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Why do growers so often get little benefit from application of calcium fertilizers? Take It In Diggin' In Diggin' In FERTILIZERS | Industry Report It's widely known that having good calcium levels in potato tubers can reduce multiple quality problems including internal rust spot, 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, says 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," says Marks. "It's this tissue collapse that contributes to internal rust spot, internal browning and premature rotting and bruising post-harvest." Marks says potatoes don't actually need very much calcium, but that the quality problems associated with calcium result from tiny localized deficiencies. 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 calcium out from leaves as an excess," says Marks. For example, if a 15.6-ton-per-acre crop of potatoes had complete loss due to internal rust spot, the actual quantity of calcium-deficient tissue (2 percent of each tuber is actually affected) is only 624 pounds per acre. The difference between the affected and healthy part of the potato is typically only 4 parts per million. "Therefore," says Marks, "the amount of calcium required to prevent an entire 15.6-ton-per-acre crop of potato from having internal browning is only 0.0399 ounces per acre. This should raise a few questions for growers: 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 calcium 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 calcium availability: transport and absorption. "Unlike most other mineral nutrients, calcium isn't phloem-mobile and can only be transported through the xylem," says Marks. "Calcium enters the plant with water and is transported upward with transpiration, where it's either absorbed and stored, or is precipitated from the leaves as excess." Understanding Transport "Calicum only moves upward," Marks stresses. "Nobody has ever witnessed or discovered a way to change this. This is why targeting and correct placement of applications is so important. Calcium applied to leaves can't correct problems in the roots. Therefore, foliar sprays of calcium fertilizers will never put the nutrient into tubers. It's physiologically impossible for the plant to move down." Understanding Absorption "Calcium is absorbed into cells using polar-auxin transport, and as auxin moves out of the cell, calcium enters," says Marks. "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 calcium to correct physiological disorders can be so ineffective," Marks continues. "It doesn't matter how much is applied; parts of the plant with low auxin levels such as tubers can't absorb it properly." How to Improve Agronomy So how can we improve tuber calcium 24 POTATO GROWER | JUNE 2019