Fixing Magnesium Deficiency

Noting that magnesium (Mg) deficiency is a problem in Florida citrus, the University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) offers the following information about its symptoms, cause and solutions.

SYMPTOMS AND CAUSE

Trees with inadequate Mg supply have no symptoms in the new spring flush, but leaf symptoms will develop as the leaves age and the fruit expand and mature in the summer and fall. Leaves that have lost most of their green color due to Mg deficiency drop freely under unfavorable conditions.

Defoliated twigs become weak and usually die by the following spring. Severe defoliation will reduce the average size of individual fruit and cause a general decline in fruit production.

In Florida, Mg deficiency in citrus is caused primarily by low levels of Mg on acidic, light sandy soils and on calcareous soils. Leaching of added Mg is particularly serious and substantially rapid when the soil pH is 4.5 to 5.0. Under such conditions, the use of dolomite to bring the pH to 6.5 will furnish Mg at the same time.

SOLUTIONS

Soil application of Mg sulfate or oxide to provide 50 to 60 pounds of Mg per acre can be successful in correcting Mg deficiency when the soil pH is adjusted.

Under calcareous soils, the amounts of Mg applied must be greater than those applied on soils low in calcium or potassium.

Foliar spray applications of Mg nitrate (3 to 5 gallons per acre) can be very effective when applied on the spring and summer flush leaves when they are about fully expanded. Magnesium should be applied regularly at 20% of the nitrogen (N) rate unless leaf analysis shows more than 0.5% Mg.

If leaf Mg deficiency symptoms occur, Mg should be applied in the fertilizer. The rate should be increased up to 30% of the N rate until symptoms are no longer present in mature leaves of subsequent flushes.

If both potassium (K) and Mg status are low, sulfate of potash-magnesia, which contains both K and Mg in the sulfate form, is a very good option.

ADDITIONAL ADVICE

It is often difficult to increase Mg and K uptake with fertilizer applied to calcareous soils. High calcium (Ca) levels suppress Mg and K uptake by citrus trees through the competition of Ca, Mg and K.

In cases where soil-applied fertilizer is ineffective, the only means of increasing leaf Mg or K concentration is through foliar application of water-soluble fertilizers, such as magnesium nitrate, potassium nitrate (KNO₃) or monopotassium phosphate. A solution of 20 pounds KNO₃ per 100 gallons of water has been shown to raise leaf K, especially if applied several times during the year.

For citrus on noncalcareous soils, nitrogen and potassium fertilizer applications with a 1:1 ratio of N to K₂O are recommended.

If leaf testing on calcareous soils reveals that high levels of soil Ca may be limiting K uptake, the K₂O rate should be increased by 25% to have a N:K₂O ratio of 1:1.25.

Source: UF/IFAS