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Good quality testing kits sold to farmers and used by experienced people with a good understanding of soil chemistry and backed up by occassional lab testing can give fairly accurate results.
Kits sold to gardeners are not necessarily of the same quality and few gardeners will have the skills to use test kits in a meaningful way.
The sensible approach is to remember that soil changes rather slowly so a lab test is valid for at least 4 years. Therefore a test now will give you the baseline that you can then use to add lime and potassium, phosphorus and possibly magnesium - once amended the soil won't change much for several years.
Nitrogen is the exception. It is released from organic materials all summer and converted into inorganic forms - these and any chemical fertiliser nitroegn get washed out of the soil by winter rains. In spring the soil is deficient in nitrogen until organic material rots or until you add chemical nitrogen fertiliser. Therefore nitrogen analysis is seldom carried out.
It is well worthwhile to have a lab test done every 4-7 years, but once the soil has been put right, all you need to do is add nitrogen. This can be in the form of compost (best), manure (good) or fertiliser (not so good but making enough compost is difficult and manure is laborious stuff to apply).
So you look at your crops and identify those that need oodles of nitrogen:
Brassicas, beetroot, salads, spinach and give them plenty of organic matter and a dose of fertiliser - say 100g per square metre of growmore or if just fertiliser 150 per square metre of growmore
Then you consider crops that need modest nitrogen and apply either organic matter or fertiliser at say 70g growmore every sq metre
Potatoes, tomatoes, sweetcorn, french and runner beans, parsnips, radishes
Finally consider those crops with low nitrogen needs:
Broad beans, peas and carrots - they can usually get by on residues from previous crops
You might have to give rather more fertiliser if your soil is thin and sandy or chalky.
If growth is not good you might water in some high nitrogen fertiliser and up the dosage of manure and fertiliser in subsequent years.
By adding nitrogen containing materials that also have potassium and phosphorus such as compost, manure or general fertiliser you will be topping up the soil reserves automatically and all should go well for years and years.
Boggy
Beware the bat-eared bogweevil
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