I test my soil annually, plus any additional soil moved in. I take a sample from three places in each bed (20 beds!), plus under the apple trees, the soft fruit, the pig pen, the chicken run, and at various places in the field depending on what I'm intending to do.
I use a professional soil testing kit intended for garden centres, market gardeners and professional growers. It's the
HI-3896. I also use a pH meter for spot checks. If you have the money (and they're not cheap), then a soils and liquids conductivity meter is very useful for telling you when soil is too dry or lacking in potassium and nitrogen).
The same company has a good why, when and how on soil testing
here.
I would have liked to have had the soil tested by a lab, as baseline to compare with the soil testing kit, but couldn't afford this.
Our soil is very acidic, varying between 5.2 and 6. In the permanent pig pen and chicken run it can dip below 5 in places, which is why the animals get rotated, with the soil limed and rested. (Although potatoes should grow extremely well when following the pigs in particular.)
Acidity between 4 and 5 is often associated with high concentrations of soluble aluminium, managanese and iron, which can be toxic to many plants. Bacteria are also hindered by strongly acidic soil, which means organic matter won't be broken as effectively or as well and that can mean nitrogen is not released.
And, of course, phosphorous uptake is best with a pH of around 6.5 while most other nutrients are best taken up between 6 and 7.
But heavy and repeated liming can make the soil alkaline (lime has a pH of 12), so you want to apply just enough lime for the plants you're intending to grow and at the right time.
So, that's why you need to combine a soil testing programme with good crop and animal rotation, with pH being adjusted at the right time. Applying lime, bonemeal, spent mushroom compost and hardwood ash moves pH upwards; applying sulphur, aluminium sulphate, sawdust, composted leaves, leaf mold and manure moves pH downwards.
If your soil is alkaline in an otherwise acidic area, it can be down to a couple of things - over-liming, leaching of lime from uphill sources, and building wastes (from concrete and mortar). So if you have a newly built house or an extension built from bricks/stone and mortar, then you may get alkaline areas near the walls and anywhere the builders made concrete/mortar or dumped their waste.
Hope this helps.
Stonehead