Drastic pH changes upset plant quality (figure 1), which is why growing media pH is a popular subject with growers. Peat mixes are not naturally well buffered against pH changes, and container growing exacerbates pH effects. Water, fertilizer regime, and even the species grown can affect pH, which is why it’s important to have a well-buffered mix. This blog explores research surrounding peat-based media pH and the effects of lime to help growers better understand liming and peat-based soilless mix performance.
Peat moss is acid (~3.5-5 pH), which is why alkaline lime is added to help raise pH to a more neutral level (~6.5-7 pH) desired by most plants. Quality peat-based mixes always contain added lime, but the variable quality of peat and lime can cause variation in mix pH.
The pH scale is logarithmic: a pH of 5 is 10 times more acidic than pH of 6 and 100 times more acidic than 7. Variation can be wide, so it is essential to create an effective mix buffer system. To do this, you must first know what causes pH variation in peat mixes. That’s exactly why Sun Gro R&D supported research at North Carolina State University aimed towards identifying and improving pH variation in peat moss and research at Martin Marietta Technologies aimed to improve horticultural lime performance. Research results allowed us to improve the performance of our professional and retail mixes.
Peat Moss Variation
Not all peat moss is the same. All fall under the genus Sphagnum, but there are several distinct species that perform differently, depending on the decomposition stage. The natural groundwater or rainwater of the peat bog can also influence pH. All these differences impact peat’s inherent acidity, but which predict peat’s lime requirement?
In late 2002, nearly 500 samples of three peat species (Sphagnum fuscum S. megellanicum, S. angustifolium) were collected from different bogs in Alberta. Alberta bogs naturally maintain the three major Sphagnum species and a hummock-hollow-type topography (figure 2). In each sample, species were tediously separated and their percentages estimated. Their decomposition rate was also determined.
We discovered that different species had pH differences. Sphagnum fuscum (hummock top) generally had a pH of 3.5 and is horticulturally the best because of its high water absorption and slow decomposition. Sphagnum megellanicum (hummock middle) also absorbs water fast and retains it well but decomposes quickly. Sphagnum angustifolium (hollows) has a pH of 4 to 5. There are also non-Sphagnum-like sedges or debris, which are generally low in pH and increase pH variability.
Peats that are highly decomposed have high pH. When the naturally undulating bog surface is made flat for harvesting, each pass of harvester picks up variation in the species and decomposition, resulting in variation in peat pH (figure 3a).
Lime costs less than 1% of the mix price, yet it can cause lots of headaches. This is largely because of lime differences (figure 5) and that most lime companies don’t understand the horticultural industry and how to manufacture a product with uniform performance.
Lime has to be water-soluble and dissolve to neutralize peat acidity. When it comes to lime in greenhouse growing, it needs to rapidly increase pH (in days if not hours) and the residual effect needs to last for 3 to 4 months. Creating a processed lime that does this is not straight forward.
Many characteristics impact the performance of horticultural lime, including:
- Particle Size: Lime particle size impacts its solubility. Smaller particles dissolve faster, but research shows that particle size accounts for only half of the reaction rate of lime.
- Dolomite vs Calcite Lime: Research shows that dolomite lime (calcium and magnesium carbonates) exhibits tremendous variation, while calcite lime (calcium carbonate) exhibits little to no variation. This indicates it is difficult to change dolomite sources and easier to change calcite sources. Dolomite lime also takes four times longer on average to react than calcite lime.
- Magnesium Carbonate in Dolomite: Increased magnesium carbonate content increasingly slows the reaction rate, but again the lime reaction rate is not fully related to magnesium carbonate content.
- Surface Area: Lime surface areas vary (figure 6) Same-size lime particles with high surface area show an increased reaction rate.
Particle size, lime type, and surface area account for 80% of the neutralization capacity of lime. These findings tell us that in addition to size and reaction rate, content and surface area must also be included in the specs to reduce horticultural lime variability. More data on other lime characteristics is being analyzed to account for 100% of the neutralization capacity.
Predicting Lime Requirement
You might expect a peat with a starting pH of 4.5 to reach a target pH of 5.8 with less lime than a peat with a starting pH of 3.5. You would think that some acidity would be already reduced by bases with the 4.5 pH peat, making the neutralization requirement lower. Surprisingly, the relation between higher starting pH and lower lime requirement is not strong, just 20%. Why? A peat with a pH of 4.5 can still have a much higher number of sites available for bases than a peat with a pH of 3.5.
Confusing? Let me explain using an analogy of a large hotel (100 rooms) versus a small hotel (25 rooms). Knowing there are 25 rooms occupied in each hotel does not tell you how many rooms are vacant unless you know the total capacity of each. When 25 rooms are occupied in the large hotel, there is still a 75% vacancy while the smaller hotel is 100% full. The pH value of each peat is comparable. Even when bases like calcium and magnesium are extracted, a peat sample can yield a greater quantity of bases (equal to a greater number of guests) and still have a lower pH (equal to a lower percent occupancy).
To predict the lime requirement of a peat, one must determine the total capacity for base saturation, and the percent saturated with bases. The correlation between increasing percent of base saturation and decreasing lime requirement is stronger (40%). The total capacity of a peat to hold bases is based on its cation exchange capacity, where cations (positively charged ions like hydrogen, calcium, magnesium) are swapped for one another.
A high cation exchange capacity generally imparts a high buffer capacity. During growing, peat with high buffer capacity will have great ability to trade other cations for hydrogen ions that are coming from plants, fertilizers, microbial action, etc. Buffered peat resists pH drifts, which is a property we desire in a mix. Our research showed Sphagnum fuscum exhibited higher buffer capacity than the other species studied.
Additionally, we generally believe that hydrogen is the source of acidity in peat. But, iron is also found in Alberta peat, and iron bonds with hydroxide in water, leaving acidic hydrogen ions in the solution. So, peat pH variations could also be due to differing amounts of iron.
So, what about the chemistry in creating a lime and peat match? What about pH drift during growing? The properties mentioned above give information to predict pH changes during growing. For example, different reaction rates of limes give information on balancing both the initial pH of a mix and pH maintenance during its use. Since the whole effect is a sum of its parts, we can select, add, or subtract relevant factors and evaluate their effects. This knowledge allows us to position the proper pieces in the pH jigsaw puzzle for the creation of great peat mixes with well-balanced pH that produce great looking plants.
~Adapted from an article by Shiv Reddy
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