Recently, I've been looking for a way to replace peat moss. I have been using it as a major component of both potting mix and seed starting mix.
I've found it difficult to really get good information on how sustainable the harvest of Canadian peat is. I fully admit that intelligent people have and will come to different conclusions about it's sustainability, based on the available information. I've decided to find something that I can be sure has a smaller environment footprint, and doesn't threaten an ancient eco-system.
Lots of people have switched to coconut coir. If it's working for you, great! You have my blessing. For me there are three reasons I'm not impressed. One is that I've seen other gardeners do comparisons between starting seeds in traditional peat mixes and in coir mixes, and the plants in coir did terribly. Downright sickly compared to the peat. Second is that I'm not convinced that shipping coir halfway around the world is more sustainable than responsibly harvested peat from my own continent. Finally, if coir can be a useful soil amendment, I have issue with removing organic soil nutrition from poor nations and further stripping their agricultural wealth.
So yeah, that's a no for me on coir.
A few years ago I heard about a peat substitute being made out of waste paper near Pittsburg. It's called PittMoss, because it's like peat, but made in Pittsburg, get it? So right away that ticks some environmental boxes for me. It's using a waste product, so not only is it not being harvested from a threatened ecosystem, it's also taking garbage out of the wastestream. And it's being manufactured practically in my backyard compared to far northern peat bogs or Asian coconut trees.
I had to try it.
My garden is my science outlet, so I love setting up good, single variable, side by side comparisions when I have a question I want to answer. I mix my own seed starting and potting mix in order to save money, have control over the ingredients, and make a lot at one time. So my standard mix is: 1/3 compost, 1/3 perlite, 1/3 peat, and a small amount of ash to correct the acidicy of the peat. Using 5 gallon buckets for the big components, an old tea cup of wood ash is enough, as pictured.
Mixed:
Then I mixed the trial version: 1/3 compost, 1/3 perlite, and 1/3 PittMoss Prime. No ash as Pitt doesn't need the pH corrected.
This is the PittMoss Prime. Up close it has some bits of color.
I started this trial on May 28th with my tomatoes. I chose well matched seedlings in terms of size and color, two of each variety. For each pair, one was planted into the peat mix, and one into the Pitt mix. A little more than 2 weeks later, on June 14th, there was a marked difference in size and fullness of the plants in each pair. The healthy plant is the peat, the stunted is the Pitt.
Hm. Looks like a lack of nutrition. So I added a top dressing of two inches of compost (under the straw) to all of the plants, Pitt and peat alike. From that point forward, the Pitt plants filled out and looked healthy. They stayed smaller than the peat, but grew at the same rate from there forward. Ok, so maybe this parallels the trouble I've seen with coir, and is evidence that peat moss contributes more nutrition than we tend to give it credit for. The next step of my comparision was seed starting. I continued to use the same peat mixture, but changed the Pitt mixture to: 1/2 compost, 1/3 perlite, and 1/6 Pitt, or 1 bucket compost, 1 bucket perlite, and 1 bucket half PittMoss Prime and half compost.
I thought for sure I had solved it--just needs a higher proportion of compost--but as I trialed seedlings side by side, they germinated at about the same time, grew to the point of their first true leaves, then while the peat seedlings continued to grow, the Pitt stalled out at this size. These are thyme seedlings that illustrate this, along with another really interesting result, which is that when I plant 4-10 seeds in each mix, I am often getting one or two more germinating in the Pitt than in the peat. So it's not bad stuff, but clearly there's something else the mix needs to be successful.
I was stymied, so wrote to the PittMoss company. I showed them the pictures. They said, "I'm going to have one of our soil scientists get back to you." Really?! A soil scientist?! Woohoo! The soil scientist was most interested in the tomato pictures. From his email:
"From the photos it is apparent that there is a significant nutrient deficiency in the blend with the Pitt Moss Prime. It appears by the purpling of the leaves that there is clearly a phosphate deficiency and possibly the small size indicates nitrate deficiency as well. The fact that the compost increased the growth rate is significant.
"As a basis for what was likely taking place remember that PittMoss Prime has no inherant nutrient supply while it has a very high nutrient holding capability once the nutrients have been applied. The blends of Plentiful and Performance have nutrients supplied in the formulation...When blending other material with PittMoss Prime it should be remembered that the high carbon content of the Prime will also require a high nitrogen addition to balance the demand...I susupect that by adding a good balance of fertilizers and or manures with some compost the end product will be similar to what we produce in the PittMoss Plentiful and Performance...If continuing to use PittMoss Prime I suggest both composts of good quality and some dehydrated poultry manure for the organic grower. Otherwise soluble plant food..."
The carbon was tying up the nitrogen! Of course! So with that knowledge, I've begun fertilizing the Pitt seedlings with a dilute fish and seaweed fertilizer once a week. This is standard practice for many seed starters, but because I use a mix with compost (a bit unusual for seed starting) I had been getting by without it. Certainly not a problem to add it into my routine now.
After a few weeks of seaweed, the seedlings look good and are growing at a better rate. In the case of my broccoli seedlings, the Pitt seedlings have completely caught up to the peat, and some are now bigger!
Blue pots on the left are the Pitt, right is peat.
Conclusions: I'll continue this comparison through my seed starting this spring, but with the addition of fertilizing the Pitt seedlings with half strength fish and seaweed fertilizer once they have their first true leaves. Based on the broccoli, I expect this to go well and think from here forward I will be using PittMoss for my seeds and container plants. Next year I plan to trial the two different proportions of the Pitt mixture against each other.
2022 Update: I did indeed do another trial, this time comparing a mix of 1 bucket each of PittMoss Prime, compost, and perlite, PLUS 2 tea cups of feather meal to PittMoss Plentiful right out of the bag. Either/both of these worked beautifully! The plants are much healthier this way and didn't need any supplemental liquid fertilizer. I think this is because as the email from PittMoss soil scientist Dr. Bethke points out above, the additional nitrogen was needed not for the plants' direct use, but to balance with the carbon content of Prime. Plentiful already contains the small amount of nitrogen I had to add to Prime, so from here forward it will be my go to, either on it's own or in the mix. As PittMoss is getting more popular, it's nice to know that if they run out of Plentiful, I can get the same results using Prime as long as I mix two cups of feather meal (or other high nitrogen organic granular ferilizer) into every 5 gallons of Prime.
If you do decide to compare one of these options to your old seed starting mix, I'd love to see your results! Please share in the comments.
