Measurements made under saturated conditions may be considered the most valid in evaluating the existing environment of the growing media. However, the drying process may hasten certain chemical reactions resulting in samples near equilibrium. Stirring the suspension is necessary to keep the growing media suspended during the pH measurement. Conversely, as the suspension becomes more dilute, pH values tend to increase. To obtain an accurate evaluation of acidity, the presence of soluble salts should be accounted for.
Salts influence ionic activities, and therefore, reduce pH values.
Differences in pH are measured in the added-salt solution suspension. This is a more precise evaluation of acidity than that measured in a substrate water suspension. Several techniques have been developed for the determination of pH. However, at present, no testing standards exist for soilless growing media. The following steps outline a procedure which is used by many growers and labs throughout Texas and the U.S. Collect Sample - Be sure to collect a sample which is representative of the entire mass of growing media in question. Remember that variation may exist between locations in the greenhouse, individual pots, as well as, different locations within the same pot.