Mobility within tree: High (symptoms in older leaves first)

Uptake Form: Mg²⁺

Function: chlorophyll

Deficiency/Toxicity Symptoms: Magnesium deficiency first appears as fading green colour at the terminals of older leaves or leaflet terminals in pinnate-leaved plants.  Chlorosis then follows interveinally giving a “herringbone” appearance.  Severe deficiency can cause marginal scorching similar to K deficiency.  Since Mg is mobile, Mg will move from leaves to developing fruit causing symptoms in leaves to worsen.  Fruit may fail to mature or drop early if the deficiency is severe.   High levels of K or low levels of available P in the soil can also result in poor uptake of Mg in a tree. Excess magnesium does not have specific symptoms.  A deficiency of K or Ca usually results in excess Mg depending on the cation balance. This may lead to bitter pit and soft scald in some apple varieties.

Magnesium levels should be checked and balanced every few years.  Magnesium, while usually applied as a spray, will also be supplemented whenever Sul-Po-Mag or dolomite lime is added to acid soil.  Magnesium sprays may be combined with sprays of B, Mg or urea.  The recommended Mg application rates to use for fruit trees is presented in Table 1.

Table 1. Possible magnesium foliar application rates.