Calcium and Magnesium – now what are they for

Getting at the details of this are quite tricky because most gardening books tell you what happens if they’re not there and leave it at that. So after a bit of sleuthing this is what we came up with.


Calcium is for healthy bones and teeth. Funnily enough it does something similar in plants.  It’s main use is to act as an ionic bridge to hold long chain molecules together to maintain the integrity of the plant cell walls and membranes. If you don’t have enough of it the cell walls are weak and prone to disease.  It also acts as part of the Cation pool in the cell vacuole (I’ll tell you more about ions and what they do at the bottom of the post) and acts as a signalling device – secretion of Calcium across a membrane can trigger other effects like hormone release etc.


A magnesium atom sits in the middle of the Chorophyll molecule so that it holds it’s shape and allows it to carry out its function of turning CO2 into Oxygen. This way of working makes it a co-enzyme and it carries out the same sort of function for enzymes in other other metabolic pathways. If you don’t have enough of it the plants have green veins and yellow leaves and you get greenback in tomatoes. Best approach to getting enough of both of these is by liming – with dolomite  if you are short of magnesium.

You normally do this after the potatoes and before the brassicas (that’s cabbages and broccoli to you) in a standard crop rotation. If you need to deal with a shortage out of sync, then do a foliar feed (spray the leaves) with a solution of Epsom Salts.

Now what’s all this about ions.

Nature sticks atoms together into molecules in two main ways. Covalent bonds in which the electrons are shared between adjacent atoms as you find in organic compounds like sugar, petrol hydrocarbons and the long chain fatty acids you find in detergents. And Ionic compounds which are basically held together with static electricity. If you think of table salt, its made up of 1  atom of Sodium which as a positive charge and one atom of Chlorine that has a negative charge. If they’re dry they form a crystal in which the charges all line up to produce a solid shape – and if they’re wet they dissolve and get away from each other. Scientists like to call positive ions cations and negative ions anions.

When we were talking about potassium, I told you that its the concentration of salts collectively in the cell vacuole (which is basically a membrane sac full of salt solution) that create the osmotic pressure which sucks up water and provides enough hydrostatic pressure to hold the plant up and make it grow.

Collectively the positive ions are Potassium, Calcium, Magnesium and Ammonium. However they are not interchangeable. If you have excess Potassium then this can inhibit the uptake of Magnesium and the plants will get out of whack.

So you can see it’s necessary to have a handle on this. You can’t test for Ca and Mg yourself with normal plant kits so you have to look out for signs of deficiencies (or get your soil tested professionally – if you want to do that let me know and I’ll put you in touch with an organisation that will do this without too much expense.) Deficiency symptoms for Magnesium we’ve talked about – green vein chlorosis and greenback.  If calcium is short you get young leaves cupping and blackening of growing points.

Next post in this series will be a rule of thumb guide to the minor minerals and then we’ll get on to something more entertaining.

5 thoughts on “Calcium and Magnesium – now what are they for

  1. Mandes at Tomato Plants

    Great stuff. Perhaps a little off topic, but would you mind if I write something about this on my tomato plant blog? I will of course, cite original source and link back to your page.

  2. Safer Smith

    I was using this article as a reference for a biology project I am working on, and I was wondering if the author could post his full name so I can cite this page correctly .

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