test results from trying different growing media for lettuce

10 years ago

So since I eat salad very frequently I started gardening for the very first time several months ago- growing my own hydroponic lettuce . Since I get no sunlight and rely on grow lights, I wanted to maximize the efficiency so each head of lettuce wouldn't cost me too much. And since I'm a dorky scientist, I ran various tests comparing different methods. Since this took me a while and it might be useful to others, I wanted to share my lettuce growing results trying different types of grow plugs.

Equipment & Methods:

1) Containers were 1 gallon translucent polypropylene food storage containers (http://www.webstaurantstore.com/4-qt-white-square-food-storage-container/407S004PP.html), spraypainted on the exterior to block out light, except for a little strip so I could monitor the nutrient solution level (see picture for example)

2) I drilled a hole in the lid for a 2" net pot at the top, with the grow plug dropped in. Since the lid wasn't opaque, I put a sheet of tin foil on top to minimize the light exposure to the roots. I grew 12 heads of lettuce, using 3 each of 4 different grow plugs, listed below along with around how much they cost each for me (bought in bulk in amounts 50-100):

Rapid Rooter ($0.32)
Quick Start - these are similar to Rapid Rooters, not sure which company makes them ($0.29)
Sure to Grow ($0.07)
Rockwool ($0.22)

3) Nutrient solution used was 1 tsp each of General Hydroponics Flora Micro/Grow/Bloom, in 1 gallon tap water, making 7-6-11.

4) Growing method used was the passive non-circulating method proposed by Prof. BA Kratky at U. of Hawaii (see: http://www.ctahr.hawaii.edu/oc/freepubs/pdf/VC-1.pdf). This is basically deep water culture, without the air pump.

5) Three seeds of Salad Bowl lettuce (http://www.ferry-morse.com/product_detail.aspx?id=7485) were planted in the grow plug, about 1/4" deep, and thinned after around 1 week.

6) The light fixtures used had four 23w T5 flourescent bulbs (6500K), and was on for 15 hrs/day. Six lettuce plants were grown under one of these fixtures. The bulbs were kept around 2" above the top of the leaves.

7) Ambient temp & humidity was 65-75 F, 55-70% throughout. Temperature a few inches under the lights was 5-10 F higher.

8) Harvest was exactly 35 days after planting the seeds. Each head was cut off flush against the top of the grow plug, and weighed immediately after harvest.

Results (weight in ounces):

Sure to Grow:
5.78
5.19
2.86 (developed a disease)

Quick Start:
6.21
5.47
2.12 (had a problem w/ the nutrient solution level lowering too fast)

Rapid Rooter:
7.83
7.37
6.10 (harvest at 34 days instead of 35)

Rockwool:
4.06
6.21
6.21

So, although it's a small sample size, it seems like Rapid Rooters had the best effect on lettuce growth, with all the other types of grow plugs being equal.

Comments (3)

PupillaCharites
10 years ago
Thanks for sharing the results of your experiment.

You're right that this would have a hard time passing a formal statistical significance inquiry, but I think it is some basic very important info. Any theory on why the trend seems to favor RR? Looking at your raw data and trusting that you forced randomized the placement of the plants (a very important part of any trial), I would guess that your lettuce is just very thirsty and I bet the RR's were the most moist, followed by rockwool and STG being dryest. The problem in generalizing any of this (and recognizing you did a great job of noting the precise growing conditions), is that the moment one specific factor changes all could reverse. It could be something as simple as water level, or more complicated like how much of a breeze is running by the leaves which will dramatically impact water evaporation from the leaves with a corresponding change in uptake requirements.

But the power of the analysis, at least for you; is if that is your growing set up, you have a great handle and a baseline to improve it.

Again - thanks for sharing. There are lots of us that don't have much to add but really appreciate learning from discussion just like yours.

BTW "I have a dream" that "someday" anyone not following the herd won't have to explain we are 'dorky' or 'geeky' just because we want to do a better job at whatever we do. Looking for fertilizers and just trying to pin down dorky things like purity or geeky things like what the recipe is for bulk commodities being sold as tonics and such...is getting tedious ;-(
LooseLeafLettuce
Original Author
10 years ago
The lettuce was grown indoors with no fan blowing air. They were placed randomly and rearranged every 5 days or so. I think Rapid Rooters have some beneficial microbes, while STG and rockwool are inert, so maybe that's why they worked better. All 4 types were actually pretty moist at harvest - I could wring them dry like a wet sponge. The main variable I didn't control for was the water level. This hydroponic method is pretty sensitive to the water level and I maintained it manually when it went too low, instead of putting a float valve in each container. I'll do that next time when I aim for a Nobel :)
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PupillaCharites
10 years ago
Sounds good. I believe your idea is pretty likely. You're probably right about no fan, worth considering it though when air masses inside circulate on their own quite slowly which can still mean an extra 5-10% lower humidity hanging over which conceivably could dramatically impact transpiration.

If you win a prize in Stockholm, instead of cigars maybe send all your posting friends some rapid rooters ;-)

One way to prove it is the microbes, is to add hydrogen peroxide as an additional test variable. If it is the microbes, that will kill them and you'll get a closer representation of the RR medium except for the bugs in it, though the additional oxygen would somewhat alter the sytem. So if we knew for sure we could just try inoculate the cheaper media since the fungi won't care.

Also if you could sacrifice some delicious lettuce, it would be most interesting to stick some in the oven on drying pans and weigh the remaining dry matter, to see if it was just more moisture, or if there is really extra nutrition. A commercial grower could care less, but as a consumer with more than I could eat, that would actually be more interesting, even at the $3/lb electric cost you were stuck with.

Nice work and thanks for the opportunity to learn what you've done.

This post was edited by PupillaCharites on Mon, Jul 8, 13 at 1:40