What is Soil pH?
Soil pH is a measurement of the number of H
+ and OH
- ions present in the soil.
What does “wet pH” mean?
This measurement is what is known on a soil test report as “wet pH” or wpH, this number is the current pH of the soil in question. Wet pH analysis is a measurement of the hydrogen ion concentration (H
+) in solution.
What is buffer pH (BpH)?
Soil test reports will also include “buffer pH” or BpH on the soil test report, where pH is measured again after being placed in a neutral solution to determine the effectiveness a liming material will have on the soil.
How is soil pH measured?
Soil pH is measured during the soil lab analysis process by placing the composite soil sample in a predetermined mixture of soil to water. Then, utilize a pH electrode instrument to measure the electric potential produced by
ions in the solution. The soil pH is a measurement of the number of H
+ and OH
- ions present in the soil.
What do the pH measurements mean?
pH in soil is measured on a scale of 1 to 14 with a neutral reading of 7.0. At a pH of 7.0 the concentration of H
+ and OH
- are in equilibrium with one another.
When soil pH is below 7.0 H
+ ions dominate. While in soils with pH above 7.0 OH
- ions are the dominant form.
Each unit of increase in pH represents a 10x decrease in H
+ concentration. These ions influence the form and reactions of essential plant nutrients in soils. Which ultimately determines their availability to the plant throughout the growing season.
What causes acidity in agricultural soil?
The pH of an agricultural soil is prone to become acidic (decrease) over time. Causative factors of acidity include atmospheric precipitation, plant root exudates, leaching of nutrients, crop removal of nutrients, and nitrification of ammonium.
Throughout the southern U.S. the nitrification of ammonium from fertilizer applications is likely the greatest contributor to observed decreases in soil pH. This is due in part to the continued increase in acreage, yield, and subsequent rates of nitrogen (N) applied to produce grain crops like corn. These contributing factors that follow crop yield trends have led many fields to require applications of lime materials more often than we are historically accustomed to.
What about lime?
The amount of lime needed to counteract the effect of N fertilizers ranges from 4 to 7 pounds of lime per pound of applied N dependent upon N source (Figure 1).
What is the optimal soil pH for row crops?
Soil pH affects nutrient availability to crops by influencing the predominate forms of essential nutrients within the soil. Optimum soil pH for most row crops ranges from 6.2 to 7.2, providing maximum nutrient availability potential to all essential nutrients (Figure 2).
What about phosphorus (P)?
Phosphorus (P) is the most pH dependent macronutrient and has the greatest availability at a pH range of 6.2 to 7.2. Potassium (K) availability is greatest above a pH of 6.0, while nitrogen (N) availability is greatest near neutrality and begins to lose stability at a pH below 6.0.
What about sulfur (S)?
Secondary nutrient sulfur (S) reacts similarly to nitrogen (N). It loses stability at lower pH (<6.0). When a soil pH falls below 6.0 the availability of the essential nutrients N, P, K, S, calcium (Ca), magnesium (Mg), and molybdenum (Mo) is severely limited and often leads to nutrient deficiency symptoms during the growing season.
What happens when soil pH rises above 7.5?
When a soil pH rises above 7.5 the availability of the essential nutrients phosphorus (P), iron (Fe), manganese (Mn), Boron (B), Copper (Cu), and Zinc (Zn) is severely limited and often leads to nutrient deficiency symptoms during the growing season. The figure below (Figure 2.) depicts nutrient availability across soil pH, the wider the bar the greater availability.
What to do when soil pH falls out of optimum range
When soil pH begins to fall below or above the optimum ranges of 6.2 to 7.2 it is recommended to take corrective action as quickly as possible to provide the maximum window for the amendment to affect the soil pH.
Again, agricultural soils are predisposed to become more acidic (decrease) over time where applications of liming material can neutralize the soil acidity and increase the pH.
Factors to consider when selecting liming material
Liming materials contain a “base” anion consisting of oxides, hydroxides, carbonates, or silicates which react with H+ ions thus neutralizing them and in turn raising the pH. When selecting a liming material, there are a couple considerations to keep in mind before selecting a one for application.
1: Fineness Factor:
describes the size of particles in the material that pass through 10 and 50 mesh sieves. Liming materials that contain smaller particles will react more quickly within the soil (Figure 3). and generally make a better liming source.
2: Calcium Carbonate Equivalent (CCE):
a measurement describing the effectiveness of materials at neutralizing acidity compared to pure calcium carbonate. The higher the CCE number the more effective the material is. When making a liming recommendation, CCE is considered to determine application rate. When utilizing Simplot Advisor, CCE is provided when a product is selected.
However, remember when observing a recommendation from outside labs that the per acre recommendation is based on 100% CCE and must be divided by the % CCE of the material used.
3: Lime materials take time to break down:
When correcting pH fall applications are imperative as most lime sources take 8 to 12 months to fully breakdown depending upon particle size (Figure 3). Incorporation into the soil profile after a lime application is always recommended and will increase breakdown of the material as well as movement into the soil profile. Maintaining proper soil pH is crucial to an effective nutrient management program. Soil pH is the basis for essential nutrient availability within the soil and maintaining soil pH from 6.2 to 7.2 ensures that applied nutrients are available to the plant.
When should I take a soil sample?
The time following harvest provides a great opportunity for soil samples to be taken and allows a timeframe for any necessary corrective action to be taken before next season’s crop. Selecting a quality lime source is paramount to ensuring a liming application is effective.
The Takeaway
Remember that liming materials take time to react within the soil profile making fall applications critical if we want to influence next season’s crop. For further information or help making in-field decisions, please consult any of your local Agronomic Sales Enablement Team members or your local
Simplot Grower Solutions Crop Advisor.
Authors
Connor Ferguson, Texoma
Justin McCoy, Southwest
Ben Lawrence, Bayou
Jeffrey Mansour, Midsouth
Click to read more about the authors
