A slight alkalinity can be cured sometimes with a little borax and manganese, but be guided at all times by soil tests when using these trace mineral elements. It doesn’t take much boron to kill a plant. Farmers in semiarid regions often use gypsum, which is calcium sulphate, to add calcium to soil that is already alkaline enough – or too alkaline. A great deal of controversy rages around the merits of gypsum or the lack thereof.
Chemical farmers don’t agree on its efficacy and organic farmers don’t agree on whether organically certified producers ought to use it. If you believe sulfur is a legitimate, natural product for organic gardeners, it is certainly one of the easiest ways to increase acidity. Two pounds per 100 square feet will lower pH about one unit.
A better way to acidify soil is with naturally acid organic materials – acid muck from swamps, oak leaves, oak sawdust, or ground up oak bark, cottonseed meal or acid peatmoss.
Ultimately, experience can tell you pretty well when lime is needed. Indeed, some gardeners never test their soil. They depend, instead, on native weed and shrub growth to indicate what the soil needs. This sometimes works but is rather risky, since weed roots often go to greater depths than those of crop plants, reaching a soil of different pH than that near the surface.
In general, however, the natural pH of a large area can often be determined by natural plant growth there, even though plots within that area can be quite different. For example:
1. Hard water in area springs and wells usually indicates abundant calcium carbonate (lime)
in the soil, and, therefore, an alkaline condition.
2. Native trees of hemlock, white pine, red spruce, oak, and black spruce in relatively large numbers usually means the soil is fairly acid.
3. Native trees of American arborvitae (Eastern white cedar) and white spruce in quantity are
usually an indication of alkalinity, especially in the subsoil.
4. Wild blueberries, most ferns, wild orchids, rhododendrons, and bayberries are all signs of acidity at crop-growing depths. Or, just watch the clover. If it germinates well and grows healthily, your soil hardly needs lime. When you plant a green manure crop of clover on one of your garden plots (which you should do anyway) lime half of the plot the year before, and not at all on the other. If the limed half grows better, you know what you need to do to the rest of the garden.