The range it covers is as follows:
Ridiculously strong alkali!
If your soil is strong enough to turn the solution white then you'll already know about it, having received prior treatment for chemical burns. You should always start this test with a violet solution. If your cabbage was cooked with London tap water like mine was then you'll start with a blue solution. Titrate it back to violet using vinegar before running the test.
In my case, the soil in the beds shifted slightly into the blue spectrum, which was surprising. My soil is slightly alkali despite being a humus-rich loam. The previous gardener must've gone berserk with the lime! Most vegetables prefer slightly acid soils, so I'll have to amend the soil before I can plant the year's crops.
Feel free to use this test at your own risk.
In other news: George seems to think the circles around the holes in the birdhouses are eyes, looking at him. Every time he looks at them from an angle where only two can be seen he raises his hackles and growls. We can now add the tree to the very long list of things that George believes have come to murder us all in our beds.