Over-informed on IPM - Episode 017: Potting Mix Malady

Show Notes

Way back in the early oughts I got myself a degree in horticulture at Virginia Tech. I’m not really sure why but I thought I would end up growing pretty nursery trees out west somewhere cool. My interests quickly turned but I’ll always remember my nursery crops class. Aside from the class when our professor brought in his childhood hoe as part of a lesson on hard work, some of the most memorable lessons included a phone call from a nurseryman to chat about the industry. On one call-in session we were talking about cull piles – how to make the decision to cull plants and then what to do with them. The nurseryman kept talking about plants getting rickets. When asked to clarify, he said rickets was just something plants get sometimes.

And getting ready to start the season and this is a great time to talk about plant rickets…actually the term we in the IPM field use is malady. This is a term used for problems that are caused by abiotic conditions. So non-living things - not an insect, or a mite, or a disease, or a mammal. The most common source of malady is usually some kind of nutrient deficiency…Sometimes malady is caused by extreme temperature, humidity, wind, UV, herbicide contamination, you name it. In this week’s episode I spoke with a colleague down in Rhode Island about something I’ll be calling potting mix malady

Andy Radin Intro

I’ve worked at URI since 2012, but I used to live in Nova Scotia and researched municipal solid waste compost in vegetables. Municipal solid waste compost is a pretty common commodity there because they have a province-wide residential composting program.

Anna: Andy’s current role has him doling out IPM advice and identifying problems with commercial growers in Rhode Island. He came across a problem…

Andy: I encountered was purported to be container media, used at a couple of farms. A couple of growers I would with were showing me their greenhouse seedling. They were growing transplants to set out and they were having trouble. Looking at their plants I was thinking I could figure out what their problem was so I asked “where are you getting this potting mix?”. They said “well…these guys…they gave us this free super sack of potting mix to try out”. So I said “your problem here is your potting mix”

Anna: What happens next in this story is why Extension is so important and why specialists like Andy are so fantastic. He got some money from the state to study this problem alongside the guys producing the problematic potting media.

Andy: So I created a systematic screening process of a bunch of different blends. Looked at particle size distribution, we measured porosity of our mixtures. We also measured porosity in other compost-based commercially available mixtures. In fact, we found out that most of these companies that are making compost-based mixtures have really poor particle size distribution profiles and really poor porosity. Kind of discovering that the way that they make them is rather unsystematic and haphazard, essentially using payloaders and cement pads and scooping and flipping and scooping and flipping. Rather than doing a really good homogenous mixing procedure. What I’m finding is that they’re overmixing and overworking this stuff.

Anna: I’m going to jump in here and cover some soil and soilless media basics. By definition soil is a material composed of minerals, living organisms, organic matter, gas, and water. By living organisms let’s remember that we’re talking about a wide diversity of macro-organisms like insects, other arthropods, annelids or worms, as well as micro-organisms like fungi and bacteria. Good guys and bad guys alike.

The term soilless media has been used for the mixes we use in potted plants and transplants. These mixes are primarily organic matter – like peat moss – with some minerals added back in – like perlite or vermiculite - and perhaps desirable fungi added in.

That balance of minerals, organic matter, air and water is important in soil as well as soilless potting media. You’ll hear Andy talking about porosity and macropore space and micropore space. Macropore space in soil is space that’s big enough so that the adhesive and cohesive properties of water aren’t strong enough to hold in place, which allows for air to take the place after rain or watering. Field capacity is the point where water has drained out of macro-pore space but there’s still plenty of water in micropore space, kind of clinging to the surface of soil particles. This the sweet spot where there’s plenty of air and water for very happy roots.

Back to Andy on the overmixing of compost-based potting mixes.

Andy: They’re overworking this stuff. Just like a good pancake batter or muffin batter, you shouldn’t overwork it. When you overwork it, you’re breaking particles down in to finer and finer bits. You wind up with much too high a proportion of particles that are of a fine scale. What those do is they start to take up all the pore space. When you’re using this media in small cells and with the watering and the drying and the watering and drying – what you end up with is a brick. Eventually you’re never actually getting water to penetrate down to the bottoms of the cells. Meanwhile the roots are starving for water and oxygen. When I was finding problems in people’s transplants, I would unpot them and say look, here’s what’s going on underneath here.

Anna: Do you have any good rules of thumb when you’re using a new source of compost or potting mix?

Andy: Well I have a newsletter article you can share – that’s readable material – but here I’ll say the problem is that most people don’t have a stack of sieves and a shaker to do the tests to find the particle size distribution of the product they just purchased. That is something I’ve offered to people here in Rhode Island. If you’re mixing up your own mix, lets say, you can bring it to me and I can do some tests on it – for a small fee – and we’ll see how it shakes out. Before you start growing plants, you can find out a lot by do a few simple tests.

One is doing the shaking thing and fractioning out the particle sizes. You really should have at least 60% of what are considered medium-sized particles, which range from ½ mm to 2 mm. Everything above 2 mm is considered coarse. Everything below ½ mm is considered fine. For several of these mixes we were getting between 30% and 55% fine particles by weight, when that should be no higher than 20%.

We also measure porosity – well what you’re measuring is the air porosity. What you’re measuring is how much air is in a mix when we’re at what you would call field capacity. Then we come up with what is micropore space, which is the amount of water that’s in the mix that the roots are really sucking up on. Some of these materials that we tested for porosity, they were really dense. They didn’t have that airspace in them. The thing is, when you’re filling a container you can’t pack it in! You’re pushing out all that air.

As I put out in my article, anyone who wants to have material tested, we can do it.

Anna: Well all right! Check out Andy’s newsletter article with more information about testing soil porosity! Something to think about when we’re trying to sleuth out problems in our transplants. Pull up those plants and look to see how happy the roots are and reach out to Andy or any of your Extension professionals if you think you have a problem with your media.

Finding a local source of organic matter is becoming more and more important to us these days so stay tuned for more on how we’re handling concerns about compost or other soil amendments. There is always herbicide contamination and soil borne pathogen inoculum to consider as well. Thanks so much to Andy Radin of University of Rhode Island and thanks to Jason Lightbown who wrote and performed our theme music.