Some notes from a fellow Cacti grower on Atmospheric pressure & how it relates to nutrient strength.
A note on altitude and how it affects the required nutrient strength for best plant performance. The transpiration rate of a plant is a measure of how much water flows through the plant over a given period of time. Since nutrients are generally taken-up with the movement of the water through the plant, this is an important concept when considering altitude effects. Transpiration rates, measured at the plant canopy, are directly dependent on the atmospheric vapor pressure. Atmospheric vapor pressure is a function of humidity and is dependent on temperature and altitude. For a given air temperature and relative humidity, as altitude increases, atmospheric vapor pressure decreases. As a result, under the same growing conditions, at altitude less water moves through the plant than at a lower altitude. When less water is moving, less nutrients are available for the plant. We compensate by increasing the strength (EC) of the feed.
If we go up 1 mile from sea level, we see a directly proportional reduction in vapor pressure. That reduction for 1 mile up is about 20%, so to say if someone is using an EC at sea level, and they go to altitude, given same humidity and temp and sun exposure, they should likely need approx 20% increase in EC. So say you like 3.0 down there, you might like 3.6 up here. Add some extra sun, and maybe it could be bumped up a bit more to accommodate the increase in photosynthesis rate. Humidity and temp will also affect vapor pressure.
Those in Colorado & other high altitude areas, what EC are you pushing for your plants? I am wondering if anyone else has noticed this.