Changing Container Substrate pH

By James Altland

This article describes the initial results of an experiment to determine the impact of peat moss, lime source and lime rate on container substrate pH. The experiment is ongoing, but the initial results demonstrate important principles of container pH management. Discussion of these results reinforce information covered in previous Digger articles (October and November 2002 issues, also posted on the Web site listed at the end of this article).

The experiment was conducted by Grazyna (Grace) Holownia, an intern at the North Willamette Research & Extension Center in Aurora, Ore. Grace is originally from Poland and came to the U.S. for a 12-month internship at J. Frank Schmidt & Son Co., a Gresham, Ore.-nursery, followed by her six-month internship at NWREC.

Grace's project started by accident. In an earlier project we attempted to determine the pH range in which Oxalis corniculata (creeping woodsorrel) grows. Pelletized lime was incorporated at 0, 5, 10, 20 and 40 pounds per cubic yard in order to grow creeping wood sorrel over a wide range of substrate pH. We used Douglas fir bark amended with 10 percent peat moss. It was assumed that with no lime substrate pH would be very low, and with 40 lbs/yd3 lime substrate pH would be very high. To our surprise, substrate pH in containers with 40 pounds of lime barely exceeded 5.0 after two months.

What happened? We checked our equipment, which was a laboratory grade pH/ion meter, and it was working flawlessly. Could adding 10 percent of peat moss to the substrate depress container pH? Was the pelletized lime dispersing in the substrate as expected? To answer these questions, Grace's objective was to determine the influence of lime rate, lime source and peat moss on container pH.

Quick review

Dolomitic lime (used in our experiment) is a combination of calcium and magnesium carbonate (CaCO₃ + MgCO₃). Both molecules react in a similar way to raise soil pH. As CaCO₃ (or MgCO₃) enters the soil, it splits into Mg²⁺ + CO₃^2-. The Mg²⁺ displaces H⁺ ions on soil colloids, and the H⁺ ions are subsequently absorbed by CO₃^2- to form carbonic acid (H₂CO³). Carbonic acid is not stable in soil and is quickly broken down into water (H₂O) and carbon dioxide (CO₂). The net effect of adding lime to soil is that calcium, magnesium or both (depending on the lime source) are added to the soil and H⁺ ions are removed. Therefore, pH is raised and the soil becomes more basic.

In scientific terms, pH is the negative logarithm of the hydrogen ion concentration in a solution. In plain English, pH is a measure of a solution's acidity. Water pH is measured on a scale of 1 to 14, where 1 equals very acidic, 7 is neutral and 14 is very basic.

Why does pH matter? When we talk about the pH of soil (or container media), we are really talking about pH of the water within the soil (in other words, the soil solution).

Soil or container pH affects the solubility of nutrients. For example, iron (Fe) is soluble when pH is between 4.0 and 6.0, but as pH rises above 6.5, iron becomes insoluble. Plants can only absorb nutrients that are dissolved in soil solution; they cannot absorb nutrients that are part of the solid soil phase. Soil solution pH affects nutrient solubility and, therefore, affects nutrient availability to plants.

For containers, pH in the range of 5.5 to 6 has been the rule of thumb; however, some will argue that pH as low as 4.5 is acceptable and, in some crops, desirable. In containers, most growers use soilless media (media without mineral soil) composed primarily of organic components (bark, peat, etc.) Organic container components inherently cause low container pH. Because mineral soils are not present in containers, aluminum (Al) toxicity is not a concern. Other minerals such as iron, manganese (Mn) and zinc (Zn) can become toxic at low pH.

Experimental setup

Buxus (boxwood) 'Green Mountain' and 'Hino Crimson' azalea were potted into substrates with different levels of lime and peat on April 13, 2004. Dolomitic lime was added at 0, 5 and 10 lbs/yd3, and in separate containers pelletized lime was added at 5 lbs/yd3. Peat moss was incorporated into the substrate at 0, 25 and 50 percent of the substrate. Containers were overhead irrigated with 0.5 inch/day, split into two cycles. Substrate pH was measured at one week and one month after potting. Substrate pH was measured using a pour-through technique, which is the recommended procedure for nursery producers (see Digger, October 2002).

Results so far...

Influence of peat moss: Adding peat moss to the substrate had no effect on container pH (see Figure 1). For each rate of lime, there was no difference in pH between those containers with no peat moss and those with 50 percent peat moss. We did not measure pH of peat moss alone; however, other references indicate it can be as low as 3.5. With no lime added, Douglas fir bark pH averaged 4.2. I would have guessed that adding peat moss to the substrate would depress pH slightly, but I would have been wrong.

Influence of lime source: By one week after potting, substrate pH in containers receiving 5 pounds of pelletized lime were 1.5 units lower than containers receiving 5 pounds of pulverized lime (see Figure 2). By one month after potting, the gap closed to just under 1 unit of pH. Based on results so far, pelletized lime might ultimately raise substrate pH to a level similar to that of pulverized lime. It is well known that lime sources with finer particle size react more rapidly in raising substrate pH.

Pelletized lime used in this study is merely a manufactured conglomeration of pulverized lime into larger pellets. These pellets are supposed to disperse upon contact with irrigation water. Pelletized lime sources like the one used in this study are slower to react and will likely take longer to raise substrate pH to a suitable level. Pelletized lime is easier to handle compared to pulverized lime. As long as an immediate pH adjustment is not necessary, pelletized lime should be as effective.

Influence of lime rate: With no lime added, Douglas fir bark (with or without peat moss) has an initial pH of 4.2. In our experiments, 5 pounds of pulverized lime raised substrate pH to 6.4, and 10 pounds raised pH over 7. One might assume that doubling the rate of lime would raise pH twice as high. Keep in mind that pH is measured on a logarithmic scale, meaning for each unit increase in pH, H⁺ concentration decreases 10-fold. Conversely, a unit decrease in pH means the H⁺ concentration increases 10-fold. Adding 5 pounds of lime resulted in an increase of 2.2 units of pH, which translates to about a 158-fold (102.2) decrease in H⁺ concentration. Adding 10 pounds of lime caused pH to rise just 0.7 units higher than adding 5 pounds. That's still a 794-fold (102.9) increase in H⁺ concentration over containers receiving no lime.

Logarithms aside, a small amount of lime will increase substrate pH 1 to 2 units. Thereafter, increasing the lime rates has less effect on substrate pH. In a recent experiment to determine pH threshold of creeping wood sorrel (different from the one described above) adding 80 lbs/yd3 lime to Douglas fir bark raised pH to 8.0. That's only one unit higher than pH observed in this experiment (and that experiment added eight times the amount of lime).

Increased lime rate and the resulting higher pH has had no apparent effect on boxwood growth so far. Increased pH has already negatively impacted azalea growth and appearance, with plants growing in higher substrate pH having less growth and more red foliar color. Azaleas are known to be pH-sensitive and to grow better in lower pH (5.0 - 5.5).

Influence of water alkalinity: Lime was incorporated into the substrate prior to potting. Thereafter, no additional fertilizers were applied. Substrate pH in containers receiving no lime rose from 4.2 to 4.9 between April 20 and May 11. The cause for this was likely irrigation water alkalinity.

Alkalinity is the concentration of carbonates (CO32-) and bicarbonates (HCO3-) in irrigation water. It is also referred to as water's buffering capacity, because water (or soil water) with high alkalinity has the ability to buffer (or resist change to) acidity. Irrigation water with high alkalinity will tend to raise the pH of soil or container solution over time, which is exactly what occurred in our experiment.

Key points

• Peat moss had no effect on substrate pH in our experiment.

• Lime with finer particle size has more rapid effect on substrate pH.

• Small amounts of lime (5 lbs/yd3) can raise pH 1 to 2 units.

• Some crops are more sensitive to substrate pH than others.

• Substrate pH will gradually increase over time when water has moderate to high levels of alkalinity.

Conclusion

Several factors influence container pH. Different water quality factors, irrigation practices and lime sources may lead to different results at your nursery. A wise man from Tennessee once said, "The difference between thinking and knowing is called checking." The only way to be sure of how fertilizer and substrate amendments affect pH of your containers is to check (measure pH) with your own equipment. Contact me if you're an Oregon grower and you want help measuring pH in your containers.

I would like to thank Hines Horticulture for donating the plant material for this study.

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