Magnesium and Tomatoes

October 3, 2015 Article

Almost every year I have planted tomatoes and looked forward to a bountiful harvest that I could eat and share around. The plants grow vigorously until they begin to fruit; then the lower leaves dry out, become brown and bitter, and fall off. It seemed to be common among nearby gardeners.

Gardening advice was that my plants had a virus. My job was to remove and discard the dying leaves and then not plant tomatoes in that spot for five years. Unless one has a very large garden and good records, the latter advice would be unusable.


After moving from Oakleigh to Burwood, I bought in compost and garden soil to grow veggies in bathtubs. Surely, my tomato problems would be over. But no, the damage appeared again.

Recently I pored over my own gardening books, finding a reference to magnesium deficiency. It is not one of the holy trinity of nitrogen, phosphorus and potash. A small amount is needed for these functions: 1 molecule in each particle of chlorophyll enables plants to use nitrogen, phosphorus and sulphur; and it’s necessary for creating proteins. If, however, the plant wants to bear fruit, quite a lot more magnesium is required. Magnesium will be pulled from the greenest place, mature leaves.

Treatment, of course, would be adding magnesium, but it will not be quick since it is absorbed more slowly than many other minerals. Hard water should contain enough magnesium for tomatoes, but soft water and rain water do not, so we city folks must add it.

Prevention is a better idea than treatment, so when preparing for next season, if you will be adding lime, make sure it is dolomitic lime, which contains magnesium. If not, find another source of the mineral. I’ve been using a dilute solution of Epson salts, but salty soils create other problems. Another option could be adding a hydroponic liquid with “major nutrients” or “plus” controlled release fertilisers.

We can improve the soil, have harvest more tomatoes.