Raise Soil pH with Lime
You can raise soil pH by applying lime. Here are some suggested amounts:
To raise soil pH of light sandy loams one full point (i.e. from 5.5 to 6.5) add 35 pounds of ground lime to 1,000 square feet. On a medium loam soil, apply 50 pounds, and on a heavy clay loam, 70 pounds. (Either agricultural lime or the fine chips used for top-dressing driveways can be used.)
Within the 6.0 to 6.9 pH range, all foods needed by the majority of shrubs, annuals, perennials and other "average" garden plants are available in the soil in soluble form, provided the foods are present in the first place. Bacteria thrive and do their vital work better in this pH range, and certain potential poisons, such as aluminum, are locked up so they cannot injure plant roots.
Pay particular attention to the above phrase, "provided the foods are present in the first place." No matter how much you raise soil pH, 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 to raise soil ph (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.
