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 yard or garden. It is also one of the most cost-effective and easiest-to-manage soil properties that can be modified to improve plant health and production.
Background Information | Importance of a Soil Test | How to Apply Amendments | 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 plant uptake or less available depending on soil pH. For instance, the micronutrient iron becomes 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 lowering soil pH rather than adding additional micronutrients.
Learn more in this article: The Importance of Soil pH.
What is Soil pH?
Soil pH is the measure of the concentration of hydrogen atoms in solution. The scale ranges 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 this when adding amendments to modify soil pH because 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 range from 4.5 to 8.2. The ideal pH range for most plants grown in Iowa is 6.0 to 7.0, but some plants, like blueberries and azaleas, prefer more acidic soils, while others, like lilac, peony, and salvia, prefer more alkaline soils. Some plants will change in appearance in response to soil pH. In bigleaf hydrangea, flowers are pink in higher pH and blue in lower.
Further complicating the issue, soil pH is not static; it can change over time due to fertilization practices, irrigation, or natural weathering.
Before you Begin, Get a Soil Test
Soil pH can be easily adjusted 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.
Learn more 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, provided the soil temperature is above 55°F.
The most effective way to change soil pH is to do it 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.
Allow the soil time to change. It may take weeks or months. When making significant changes to soil pH before planting perennial crops, it is best to amend the soil, wait 6 months, and recheck the 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 longer, since 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 as deeply as possible without damaging the roots. Water thoroughly after the application. When applying amendments as a top dressing, plant response will take longer and depends on 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 reduce the time required to adjust soil pH while protecting plants from overapplication, which 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; 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 very quickly in soil because it undergoes only a chemical process. The change in pH happens within days or weeks. However, aluminum sulfate is not an ideal amendment because it 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 sometimes used by commercial growers because it is somewhat soluble and can also serve as an annual nitrogen source. It is intended as a safeguard to help keep soil pH within the desired range. It is not an effective means of reducing soil pH pre- or post-planting. Other fertilizers, such as diammonium phosphate, monoammonium phosphate, and urea, are acidifying agents that may decrease pH over time or help keep pH down on naturally high-pH soils. These fertilizers should not be used to lower soil pH, but they can cause a gradual decrease in soil pH when used extensively, as in agricultural fields.
Canadian 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 the amount of pure elemental sulfur required per 1,000 sq ft to decrease soil pH based on the existing pH, target pH, and soil type.
To determine the amount of amendment needed for your yard or garden (which is not likely exactly 1,000 sq ft), take the amount of amendment needed for 1,000 sq ft multiplied by the size of your yard or garden, and then divide by 1,000.
| Soil pH | Soil Type | |||
|---|---|---|---|---|
| Current | Target | Sandy Loam (lbs/1,000 sq ft.) | Loam (lbs/1,000 sq ft.) | Clay Loam (lbs/1,000 sq ft.) |
| 5.0 | 4.5 | 4 | 13 | 18 |
| 5.5 | 4.5 | 8 | 26 | 37 |
| 6.0 | 4.5 | 12 | 38 | 53 |
| 6.5 | 4.5 | 15 | 50 | 70 |
| 7.0 | 4.5 | 19 | 65 | 88 |
| 7.5 | 4.5 | 23 | 75 | 104 |
| 7.0 | 6.0 | 8 | 15 | 18 |
| 7.5 | 6.0 | 16 | 27 | 31 |
| 8.0 | 6.0 | 31 | 44 | 53 |
| 8.5 | 6.0 | 48 | 65 | 74 |
| 7.0 | 6.5 | 2 | 4 | 74 |
| 7.5 | 6.5 | 11 | 20 | 23 |
| 8.0 | 6.5 | 28 | 27 | 46 |
| 8.5 | 6.5 | 46 | 60 | 69 |
Increasing the Soil pH
Lime
The pH of acidic soil can be raised by adding lime. Most lime in Iowa is a mixture of calcium and magnesium carbonates, though the ratios vary. Lime is slow-acting but relatively inexpensive and safe to use. Hydrated lime is more reactive and raises soil pH faster than lime; however, it is dangerous to handle.
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 uses the buffer pH figure, not the soil pH. 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 1,000 sq ft, to increase soil pH from its present level to 6.5 or 6.9 to a depth of 6 inches. When bulk agricultural lime is used, additional adjustments are required to correct for particle size and purity.
To determine the amount of amendment needed for your yard or garden (which is not likely exactly 1,000 sq ft), take the amount of amendment needed for 1,000 sq ft multiplied by the size of your yard or garden, and then divide by 1,000.
| Buffer pH | Target Soil pH = 6.5 (lbs/1,000 sq ft.) | Target Soil pH = 6.9 (lbs/1,000 sq ft.) |
|---|---|---|
| 7.0 | 0 | 25 |
| 6.9 | 0 | 44 |
| 6.8 | 14 | 62 |
| 6.7 | 30 | 80 |
| 6.6 | 48 | 101 |
| 6.5 | 64 | 119 |
| 6.4 | 80 | 138 |
| 6.3 | 96 | 16 |
| 6.2 | 115 | 177 |
| 6.1 | 131 | 195 |
| 6.0 | 147 | 213 |
| 5.9 | 163 | 232 |
| 5.8 | 181 | 253 |
| 5.7 | 197 | 271 |
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 in soil pH. It is often used as a fertilizer to supply additional calcium or sulfur to the soil without altering pH.
More Information
- Managing Garden Soil
- How to Interpret Soil Test Results
- Introduction to Soil Science
- Managing Soil pH in Horticulture Crops from the Iowa State University Extension and Outreach Small Farm Sustainability Team
- Micronutrient Deficiencies of Trees
- Soil pH Preferences for Selected Landscape Plants, University of Minnesota (PDF)
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