Digging into substrates

Selecting the right substrate can be vexing for growers. Will rockwool help your leafy greens grow? Can stabilized peat shave a few days off your production schedule? Is phenolic foam right for your hydroponic system? When growers started posing these questions to Michael Evans, professor in the department of horticulture at University of Arkansas, he realized he had his next research project.

Evans has been conducting research involving hydroponics and substrates for years and plans to continue his work through at least 2016. We talked to him to get an update on his work and get his advice for growers.
 

Produce Grower: Your most recent project involved a thorough examination of different substrates in a hydroponic system. Can you tell us about that project?

Michael Evans: This was a study that went over an entire season. It went from fall, in October, through the end of April. The study was repeated three times. We limited ourselves to evaluating these materials in a sub-irrigation system only. We used flats or commercial sub-irrigation trays you can find on the market, that are designed to put, for example, Oasis cubes or rockwool slabs in. Three replications, two systems, eight substrates.
 

PG: And what did you find?

ME: If you look just at what we did, then the rockwool cubes had faster germination in the early stages of growth. We looked at two types of rockwool cubes, one with a top groove and one with a bottom groove. Getting up to the one, two and four leaf stage, seedlings develop much faster in the rockwool. Seedlings also developed faster in Grow-Tech’s stabilized peat, FlexiPlugs, than in some of the other material. In this one experiment, we saw those differences.
 

PG: How can growers determine the best substrate for them?

ME: Whether one substrate is better than the other will depend on a number of factors, including: your climate, whether you’re sub-irrigating or top-irrigating, whether you start with fertilizers right away or if you delay a few days, etc. You might find, in your situation, if your methods are different than ours, that you have very different results. Maybe an Oasis Cube is the best substrate in your situation.
 

PG: What is the message you want growers to take away from your research?

ME: The message is that these substrates don’t perform the same. You can see real significant differences. What it tells growers is that they ought to take the time to do these kinds of experimenting in their operation, under their environmental and cultural conditions, to see what substrate works best for them. As you saw with us, you could potentially shave seven days off production time with the right substrate. If you know that kind of time savings is possible, it shows that it’s worth the grower’s time to experiment with substrates in their situation.
 

PG: Are you conducting further research on these substrates?

ME: We’re going to do an experiment coming up that will sort of repeat what we did here. The difference will be that we’re going to sub-irrigate and top fertigate. We want to see the difference in the results, and that was a request we got at Cultivate’15.

We’re also going to grow these crops out, so when the seedlings hit the size that they’re able to go to the NFT-trough, then we’re going to transfer them. So in other words, we want to look at the impact of the substrate all the way through to production, to finished crops. Those will be our big changes to our substrate research this year.

Another aspect of substrates and growing that we’ll be looking at is the usefulness of a nursery phase. There are two ways that greens are done: more or less. You go into what’s called the propagation phase. That’s often in flats or commercial propagation trays. Then, at that point, it can go one of two routes, usually.

Route 1: They leave those seedlings in there until they get up to four leaves. Depending on your substrate, it can get crowded. Even though the flat is the same size, you may have 104 or as many as 276 per flat, meaning they can be more or less crowded depending on the number of holes. Then I’ll transfer over to final spacing in my NFT or DFT system.

Other people will let them stay in the germination phase until they have a full, expanded leaf. Then they’ll transfer them over to the NFT-troughs, but the troughs or gulleys have much closer hole-spacing because the plants are smaller. They call that a nursery phase. They may leave them in there for seven days and then take them out and put them in the final spacing troughs, into productions. Some growers will let them get crowded. Other people will let them grow a leaf, put them into a nursery phase and then into a production phase.
 

PG: How will that research help growers?

ME: You’d think with the nursery phase that you might be saving on space because you’re spreading the plants out. But you’ve also got some cost because you’re handling them twice instead of once. So there’s additional labor involved, there’s potential damage to the plant. Anytime you handle something, there’s potential for disease transfer. A lot of growers have said that transfer isn’t worth it.

But then they’ve wondered, to get over the spacing issue, could I use a 104-tray and keep them in germination longer? A lot of people who use the nursery phase use 276-trays. Is it better to skip the nursery phase and use a bigger cube and leave them in the propagation phase longer and then jump straight? We don’t know the answer yet. We’re going to be looking at the substrates. Is it better to use a 276 cell, so that they’re very dense in propagation, and go to the nursery? Or can you use a 104 and leave them in propagation longer? Can you use a 167? Do you lose that much if they’re crowded for five or six days? We’re going to be looking at the economics of it all. We want to know how growers can save money.
 

PG: When will you present your findings?

ME: We may wait to start the top-fertigation substrate study until late September and the nursery phase study could take two years. I’d be open to presenting some preliminary results at Cultivate’16.

This interview was edited for length and clarity.