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Adjusting Soil pH

Peat moss
Add peat moss to lower soil pH
Add peat moss to lower soil pH

The results of a soil pH test will indicate how acid or alkaline your soil is. Organic soil amendments are the best way to adjust soil pH. Adjusting soil pH is not an exact science and takes time.

A pH test number that is more than 0.5 on either side of the optimal pH number for the plants you want to grow will require a soil amendment or additive to adjust the pH. A pH test number that is within 0.5 of the optimal pH number for the plants you want to grow does not require soil amendments.

The quickest way to change pH is to add either agricultural sulfur (powdered sulfur, aluminum sulfate, or iron sulfate) to make alkaline soils more acid or agricultural ground limestone to make acid soils more alkaline. The best long-term way to improve soil pH is to add other slower, but longer acting organic materials (see the lists below).

The amount of additives or amendments necessary to correct soil pH varies according to soil texture. Sandy soil requires the least amount of additive, loamy soil a bit more, and clay soil the most. To determine the texture of your soil, wet a bit of garden soil and rub it between your fingers; if the soil feels gritty it is sandy; if it feels smooth like flour it is loamy or silty, if it feels sticky or slippery it has high clay content.

If you are adding agricultural sulfur or ground limestone to your garden, follow the recommended application rate listed on the package or bag. The amount will vary according to the type of soil in your garden and the number of square feet you are amending.

After adding sulfur or limestone or any soil additive to your garden re-test the soil in 40 to 60 days. Expect small changes to the pH—0.5 to 1 unit at most. Allow at least five or six weeks between the applications of soil amendments. Work steadily towards achieving the pH that will be ideal for plants you want to grow. Changing the soil pH by one unit each year is reasonable.

A good time to begin adjusting soil pH is in the Fall; check the progress again in the Spring. If you begin the process in Spring, start early–at least three weeks before planting. This will allow soil additives to begin to break down and start working. Check the soil pH regularly—twice a year is ideal.

Organic materials to adjust soil pH:

 

Adjusting Alkaline Soil:

The best way to lower soil pH to neutral 6.0 to 7.0 is to add naturally acidic organic materials:

  • Aged sawdust
  • Wood chips
  • Leaf mold
  • Peat moss
  • Cottonseed meal
  • Pine needles
  • Coffee grounds
  • Fresh manure
  • Oak leaves
  • Ground up oak bark
  • Pine needles

If your garden is in a region of low rainfall, a high pH could be the result of accumulated salts. Flush salts below the root zone of sensitive plants such as beans, carrots, onions, and peppers by watering regular non-saline water. Also add compost, mulch, or leaf mold.

The best time to correct soil alkalinity is in the Fall; that will allow organic materials time to decompose (organic materials tie up nitrogen while decomposing).

Adjusting Acid Soil:

To raise soil pH of acid soil is add any of these long-acting organic materials:

  • Bonemeal
  • Ground clamshells
  • Ground eggshells
  • Ground oyster shells
  • Hardwood ashes
  • Fine-ground agricultural lime: dolomitic lime (if the soil test reveals a magnesium deficiency) or calcitic lime (if the soil test reveals a calcium deficiency). The general guidelines for applying limestone to increase pH by one unit across 100 square feet: 3 pounds of sandy soil; 5 pounds on sandy-loam; 7 on loam; and 8 pounds on clay soil.

The best time to correct soil acidity is in the Fall. If you want to correct soil acidity in the Spring do as at least three weeks before planting; that will allow time for soil additives to begin to work.

Related Articles:

Understanding Soil pH

How to Test Your Soil

 

Written by Stephen Albert

Stephen Albert is a horticulturist, master gardener, and certified nurseryman who has taught at the University of California for more than 25 years. He holds graduate degrees from the University of California and the University of Iowa. His books include Vegetable Garden Grower’s Guide, Vegetable Garden Almanac & Planner, Tomato Grower’s Answer Book, and Kitchen Garden Grower’s Guide. His Vegetable Garden Grower’s Masterclass is available online. Harvesttotable.com has more than 10 million visitors each year.

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