Pay particular attention to the above phrase, "provided the foods are present in the first place." No matter how pH is juggled up or down, it cannot make available any food element that is not present. For example, plants may show by certain signs that they are not taking up iron from the soil. If the pH is high, we might suspect that iron is present but locked up in insoluble form. If, however, plants still show a deficiency of iron after sulfur has been applied to lower the pH, then we know that iron is lacking and must be supplied in a form plants can absorb.
Because plants tend to remove calcium from the soil as they grow, which in turn lowers pH, lime is closely tied in with our use of the pH theory. To a considerable degree, proper lime application (assuming supplies of plant nutrients are ample) becomes the key to our success with garden soils. This does not mean that the indiscriminate use of lime year after year is the right way to run a garden. Too much alkalinity can do as much harm as too little. This is why no "rule of thumb" can be set up that will work all the time in every garden. The only safe guide is an actual test of soil reaction.
MAKING THE pH TEST
There are several methods of making a pH test. The most accurate, and one that will probably be used if you send soil samples to your state agricultural experiment station, is an electrical "bridge" which checks the reaction by electrical resistance. This is an expensive piece of apparatus and one that few amateur gardeners are likely to buy.
While this device gives extremely true pH readings, I strongly recommend that gardeners use home test kits despite the probability of less accurate results. There are a number of reasons why. First, since most stations charge for each sample submitted, the economy-minded gardener's usual practice is to mix soil from several sites into one sample and submit it for an "average" test. If the soil throughout the garden is uniform, this average test method is satisfactory.
Such a situation, however, is quite unusual. For example, black dirt used as topdressing over backfill around most speculative (development) houses may come from piles of earth scraped from half a dozen different sites.
My own vegetable garden, while on land graded nearly a hundred years ago, is an example of how much the soil in one plot can vary. In one area it lies over an old creek bed that was filled-in in 1868 to make a level building plot.