How To Change Your Soil's pH

Iowa soils are very diverse and so are the chemical characteristics that make up these soils.  Soil pH is one property that can vary widely across the state both naturally and due to how we manage the field or garden.  It is also one of the most cost effective and easy to manage soil properties that can be modified to improve plant health and crop production.

Steps to Decrease Soil pH  |  Steps to Increase Soil pH  |  More Information

Why Soil pH is Important?

Soil pH affects the availability of nutrients for plant uptake.  They may become more available for pant uptake or less available depending on soil pH.  For instance, the mircronutrients iron, manganese, and zinc become more available as pH decreases but molybdenum becomes less available.  In Iowa, micronutrient deficiencies such as iron or zinc are often due to above optimum soil pH and can be corrected by reducing the soil pH rather than adding additional micronutrients to the soil.

What is Soil pH?

Soil pH is the measure of concentration of hydrogen atoms in solution.  The scale has a range from 0 to 14.  Less than 7 is acidic, 7 is neutral, and greater than 7 is considered alkaline or basic.  As the soil pH increases, the number of hydrogen atoms decreases.  However, the scale is not linear.  Soil with a pH of 5.5 is 10 times more acidic than soil with a pH of 6.5 and 100 times more acidic than soil with a pH of 7.5.  It is important to remember the scale is not linear when adding amendments to modify soil pH.  When modifying soil pH, doubling the amount of sulfur or lime will not double the change in soil pH.

Soil pH in Iowa

Most soils in Iowa have a pH between 5.5 and 7.5, however Iowa soils can be as low as 4.5 or greater than 8.2.  The ideal range for most plants grown in Iowa is 6.0 to 7.0 but some plants like blueberries and azaleas prefer more acidic soils and others like lilac, peony, and salvia prefer more alkaline soils.  Further complicating the issue, soil pH is not a static condition; it can change over time due to fertilization practices, irrigation, or natural weathering.  Because of the broad range of soil pH found across Iowa and the varying needs of plants, gardeners and farmers often must adjust the soil pH for optimum plant growth and production.

Before you Begin, Get a Soil Test

Soil pH is easily modified in most soils using sulfur or lime.  However, before attempting to modify the soil, collect a soil sample to determine the existing soil pH and buffer pH.  These soil properties are essential components to making informed decisions about amending soil pH.  Directions for collecting a soil sample and getting a soil test can be found in this article: Soil Testing Resources for Home Gardeners

How to Add Amendments to the Soil

Amendments used to change soil pH can be applied in the spring or fall. The soil temperature should be above 55°F when treating the soil.

The most effective way to change soil pH is to make any changes before planting.  Lime and sulfur, amendments used to increase and decrease soil pH, respectively, are not water soluble and need to be mechanically incorporated into the soil with a tiller, shovel, or disk to a depth of 6 to 8 inches.  Once plants are in the ground, it is nearly impossible to make further corrections without disturbing the root system, nor is it reasonable to amend the soil below a 6 to 8-inch depth.  Allow the soil time to change.  Changing soil pH is not instantaneous and may take weeks or months.  When making significant changes to the soil pH before planting perennial crops, it is best to amend the soil, wait six months, and recheck the soil pH to verify it has reached the desired range. If it has not, additional amendments may be necessary.

If plants are already in the ground, adjusting the soil pH will take much more time since the amendments cannot be fully incorporated into the top 6 to 8 inches of soil.  Apply the amendment to the top of the soil in the plant's root zone.  Rake or incorporate it in as deep as possible without damaging the roots.  Water thoroughly after the application.  Topdressing is not as effective as incorporating the amendments into the soil. Plant response will take longer and depends upon soil moisture, application timing, and plant growth. Because the amendment is only being incorporated into the top 1 or 2 inches (rather than 6 to 8), the application amount should be split into several applications over several years.  This will slow the time it takes to adjust the soil pH but protect plants from over-applications that can cause damage or death.

Decreasing the Soil pH

Elemental Sulfur

Elemental sulfur and aluminum sulfate are the most common amendments used to decrease the soil pH.  Elemental sulfur is the safest option to decrease soil pH; it is relatively inexpensive and available via local agriculture suppliers and garden centers.  Unfortunately, it is slow to react.  Elemental sulfur must go through two processes, a biological process and a chemical process, before soil pH is decreased.  This often takes 3 to 6 months of warm soil temperatures when soil biology is active.  Aluminum sulfate reacts in the soil very quickly as it must only undergo a chemical process.  The change in pH happens within days or weeks.  However, aluminum sulfate is not an ideal amendment because requires more material than elemental sulfur to reduce the soil pH and aluminum is toxic to plants.

Ammonium Sulfate

Soils that are naturally high in pH or highly buffered will tend to return to their natural state.  Ammonium sulfate is sometime used by commercial growers on these soils because it is somewhat soluble and may also be used as an annual nitrogen source.  It is intended as a safeguard to help hold the soil pH down within the desired range.  It is not an effective means at reducing the soil pH post planting nor should it be used to decrease the pH preplant.  Other fertilizers such as diammonium phosphate, monoammonium phosphate, and urea are acidifying agents that may decrease pH over time or help hold pH down on naturally high pH soils.  These fertilizers should not be used to decrease the soil pH, but are often responsible for the gradual decrease in soil pH in commercial agriculture fields.

Sphagnum Peat Moss

Sphagnum peat moss is often suggested as a soil amendment to decrease soil pH.  However, most peat moss found in garden centers is neutral or slightly acidic.  Only Canadian sphagnum peat moss has a low pH of 3.0 to 4.5 and will effectively reduce soil pH.  Canadian sphagnum peat moss is best used in addition to sulfur applications when building raised beds for acid-loving plants or when large pH adjustments are required.

Calculating How Much Amendment is Needed

Three pieces of information are required to determine how much sulfur is necessary to decrease the soil pH:  

• Current pH (from a soil test)
• Target pH (what you want the pH to change to)
• Soil type (often noted in soil test)

Use the table below to determine how much pure elemental sulfur is required per 1000 sq ft to decrease the soil pH based upon the existing pH, the target pH, and soil type.  

Pounds of sulfur required to decrease soil pH to a depth of 6 inches per 1000 sq ft. 

Increasing the Soil pH

Lime

The pH of acidic soil can be raised by incorporating lime into the soil.  Most lime found in Iowa is a mix of calcium and magnesium carbonate although the ratios will vary.  Lime is slow acting but it is relatively inexpensive and safe to use.  Hydrated lime is more reactive and will increase the soil pH faster than lime, however it is dangerous to work with.

Calculating How Much Amendment is Needed

Two pieces of information are required to determine how much lime is necessary to increase the soil pH:

• Current buffer pH (from a soil test)
• Target pH (what you want the pH to change to)

Note that the lime table requires buffer pH and not the soil pH from a soil test.  Buffer pH results are only provided on a soil test report when liming might be needed as determined by the soil lab.

The table below lists lime recommendations in pounds of pure fine calcium carbonate per 1000 sq ft to increase soil pH from its present level to pH 6.5 or 6.9 to a depth of 6 inches.  When bulk agricultural lime is used, additional adjustment are required to correct for particle size and purity.

Pounds of lime required to increase soil pH to a depth of 6 inches per 1000 sq ft.

From Table 14 of extension publication Crop Nutrient and Limestone Recommendations in Iowa PM 1688 via the Iowa State University Extension and Outreach Store.

When No pH Change is Desired

Gypsum

Gypsum is a calcium and sulfur fertilizer that produces no net change on soil pH.  It is often used as a fertilizer to supply additional calcium or sulfur to the soil when no pH change is desired.

More Information

Commercial fruit and vegetable growers are encouraged to see Managing Soil pH in Horticulture Crops from the Iowa State University Extension and Outreach Small Farm Sustainability Team