Old concrete posts

Question

We moved into a 1950's bungalow 18 months ago and first job in the garden was to take down unsaved block wall but every 9' or so was concrete posts held up by concrete/cement that was blue in colour we have since planted a hibiscus hedge (I have planted 18 in total) but the ones that ended up close to the old post holes have died I also planted a large fern in a hole that contain this stuff that to started to die we removed iT and it has picked up again any ideas please what was in the concrete/cement and is there anything we can do to stop it

Answer 1

I suspect your issue is relating to soil acidity and maybe also soil type.

Hibiscus and ferns both prefer to grow in a neutral to slightly acid soil of pH 6.5 to 7.0 (soil acidity) and a free draining sandy loam (soil type).

Concrete made in the 1950's could contain all manner of material, especially if homemade on site. One material it will most certainly contain is cement and cement is made from lime or calcium oxide.

In soil, both lime and calcium oxide create an alkaline soil - pH greater than 7.0 - which is ok for some plants but not hibiscus or ferns. It's highly likely the soil around the concrete is alkaline.

I'd recommend removing the concrete posts and concrete footings if at all possible. They may be of low quality manufacture and therefore may be leaching lime into the surrounding soil. If removal is not possible, then you can treat the soil.

Alkaline soils can be treated to balance pH.

There are a few methods to effect this change but before I write about that, I'd advise you purchase a cheap soil pH test kit. These can be purchased online or from a garden nursery and some hardware stores. Test your soil in and around the concrete post holes to be certain that is the cause of the problem.

Soil treatment...

To acidify soil, I use powdered sulphur. This should come with a health warning as some people suffer allergic reaction to sulphur.

When converted by bacteria to sulphuric acid, sulphur lowers soil pH and increases plant root access to nutrients.

Sulphur also plays a critical role in the formation of plant tissue proteins and vitamins and so it's good for general plant health.

A method to help balance pH without use of sulphur, is the generous use of compost (but not fresh mushroom compost, which un-composted is alkaline). Some gardeners prefer to dig compost into the soil but I prefer to let the worms do this for me - they are after all perfectly designed for the task.

Place an organic compost at least 10cm / 4in but preferably 15cm / 6in thick across the area of your garden. Water in thoroughly.

This method, while more gentle, takes longer to achieve the desired results.

Note to be careful to keep the compost away from the trunks of each plant. Depending on the type and age of the compost, sometimes this fresh material can "burn" the plant bark.

Compost will also improve soil structure and therefore improve drainage, which as mentioned earlier may also be a problem.

If you'd like some more advice on soil type, update your question to include some information on the soil (depth, structure, appearance) along your fence line, with a couple of photos if possible.