Alkaline soil occurs most generally where rainfall is low. In order to render these soils more acidic, several organically-based methods can be used. Follow the steps outlined below to lower the soil pH in your garden and grow fruit, vegetables and ornamental plants that prefer acid soils.
Step 1 - Determine the pH of Your Garden Soil
Test the garden soil at several places with the soil pH test kit. Dampen the soil where you want to take samples. Scoop out soil with a sterilized hand spade. Take soil samples from several locations in your garden, place them in the soil sample jars, labelled by their location and date obtained, and test them with the soil pH testing kit. Follow the pH testing instructions carefully.
Some spots in your garden may be more or less alkaline than others. If your soil pH tests at above 7.5 there may be an abundance of calcium carbonate in the soil. When this molecule is present in such quantity, reducing the soil alkalinity is very difficult. You may need to irrigate it for a few months after applying organic compost and mulch to lower the calcium carbonate content.
Step 2 - Prepare the Soil for Organic Correction of pH
Mark the area of your garden where you want more acidic soil with a border of wooden pegs, corded together. Remove all existing plants whether annuals or perennials, and rototill the soil thoroughly. Water the soil area to saturation and rototill it again.
Step 3 - Add Organic Compost
Add organic compost to the soil to a depth of 2 inches across the whole bed. Rototill this compost into the soil until thoroughly mixed. Organic compost in soil will help it retain its new pH level after the soil has been corrected.
Step 4 - Add Sphagnum Peat Moss
For a small garden plot of up to 100 square feet, add a 2-inch blanket of organic sphagnum peat to the top of the soil. Use the rototiller to work this peat into the top 8 inches to 1 foot of garden soil in the early spring, just before planting.
Step 5 - Add Sulfur for Large Garden Areas
For garden soil enrichment in an area over 100 square feet, elemental sulfur is safe, cost-effective and thorough, although slow compared to peat moss. Available in granular form, elemental sulfur is easy to apply to a large soil area. Use 3/10 pound of granular sulfur for each 10 square feet of soil. Mix it in well with the rototiller into the top 12 inches of soil.
Wait three months between applications of sulfur on garden areas over 100 square feet. You can also use aluminum sulfate or iron sulfate to provide sulfur to acidify the soil, at a maximum ratio of five pounds for each 100 square feet.