Vpd vs humidity?

VPD is calculated from temp and RH ... higher humidity will reduce VPD, all other factors remaining the same. higher temps will increase VPD, all other factors remaining the same. you want a certain range of VPD based upon stage of life cycle the plant is in... low vpd for seedlings and clones, for example, then full-bore with mature plants with much higher VPD. VPD will very strongly correlate to how much the plant 'drinks.' The rate of water intake can impact the concentration of fertilizer you want to feed... if elevated, you need a lower concentration. just think of the amount of mass of nutrients that are beign taken in. faster rate of higher concentraion means higher mas of nutrients... the plant can only use so many grams of N or Ca et al per day. I would shoot for an intended VPD while staing between 40-60% RH. Avoid > 30c. this is not a hard line in the sand.. some are perfectly fine at 32C with no signs of heat stress. Tht doesn't necessarily mean it is "perfect" but at the very least the issue is so small it's hard to discern. None of this stuff will polish a turd. bad genetics will always be bad genetics -- from the point of view of entire plant or a more narrow assessment of a specific trait. how much does an optimal VPD impact results vs slightly off? i doubt it's huge, given a common sense range of VPD. i think the biggest thing is, if you don't tightly control this stuff, it's useful to preemptively react to it. E.G. - vpd is sky high, you should probably dial down concentration of fertilizer. avoid the tox symptoms that would develop over days or weeks... hopefully these sorts of things are slow moving... that means your formula and perceptions of how to fertilize is decent.