If you live in a dry, rocky region, chances are your garden soil is naturally alkaline. Most plants prefer a pH level of slightly acid to neutral, or 6.0 to 7.0. But a garden with soil that registers at 7.5 and above is considered alkaline. Plants growing in alkaline soils have a harder time taking in nutrients, especially iron, phosphorus and manganese. To correct high-pH soil, you'll need to determine just how alkaline it is, then work a correcting agent into the garden.
Nail the Numbers
To determine how much of an amendment you'll need to use, it's important to know exactly what your soil's pH is. You may prefer to send a sample of your soil to a university extension service or a professional soil testing service. This option gives you the pH, but provides you with recommended treatment, based on other data about other soil conditions in your garden. If you prefer to test the soil yourself, a basic soil testing kit or a portable testing meter from a garden center will give you a basic pH reading.
Calculate the Corrector
Elemental sulfur, a powdery material, will correct alkaline soil. As a general guideline, you'll need 1 pound of sulfur for ever 1 point you need to lower the pH, per 100 square feet. In practical terms, this means that a garden bed of that size which has tested at 9.0 would require 2 pounds of sulfur to reach the neutral zone of 7.0, while a 200-square-foot garden would require 4 pounds.
Figure Other Factors
Although the 1 pound per 1 point per 100 square feet is a useful guideline to correct alkaline soil, other factors can affect that number. If you've sent your soil sample to a lab, the printout you receive will give you recommendations based on other factors about your soil, such as soil type. If the soil is on the sandy side, less sulfur is needed than would be for loamy soil. On the other hand, hard-packed, clay soil often needs more sulfur than the usual guidelines indicate.
Apply the Amendment
Because it takes time for elemental sulfur to effectively lower your garden's pH, it's best to apply it at least two months before the growing season, or as soon as you can work the soil. For an average-size garden, hand-broadcasting is the best method for applying elemental sulfur. Use garden gloves to handle the material, and scatter it evenly over the area you need to correct. Covering your mouth with a dust mask is also a good idea. After you've broadcast the elemental sulfur, use a spade or hoe to work it into the top several inches of the soil.
Assess the Alternatives
Peat moss is a traditional material for correcting alkalinity, although the peat's acidity can vary and it's harder to calculate how much you need. It's also bulkier and more expensive than elemental sulfur. Reserve it for correcting small beds. Also somewhat pricey are ferrous sulfate or aluminum sulfate. The advantage of these two compounds is that they work immediately. If you were unaware of an alkalinity problem until your plants started developing problems, for example, ferrous sulfate or aluminum sulfate may be worth the higher price tag.