Why so much emphasis on soil pH ?
By 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 , that 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 .
Why Test Soil ?
• 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 .
Figure 1
( continued on p . 15 )
MLP Newsline | Summer 2022 13