C. A. Conover 1
CFREC-Apopka Research Report RH-96-2
During foliage plant production, growers often monitor certain factors affecting plant growth in order to produce high quality crops. One factor indirectly affecting plant growth is pH of the growing medium. A pH of 7 is considered neutral, a lower pH is acidic and a pH above 7 is alkaline; but producers of container grown plants need to understand just what these numbers represent and how the pH of the growing medium can indirectly influence foliage plant growth. We know that pH is defined as the measurement of the negative log of the hydrogen ion [H+] or hydroxyl ion [OH–] concentration. If we compared the pH of two potting mixes and one had a pH of 4.5 and the other a pH of 6.5, we would know that the medium with the lower pH contained more [H+]’s.
Since pH is a logarithmic scale, the difference in the number of [H+]’s between soils with pH’s of 4.5 and 5.5 is much greater (one hundred times greater) than the difference in the number of [H+]’s between soils with pH’s of 5.5 and 6.5. Therefore, many more [H+]’s are needed to lower the pH from 5.5 to 4.5 than are needed when lowering pH from 6.5 to 5.5.
Most growers who notice a decrease in medium pH of container grown foliage plants may wonder what causes this and why it can sometimes happen so rapidly. The main cause of decreasing pH of the “soilless” growing media favored by the foliage plant industry is related to the fact that these media are organic in nature. Decomposition of organic matter results in the formation of both organic and inorganic acids. These acids cause the basic ingredients of the potting medium, such as calcium, to dissolve and eventually leach out, which then results in lowered growing medium pH. Fungi, which utilize the organic components (carbon) of potting medium as their food source, also significantly contribute to acid formation.
Often, another major source of acids are the fertilizers applied for plant growth, with different nutrient sources responsible for eventually causing high or low acidity. For example, one 20-20-20 fertilizer needs 597 lbs of CaCO3/ton to neutralize its acidity, while a 20-7-9, a fertilizer with different nutrient sources, requires only 273 lbs. Most fertilizers utilized in the foliage industry are acid forming, and the higher the fertilizer application rate used, the more acidity created.
Although most plants will grow within a wide pH range, 3.5 to 8.0, for many years the suggested range of pH values for growth of good quality foliage plants was 5.5 to 6.5. This recommendation originated back when most ornamental plants were grown in mineral or organic soils or soil-containing mixtures. In soils, this is the pH range where nutrients required for foliage plant growth, such as nitrogen, phosphorus, potassium, sulfur, calcium and magnesium, are most available for plant uptake. In addition, within this pH range, potentially harmful micronutrients these soils may contain, (iron, manganese and zinc, as well as the element aluminum) are mostly insoluble or only slowly become soluble and, therefore, less available to plants.
As growing medium pH decreases, the compounds containing these micronutrients dissolve and basic components such as calcium are leached, so that the amount of aluminum and the micronutrients iron, manganese and zinc available to plants is greatly increased. However, soil is rarely a component of the various types of modern media used to produce foliage plants. Aluminum is not a significant component of artificial growing media and additions of iron, manganese and zinc are specifically controlled when artificial media are produced. The chance of a micronutrient toxicity problem developing as pH decreases is unlikely for plants grown in a good quality artificial growing medium. When micronutrient products are applied to foliage crops at the rates recommended by manufacturers, micronutrient levels in the medium are sufficient for healthy plant growth, but below levels needed to create toxicity problems.
We have conducted a large number of experiments over the years on various aspects of growing medium pH. For many years, we have suggested that dolomite be incorporated into peat and peat-bark based potting mixes before use, at the rate of 7 lbs/yd3, to help adjust and maintain the growing medium pH at the recommended level and to provide calcium and magnesium needed for healthy growth. None of the research we have conducted indicates these dolomite incorporation rates should be changed, especially in view of the fact that our research has shown that it is very difficult to increase pH of media already being used to grow a foliage crop without damaging plants.
Use of higher dolomite incorporation rates is widespread in the industry, but we feel that this may cause more problems than it helps. Further additions of dolomite produce little change in pH but do create a much higher base saturation of the medium, which often results in decreased availability of necessary micronutrients.
When most foliage crops were grown in soil, the recommended pH range was 5.5 to 6.5. This recommendation is still a valid recommendation for any producers still using soil to produce foliage plants. As the industry shifted from use of soil to various artificial media, use of artificial media also increased at our research facilities.
For the past twenty years, most plants used in our research were grown in artificial media. To reflect this change we recommended a pH range of 5.0 to 6.5 as best for foliage plants (Conover and Poole, 1990). However, during research on nutrition of foliage plants, pH has often been observed to decrease over time. Often, at the end of a crop production schedule or termination of the experiment, pH would be lower than 5.0. Media pH’s as low as 2.9 have been observed, and a range of 3.5 to 4.5 has not been uncommon and has been associated with foliage plant crops of high quality. In examining our data, we have not been able to associate any problems with pH’s as low as 3.5 to 4.5 at the end of the crop cycle, and therefore we are revising our suggested pH range for foliage plants from 5.0 to 6.5 to a new range of 4.0 to 6.5.
We have previously published information on suggested levels of dolomite for use in artificial media as well as ways to raise pH of media containing growing plants. The data found in these publications on dolomite levels, as well as ways to safely raise pH, if desired, are still valid. In addition, in two recent reports we have made available results of new research on adjusting pH and effects of using different dolomite sources. The primary purpose of this report is to inform growers that we have not observed plant problems due to a pH range of as low as 3.5 to 4.5 occurring at the end of the crop cycle on foliage plants used in our research, when plants were grown in various types of peat, bark or peat-bark based artificial media. We feel that growers using such media need to reconsider their attempts to adjust pH’s below 5.0 but above 3.5, especially if these efforts might negatively affect marketability of the foliage crop.
Professor of Environmental Horticulture and Center Director, Central Florida Research and Education Center, 2807 Binion Road, Apopka, FL 32703.
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