A while ago I started a cos lettuce grow-off between three different types of container growing. And how did it work out? Well, all three methods ended up working pretty well, although my possibly biased impression is that the lettuces planted in the soil-less mix (50-50 vermiculite and perlite, left in the pictures) and Mexican style SWC (right in the pictures) both seem to do slightly better than those in he 1 part perlite 4 parts potting soil mix (in the middle). The pictures below were taken about 3 weeks ago. This is not a completely scientific comparison since I have been harvesting lettuce from them.
Soil-less mixture far left, 20/80 perlite/potting soil in middle, Mexican style SWC far right.
Soil-less mixture far left, 20/80 perlite/potting soil in middle, Mexican style SWC far right. Notice the smaller cos plants growing in the windowsill. Even in 7.5 cm pots a number of edible leaves were yielded :)
The trouble with hydroponics
About a week ago the lettuces in the soil-less mix suddenly appeared quite limp and weak. I measured the Ph of the nutrient mix in the reservoir and it was about 4.8. A bit low! For interest sake I checked the Ph in the soily container, and it was similarly low, but the lettuce growing there was fine. After leaching with a good quantity of fresh water, the lettuce in the soil-less container perked up almost immediately.
This demonstrates quite nicely the vicissitudes of 100% soil-less hydroponic growing. You actually need to manage the nutrient Ph etc. actively, since you don’t have the luxury of the soil buffering the Ph for you. I had been watering both the soil-less and soily container using hydroponic nutrient solution without bothering to regulate the Ph in the reservoir, but trouble only arose in the soil-less container. This is not an indictment of hydroponic growing, but simply an observation that if you go hydroponic, you really do need to do it properly :)
Happiness and Joy
I must say that I have been pleasantly surprised by the quantity and quality of the lettuce that I have been able to harvest. I have made several (10+) salads already, and they still seem to be growing well. It seems that Cos lettuce grows even more easily than rocket :)
I'm interested in the cost of each of the growing mediums. If the plants do similarly well, then surely it makes sense to just go with with whatever is cheapest?
Well, the mexican style SWC would seem to be cheapest. If I remember correctly perlite costs somewhere between R5-10 per litre, vermiculite about half that, and potting soil about R1/litre. The soilless mix is obviously the most expensive, but since you can re-use that it may work out cheaper in the long run for certain kinds of crops.
What I read on the interwebs seems to indicate that the Mexican style SWC doesn't work well for most fruiting plants (e.g. tomato), but if your crop grows well in Mexico, that is probably the best option :)