Double, double toil and trouble — no, not a recipe from the Scottish play, but three soil amendments that every gardener should know. These are typically worked into the soil when the garden bed is prepared for planting. Follow the links for more information.
In areas with heavy rainfall, calcium and magnesium are washed out of the soil, making it more acidic as well as removing these necessary elements. On the pH scale, 7 is neutral, 6 or less is acidic, and 8 or more is alkaline. Soil that is of the incorrect pH for a particular plant has a negative effect on the plant’s growth; for example, it can make it more difficult for the plant to take up nutrients from the soil. Limestone, or lime, is added to soil to raise the pH, i.e. to make it less acidic, or more alkaline. It also prevents blossom end rot in tomatoes, peppers, and eggplant by providing calcium.
Limestone is available in two types: calcitic limestone that contains calcium carbonate (CaCO3), and dolomitic limestone that contains appreciable amounts of magnesium carbonate (MgCO3) in addition to the calcium carbonate. If your soil test shows that you need magnesium as well as calcium, then you should use dolomitic limestone. You can’t just add chunks of limestone rock to your garden, however, as the limestone won’t be absorbed for many years. Finely ground or powdered limestone will work fairly quickly, but it is messy. The best choice for home gardeners is limestone that has been powdered then pressed into pellets. A soil test will tell you how much lime to use; otherwise, the bag usually has instructions on how much lime to add for a given change in pH. I added lime to garden bed number four (where I planted cabbages and their relatives) this past spring.
Sulfur (also spelled sulphur) is used in the opposite case; it is applied to soil to lower the pH, i.e. to make it more acidic, or less alkaline. Sulfur is also a necessary element for plants. Some parts of the world have alkaline soil, but as they are usually dry and less populated, I suspect most gardeners live in areas that have acid or neutral soil. You might still need to use sulfur even if your soil is not alkaline, as plants such as blueberry and azalea require a pH of around 4.5 to 5.0 for best growth. Also, soil that is near concrete may be more alkaline than the rest of the soil in your garden. If you grow some varieties of hydrangea, you need a pH of 5.5 or less if you want them to produce blue flowers; if the pH is above 6.5, the flowers will be pink. I have spread sulfur around my blueberry and hydrangea bushes in the past.
Metal-containing sulfur compounds such as iron sulfate or aluminum sulfate can be used to lower the pH and will do so more rapidly than elemental sulfur. I feel that they are not as safe to use, except perhaps in the case of hydrangea bushes where aluminum is the source of the blue flower color. A better option is ammonium sulfate ((NH4)2SO4), which will lower the pH and provide some nitrogen as well.
Gypsum is claimed to loosen clay soils, but this is only recommended for heavy clay soils or soils that contain high concentrations of sodium (Na+) salts, i.e. in coastal areas. Gypsum (calcium sulfate, CaSO4) does provide both calcium and sulfur (see above), two necessary elements for plant growth. I added gypsum to garden bed number three (where I planted tomatoes, eggplant, and peppers) this past spring as a source of calcium to prevent blossom end rot, but after reading more about its potential negative effects, I may stop using it. If you live in an area with soil that is neutral or alkaline but lacks calcium, gypsum is one way to provide it without altering the pH. It also can help loosen very heavy clay soil. It is available as a pelleted powder, similar to limestone.