Soil pH and Measurement

By Robert Howell

Everyone who has grown chrysanthemums has heard the term "pH." What is pH anyway? It is a measure of the acid/alkalinity of the substance being discussed, most likely plant growth media, but could also include analyses of plant tissues.

What agents contribute to the pH of a material? The hydrogen ion (H) from the disassociation of water molecules is one source. The free hydrogen unites with carbon dioxide (CO2), generated by soil borne microflora to form a weak acid (carbonic acid).

Other contributors to lower pH values would be ammonia (NH4)-type fertilizers, which would yield hydrogen ions. Sulfur compounds, both fungicides and fertilizers, can unite with hydrogen to form sulfuric acid (H2SO4) and nitrates unite with hydrogen to form nitric acid (HNO3). Think, acid rain taking place in the soil. We all know that both nitrogen and sulfur compounds are essential for good plant growth. To counteract the effects of too much nitrogen or sulfur on low pH values we add calcium and or calcium plus magnesium as lime.

Why should we be concerned about pH values? Plant nutrients have to be solubilized in order to be available and taken up by plants. Solubility of minerals is related to pH. Manganese and iron are essential plant nutrients but at a pH of 5 and below they are very soluble and can be phytotoxic. By using lime the pH can be raised to 6.5, but still enough of these two minerals would be available for mums. Aluminum, nonessential for plant growth but prevalent in clay soils, is very soluble at pH of 4 and is very toxic to many plants.

You may wonder why that with a peat based media a pH of 5 is acceptable but with a soil-based media the pH should be 6.5-6.7 for mums. Peat is nearly devoid of a significant quantity of minerals and those that are present mineralize very slowly. In soil-based media the potential for toxic levels of certain minerals is present so raising the pH avoids a potential problem. (One can raise the media pH too high -- near 7 -- and cause nutrient deficiencies.

If peat-based medias are nutrient deficient, how does one correct for this? There are both water soluble and slow release fertilizers available that contain microelements in sufficient quantities to meet plant needs.

The nice thing about pH is that it can be ascertained quite easily with available devices. Costs of pH meters range from $20.00 to hundreds of dollars. Most if not all measure pH values by conductance between two dissimilar metals. I find that the $20-25 ones determine the pH of soil solutions as well as the more expensive meters.

The cheap pH meters generally have an 8" probe. At the terminal end is a piece (about 1") of dis-similar metal. When the probe is exposed to humidity or water the metallic surfaces become oxidized, therefore the meter does not perform well. The probe needs to be polished. I suggest that you discard the pad that comes with the meter and buy some 4-0000 grade steel wool and use it frequently to polish the probe. Buffer solutions of pH4 and pH7 are available to check your pH meter readings.

Most any garden center carries pH meters, including Gempler's: P.O.Box 44993, Madison, WI 53744 (www.Gemplers.com). This company also sells soil test kits to determine nitrogen, phosphorus, and potassium levels.