This is something I'm presented with all too often.
We always recommend that customers have their soil pH tested before planting, especially on new top soil bought from landscape supply yards. For some reason, one prominent supplier in my area is content in selling a product not fit for purpose, and has done so for decade now. Plants grow best with a neutral to slightly acid soil pH. Some plants such as Camellia's and Azalea's prefer it more acidic again.
The picture above is on the other end of the pH scale, being highly alkaline. Plant's will not grow and will slowly decline till death if soil pH is as high as that. Although this condition can be corrected with appropriate products, it can be a expensive and time consuming process of application and re-testing until lowered. One has to question why a customer should need to spend more money to make the soil actually usable. These garden soils are sold as "premium garden mix", well you won't have a garden at all with that pH.
For reference, here is a pH scale.
As you can see, the sample above is beyond the highest number on that chart, most likely closer to a pH of 13.
It can be hard to explain to customers who are seeing their plants going backwards that the problem most likely is due to the soil. In their minds, they bought great looking soil, and paid a premium price for it. Once they see the colour of the sample, and where is
should be, it starts to make sense.