Focus on nutritional elements: magnesium

27 february 2018

A plant needs magnesium (Mg) for beautiful green leaves. The nutritional element is also necessary to ensure a proper functioning of the enzyme in the plant that produces protein. In ‘Focus on nutritional elements’ this month, everything about magnesium.

What is the function of magnesium?

Magnesium is taken up by the plant roots as magnesium ion (Mg2+). Magnesium is mobile within the plant. It is an important building brick of the green leaf colour of plants (the chlorophyll). This green colour is in the so called chloroplasts. Magnesium is also indispensable for the enzyme in the plant that makes sure that proteins can be made out of amino acids. The level of magnesium in dry matter varies from 0.1 to 3 percent.

In what shapes does magnesium occur?

Most potting soil basic fertilizers contain a small amount of magnesium. Just like calcium and other cations, magnesium is bound to the adsorption complex. That happens to a significantly lesser extend to magnesium than to calcium. In a potting soil, the level of soluble magnesium is determined by the quantity of dosed fertilizers. The source of magnesium mainly comes from lime, if it contains magnesium. In unprocessed coir pith, magnesium is adsorbed next to calcium, causing it to be poorly available to the plant without further fertilizing. Coir products (substrate components) under the RHP quality mark need to be processed to obtain a proper nutritional balance.

What is the effect of magnesium?

In the past there was regularly a magnesium deficiency in cultures. Especially tomatoes planted outdoors suffered from this. Because of magnesium deficiency,  leaf turns yellow/green to yellow between the nerves (caused by the decomposition of chlorophyll), while the nerves themselves remain green. This phenomenon is called chlorosis and, when there is a magnesium deficiency, expresses itself mainly in the oldest leaves. In monocotyledons magnesium deficiency is expressed in a spotted appearance ('tigering'). The uptake of magnesium is obstructed by a low temperature of the growing medium and by a high potassium concentration. Magnesium surplus is expressed as salt damage. This does not often occur.

RHP magnesium gebrek tomaat Cordyline

Magnesium deficiency in tomato and Cordyline.

What is the RHP norm for magnesium?

Magnesium is in most potting soil basic fertilizers, but mainly comes from lime if it contains magnesium. The magnesium level of RHP certified products is checked among other things before use in a substrate. Lime is a mineral; this is also checked for contamination with, among others, trace elements. For substrates with the RHP Horticulture quality mark, the substrate producer and grower decide together what the magnesium level should be, suitable to the culture.

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