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pH? Widely Used, Little Understood

pH is defined as the negative log of the hydrogen ion (H+) concentration on a scale from 0 to 14, 7.0 being the neutral point, with less than 7.0 being defined as “acidic” and greater than 7.0 as “alkaline.”  The optimum pH ranges for plant root function growing in a mineral soil, an organic soil, a soilless organic rooting medium, or a hydroponic nutrient solution are not the same. In addition, there is no one explanation for the pH effect on plant roots growing in these four media. A pH range of 4.5 to 8.5 is the normal encompassing range associated with these rooting media with plants functioning best in “acidic” environments

The plant root has a means of “buffering” itself from the surrounding rooting medium, that “buffer zone” is known as the rhizosphere, a thin cylindrical zone encircling the root that teems with physio-chemical activity. The pH of this zone is usually less than that of the surrounding rooting medium, contains an active microbiological population, referred to as mycorrhizae, that is fed by substances released from the root, and is an area where elemental ions interact to be either absorbed by the root, or by precipitation and/or chelation made unavailable for root absorption. This latter process provides “protection” to the root, avoiding possible chemical injury, and by keeping substances from entering the plant that can be detrimental to functions within the plant. Plants that grow well in alkaline soils (pH>7.0) create rhizosphere environments that can cope with properties associated with alkalinity. The use of mycorrhizae root treatments have been useful when plants are being grown under less than optimal soil pH and in soils with toxic concentrations of heavy metals.

The commonly used pH determination procedure is with the use of a calibrated pH meter equipped with either separate glass and reference electrodes, or with a combination single body electrode. Today’s pH meters are rugged and easy to use. Calibration, using a two-point procedure, requires what are called “buffer solutions” of known pH that will bracket the pH range of what is expected for the unknowns. For acid range determinations, one of the two buffer solutions would have a pH of 4.0, the other pH 7.0. For alkaline range determinations, pH buffers 7.0 and 10.0, respectively, are required. The procedures for calibrating a pH meter are provided with the meter and should be carefully followed. Temperature compensation is required if pH determinations are being made above or below normal room temperatures (65-75oF).

Once the pH meter is calibrated, a “reference sample” of known pH of the same matrix as that of the unknowns is needed to verify the pH meter calibration. For example, when determining the pH of either a soil or soilless mix, have a soil or soilless mix available that has an already determined pH. If not commercially available, have someone using their own pH meter, prepare a reference standard. Self generated standards based on repeated pH measurements of a bulk sample using the same pH meter as for making pH measurements on unknowns does not make for a suitable “reference” standard.

The pH of water when ion free, or with ions present that would interfere with the pH glass electrode, is not easily made. Therefore, an accurate pH of “pure” water is extremely difficult to determine since there must be electrical conductivity between the glass and reference electrodes. In addition, a pH value obtained for ion-free water is a meaningless value since the concentration of hydrogen ions is very low and fluctuates based on storage conditions.

The water pH of a soil or soilless mix is determined in a slurry-ratio mix of water and soil or soilless mix, the
ratio specified based on the pH interpretation scale used. For most soils, the ratio between water and soil is 1:1 on a volume basis, and that for an organic soilless mix at the water saturation point. The pH value determined is the result of three interacting factors, the concentration of hydrogen ions in solution, those hydrogen ions in equilibrium between the colloidal faction and that in solution, and the “proton” effect of the colloids presence. Removing the solid phase by filtration, the pH of the filtrate will be higher (can be as much as a whole pH unit) than that of the slurry, the pH difference being related to the physio-chemical properties of the solid phase.

For those soils with low colloidal content, and/or when the ion content in solution is low, pH meter readings are difficult to make since the meter does not quickly settle on a fix point. To overcome this difficulty, the pH determination is made in a salt solution, a procedure that is gaining in use. A “salt pH” determination is made in a 1:1 volume slurry of soil and a salt solution of either one hundredth molar calcium chloride (0.01M CaCl2) or one normal potassium chloride (1N KCl). In such a matrix, electrical conductivity is ensured and the pH reading quickly stabilizes and remains constant. The “salt pH” value obtained depends on the characteristics of the soil (its colloidal content properties) and which salt solution is used. A half a pH unit lower in 0.01M CaCl2, and as much as a whole pH unit lower in 1M KCl, is what can be expected for a “salt” pH value as compared to the water pH value. A “salt pH” determination is only for mineral acid soils.

The optimum mineral soil water pH range for best plant growth is from 5.6 to 6.5, although there are exceptions for particular soil types and plant species. Below pH 5.5, the “availability” of the essential elements phosphorus and magnesium declines, while the concentration of aluminum, manganese, and for some soils copper, begin to advance into the toxic range. Interacting elements, such as phosphorus and aluminum, result in the formation of complexes that reduce phosphorus availability. As the water soil pH advances above 6.5, the “availability” of copper, manganese, iron, and zinc declines, and interaction among the major elements, calcium, magnesium, and potassium, begins to impact root absorption of both magnesium and potassium. For most soils, the availability of phosphorus also declines with increasing pH.

For an organic soil, the optimum pH range is from 5.2 to 6.2, although this range may vary depending on both soil and plant characteristics. The same influence that pH has on elemental status, whether in solution, precipitated or chelated, exists as in a mineral soil with the organic phase becoming a significant factor depending on its content and physio-chemcial properties. The detrimental impact of increasing alkalinity (pH>6.2) on the plant root function is greater than that for increasing acidity.

For an organic soilless rooting medium, the optimum pH range is from 5.2 to 5.8, although this range may vary depending on the major ingredients in the soilless mix, i.e. peat, pinebark, perlite, vermiculite, etc. and plant characteristics. The detrimental impact of increasing alkalinity (pH>6.0) on the plant root function is greater than that for increasing acidity.

For a nutrient solution, the optimum pH range is from 5.2 to 5.8, although this range may vary depending on how the nutrient solution is used and plant characteristics. In a standing-aerated nutrient solution growing system, the pH of the nutrient solution will decline as the result of root respiration and the removal of elements from solution by root absorption. In re-circulating nutrient solution growing systems, pH adjustment of the nutrient solution is normally required to maintain that pH range that keeps elements in solution and roots functioning.

With root activity, the rate and extent of pH change will depend on the rooting medium’s “buffer capacity.” For the solid rooting media, that “buffer capacity” is related to the colloidal fraction, whether inorganic (clay) or organic (colloidal and humic substances), and their physio-chemical properties. Elemental removal by root absorption of the “alkaline” elements, calcium and magnesium, will also contribute to the acidification of the rooting media.

The pH of a rooting medium is in some ways, a meaningless value, unless it can be related back to what that value means in terms of its effect on both plant root function and composition of the solution that surrounds the root, which in turn determines root adsorption “availability” of elements, both essential and toxic. That is why a pH determination, besides the requirements needed to make an accurate measurement, is an equally difficult value to interpret.

J. Benton Jones, Jr. has a PhD in Agronomy and is the author of several books including Hydropopnics: A Practical Guide for the Soilless Grower and Tomato Plant Culture. Dr. Jones has written extensively on hydroponic growing and has been outdoor vegetable gardening employing sub-irrigation hydroponic growing systems (GroSystems.com) and using domestic water for making his nutrient solution.