When you go to buy garden soil, ALWAYS pH test it before you pay for it. If the business refuses, take your money elsewhere.
This sample below is from a newly created garden with soil sold by a local landscape supply yard. The plants have started going backwards and look very unwell.
That reading is far, far too high for any plant to grow in and no amount of Seasol or fertilizer will fix it.
For the most part, plants require a slightly acid, neutral or slightly alkaline soil pH. Gardenia's, Magnolia's and Camellia's require slight acidity. Lavender's, Lilac's and Buddleia's will tolerate some alkalinity.
When the soil is so far out of range like above, plants can no longer make use of the nutrients available to them. As the roots attempt to move beyond the potting mix they were grown in, they become stunted and impacting how they absorb water.
Both extremes can be remedied, lime increases soil pH and Alum or Sulphur decreases it. Alum is very aggressive so should be used with care, but it will give the fastest drop. From there, fine tune with Sulphur.
The reasons for frustration regarding this topic are this -
-A customer will come to us wondering why their plants, which may or may not have been purchased from us, are not growing and/or looking very sick. Our first response is to bring us a sample of the soil to pH test. Nine times out of ten, the pH level will be highly alkaline.
This then leads to the question if the fresh topsoil was added at the time of planting, again the response mostly yes. It's then up to use to explain why it's not poor quality plants they have bought, rather the soil they planted it into.
-The customer then has to spend more money to make the soil actually suitable for growing plants. Topsoil is not a cheap purchase anyway, and often customers will be angry for buying something so unusable and then having to spend more on top of that.
-There about four landscape supply yards in my area. Three of those sell soil highly alkaline and one completely ignorant of what they are selling.
I have managed to find out what the induvial components are in what they call "Premium Garden Soil". The basic ingredient is from a company that uses household green waste and turns it into compost. The second element is cow manure, the third is sand. Well rotted cow manure is generally pH neutral, so either the sand or compost, if not both, is likely to be alkaline.
Despite this soil looking and smelling good, the stuff is completely not fit for purpose. And yet, these suppliers continue to sell this stuff and turn a blind eye to what it is doing.
As mentioned, prior to planting it's best to do some pH testing of your existing soil and any soil brought in. Effective pH test kits are not too expensive, if you don't have access to one then take samples to your local nursery (not Bunning's) (Also avoid those cheap pH meters that are available, they are not very accurate). Doing this prior to spending money on plants will make it easier to remedy a pH imbalance.