Water requirements

Everything I've seen about Kratky so far treats the water requirement as a simple function of the tub size — whatever tub you're using, fill it deep enough to wet the bottom of the growing medium. This is of course practical, but completely avoids the question of predicting how big a tub to choose in the first place. To do that, we need to know how much water a given crop will need over its lifecycle.

Kratky’s original paper does report needing 12-15 liters per kg of lettuce harvested, but this is after-fact reporting and (with the exception of Green Ice and Green Mignonette lettuce varieties) doesn't shed much light on how to predict a crop's water needs.

I've also seen statistics for crop water usage, but these have all been with relation to crops that were in some kind of soil that was exposed to the air. In these cases, evaporation from the soil would significantly increase the water requirements, so I don't think they offer much insight to the requirements of a more contained system like Kratky.

Conventional Kratky wisdom is to check the water levels regulary and top them up as needed, but I'd prefer a methodology that models and predicts water needs more accurately.

I haven't found any Kratky-specific rubrics yet, but a first approximation occurs to me: the container should be at least deep enough to contain the expected root depth of the crop and have a lid size at least equal to the recommended spacing for seedlings. So a container for a single specimen that needs to be planted on a 4" spacing would have a lid 4"x4", while a container for 12 plants at 3" spacing would measure 9"x12".

At the very least, sizing containers this way should provide sufficient volume for the root structures to not be cramped. However, it ignores the possibility that growing in water will have different space requirements than the norms established by a history of growing in soil. It also assumes that the volume defined by its root network is identical to the volume of water it will need over its lifespan. There is no reason to believe that this is the case, but it at least provides a starting point, if no other (better) rubric comes up.

So for now, I think that will be my methodology. And of course, I'll be tracking water usage in my tests, so we'll eventually have water data for the crops that become part of the calorie fountain system.