New England Blade – Jason Lanier, UMass Extension Turf Specialist

Soil pH is a measure of acidity, quantified by the number of free hydrogen ions (electrically charged atoms, denoted by H+ in this case) present in the soil “solution” (the liquid fraction of the soil in which nutrients and other compounds are dissolved). It’s a bit counterintuitive, but a lower pH value indicates higher acidity (more hydrogen ions in the soil solution). pH values below 7.0 are in the acidic realm, 7.0 is considered neutral, and above 7.0 is alkaline, or basic (less hydrogen ions in the soil solution). Most soils in New England naturally trend towards the acid side and a typical pH value for un-limed northeastern soils is around 5.0, indicating a significant degree of acidity.

A common misconception is that soil acidity is a tangible, harmful agent that negatively impacts plants in a direct sense. This is not nearly as consequential, however, relative to possible indirect effects; interconnected effects that have implications for the entire plant/soil system. For example, maintaining pH in a favorable range is central to effective nutrient management, as pH is one of the most significant drivers of soil nutrient availability (Fig. 1).

As shown in the figure, the primary nutrients nitrogen, phosphorus, and potassium are most available within the so-called “neutral range” of 6.0 to 7.0, as are the important secondary nutrients magnesium and sulfur. Essential micronutrients such as boron and copper are also readily available when the soil is in a slightly acidic state. In many cases, good pH management is all that is needed to promote sufficient micronutrient availability. These are some of the main reasons why, generally speaking, many agricultural and horticultural soils have a target pH of around neutral to slightly acidic.

When pH is skewed, however, plants can suffer from nutrient deficiencies and associated problems with growth, appearance, and function. On the other end of the spectrum, availability of elements like aluminum can become excessive and reach plant-toxic levels when the soil becomes too acidic. Moreover, numbers and activity of soil microorganisms can be reduced when pH is too high or too low, further affecting nutrient release from organic decomposition and other meaningful, positive contributions from those populations.

The natural tendency of our soils for lower pH is most easily managed by regular liming. Applications of calcitic (calcium carbonate) or dolomitic (calcium magnesium carbonate) limestone create a reaction that removes hydrogen ions from the soil solution, thereby raising pH over time. Liming materials also supply essential calcium, which can be less available even at favorable pH levels. The best guide for how much lime and how often? Soil testing. Soil testing to inform liming needs is affordable and freely available from multiple labs, including the UMass Soil & Plant Tissue Testing Lab. For more information on adjusting soil pH from the UMass Lab, go to: https://ag.umass.edu/soil-plant-nutrient-testing-laboratory/fact-sheets/adjusting-soil-ph

An important note for the landscape is that not all plants perform best at a pH level in the neutral range. So-called acid loving plants include blueberries, hollies, azaleas, rhododendrons, dogwoods and many conifers. Again, it comes down to nutrient availability – most of these plants perform best with higher levels of the micronutrients (often iron) most available at lower (acidic) pH levels. Acidifying fertilizers such as ammonium sulfate are commonly used to maintain a lower soil pH and enhance the quality of these unique species

Hort Notes 2022 Vol. 33:3

Why Test Soil? – Here are five good reasons!

• Nutrient Levels – Test results provide you with soil nutrient levels and fertilizer recommendations when needed.
  
• Soil Acidity – Soil pH and exchangeable acidity are measured for the determination of lime requirements.
  
• Environmental Protection – Test results identify areas with excess nutrients that can pollute local waterways.
  
• Economics – You buy soil amendments only when needed, avoiding unnecessary spending.
  
• Health – Soil testing can alert you to elevated levels of lead or other toxic heavy metals.
  

Soil tests do not identify plant growth problems associated with soil drainage, insects, plant diseases (whether soil-borne or not), weeds, winter injury or the misuse of pesticides.

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