Tech Notes Issue 18

pH: "The Power of Hydrogen" and What it Means

in Turfgrass Management.

pH measures the acidity or alkalinity of a solution. More specifically it is the measure of the hydrogen ion (H⁺) concentration. Danish Chemist S. P. L. Sørensen introduced the term pH for a shorter version of "power of hydrogen". The pH scale is 1 to 14 with 7 being neutral. At a pH of 7 the H⁺ concentration is equal to the hydroxyl (OH^-) concentration. pH's less than 7 are considered acid (> amount of H⁺), while values higher than 7 are basic (> OH^-). Since pH is a logarithmic scale the H⁺ concentration difference between a pH of 5 and 6 is 10-fold. Thus, moving from a pH 7 to a pH 5 is 100 fold increase in H⁺ concentration.

The greatest impact pH has on soils is nutrient availability. pH can influence the balance of  cations and the nutrient's chemical form. In addition pH influences, and alters microbial activity associated with the transformation process of nutrients like nitrogen and sulfur.  Calcium availability can decrease with increasing acidity but the likelihood of a deficiency is remote. As pH values become more acidic - nitrogen, phosphorus, potassium, sulfur and magnesium become potentially less available. As pH becomes more alkaline iron, manganese, and phosphorus potentially become less available. From a sampling procedure, thatch is not normally considered a "soil" in the sense it is not considered in soil testing. Thatch can vary in pH and be quite different from that of the underlying soil. Thatch is more reflective of the pH of the irrigation source, rainfall, and type of fertilizer used (ex. acidifying-types). Management difficulty can arise where thatch is normally excessive and not normally considered in the soil test recommendations.

Given the properties of thatch, relatively porous, poor nutrient retention, the ability to impact thatch pH is great, which can have a dramatic impact on the turf system. For example, a calcareous soil that has a significant thatch layer where lowering the soil pH can result in a significant divergence in pH. If the soil tests have not accounted for thatch, lowering the pH through sulfur applications may not reflect the pH change that is occurring in thatch. In this example, repeated applications of sulfur may not lower the soil pH much, but drastically lower the thatch pH. I have observed where the soil pH may remain relatively unchanged but the pH of the thatch from sulfur applications reduced into the range of 2.8.

As with any value or test, pH can serve as an indicator of a turfgrass systems or health by providing signs of nutrient availability, and competitive ability of the turfgrass system. But like any test you need to know how the measurement was taken and the soil conditions used in the test.