrations in plant sap high enough to inhibit calcium flow into the developing fruit embryo, regardless of calcium concentrations already accumulated in the plant. The context for these observations is our use of systematic sap analysis to monitor nutrient movement within a plant throughout the entire growing season.

Assessing Nutrient Content
Since 2011, we have been using recently developed plant sap analysis technology to measure the nutrient content of plant sap in leaf tissue. Sap analysis is substantially different from petiole analysis, which you may be familiar with, and provides a more accurate indicator of nutrient levels within plant sap.

Differential testing is used in collecting leaf samples for sap analysis. Samples are collected and measured from the old and new leaves separately. These samples are collected every two weeks throughout the growing season, until leaf drop in the fall, to observe nutrient movement within the plant of all the mobile elements. Plants may be storing a surplus of nutrients in the older leaves while keeping the new growth at optimal levels or, conversely, they may be sabotaging their older leaves and moving nutrients away from old leaves and into new growth or the fruit.

This testing method enables us to observe how plants are partitioning all the mobile nutrients throughout the tree frame, and to predict potential deficiencies or excesses long before they express as visual symptoms. I have evaluated thousands of sap analysis samples from many different crops in the last six years. This process gives insight into nutrient flow within plants throughout the growing season, and actionable real world information on how to resolve fruit quality challenges from a very practical perspective.

The Big Difference with ‘Honeycrisp’
While working with sap analysis, there was a startling difference between the nutritional profile of ‘Honeycrisp’ apples and apple varieties that were less susceptible to bitter pit. ‘Honeycrisp’ fruit contained potassium concentrations as much as two or three times that of apple varieties grown right beside them in identical soil types with identical fertilization practices. The same trend was observed on other crops that are exceptionally susceptible to calcium disorders, such as chili and bell peppers, ‘Braeburn’ apples, and others. These cultivars and species have a clearly visible predisposition to hyper-accumulate potassium.

The antagonistic relationship between potassium and calcium in plant tissue is well documented and is strikingly visible when using sap analysis. When plants contain high concentrations of potassium, foliar or soil applications of calcium are ineffective at producing the desired crop response. This explains why some growers have been making intense calcium applications with limited or no crop response. The use of sap analysis helped us understand that ‘Honeycrisp’ apples do not have a calcium deficiency problem. They have a potassium excess problem that is expressed in the plant as an inability to absorb calcium.

To manage this crop characteristic of excessive potassium absorption, the obvious first step is to reduce, or in many cases entirely eliminate, all soil and foliar applications of potassium. Even a very small application — as little as 8 ounces of actual potassium per acre applied as a foliar during fruit development — can reduce calcium mobility into the fruit. This is especially true during critical early periods of fruit development, such as the cell division stage immediately after pollination.

However, with a number of growers we have worked with, simply stopping potassium applications has not been enough. If the soil contains a generous supply of potassium either from historical applications or from high native levels in the soil’s geological profile, plants continue to accumulate potassium even without further applications.

A Promising Experiment
For several years, we were uncertain how to manage this situation. Then we had a fascinating experience with potassium absorption challenges in tomatoes. With a large group of East Coast tomato growers, we discovered that by making foliar applications of manganese, specifically in the reduced form, potassium absorption was very well regulated in the crop.

Manganese seems to function as an effective thermostat for potassium absorption and translocation within the plant. It can both up-regulate and down-regulate potassium absorption, as well as modulate potassium translocation into the fruit. When trees have a generous supply of manganese in the proper form, potassium does not move into the fruit as rapidly as when the tree contains a surplus of potassium, allowing calcium to move into the fruit more readily, and helping to prevent bitter pit challenges. More than 90% of all the orchards we have worked with did not have adequate manganese to provide this potassium regulation effect, or the manganese being applied was in a form that the trees could not absorb and metabolize.

How to Apply This to Your Orchard
In practical application in the field, these four issues need to be addressed:

  1. You need to measure where these nutrients are to understand where the sticking points may be in your operation. Consistent plant sap analysis is very inexpensive when you consider improved crop value and saved fertilizer costs.

  2. Stop all potassium application until you are certain you need it, as measured by an analysis.

  3. You will likely need to continue calcium applications as potassium concentrations begin to drop.

  4. To help regulate potassium inside the plant, you probably will need to find manganese chelate in the reduced form and add it to your foliar mix.


Growers have been able to substantially reduce bitter pit in apples, cork in pears, and other calcium-related disorders in a single growing season by understanding and addressing these nutrient interactions.

bottom of page