Fertilizing wicking mounts - Orchid Board

Ray · #1 11-06-2020, 08:25 AM

Mineral deposits occur when the solvent (water) evaporates. How the solution is delivered makes no difference whatsoever.

What is different between wicking the solution and misting it is the volume of solution delivered. Misting delivers less solution, so less solute, so there will be less buildup - but with less nutrition being delivered to the plants, as well.

What’s important to the plants is the “parts” (mass) of nutrition delivered, not the “parts per million” (concentration).

In traditional pot culture, we really have no idea how much solution the plants take up when we feed, but in S/H culture and with your wicking mounts, where there is a constant supply of solution, it has to be greater, so we compensate by decreasing the concentration.

That’s the “supply side” view, but I guess we can also look at it from the “demand side”.

For a plant to gain a pound of mass, it must incorporate about 200 pounds (~90-91 kg) of water and about 5 g of fertilizer. Let’s say we expect a plant to add a pound of weight in a 4-year period - maybe a decent guess for a mature phal. That would mean the plant - on average - must absorb about 0.024 g (24 mg) of nutrition (and about 440 g of water) per week. If 100% of what we deliver is absorbed, that suggests the “ideal solution” would be 24 mg/0.44kg=55mg/kg=55 ppm. (Note: that is true TDS, not ppm N.)

Except we know full well - but, unfortunately, cannot quantify - that the plants don’t absorb nearly that much of what we provide, as most of the solution retained in the medium is lost to evaporation. (The solids don’t evaporate and are still available for uptake as long as they’re in solution.)

So now we have to make some assumptions about percentages evaporated and absorbed.

Reminder: Our example plant requires 24 mg of fertilizer and 440 g of water (about 15 fluid ounces or 0.116 gallons) per week

If we assume 90% of the water evaporates and all of the remaining solution is absorbed (doubtful), that means we can provide that 24 mg of nutrition in 0.44/0.1=4.4 kg of water, making it a 5.5 ppm solution. If 99% evaporates, we only need a 0.55 ppm solution.