Improve tuber calcium levels
(Sponsored) Although improving tuber calcium levels improves quality, growers often get little benefit from application of calcium fertilizers. David Marks gives an expert view on why.
It’s widely known that having good calcium levels inpotato tubers can reduce multiple quality problems including Internal Rust Spot (IRS), internal browning and hollow heart. Calcium also plays a role in reducing susceptibility to bruising and post-harvest diseases. So, what exactly do plants do with calcium? Its main function within cell walls is to give cell wall rigidity and strength, explains David Marks, calcium expert and managing director of Levity Crop Science.
“The main symptom of calcium deficiency is the disintegration of cell walls and the collapse of affected tissues. It’s this tissue collapse that contributes to IRS, internal browning, and premature rotting and bruising post-harvest,” he says.
Potatoes don’t actually need very much calcium, the quality problems associated with calcium result from tiny localized deficiencies, he says. But these minor deficiencies, in terms of the portion of tissue affected, can make crops unsellable.
“While tubers may have small areas of calcium deficiency, the rest of the plant rarely suffers any shortage at all and is often precipitating Ca out from leaves as an excess,” explains David Marks.
For example, if a 15.6 tons/acre crop of potatoes had complete loss due to internal rust spot, the actual quantity of Ca-deficient tissue (2% of each tuber is actually affected) is only 624 lbs/acre. The difference between the affected and healthy part of the potato is typically only 4 ppm. “Therefore, the amount of calcium required to prevent an entire 15.6 tons/acre crop of potato from having internal browning is only 0.0399 oz./acre. This should raise a few questions for growers,” he says.
“Why are small parts of the tuber deficient when the area right next to them isn’t? Why are these small areas of tissue deficient in Ca when there’s no whole plant deficiency and why doesn’t applying large amounts of calcium reverse the deficiency?”
In order to answer these questions, it’s important to understand how calcium behaves in a plant, he says. There are two factors to be considered in plant Ca availability; transport and absorption.
“Unlike most other mineral nutrients, Ca isn’t phloem mobile and can only be transported through the xylem. Ca enters the plant with water and is transported upwards with transpiration, where it’s either absorbed and stored, or is precipitated from the leaves as excess,” explains David Marks.
“Ca only moves upwards. Nobody has ever witnessed or discovered a way to change this. This is why targeting and correct placement of applications is so important. Ca applied to leaves can’t correct problems in the roots. Therefore, foliar sprays of Ca fertilizers will never put the nutrient into tuber. It’s physiologically impossible for the plant to move down,” he stresses.
“Ca is absorbed into cells using polar-auxin transport, and as auxin moves out of the cell, Ca enters. Parts of a plant that are low in auxin can’t absorb the nutrient effectively, regardless of how much is available.”
High auxin-producing areas include new shoots, new flowers, and new leaves. Low auxin-producing areas include fruits, roots and tubers. “This is why applying Ca to correct physiological disorders can be so ineffective. It doesn’t matter how much is applied, parts of the plant with low auxin levels such as tubers can’t absorb it properly,” he says.
How to improve agronomy
So how can we improve tuber Ca levels? Part of the answer is targeting the tuber zone, believes David Marks. “Don’t apply it to foliage and expect it to get to tubers. For best results it must be placed near the stolon roots inside the tuber zone.
“Use Ca in an available form, and don’t confuse it with liming agents. It’s easy to think Ca status will be improved where liming agents are used to adjust pH, where in reality, they supply very little (the Ca in lime is only 1 in 10,000 available).
“Time applications to when tubers can absorb it. Tubers produce very little auxin once they start growing, so to get conventional Ca sources into a tuber it really needs to be done during the cell division stage. Once tubers reach 5mm in size there’s very little new cell formation, and auxin levels decline. For Ca to be able to get in the tuber it needs to be available between hook eye and 5mm tuber size,” advises David Marks.
Finally, he advises growers who use more advanced Ca products that have been shown in field trials shown to lift in-tuber calcium levels. One such technology is LoCal, known as Cell Power® Calcium Gold and Cell Power® Calcium Platinum in the U.S., developed by scientists at Levity it stimulates Calcium transport into low auxin tissue. David explains “Levity has done a lot of research on fruit crops like apples, and we have had consistently good results correcting the physiological disorder Bitter Pit using low rates at the right timings. We have shown that 100 fl. oz. of a product that the fruit can absorb is far more effective than 31 gallons of standard calcium.” So, what does that mean for potato growers? David adds, “to get Ca into tubers we need to hit the tuber zone. Our approach is to use a product that combines Ca with LoCal (in the US we recommend Cell Power® SizeN® Calcium) soil applied at tuber initiation through bulking. This approach puts calcium in the right place, in the right form and unlocks the ability of the tubers to absorb it, where they would otherwise be unable to.”
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