Coir vs Peat vs Perlite

I learned only recently that the UK government is banning the sale of horticultural peat to amateur gardeners by 2024. This is just over a year away. Very soon.

Coconut Coir

To be honest, I stopped using peat based potting mixes a couple years ago in favour of coconut coir based ones. Coir is marketed as a more sustainable alternative to peat. It is quoted frequently that this is because it is a waste material from the coconut industry. Coir is basically the shredded, rinsed, and dried pith from the husks of coconuts. Light and water retentive, like peat.

I've always wondered about the validity of this claim. Does coir genuinely possess a smaller ecological footprint than peat, or have I just been beguiled by the advertising?

I dearly hope the claims are true because, if they aren't, banning the sale of peat in the UK will achieve nothing on a global scale. It will merely shift problems from one place to another. Tricky! As I say, let's hope the advertising claims are backed up by the genuine science.

I finally took the time this week to do some digging, no pun intended, into the scholarly literature on this subject. Here are my findings.

First of all, I quickly discovered that little literature actually exists on this topic. Bizarre! 

Most research instead focuses on comparing the horticultural merits of coir verses peat as a growing medium for a variety of crops. I won't go into the details of these here. It suffices to say, both have their own pros and cons and should not be viewed as 100% equivalent to one another. I'll let you do your own research on this if you are so inclined. It is quite interesting.

Digging deeper, I finally hit up one fascinating article of interest published October 2021 in The International Journal of Life Cycle Assessment. It assesses the environmental and social impacts of producing peat, coir, and perlite for inclusion in horticultural growing media. We've struck gold! 

Specifically, the researchers assessed peat from Germany, coir from the Philippines, and perlite from Turkey all imported into Spain for urban rooftop farming. When examining the environmental impacts of these three supply chains, the researchers considered product production as well as transportation to produce nice comprehensive life cycle assessments for all three.

The transportation element of coir is the bit that has niggled me the most. Peat is harvested right here in the UK and close by in other parts of Europe. In contract, coconuts only grow in tropical / sub-tropical regions so coir must travel long distances to reach my doorstep. Spain has very similar transportation requirements for these products as the UK, so what did this research find?

The assessment of environmental impact was broken into eight categories with the three products ranked in relation to one another against them. Here is a summary.

  • Global Warming
    • Most harmful - Perlite
    • Medium - Peat
    • Least harmful - Coir
  • Terrestrial Acidification
    • Most harmful - Perlite
    • Medium - Coir
    • Least harmful - Peat
  • Freshwater Eutrophication
    • Most harmful - Perlite
    • Medium - Peat
    • Least harmful - Coir
  • Marine Eutrophication
    • Most harmful - Perlite
    • Medium - Coir
    • Least harmful - Peat
  • Ecotoxicity
    •  Most harmful - Perlite
    • Medium - Peat
    • Least harmful - Coir
  • Land Use
    •  Most harmful - Peat
    • Medium - Perlite
    • Least harmful - Coir
  • Fossil Resource Scarcity
    • Most harmful - Perlite
    • Medium - Peat
    • Least harmful - Coir
  • Water Consumption
    • Most harmful - Perlite
    • Medium - Coir
    • Least harmful - Peat

We can see coir is ranked least harmful in five  out of the eight categories, including global warming, making it overall the least damaging to the environment. PHEW! That is good news!

Unexpectedly, however, that finding was not the most striking aspect of this article for me. Instead, that honor goes to the perlite. Just look at it! It ranked most harmful, in seven out of the eight categories. Seven of them! By a whopping margin I should add.

Why is this never talked about the way peat is? The article explains that perlite is mined from open pit mines and as such is a non-renewable resource. Additionally, to make the individual particles expand into the fluffy snow-like substance we recognise in our potting soils, they are super-heated, which of course requires huge energy inputs. 

My eyes have opened. In short, like my switch away from peat, I am also now going to do the same with perlite. In fact, I've never really liked the stuff. I find those little white specs unsightly in my potting mixes. Also, I find coir lighter and fluffier than peat so I'm not sure how much perlite really benefits my potting mixes nowadays. I'm going to trial just cutting out perlite without a replacement and see how that goes. I'll make sure to keep you updated.

Comments

Post a Comment

Please feel free to comment. Thank you.

Popular posts from this blog

Vegetable Growing Calendar

Image
I've always liked those little gantt chart style sowing calendars you see every so often. You know the ones I'm talking about, those little calendars that show you when to sow and harvest your vegetables, if everything goes according to plan of course. I always look at these calendars and think, gosh this has such potential... But, this one is too difficult to read, that one is missing the vegetables I like, and these ones I'm never going to have in my allotment shed or garage or greenhouse when I need them. Yet despite all that, they still have such potential, if only... Well, this month I took the plunge. Taking inspiration from all the others, I built my own.  I published it here: Vegetable Growing Calendar freely available to all. I'll let it speak for itself.  Before leaving you to explore, I should mention that over the winter you will notice the list of varieties in there grow. This project has encouraged me to slowly begin working my way through my massively di
Read more

Allotment Waterlogging Mitigation

Image
In the last month, our City Council offered us a different allotment plot, one less windswept and possessing a thriving Allotment Association. Hurray! The timing of this couldn't be better from the point of view of observing the pressures the ground faces from Devon's wet winter weather. The shallow sloping plot possesses a thin layer of heavy clay soil over what appears to be shillet. Bad for drainage, it must be said. After rain, the site becomes a quagmire of mud with pools of standing water. Not ideal, but equally not insurmountable. The plot design must mitigate waterlogging while catering for the periods of dry summer weather we occasionally, yes occasionally, get too. This clearly requires careful consideration. After contemplating this question over several nights of fitful sleep, this is the plan I'm actioning. A photo of my progress to date describes it nicely. As you can see, it's a lot of graft, but hopefully worth it. The approach consists of building up a
Read more

Pea and Bean Weevil Mitigation

Image
I'm working a new allotment plot this season and, as I normally do, I planted broad beans. I watched the little seedlings grow, but my glee quickly turned to concern when I noticed a curious problem develop. All the young leaves acquired these little notches along their edges. Below, you can see what I mean, like little peck marks, but they seemed too uniform and neat for pigeon damage... I felt puzzled. So, as I do, I commenced some research and RHS quickly offered the answer: pea and bean weevils ( Sitona lineatus). The leaf damage is caused by adult weevils. They overwinter in field margins and then fly in spring seeking out legumes, preferably beans and peas, on which to feed. Once fed, they lay their eggs in the soil around these same plants. Their larvae then feed on the plant's nitrogen fixing root nodules (housing rhizobia ). To quench your curiosity, you can see clusters of rhizobia filled root nodules growing on the broad bean roots below. My natural next question wa
Read more