Determining status of trees and soils
The magnesium and trace element status of fruit trees should be determined by chemical analyses of mid-shoot leaves sampled in January and February. Standards are available to interpret these results. Comparative sampling of the youngest fully emerged leaves (approximately five leaves back from the growing tip) from healthy and unhealthy trees can be used at any time of the growing season.
Misleading results may be obtained, if leaves have been sprayed with foliar nutrients and/or fungicides. Soil analysis does not provide a reliable guide to the trace element status of the soil for fruit growing. This is particularly so for analyses of surface soil to a depth of 10 or 15 centimetres. Most feeder roots are found between 10 and 30 centimetres deep, depending on soil type and rootstock. Tree roots, however, will explore a large volume of soil in search of nutrients and water.
Corrective treatments
Correcting trace element deficiencies in trees using both soil and foliar-applied sources can be difficult and it may take several years before results may be apparent in tree growth on mature trees.
Spray treatments
Foliar sprays may be an effective way of correcting some trace element deficiencies. Sprays are best applied in early to mid-summer, with three to four applications required each year. The rate and number of applications will depend on the product specifications and the degree of deficiency of the trace element to be corrected.
Dormant sprays, using higher rates of chemicals, are useful supplements where leaf analysis has identified a severe deficiency. Dormant spraying is best carried out in July to early August, preferably before or two weeks after application of a winter oil spray.
Generally it is not recommended to apply any trace element sprays in the first four weeks of fruit development. Growers must read labels carefully, as some products applied at the wrong growth stage may result in fruit russeting.
The following recommendations are for sulphate-based trace element sources. The recommendations should be used as a guide only, and their effectiveness in improving tree growth monitored regularly after product application. Spray grade formulations of any sulphate should always be applied in foliar sprays in preference to fertiliser grade products.