Lowering Soil pH
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Most plants thrive in neutral to slightly acidic soil conditions with a pH between 6 to 7. Most soils in West Virginia are slightly acidic, but there are still instances where there is a need to make certain adjustments for the good of the plants. Plants, such as blueberries, azaleas, rhododendrons, magnolia, Japanese pieris, hydrangeas, daffodils, nasturtium or gardenia, that grow best under acid conditions require amendments to lower pH. Making soil amendments is important because pH is one of the most crucial factors in plant nutrition.
A plant’s nutrient uptake depends on many factors, including plant species and soil properties. Soil properties determine potential fertility level, solubility, available organic matter, mineralogy, texture, moisture content, response to temperature and the sun, air and oxygen relation, and soil water evaporation rate, among others. Soil acidity is used to express the quantity of hydrogen (H+) and aluminum (Al+3) cations in soils. Measured soil acidity level depends on the concentration of H+ ions in the soil solution. The pH scale goes from 1 to 14, with 1 being extremely acid and 14 extremely alkaline. In general, soil pH ranges from 3 to 11 and directly influences the availability, mobility and uptake of nutrients by plants through its impact on chemical composition of nutrients and soil microorganisms’ activity.
Determining the need for lowering the soil pH
Knowing how to recognize plants’ signs and symptoms will provide an insight into
the state of their nutrition. If soil pH is outside the acceptable range for the
optimal plant performance, visual symptoms will indicate whether there is a lack
off or a toxic excess of certain elements. Acid-loving plants grown in high pH
soils will develop visual symptoms of chlorosis or distinct yellowing of the leaves
between the veins and narrow green bands along the veins (Fig.1). To prevent this
from happening, soil pH must be kept within the optimum range, ensuring adequate
nutrient supply in the soil solution.
Figure 1. Severe chlorosis due to iron deficiency in blueberry and oak.
Processes leading to lowering pH
The first step is to send the soil sample to the WVU Soil Testing Lab to determine the soil pH. It is important to know that the amendments needed depend on soil characteristics. It is more difficult to acidify soils with high organic matter and high clay content, soils with free lime content, and soils with high cation exchange capacity and high buffering capacity. It is important to realize that there are several different types of acidity. Your soil’s acidity could be active, exchangeable (sometimes called replaceable), and residual soil acidity. The active acidity is the acidity of the soil and water solution showing the increased presence of the hydrogen ions (H+) outnumbering the hydroxyl (OH-) ions. A higher amount of carbon-dioxide (CO2) in the presence of water or moisture in the soil forms carbonic acid. This acid breaks down, releasing H+ and HCO3 -, which increases the hydrogen ion concentration in the soil solution and acidifying the soil.
H2O+CO2↔H2CO3↔H++HCO−3
Depending on the cation exchange capacity and composition of the exchangeable basis (Ca, Mg, Na), carbonic acid formed by the reaction between CO2 and water in the soil can be neutralized by exchangeable basis, creating calcium, magnesium or sodium carbonates. The reaction between the calcium carbonate or an absorbing complex saturated with calcium and carbonic acid will produce water-soluble calcium bicarbonate, Ca(HCO3)2.
CaCO3 + H2CO3 = Ca(HCO3)2
Calcium carbonate undergoes hydrolysis in the soil, producing very weak carbonic acid and a strong basis.
CaCO3 + H2O→Ca2++HCO−3+OH−
The same processes occur if there is soil particle saturation with sodium (Na), creating sodium carbonate that hydrolyzes into a very strong base, NaOH, and weak carbonic acid, creating more alkaline solution.
The laboratory soil water pH test gives information on the active acidity, but it does not measure the reserve acidity formed by the residual and exchangeable acidity.
Exchangeable acidity or salt replaceable acidity is correlated to the aluminum and hydrogen ions absorbed on the cation exchange sites (on the soil particle) that are flushed out with salts, like potassium chloride (KCl), replacing the hydrogen cations with potassium on the soil particle, releasing the hydrogen cation into the solution.
Because of this displacement, the soil solution is acidified. Besides exchangeable hydrogen, highly acidic mineral soils contain exchangeable aluminum, which can pass into the solution while interacting with neutral salts.
In the solution, aluminum chloride (AlCl3) undergoes hydrolytic dissociation, yielding strong acid and a weak base.
AlCl3 +3H20 = Al(OH)3 + 3HCl
Exchangeable acidity could be defined as acidity that is created by exchangeable hydrogen and aluminum ions extracted from the soil with neutral salts.
The exchangeable cations are in equilibrium with the cations present in the soil solution. As the cations are being removed from the soil solution, they are replaced with the adsorbed or exchangeable cations (on the surface of the soil particles) and become available for the uptake by the roots.
Residual acidity is correlated to aluminum, iron and hydrogen ions. When a soil is treated with a neutral salt solution, not all hydrogen ions pass from the soil particle into the solution. It is more tightly bound to the soil particle and could be released when treated with a solution of hydrolytically alkaline salt. The alkaline reaction of the solution of the salt or a stronger base, driving pH above 8, is the main reason why the exchangeable hydrogen is pushed out of the salt into the solution.
Potential acidity depends on the presence of the exchangeable hydrogen and aluminum ions.
Acidic soils, particularly highly acidic mineral soils, contain exchangeable aluminum (Al) that is pushed into the soil solution through hydrolysis reactions, forming hydrogen ions (H+).
Hydrolyses by Aluminum
Al3+ + H20 = Al(OH)2+ + H+
Al(OH)2+ + H20 = Al(OH)2+ + H+
Al(OH)2+ + H20 = Al(OH)O3 + H+
The end result is a strong acidifying effect on the soil.
Altering pH towards the lower, acid range could be accomplished by using several available products. They include elemental sulfur, aluminum sulfate, iron sulfate, ammonium sulfate, urea, ammonium nitrate, ammonium chloride, mulches and sphagnum peat moss.
Organic mulch, sphagnum peat moss and sulfur need to be incorporated into the soil to the depth of 8 to 12 inches before planting. Breakdown of the organic matter and conversion of nutrients to their available or water-soluble forms are greatly depending on the soil type, texture, temperature and aeration.
Aluminum sulfate and iron sulfate are oxidized forms of sulfur and can lower soil pH faster than elemental sulfur. However, application of these fertilizers requires a significantly higher rate to be effective, about six to eight times, making them more expensive (Table 1).
The other concern is that these two fertilizers in excess quantities can be phytotoxic.
Application of fertilizers that contain nitrogen or ammonium has a strong acidifying effect; they have a physiologically acid reaction (Table 1).
The oxidation of ammonium NH4+ to a final product of NO3– is facilitated by bacteria. In the irreversible reaction shown below, it releases H+ and consequently acidifies the soil.
2 NH4++4O2→2 NO3–+2H2O+4H+
Table 1. Pounds of fertilizer needed to equal effectiveness of one pound of sulfur.
| Fertilizer material | Pounds of fertilizer needed to equal 1 pound of sulfur |
| Sulfur | 1 |
| Aluminum sulfate | 6.94 |
| Iron sulfate | 8.96 |
| Ammonium nitrate | 5.91 |
| Urea | 3.71 |
| Di-ammonium-phosphate | 4.45 |
To substitute amounts of sulfur in Table 2 and Table 3 with any of the fertilizers presented in Table 1, multiply with the pounds of fertilizer needed to equal one pound of sulfur.
The process of bio-oxidation of nitrogen in the added Ammonium sulfate, produces nitric acid, sulfuric acid and water.
(NH4)2SO4 + 4O2 = H2SO4 + HNO3 + 2H2O
These acids are neutralized to a certain extent by the bicarbonates present in the soil solution and the cations absorbed on the soil particles. The neutralization of the inorganic acids and breakdown of the bicarbonates and displacement of base from the soil particle (absorbing complex) is done by hydrogen. Excess of hydrogen ions in the soil solution lowers buffering capacity and increases acidity.
2HNO3 + Ca(HCO3)2 = Ca(NO3)2 +2H2CO3
H2SO4 + Ca(HCO3)2 = CaSO4 + 2H2CO3
Probably, the safest way to acidify the soil is by adding granular sulfur. However, this is not the fastest acting amendment. Sulfur undergoes a slow process of sulfur oxidation in which the elemental sulfur is transformed into sulfate thanks to intense microbial activity. During that process, hydrogen ions are released into the soil solution as well as sulfuric acid that lowers the pH. It takes about a year for the process of sulfur oxidation to complete and reduce the soil pH. The oxidation of elemental sulfur (S) in neutral or alkaline soils to a final product of SO4+ is shown below, and it releases H+ and therefore acidifies the soil.
2S+3O2+2H2O→H2 SO4+4↔SO2−4+2H+
Microbes are living organisms and the intensity of their activity is mainly a function of soil temperature (must be 55 F or above), moisture, illumination and aeration. Under water-saturated or flooded soil conditions, anaerobic bacteria would convert sulfur into hydrogen sulfide (H2S - smells like rotten eggs), which is phytotoxic and will kill the roots. The best timing for sulfur or sulfur-containing fertilizer applications is in spring, when the soil temperature is suitable for the bacterial activity.
Attempts to change pH drastically, from the values 8 and above to desired levels of 4.2 to 5, may not be economically feasible. Besides, soils with a pH above 7 contain some undissolved calcium and magnesium carbonates that immediately neutralize the acid produced by sulfur oxidation.
In the tables presented below (Table 2 and Table 3) are amounts of sulfur necessary to change pH to a desired value depending on the soil current reaction and its texture.
Table 2. Amount of sulfur required to lower the soil pH for growing blueberries in pounds per acre. Source: The Mid-Atlantic Berry Guide for Commercial Growers 2013-2014.
Table 3. Amount of sulfur required to lower the soil pH for growing blueberries in pounds per 100 square feet. Source: Adapted from The Mid-Atlantic Berry Guide for Commercial Growers 2013- 2014. Recalculated by M. Danilovich to show the amount of sulfur needed to lower present pH to desired velues in pounds per 100 square feet.
Table 4. Pounds of Ammonium sulfate per 100 square feet to lower pH to recommended level. S ource: Adapted from the Mid Atlantic Berry Guide for Commercial Growers 2013-2013. Recalculated by M. Danilovich by using 2.83 pounds of ammonium sulfate as an equivalent for 1 pound of sulfur (Table 1).
The Soil Fertility Handbook provides a nice table with recommendations for blueberries that shows how much sulfur is needed to lower pH for 1 pH unit depending on a soil texture. The following example is based on their model.
Example: Sulfur recommendation for blueberries
The soil type is loamy silt. The soil test indicates that the pH is 6.2. The soil pH should be lowered to 4.5.
6.2-4.5=1.7
The soil pH needs to be lowered for 1.7 units.
Take the values for silt from Table 3 that are closest to the pH revealed by the soil test. In this case, it would be 6.5.
To bring the pH from 6.5 to 4.5, or lower it 2 units, it takes 13 pounds of sulfur. Therefore, lowering the pH 1.7 units will take X pounds of sulfur.
For 2 units: 13 pounds = 1.7 units: X pounds
X=(13*1.7)/2=20.8/2=10.4 pounds
So, in this scenario, to go from pH 6.2 to 4.5 it would take 10.4 pounds of sulfur.
Sources:
The Mid-Atlantic Berry Guide 2013-2014 – 034_AGRS097=1
Kluepfel, Marjan; B. Lippert; J. Williamson: Changing the pH of Your Soil. https://hgic.clemson.edu/factsheet/changing-the-ph-of-your-soil/ , 2012
Soil Fertility Handbook. Ontario Ministry of Agriculture, Food and Rural Affairs. Queen’s Printer for Ontario, 1998
Hart, John: Fertilizer and Lime Materials. Archival copy. For current information, see OSU Extension Catalog: https://catalog.extension.oregonstate.edu/em9060 ; https://catalog.extension.oregonstate.edu/em9057
B. A. Yagodin, Editor: Agricultural Chemistry, Vol. 1. Mir Publishers Moscow, 1984
Skousen, Jeff and L. McDonald: Land Reclamation: Liming Principles and Liming Products. WVU Extension Factsheet, November 2005.
Author: Mira Bulatovic-Danilovich, Consumer Horticulture Specialist, WVU Extension Agriculture and Natural Resources
Special thanks to Eugenia Pena Yewtukhiw and Edward Rayburn for reviewing this information.