A Consideration of Peat Quality As It Affects CP Horticulture

by William Dawnstar

CP horticulturalists employ many different substances in their mixes such as are appropriate to different needs of different genera, but one common element in most of them is sphagnum peat. Peat is the base line to which varying amounts of various substances are added to make that “perfect mix” in which our plants will thrive.

Sphagnum peat is ideal for CP, because it is naturally low in nutrients. But, this is not to say there are no nutrients in peat; they are there, but they are locked up and unavailable to plants which have evolved with a preference for low nutrient niches. Sphagnum bogs are an example of a nutrient poor habitat, and it is from these bogs that peat we use in our mixes is harvested.

In this discussion, I hope to show that peat is not a uniform product, but varies in terms of mineral content depending on where harvested, and explain why this is so. I feel it is an important consideration, considering that general advice states that the mineral content in substrates for CP should be well below 100 parts per million (PPM).

I will also suggest a protocol for improving the quality of the mixes used in CP cultivation that employ peat.

Peat is formed by partial decomposition over long periods of time of the sphagnum moss in a bog. As successive seasons produce new layers of growth, the previous season’s biomass becomes compacted more and more. Peat in a bog can be many meters deep, with only the top most layers in active growth, and the deeper the peat, the older the bog is.

The reason peat is low in nutrients is because the nitrifying bacteria that break down organic matter in other habitats are not present in a bog, especially the upper layers. This is due to a combination of mutually reinforcing factors: topographic air flow as a result of glacial furrowing has maintained cooler temperatures since the ice age, discouraging Nitrobacter while fostering naturally acidic conditions and a pytosociology that in turn maintains that acidity. In bogs, decomposition happens predominantly in lower layers, where anaerobic processes are favored. The conclusion is the deeper the peat, the greater the decomposition over time, and the greater the nutrient content.

The upper layers of the bog are not compacted, and loose and open growth of the live moss allows for good oxygenation, and discouraging anaerobic decomposition. Here, the nutrient levels are the lowest, culminating in the live Sphagnum which has virtually no nutrients present available for plant growth.

When a bog is harvested for its peat, it is done in successive layers. The upper layers of peat from any given bog will be found to be the best for CP cultivation. In the U.S. and Canada, the pear reserves are extensive, and not yet deeply mined and the peat is of good quality. In Europe, the scenario is different. The peat reserves there are more depleted, and the peat harvested now is from much deeper strata.

Three independent experimenters reported their results using a TDS meter to test the runoff water from Canadian peat, which as I have mentioned tends to be of superior quality for our needs. In no case were the readings below 100 PPM, and ranged from 350-1000 PPM. Although this was not a rigorous scientific experiment, I feel it does support my observations that too high a mineral content in the mix favors all sorts of negative possibilities: bacteria, algae, moss, and fungus in a process that culminates in detrimental insect attack.

If the nutrient content is high, then those anaerobic organisms present in the peat will proliferate, especially in compacted and overly wet mixes. They begin to do their work of breaking down the peat, fixing carbonates and nitrates which is a function of cyanobacteria. Initially, plant growth may be normal in such a mix, but as the process continues, nutrient salts accumulate over time. Algae can proliferate using these nutrients, producing the agar that is a perfect substrate for fungal colonization. Once a mix becomes this rich, fungus gnats can infect the pot, and root nematodes can also proliferate. It is a gradual downhill slide. In my own and others experience, such infected pots the plants never thrive, although they may persist.

I found more support for my theory in another long-term grower who also arrived at this same observation in the course of his own experience with growing these plants. This is the protocol we are using. We no longer test our peat, but follow this protocol in all instances.

Fill a five gallon bucket halfway with peat. Cover this with pure water, and break the peat as much as possible by hand kneading it beneath the surface. Allow the bucket to stand for 1 week. Then squeeze a double handful of the now wet peat into a ball, and squeeze the water out. Place the ball into another bucket, and repeat until you have a quantity sufficient to your needs. You can’t make too much!

Since the initial runoff water showed so high a reading, in some instances ten times the recommended limit, it was decided to perform the rinsing twice. This put the runoff water reading closer to 30 PPM, much more acceptable!

Sand and perlite is also treated the same, and is rinsed until the water runs clear. If pure water is at a premium, tap water serves for the rinse. I follow it with a final rinse in rain water. The aim is to remove as many trace nutrients as possible.

Since most of my common use is for a 50/50 peat sand mix, I make up this proportion with these rinsed materials, and immediately fill as many pots as I will anticipate needing in a season. For me, this is a large number. These pots of mix are then stored outside on a wire rack, allowing the rains of the season to pour through them, and also leaching out remaining salts. The older pots made are the first selected for use.

Some may ask why not just use LFS? My reply is both for the cost involved, but also because the plants I focus on (Drosera, mainly) in fact do prosper from the low levels of nutrition that is found in peat, but absent in LFS.

Even with the utmost care regarding rinsing, problems still arise in some 10-20% of my pots. Nature refuses to follow the rules. Also, pots of mix age: the older the mix, the more likely that problems will arise over time. For this reason I generally repot routinely every other season, recycling the mix of course. Remember, peat is a non-renewable resource! I encourage experiments with non-peat based mixes, since they will be a needed inevitability in the future as the existing peat sources are depleted.

Optimal growing means optimizing all the conditions that you can. Small differences can produce big results in terms of overall success. If you are experiencing pots in which mosses, algae and fungi are a common occurrence, I suggest you consider this as a possible way to improve your culture.