For being a little word, the term pH has a big meaning to growers. While pH is only one of many numbers appearing on a soil test result sheet, mix pH is one of the fundamental aspects of plant nutrition. Because growing mix pH is affected by several factors, pH management can be the cause of much confusion and frustration. An understanding of cause and effect can help clarify growing mix pH issues.

When considering a mix, one of the first questions asked is "what is the pH? This seemingly simply question is not easy to answer. Many commercial mix companies, including Fafard, make a statement similar to this: "pH after wetting is 5.5 to 6.5." However, this is just the starting point pH and is not the whole answer to the question.

The starting point pH is what should be expected two to three days after watering. Inexperienced growers might expect that this starting point pH is locked in for the duration of the crop. This assumption can lead to serious problems because the pH can change. Other factors such as water quality and fertility have a big effect on pH. The amount of lime added to the mix does set the initial pH, but other factors can have more long-term influence.

By now everyone should know that water quality has a huge effect on growing mix pH. Water contains various dissolved minerals, including one called bicarbonate. Bicarbonate is a mineral that, like limestone, can raise the pH of the growing mix. At each watering, a fresh dose of bicarbonate is added to the growing mix. If the water's bicarbonate level is too high, repeated waterings will cause the mix pH to increase. Some waters contain very low bicarbonate levels, and repeated irrigation can actually cause the pH to drop.

Type of fertilizer used also has a major impact on mix pH. Fertilizers that are made with urea or ammonium nitrogen sources are considered acidifying while those made with nitrate nitrogen sources are more neutral or can cause the pH to rise. Examine the label---if the predominate nitrogen sources are urea and/or ammonium, the fertilizer can lower the growing mix pH. When nitrate is the predominate nitrogen source, use of the fertilizer can raise mix pH. Soluble fertilizers indicate pH effect with a statement of "potential acidity" or "potential basicity" included on the fertilizer label.

Fertilizer and water alkalinity affect pH over the long term. However, daily pH fluctuations can occur because of the relationship between pH and electrical conductivity (EC).

Soluble salts have a depressing effect on pH. The pH will be lower after fertilizing and higher after leaching. Growers who use periodic soluble fertilizer applications followed by clear waterings can see pH swings, depending on when the samples are taken. Because of this, growers experienced in pH tracking know that the samples should always be taken at the same place in the watering/fertility cycle. Growers who constantly feed do not see such wide swings. The concentration of fertilizer used affects pH. All else being equal, crops fed at higher rate will have a lower pH value. Poinsettia and mum growers who stop fertilizing during the final weeks often see the pH climb into the 7.0 range during this time.

A fourth factor also affects pH, one that many growers do not consider---namely, the plant species being grown. Several commonly grown crops acidify the growing mix through root secretions. Geraniums are the most conspicuous acidifier o consider stock plant growers and the problems they have keeping pH from dropping, especially in the last month before cuttings are taken. Similar problems are also seen with the foliage plant pothos.

So, we have four factors influencing the pH--- the amount of lime in the mix to start with, the water, the fertilizer, and the crop grown. In almost all cases, the amount of lime in the mix has the least long-term term affect. This means that the starting point pH can be meaningless to many growers. This is why many growers have a pH-monitoring program.

A monitoring program has two phases. Initially, the purpose is to identify how the combination of mix, fertilizer, water and plant influence pH during the crop cycle. If the trend is for the pH to go above or below the acceptable range, corrective measures can be taken to manage pH within the desired range by changing adjusting mix lime rate and/or by treating the water. The second phase involves periodic but less frequent testing to make sure that the pH management program stays on track.

Growers who start an intensive pH-monitoring program are often dismayed by the results. When testing numerous samples from a crop, the average pH might be right on target at 6.0. However, the individual test results comprising the average could range from 5.6 to 6.4 even from uniform looking plants that are treated as a unit. This deviation from the average increases as the crop ages. With younger crops, the results will be closer to the average than with older crops. This is why I tell growers not to get too excited by the test results from 1 or 2 pots and never to make pH management decisions until a thorough set of samples has been tested.

Growing mix pH management is greatly simplified once trends are identified through tracking. If a grower has found that the trend is for the pH to climb, he can make changes early on before the pH is way out of the desired range. Growing mix pH is most easily changed in small increments. It is much easier to change the pH from 5.5 to 6.0 than from 4.5 to 6.0.

Growing mix pH is an important cultural factor. Understand that pH is not a set value, and that, depending on conditions, over time pH can climb, drop, or waiver up or down. Trends should be identified through pH tracking, and then appropriate changes made to prevent pH from moving outside of the desired range. Mix pH problems can be avoided by identifying how the combination of growing mixes, water, fertilizer and the plant influence pH during the term of the crop.