Use of Sulfur to Lower Soil pH
It is difficult to make exact recommendations for amounts of chemicals needed to lower soil pH. Light soils require lesser and heavy soils need greater quantities of acidifying agents. A soil high in organic matter has a different requirement than one low in organics. If the organic matter in the soil has been reduced to humus, the "buffering" effect of the humus usually increases the amount of pH alteration material needed.
To lower soil pH of light sandy loams one full point (i.e., from pH 6.0 to 5.0) as a guideline the use of sulfur, 10 pounds of dusting sulfur per 1,000 square feet, should be about right. In medium loam soil, add 15 pounds, and to heavy clay loam, 20 pounds. (Ordinarily dusting sulfur is perfectly satisfactory; no need to pay a premium for special grades.)
The only sensible way to solve the problem is to treat the soil and recheck the pH reading after two weeks, after a month, and again after two months. If not enough material was applied, simply add more. If too much, there is no harm in using lime to undo the effects of the use of sulfur to lower soil ph.
My gardening preference is for plants classed as circumneutral at a pH range of 6.0 to 6.9. Within this range all the food elements they need are available in highest concentration except perhaps for iron, zinc and copper, which are, however, present in large enough amounts for normal growth.
Organic matter, a vital soil ingredient, has an important effect on pH. When present in the soil in generous amounts it "buffers" the bad effects either of a too alkaline soil. For this reason, plants growing in a soil high in organic matter will often do well even though the pH reading is nearly a point either way from the ideal range. Most plants commonly grown in gardens do best within a pH range of 6.0 to 6.9. Only those which require an acid soil (rhododendrons and blueberries, to name two, usually called ericaceous plants) require a lower soil pH.
Here, then, is a key to better plant growth - keep the soil pH between 6.0 and 6.9 and keep up the organic content.