TECHNICAL
Calcium management in apple orchards Most orchardists are aware that good calcium levels in their fruit is important. By Dean Rainham : AgFirst horticultural consultant, Hawke’s Bay Calcium is important for cell wall integrity. Without sufficient calcium, cells become leaky, more prone to disease and can collapse and die. Fruit with adequate calcium status improves the longevity of storage after picking. The incidence of bitter pit and blotch is associated with low fruit calcium status (Figure 1 ) and it usually expresses in storage. There are several factors that increase the risk of calcium related problems in apple fruit, it is never just about one risk factor. Understanding calcium movement and distribution in the tree and what influences this allows for better management to mitigate the risk of both bitter pit and blotch incidence. This article outlines calcium physiology in the tree, discusses the risk factors and poses some practical management responses. Calcium availability & uptake First and foremost, it is important to have enough calcium in the soil. A soil test will identify this. Calcium needs to occupy at least 65% of the soil base exchange sites. Maintaining the pH (acidity/alkalinity) between 6.5 and 6.8 will ensure the calcium in the soil is in a plant available form. Calcium must come into the soil solution and then in contact with the root surface for plant uptake to occur. When plants transpire water flows to the root surface and calcium is transported in this mass flow. Good water management is therefore vital to achieve good calcium status into the tree. Trees under water stress may also suffer from a shortfall in calcium. Conversely, if there is a sustained period of too much water, anaerobic soil conditions can develop which will severely reduce root function and therefore calcium uptake is compromised. Ensuring good nutritional balance with other competing cations (positively charged ions) such as potassium and magnesium in the soil is important. High amounts of these
Figure 1 Bitter pit in apples
competing cations, especially potassium, will reduce calcium uptake. Calcium, being a cation, cannot exist alone in soil solution without an anion (negatively charged ion) such as phosphate, nitrate and sulphate. These anions must also be balanced in the soil. Calcium is primarily taken up by newly developed roots. Thus it is important to maintain a soil environment that sustains a healthy root system and promotes new root growth especially in the spring. Good drainage, nutrient and water management along with organic matter, encourages a healthy and thriving soil ecosystem. Be careful with the overuse of chemicals which lowers biodiversity in the rootzone, impacting on root growth and nutrient availability. Calcium movement and partitioning Calcium is translocated through the plant in the transpiration stream via the xylem vessels (Fergusson et. al.). Calcium will therefore be deposited to plant organs that have the strongest demand for water; this is principally the leaves. New developing leaves with high transpiration rates are the strongest sink for calcium. The demand for calcium by the leaves is over 75 times more than the fruit (Fergusson et .al). Excessively vigorous trees are usually the primary reason for low fruit calcium levels and increased risk that bitter pit or blotch is expressed in post-harvest storage. Water flow and therefore calcium movement to the developing fruit is a little more complex. During early fruit development the surface to volume ratio of the fruit is high. This is favourable for maintaining the transpiration stream The ORCHARDIST : DECEMBER 2021 61
