Soil pH in The Home Garden

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 crop production inputs.  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.

 

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.

 

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.

 

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.

 

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 can be found in the extension publication Soil Sample Information Sheet for Horticulture Crops ST0011 via the Iowa State University Extension Store

 

It is absolutely necessary to make soil pH 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.  This is not an instantaneous process 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 6 months, and recheck the soil pH. 

 
Decreasing the Soil pH

 

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.

 

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 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.

 

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), and soil type.  Table 1 indicates 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. 

 

Table 1: Pounds of sulfur required to decrease soil pH to a depth of 6 inches per 1000 sq ft. 


















Soil pH Soil Type
Current Target Sandy Loam Loam Clay Loam
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

 

The pH of acidic soil can be raised by incorporating limestone into the soil.  Most limestone found in Iowa is a mix of calcium and magnesium carbonate although the ratios will vary.  Limestone 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.

 

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) and 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. Table 2 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.

 

Table 2: Pounds of lime required to increase soil pH to a depth of 6 inches per 1000 sq ft.

 











Buffer pH Target Soil pH Buffer pH Target Soil pH
  6.5 6.9   6.5 6.9
7.0 0 25 6.3 96 16
6.9 0 44 6.2 115 177
6.8 14 62 6.1 131 195
6.7 30 80 6.0 147 213
6.6 48 101 5.9 163 232
6.5 64 119 5.8 181 253
6.4 80 138 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.

 
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.

 
Summary

 

Modifying soil pH is a slow process that requires patience.  Always begin the process with a good soil sample to determine the existing soil pH, buffer pH, and cation exchange capacity.  Apply the required amendments and incorporate into the soil as outlined above.  If planting perennial plants, recheck the soil pH three to six growing season months later to verify that the soil pH has reached the desired range.  If it has not, additional amendments may be necessary.

 
Notes

 

Commercial fruit and vegetable growers are encouraged to see Managing Soil pH in Horticulture Crops at IowaProduce.org.

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Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Horticulture and Home Pest News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on February 12, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.