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How to Lower pH in Garden Soil

Successful gardening is as much about chemistry as botany. Soil chemistry, for example, has an enormous influence on plant health. Soils maybe acidic -- also called sour -- neutral or alkaline -- sometimes called sweet. Unlike humans, very few plants are afflicted with a sweet tooth. Without mildly acidic to mildly alkaline soil, they can't absorb enough nutrients for healthy growth. Knowing how to lower soil pH when necessary gives you a nearly limitless choice of plant possibilities.

Tip

Soil pH is measured on a a scale of 0 to 14, with 0 being the most acidic, 7.0 being neutral and 14 being the most alkaline. Because the scale uses logarithmic units, soil with a pH of 6.0 is 10 times more acidic than neutral soil -- pH of 7.0 -- and 100 times more acidic than soil with a pH of 8.0. Even a small dip in a soil's pH number can make a big difference to plants.

Acid-Lowering Soil Amendments

Lowering your soil pH won't happen overnight, but the type of amendment you choose for the job influences the speed of the soil reaction. Organic amendments take the longest, because they react biologically with the soil,

Sphagnum Peat Moss

It may take sphagnum peat moss several years to lower soil pH significantly. The wait is worth it, however, because peat moss also improves the soil's nutrient-holding ability and drainage. Use it at the rate of 2 1/2 pounds per square yard of soil.

Elemental Sulfur

Organic elemental sulfur needs several months to lower soil pH. Soil bacteria convert its sulfur into sulfuric acid, so it works most quickly in warm, moist soil. Applied in spring, it reacts with the bacteria through the summer.

Your soil's type dictates the amount of sulfur to use. To lower the pH of loam soil one full unit, use 2 pounds of elemental sulfur for each 100 square feet of soil. For heavier clay soil, increase the sulfur to 3 pounds per 100 square feet. For sandy soil, lower it to 1 1/3 pounds.

Tip

Because the soil's pH begins to increase as soon as the soil bacteria exhaust the elemental sulfur, two applications of 1 pound per 100 square feet in early spring and early summer are more effective than a single one. In a warm-winter climate, space four applications of 1/2 pound per 100 square feet every three months.

Aluminum Sulfate

Because aluminum sulfate lowers pH as soon as it dissolves in the soil, it's a tempting alternative to peat moss and elemental sulfur. The problem is that amounts higher than 5 pounds per 100 square feet of soil damage plants. If your soil needs more, split the amount into two or more applications as you would for elemental sulfur.

To lower the pH of loam soil one full unit with aluminum sulfate, use 12 pounds per 100 square feet. For clay and sandy soils, use 18 and 8 pounds per 100 square feet, respectively.

Applying the Amendment

Sphagnum Peat Moss

Mark the soil's perimeter with the wooden stakes.

Loosen the top 6 inches of soil with the rotary tiller.

  • Because aluminum sulfate lowers pH as soon as it dissolves in the soil, it's a tempting alternative to peat moss and elemental sulfur.
  • The problem is that amounts higher than 5 pounds per 100 square feet of soil damage plants.
  • If your soil needs more, split the amount into two or more applications as you would for elemental sulfur.

Spread the tarp along the soil's edge. and pile it with the appropriate amount of peat moss for the staked area.

Spade peat moss from the tarp onto the soil, distributing it as evenly as possible.

Till the peat moss thoroughly into the loose soil.

Elemental Sulfur and Aluminum Sulfate

Before applying either one of these amendments, stake the area and loosen the top 6 inches of soil as you would to apply peat moss.

Empty the appropriate amount of amendment into the spreader's bucket.

Adjust the spreader's meter to the amendment label's recommended setting for the area you need to treat.

  • Spread the tarp along the soil's edge.

Turn the spreader on and push it in front of you in a straight line, walking from one end of the soil area to the other. Squeeze its trigger to release the bucket's contents as you go.

Turn the spreader so its outside wheel overlaps the inside track of the previous path, and return to the opposite end of the soil bed. Repeat until the spreader's bucket is empty.

Work the sulfur or aluminum sulfate into the soil with the rotary tiller.

Tip

Many garden supply stores rent drop spreaders and rotary tillers.

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