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.
- Capacity – Plant cells have a limited capacity to hold calcium.
- Transport – Calcium cannot move against transpiration flow.
- 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. 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.
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.
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).
So how do we apply this to blueberry?
Lets 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 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.
A lot of focus has been placed on calcium form in blueberry (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 can not 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 maximised and help to reduce the impact of growth flushes (roots are the site of synthesis of cytokinins, and improving plant production of them helps to balance auxin influenced growth).
Secondly we can time and place calcium applications to best effect. Fruit absorb 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, allowing fruit to absorb calcium during growth stages and growing conditions where applications of conventional calcium fertiliser would be ineffective.
Products like Albina contain LoCal technology and can be used to get calcium into fruit where the natural ability to absorb is low. Giving farmers a low dose tool to improve fruit quality and firmness.
To give good root growth and better growth habit apply Lono at 5L per Ha, 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 Albina. Albina applied at 1L per Ha at flowering, with a couple of follow up applications will be more effective than more frequent applications of standard formulations.