Joe Hannan
horticulture field specialist
Iowa State University Extension and Outreach
jmhannan@iastate.edu
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.
Elements do not exist in the soil in pure forms, but instead are found as positively charged cations or negatively charged anions. Each element exists in several different chemical forms within the soil but only a few are able to be taken up by plants. Which chemical form an element is in is determined, in part, by soil pH. As pH changes, so does the availability of each element to be taken up by plants (Figure 1). 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
Figure 1. Effects if soil pH on nutrient availability.
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, it is often necessary to adjust 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, buffer pH and cation exchange capacity (CEC). 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 and Outreach Online Store. It is absolutely necessary to make soil pH changes before planting. Lime and sulfur are not water soluble and need to be mechanically incorporated into the soil with a tiller, shovel, or disk to a depth of 6 inches. Once plants are in the ground, it is nearly possible to make further corrections. In addition, it is not 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 depending on product applied.
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. Apply additional amendments if necessary. Because perennial crops are long lived and difficult to establish, it is critical to ensure the soil pH is correct before planting.
Decreasing the Soil pH
Elemental sulfur, aluminum sulfate, iron sulfate, and ammonium sulfate are common amendments used to decrease the soil pH. Elemental sulfur is the preferred amendment to decrease soil pH as it is relatively inexpensive, safe to use, and available via local agriculture suppliers and garden centers. Unfortunately, it is slow to react. Elemental sulfur must go through two processes before decreasing soil pH. It must go through a slow biological process followed by a rapid chemical process. 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. It requires significantly more product than elemental sulfur and aluminum is toxic to plants. Iron sulfate, also known as ferrous sulfate, is more costly to use than elemental sulfur and requires eight times more product to decrease the soil pH. The iron sulfate reaction is a fast, chemical reaction similar to aluminum sulfate. This salt disassociates into iron and sulfuric acid. The iron binds to the clay or precipitates out of the soil solution providing little to no fertility value and leaving the sulfuric acid to neutralize the soil. Elemental sulfur, aluminum sulfate, and iron sulfate must be mechanically incorporated into the soil to effectively change the soil pH as none of these products are water soluble and will not move into the soil profile on their own.
Ammonium sulfate is sometime used by commercial growers on soils that are naturally high in pH and have already been modified for crop production. Soils that are naturally high in pH or highly buffered will tend to return to their natural state. Because ammonium sulfate is somewhat soluble, commercial growers may use it as an annual nitrogen source and as an added 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.
Spaghnum 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 spaghnum peat moss has a low pH of 3.0 to 4.5 and will effectively reduce soil pH.
Three pieces of information are required to determine how much sulfur is necessary to decrease the soil pH: current pH, desired pH, and CEC. Some soil labs do not report the cation exchange capacity but an estimate of this value can be found in the soil survey data for each soil type across the state. The Web Soil Survey provides soil data and information produced by the National Cooperative Soil Survey. It is operated by the USDA Natural Resources Conservation Service (NRCS) and provides access to the largest natural resource information system in the world.
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 the cation exchange capacity. Sandy soils have a CEC of 1-10 meq+/100g. Loam soils have a CEC of 5-15 meq+/100g. Clay loam soils have a CEC of 15-30 meq+/100g while clay soils have a CEC of greater than 30 meq+/100g. For best results, sulfur should be broadcast across the soil surface and incorporated to a depth of six inches. For application rates greater than one ton per acre, split the application over two years.
Table 1: Pounds of sulfur required to decrease soil pH to a depth of six inches per 1000 sq. ft. Highbush Blueberry Production Guide NRAES-55.
Pounds of Sulfur per 1000 sq ft Required to Modify Soil pH to a Depth of 6 inches
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 of varying ratios. Limestone is slow acting but 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 soil pH: current buffer pH as determined from a soil test and target pH. Table 2 lists lime recommendations, based on SMP Buffer Test, given in pounds of pure fine calcium carbonate (CaCO3) to increase soil pH from its present level to pH 6.5 or 6.9 to a depth of 6 inches per 1000 sq. ft. Note that the lime table requires buffer pH and not soil pH from a soil test. Buffer pH results are only provided on a soil report when liming might be needed as determined by the soil lab. When bulk agricultural lime is used, additional adjustment is required to correct for particle size and purity. This is known as the Effective Calcium Carbonate Equivalent (ECCE). To calculate the quantity of limestone to apply, divide the rate from the chart below by the percent ECCE.
Table 2. Lime recommendations given in pounds of pure fine calcium carbonate (CaCO3) to increase soil pH from its present level to pH 6.5 or 6.9 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 Online Store.
Troublesome Soils
Some soils are very difficult to change the soil pH or cannot be changed. Soils with high cation exchange capacities such as clay loams often require prohibitively large quantities of lime or sulfur to effectively change the pH. These soils will naturally return to their previous soil pH if not constantly managed with additional lime or sulfur applications. In these situations it is best to find plants that are better suited for the natural pH of the soil. In situations where no alternative site is feasible, pH induced micronutrient deficiencies can be managed with foliar applications. Boron, iron, manganese, and zinc are often applied to specialty crops in Iowa where soil pH cannot be amended. Before applying foliar nutrients, foliar sample to verify need. Some crops species and cultivars within species are better suited to adverse soil conditions than others are.
Across the Western Iowa Missouri river valley and the Northeast Iowa Mississippi river valley as far south as Dubuque County, many of the soils have limestone bedrock very close to the soil surface. The limestone provides a natural supply of calcium carbonate to the soil. This carbonate maintains the soil pH at approximately 8.2. These soils are known as calcareous soils and cannot be reduced regardless of the amount of sulfur applied to the soil.
Soils that have previously been limed may have unused calcium carbonate in them. To decrease the soil pH of a soil with unused calcium carbonate, the unused lime must also be neutralized with sulfur. To test if a soil has free calcium carbonate in it, a fizz test should be conducted to determine how much free carbonate is present. This is done by applying a few drops of household vinegar to the soil and listening and watching for bubbling. Non-calcareous soils can have additional sulfur applied to neutralize the free carbonate in addition to the sulfur required to decrease the soil pH as listed in table 3.
Table 3 from Acidifying Soil for Crop Production: Inland Pacific Northwest. Oregon St. Univ. Ext. Publ. EM 8917-E provides an estimate of how much free carbonate is in the soil and how much additional sulfur should be applied on an annual basis to neutralize the free carbonate.
Gypsum
Gypsum, also known as calcium sulfate dihydrate CaSO . 2H2O, is a calcium and sulfur fertilizer that produces no net change to soil pH. It is often used as a fertilizer to apply additional calcium or sulfur to the soil when no pH change is desired.
Summary
Modifying soil pH is a slow process that may take several years. Always begin the process with a good soil sample to determine the existing soil pH, buffer pH, and cation exchange capacity. Amend the soil as outlined above and, 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. In some situations, soil amendment may not be feasible. If plants display signs on micronutrient deficiency, foliar applied micronutrients can be used to correct the deficiency.