Soil for Magnolias

Recently, Anthea wrote to me about the problem of growing her Chinese Magnolia where soil is alkaline.
I was just reading your article on changing soil ph and was hoping you could give me an idea on how to fix a problem I have with my magnolia x soulangeana.
I bought the tree approx 4 years ago and, although it has grown well and has good leaf coverage, it has never flowered. My thoughts at first was that it was still too immature to do so, but I have since discovered that the ph level of our soil is very alkaline (we live on the side of an extinct volcano towards the coast). Over the last year, I have been trying to reduce the alkalinity of the soil with a general garden sulphur, but this does not seem to have worked. Once again, no flowers/sepals this year. I have checked the soil ph again, and it is still as alkaline as it was a year ago. I do not want to be as drastic as to uproot the tree and replant in an acidic base soil as I am aware that magnolias don’t take kindly to transplants but I am at a loss as to what else I could do. Do you have any suggestions as to how to rectify this problem? I live in Mount Gambier in the south east of SA. Any help/advice would be greatly appreciated.

It is a long job to reduce very alkaline soil with sulphur alone Anthea, and when soil is very alkaline plants can’t absorb the nutrients they need to produce flowers (or fruit, in suitable species).

I would try putting some aged cow or horse manure on the soil surface around the tree (keeping it well clear of the trunk) after a thorough watering, and covering the manure with about 5 cm of organic mulch to keep it damp. As the manure (and mulch) break down they will release hydrogen into the soil. The electrically charged hydrogen ions will replace the calcium ions in the soil and the pH will drop. It won’t happen overnight, but it will happen. One of the problems on dairy farms is that the soil becomes quite acidic due to the constant manure deposits.

Then, when the tree is in leaf, spray the foliage with organic seaweed extract diluted to weak black tea colour. Seaweed is high in potassium that plants require for good flower formation, and it also contains a range of trace elements that plants need but can’t absorb from alkaline soils. This may be enough to assist flowering next season. I would spray as soon as leaves form and again in early summer.

By the way, volcanic soils are usually rich in nutrients and Plants usually grow well in them when pH is adjusted. As the pH problem is likely to affect your entire garden, it might be worth your while to invest in a pH test kit and and make annual adjustments to your soil where necessary.

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