Soil testing is a necessary practice that allows us to better understand our landscape’s full potential, particularly in regard to soil acidity, also known as soil pH. Understanding soil pH and its relationship to healthy landscapes helps us determine how to manage soils according to plant needs and achieve the highest level of agronomic performance.
Soil pH 101.
Soil pH, or soil acidity, is the amount of hydrogen (H+) ions in the soil solution. The actual number of H+ ions is very large, so a pH scale is used to provide a convenient way to express acidity and alkalinity of soil solutions. The pH scale simplifies the acidity status of soils: values below seven are acidic and values above seven are alkaline. The farther away from a measurement of seven (neutral), the more acidic or alkaline the soil.
Measuring and understanding soil pH is important because the H+ ion is highly reactive and greatly influences the chemical properties of the soil and soil solution. Soil pH determines the solubility of nutrients and their availability to plants and microorganisms. At varying ranges, some nutrients may become bound in chemical combinations that render them unavailable, while others may become solubilized and produce injury due to toxicity.
Aluminum and manganese are well documented for producing toxicity to some plants as pH values approach 5 and below, while phosphorus is bound and unavailable. Iron and manganese become increasingly unavailable to plants near neutral and above a pH of 7, and can result in yellow vegetation know as chlorosis.
Plants that are growing in soils with pH values that are not near the appropriate ranges will appear weak, have poor color and will not respond to applied nutrition.
The correct pH for a landscape can vary because plants and microorganisms adapt to function and thrive in certain pH ranges. Ideal pH ranges differ for many organisms, but generally, the most hospitable soil pH range for plants is 6 to 6.8, or just below neutral. At this range, nutrients are soluble and therefore available to plants. If a soil has a low pH, you should elevate the level to an agronomic range between 5.5 and 6.5. If the pH status is outside of this value range, plants can become stressed and suffer.
It is easy to find information on the proper pH range for your landscape plants. University and extension agencies have researched most plants and have accurate information that can guide you to a program for optimal pH conditions.
A pH meter will give the pH measurement of a solution. The pH of a soil is usually measured in a slurry of soil and water. In soil testing laboratories, one part of soil is mixed with one or two parts of water. Then hydrogen electrodes are immersed in the stirred suspension.
A pH meter must be calibrated with a series of solutions of known pH. Colorimetric indicators are also a method of determining pH. Solutions or paper containing a universal color indicator can be used to compare with a color chart to establish the pH of the solution.
Soil samples should be collected in a manner that will give an average of the soil pH for a specific, homogeneous area. Take soil cores to the depth of the root zone, typically 4 to 6 inches, and thoroughly mix them in the sample bag. You should not include soil cores from different areas, but instead separate the samples for measurement later.
This process will allow you to map out the different soil types and pH values of the landscape.
Since many landscapers use fill dirt to develop the final grade before planting, you might have varying soil types with different pH values.
Time to sample.
It is important to know the soil pH at the initiation of service to a property so that adjustments can be made from the start along with proper communication of pH status to the property owner.
Soil pH measurement at the time of initial contact and before the agreement is consummated is recommended so that the relationship with the customer begins with full knowledge of this influential soil characteristic.
Sampling and measuring soil pH once a year or when a problem arises will help ensure a long, trusting relationship. Soil pH values can change with time and identifying trends will allow for proactive application of treatments before catastrophic failure.
Adjusting soil pH is like steering a very large ship. An ocean liner will not change direction quickly upon turning and a soil’s pH will not adjust quickly either. However, with proper applications of amendments and follow up measurements, the soil will become hospitable to a healthy landscape.
The pH of irrigation water, the deposition and breakdown of plant debris and the soil mineralogy can buffer soil pH, which means the soil is resistant to change. As the pH is neutralized, more acidity or alkalinity is replenished in the soil solution from these other external influences in the landscape.
Raising soil pH can be accomplished by the addition of dolomitic lime. Dolomitic lime contains calcium and magnesium which are both macronutrients, plus carbonate to neutralize acid. The lime should be fine textured, but a pelletized lime is also acceptable for ease of application.
Liming materials are only sparingly soluble, therefore the rate and extent of dissolution in the soil are dependent on the amount of surface area in contact with the soil solution. You should follow a laboratory recommendation for rate or apply 50 pounds per 1,000 square feet and follow up with additional measurements in six months.
Lowering soil pH may be necessary to improve plant health and vigor. You can accomplish this through the utilization of acid forming fertilizers such as ammonium sulfate or urea.
Landscapes can be treated with elemental sulfur. Sulfur is oxidized by microorganisms and produces acid and sulfate ions.
You should follow label recommendations and rates but a typical application is four pounds of granular sulfur (not sulfate) per 1,000 square feet evenly watered into the soil. These microbial reactions are temperature-dependent, so warm weather will increase the reaction speed.
Soil acidity or alkalinity, expressed as pH, is a master variable that greatly affects chemical and biological properties of soils. Proper management of soil pH can be a major determinant of landscape quality.
The author has a Ph.D. in soil chemistry and is a field training manager with Massey Services, a pest prevention, termite protection and landscape care company based in Orlando, Florida.