ppm ferts for cloudforest orchids

[Tonio]

for my wabi kusa wall project I try to find the right amount of ferts.

Among other plants on the wall some cloud forest orchids will grow. What I understand from reading here and on the www is that they like to be fertilized on the lean side.

But I need some advise on how low I should gow.

In my setup plants are always exposed to the tank water and the ferts in it.

Once a week I change about 30% of the water, minerals and fertilizers will be added to a certain level to the fresh water before it is added. By only fertilizing the new water there can't be fertilizer build up over time. the levels I add are the maximum levels.

what I have now:

NO3 6 (N 1.35538)
PO4 0.5 (P 0.16307, P2O5 0.37367)
K 6.52567 (K2O 7,86147)
Ca 4.37853
Mg 2.81696
S 2.17
Fe 0.2
Cu 0.01067
B 0.02667
Mn 0.05333
Mo 0.00267
Zn 0.01067
Na 0.04333
Co 0.00013
Cl 0.01433
Al 0.00003
Ti 0.00003
Ni 0.00003
I 0.00003

HCO3 10.15
Acetate 12.44

TDS 37 (calculated)
GH 0.67 DH
KH 0.47 DH

Iron and other micro nutrients will be OK I think.
My concerns or with the macro nutrients.
Nitrogen is to low maybe?
Potassium is quite high (compared to NP) the majority is coming from potassium bicarbonate, and I need the bicarbonate part to buffer pH if dosing co2. The alternative is sodium bicarbonate. And sodium is useless.

[Ray]

I’d say you’re on the low side, based up some reading and my own experience with S/H culture.

Analyses of the water cascading through the forest canopy and trickling down host tree trunks has shown TDS levels in the 15-25 ppm range, and almost all of it is nitrogen.

In a semi-hydro environment, I routinely use 25 ppm N.

As far as “mineral buildup” is concerned, I think it would be wise to periodically change out all of the water. I don’t think any of us has any idea how fast plants take up nutrition, but keep in mind that as they absorb nutrient ions, they emit a compensatory one, so the water really isn’t getting “purer” over time, plus as the water evaporates, anything in it gets concentrated.

[Tonio]

Thank you Ray,

I agree at least N should be a bit higher.

But again keeping TDS in check, (opinions about what TDS should be very a great deal though) having nutrients in a reasonable amount and keep al the lifeforms involved happy is quite difficult.

for example, 25ppm N translates in 110.75 ppm NO3. For aquarium fish levels above 30 ppm NO3 are not recommended. Most fertilizers use ammonium nitrate (NH4NO3) but ammonium(and nitrite) will be lethal at only a few ppm. So nitrate fertilizers like calcium/potassium/magnesium nitrate are the only options.

Based on your comment I read some research on rainwater chemistry in rain forests. A few interesting finds:

cloud water contains much more dissolved ions then rainwater. because there is less water in there. the mean conductivity of cloud water in Puerto Rico was 74,2 uS/cm. Rain water in the same location 29.8.
source

In the western amazon forest in Brazil, mean NO3 in rainwater varied between 5 and 30 umol/l or 0,31 -1,86 mg/l NO3 or 0,07 -0,42 ppm N. Source

in rainwater in Puerto Rico total N was more or less similar, in cloudwater the mean total N was much higher; 0,96 ppm N. Still not much but significantly higher.

variations in many nutrients are huge though so maybe it is not so critical anyway.