Soil pH: Do You Know How Acidic Your Soil Is?
This is Part One of a three-part series taking a deep dive into soil pH
Soil pH is an important soil chemical property that affects virtually all chemical and biological processes in the soil, including nutrient availability. It is a property analyzed in most basic soil tests and is shown in a soil test report.
Knowing soil pH levels is critical
Maintaining the soil pH at optimal levels helps to maintain soil quality and overall productivity. A standard soil test can provide growers with the information they need to understand the pH value of their soil and where potential issues may occur.
Effects of soil acidity are variable dependent on a number of other factors, but these are the most common and serious:
- Aluminum and manganese can become toxic at low soil pH because of increased solubility.
- Strongly acidic soil reduces microbial activity and slows organic matter decomposition.
- Activity of symbiotic N-fixing bacteria is optimum at or near neutral pH and decreases with greater soil acidity.
Soil pH can dramatically affect crop nutrient availability in several ways:
- Phosphorus availability is negatively impacted by both high and low pH.
- High pH can reduce micronutrient availability.
- Reduction in plant root growth limits nutrient uptake.
- Calcium (Ca) and magnesium (Mg) are often less abundant in strongly acid soils leading to potential deficiencies.
- High pH can increase volatilization potential of surface-applied urea.
- Soil pH can vary, even in different parts of the same field.
An example of a pH-induced nutrient deficiency is iron chlorosis, exhibited as interveinal chlorosis or yellowing, between the veins of the new leaves. It occurs in alkaline soils and indicates poor soil-iron availability caused by high pH. The iron deficiency reduces chlorophyll needed for photosynthesis, which impacts growth and yield.
On the other end of the scale, low pH often leads to aluminum (Al) toxicity which is considered a major constraint for crop production in acidic soil around the world. When the soil pH is below about 5.0 to 5.5, aluminum minerals dissolve releasing aluminum cations (Al3+) into the soil solution. Soluble aluminum reaches levels toxic to plants and their roots and impairs root cell division and elongation. Stunted root growth causes reduced water and nutrient uptake. (3) Soils in this pH range are also often deficient in calcium, a nutrient that can help mitigate the effects of aluminum.
Optimum soil pH can vary among crops but is generally in the slightly acid range for many crops, about pH 6.0 to 7.0. When the soil pH is in this optimum range, microorganism types and numbers are optimized, nutrient availability is optimized and most crops can thrive. Knowing a crop’s optimum pH can help a grower make management decisions to provide the best conditions for a successful crop.
Optimum soil pH levels for selected crops
At optimum levels, microorganisms such as fungi, bacteria and actinomycetes are optimized. Actinomycetes, an organism that thrives at neutral to slightly alkaline pH, play a major role in organic matter cycling. They inhibit the growth of several plant pathogens in the rhizosphere, and they decompose complex mixtures of compounds in dead plant, animal and fungal material resulting in the production of many extracellular enzymes which are conductive to crop production. (4)
Causes of soil acidity
There are several causes of soil acidity or pH imbalances; some are within the growers’ control, others are not.
Soil acidification is a naturally occurring process. Soil parent material, weathering, native vegetation, climate and precipitation, soil age, and farming practices all impact soil pH. The decomposition of organic matter, leaching of basic cations (such as calcium and magnesium), rainfall (which is naturally acidic), and plant respiration all contribute to the decrease in soil pH. Any fertilizer that includes ammonium as a nitrogen source will undergo nitrification – the biological oxidation of ammonium to nitrate by microorganisms – leading to a decrease in soil pH from the liberation of hydrogen ions (H+) from the ammonium. Some soil minerals, like limestone or certain other salts, are alkaline in nature; their presence leads to alkaline soil or high pH.
Growers can influence soil pH by certain management practices. Crops grown, crop rotation used and fertilizer use (especially nitrogen), are all ways growers can influence the pH of the soil.
A Montana State University Extension fact sheet explains the role nitrogen fertilizer plays in soil acidification. “The primary agronomic cause for low soil pH is use of ammonium-based nitrogen (N) fertilizers (e.g., 46-0-0, 32-0-0, 28-0-0) at rates that exceed crop N uptake. When the ammonium gets converted to nitrate, hydrogen ions (H+) are released, lowering soil pH in the topsoil. If nitrate is taken up, plants release basic anions (OH– and HCO3–) to maintain their charge balance. Conversely, if nitrate leaches, these basic ions are not released and are then not available to balance the acidity coming from ammonium to nitrate conversion.” (5)
Proper soil pH is critical for healthy plant growth, healthy soils and balanced plant nutrition. A proper understanding of soil pH, its causes and its chemistry can help a grower manage this critical soil property. There are solutions to this issue, including liming and nitrogen management.
The next two articles will look at the chemistry of soil pH and impacts on soil quality and nutrient availability, as well as remediation of soils with low or high pH levels and liming.
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