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Ins and outs of Magnesium in plant cultivation

Magnesium (Mg) is sometimes a forgotten element. In many areas of North America, adequate Mg is available in the groundwater used for irrigation. Concentrations of 25 to 50 ppm Mg are sometimes available and provide adequate levels for plant growth. In addition, supplemental Mg is also supplied via the dolomitic limestone used to adjust the substrate pH. But not all growing locations are blessed with a supply of Mg.

Interveinal chlorosis (yellowing) expands between the veins of the older leaves as magnesium deficiency symptoms progress (Alocasia ‘Maharani’).

Deficiencies can occur due to a number of factors. In areas without Mg in the irrigaKon water, of course the free source is lacking. We have observed in these locaKons that the level of Mg supplied by the dolomiKc limestone will provide adequate levels of Mg through 50 to 75% of the season, aXer that Kme, symptomaKc leaves start to be noKced (Fig. 1). This is especially true during warm springs when the plants are irrigated more frequently and the Mg is most likely leached from the substrate.

Magnesium uptake is also affected by other elements. In general the recommendaKon is to target a 4:2:1 raKo of potassium (K) to calcium (Ca) and Mg. This helps avoid antagonisms that limit the plant’s ability to uptake adequate levels of any one of these elements. AntagonisKc situaKons are commonly observed with crops such as tomatoes in which high levels of Ca are provided to avoid blossom end rot or with geraniums in which high levels of dolomiKc limestone or flowable lime are added to avoid pH drop of the substrate. These elevated levels of Ca limit the uptake of Mg and plants often develop deficiency symptoms.

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