April 30, 2020 4:33 pm Published by Donna

It is well known that good calcium levels in blueberry fruit improves quality and shelf life.  However, results from most calcium fertilizers can be erratic.  This article by Dr. David Marks of Levity Crop Science discusses how it is the crop’s physiology that limits calcium in fruit rather than lack of availability and discusses how growers can minimize this.

The three rules of Calcium Nutrition

To best understand why calcium applications to blueberry are inefficient, we need to understand the three basic rules of plant calcium metabolism, and then apply them to blueberries, this can then help growers understand how to improve agronomy. 

So, what are these rules:  

1. Capacity – Plant cells have a limited capacity to hold calcium.
2. Transport – Calcium cannot move against transpiration (the movement of water) flow.
3. Absorption – Calcium can only be absorbed where auxin is present.

Unlike other nutrients like potassium, plants cannot store excess calcium. Indeed, having too much calcium in cells is detrimental therefore when the holding sites for Ca in cell walls are full plants transpiration out any extra Ca being absorbed as calcium oxalate.  Often times plants that are calcium deficient in fruit are still transpiring out calcium from leaves.
Calcium is not phloem mobile, meaning that for plants to transport calcium it must move through the plant with water.  Water movement in plants is governed by transpiration and moves in one direction i.e. from roots towards shoots, with the areas of most water loss receiving the biggest flow of water and therefore the biggest supply of calcium.
Not all parts of plants can absorb calcium to the same level, therefore we see local deficiencies in fruit crops.  Calcium absorption in plant cells is linked to ‘polar auxin transport, therefore, parts of plants high in auxin absorb calcium easily, if available, and parts of plants low in auxin absorb calcium sparingly, no matter how much is supplied.

So how do we apply this to blueberries?
Let’s look at the physiology of blueberries and see how the three rules of calcium interact with it.

In common with most crops, blueberries struggle to get good calcium levels in the fruit. This is due to the fruit being a low auxin tissue and therefore a poor sink for calcium. When the fruit is young and small (<2mm) it is in the cell division stage where the new cells that will become the fruit are being created and at this point auxin levels are good, however, as the fruit starts to increase in size cell division is no longer occurring.  The cells are instead expanding, and auxin levels are lower.  This means as fruit increase in size the ability to absorb calcium decreases.

Blueberries lack the capacity of other plants to process nitrate, so leaf build-up is rapid following soil nitrogen application, whatever the form applied.  This leads to excessive auxin hormone production in leaves.  It is this buildup of auxins that lead to growth flushes.  During growth flushes strong calcium sinks form in the shoots and leaves, which is also the main destination for water transport.  This means that it is the shoots that have both the highest throughput of calcium, moving with water towards shoots, and the highest absorption of calcium due to high polar auxin transport.  The capacity however is limited, and during growth flushes, calcium will be precipitated out of leaves even if fruit is deficient.

During growth flushes blueberry fruit receive limited calcium throughput and have limited calcium absorption ability that makes getting calcium into fruit tricky.

Calcium problems in blueberries is driven by physiology
Once we understand the physiology of calcium in blueberries, it becomes clear that low calcium in fruit has little to do with fertilizer, and a lot to do with the crop itself.

If we apply calcium to the soil it will largely bypass the fruit, instead moving mostly towards foliage, where there is higher transpiration and particularly during growth flushes, higher capacity to absorb.  Rather than move excess calcium supply to the fruit, the leaves just precipitate it out.  Fruit calcium is not a whole plant deficiency.

If we apply foliar calcium this may get calcium to the fruit, but apart from a short period when young, the capacity to absorb it is low and most of the calcium applied will not get in.

A lot of focus has been placed on calcium form in blueberries, nitrate vs chloride etc., but this is largely a red herring as the form has no influence on absorption.  The level of calcium that makes the difference between good quality fruit and fruit with poor shelf life is very small (2-3 ppm or 2-3g per MT of fruit), but the quantity of calcium applied to improve the fruit is very high relative to the need.  Fruit farmers apply far more calcium than is required to fix the problem, but most is wasted as the plant cannot use it where it is needed.

So how can we improve calcium in the fruit?
There are a few things growers can do that will help.  Firstly, we can place emphasis on creating a good and constantly growing root system.  This will ensure that transport of calcium is maximized and help to reduce the impact of growth flushes.  Roots are the site of synthesis of cytokinin’s, and improving plant production of them helps to balance auxin influenced growth.

Secondly, we can time and place calcium applications to be most effective.  Fruit absorbs calcium best if it is applied directly between flowering and 2-3mm fruit size, so foliar applications made during this period will be more effective than applications made later in the season.

Thirdly we can use technology that improves calcium absorption. LoCal is a technology developed by Levity that allows calcium absorption in the absence of auxin, products like Cell Power® Calcium Gold, and Cell Power® Calcium Platinum containing LoCal technology and should be used to get calcium into fruit where the natural ability to absorb is low.

To give good root growth and overall better growth habit apply Cell Power® SizeN® at 2 quarts per ac, commencing at bud break and making three more applications at three-week intervals.  This will keep roots actively growing, encourage a good growth habit, and ensure the plant focuses on fruit production.

To improve fruit calcium levels consider using a calcium that the fruit can absorb properly, like Cell Power® Calcium Gold, and Cell Power® Calcium Platinum at 1 to 2 pints per ac.  Application at flowering, with a couple of follow up applications will be more effective than more frequent applications of standard formulations.

For more information on products please contact OMEX® Agrifluids, Inc., at www.omexusa.com, OmexUSA@Omex.com or call 559-661-6138.

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