Calcium is increasingly recognised as a key to mango production in the Northern Territory for both yield and quality. Observation on yields indicates some of the low yields and fruit drop problems may be the result of low calcium levels and we are becoming more convinced that calcium is essential for good quality, particularly good skin colour. The evidence at the moment is only by observation. Recent research from Africa indicates that mangoes use a lot of calcium – 10 times that of phosphorus and magnesium.
Leading growers in North Queensland have long recognised the importance of calcium. Research by DPIF in the Northern Territory has indicated that growers’ opinion that calcium is important is possibly correct. Most Northern Territory soils including Katherine are very low in calcium – around one quarter of recommended levels. Soil levels should be at least 1000 ppm; most Northern Territory soils are only 250-300ppm.
Not only is the amount of calcium important but the ratio of calcium to magnesium. This should be at least 5:1 calcium to magnesium (expressed as meq/100g) The quality reports have shown the best coloured fruit come from orchards high in calcium, low in magnesium. Unfortunately many Top End soils including Katherine have ratios less than 2:1 (expressed as meq/100g).
Magnesium is present in many Northern Territory water supplies and for mangoes this is a problem. It makes fruit green and soft. The amount of calcium and magnesium in water supplies is variable so soil testing is crucial to get levels right. Test your soils to ensure you have at least 1000ppm calcium and your calcium:magnesium ratio is at least 5:1. NTHA provide a free interpretation service to members.
Calcium can be applied as lime, gypsum or dolomite. Dolomite generally is not recommended for mangoes as it is high in magnesium. Lime is used to increase pH and gypsum is used to increase calcium where pH is OK or high. Foliar sprays of calcium around flowering and fruit set have also been used to improve yield and quality. Current indications are that calcium should be applied at flowering (soil and foliar), after harvest (optional), and especially at the end of the wet season after the heavy rains to set the tree up for good calcium levels before flowering.
The amounts of lime or gypsum required on most Top End soils are large. The recommendations to date have generally been too low. They fail to take into account wet season leaching and the negative effects of high magnesium in most irrigation water. Mangoes appear to use a lot of calcium. Most growers have taken 2-3 years to get their calcium levels and their calcium:magnesium ratio right. Applications of 20kg/tree, three times a year for 2-3 years are not uncommon so get your soil tested. Leaf analysis can be used as a guide but can be misleading as calcium increases naturally as leaves age. It is important to realise that other elements also have to be corrected to get the maximum benefit.
Boron is essential for the best calcium response and should be applied at up to 50gms per mature tree. It is also important to have phosphorus, zinc and potassium levels right. The bottom line is get your soil tested. The best time to test soil (in Northern Territory) is December to February before the main time for application in March.
Note from Sheryl: Refer previous newsletters on talks by Peter Young and Robert Pulverenti
Robert says to only put on 5g per m2 per canopy and water in very well.