It is well known that good calcium levels in blueberry fruit can improve quality and shelf life, but results from calcium fertiliser can be erratic. Here David Marks of Levity Crop Science discusses why it is the crops physiology that limits calcium in fruit rather than lack of availability and discusses how growers can adapt agronomy to this.

Three rules of Calcium

To understand why calcium applications to blueberry are inefficient we need to understand three basic rules on plant calcium metabolism, and then apply them to blueberry. This can then inform how to improve agronomy.

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

Capacity

Unlike other nutrients like potassium, plants cannot store excess calcium. Indeed having too much calcium in cells is detrimental. when the holding sites for Ca in cell walls are full plants precipitate out any extra Ca being absorbed as calcium oxalate. Oftentimes plants that are deficient in fruit are still precipitating out calcium from leaves.

Transport

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 from roots towards shoots, with the areas of most water loss receiving the biggest flow of water and therefore the biggest supply of calcium.

Absorption

Not all parts of plants can absorb calcium to the same level, this is why 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 mow much is supplied).

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 build up of auxins that lead to growth flushes.

During growth flushes strong calcium sinks form in the shoots and leaves, which is are 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 blueberry are driven by physiology

Once we understand the physiology of calcium in blueberry it becomes clear that low calcium in fruit has little to do with fertiliser, 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 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.