How Much Fertilizer in RO Water?

[lycaonpictus]

I've read conflicting reports on whether or not you need to use LESS fertilizer in RO water than you would in "normal" water. Some places says definitely, others say it doesn't actually matter.

Just wondering what the general experience has been on the Orchid Board.

[King_of_orchid_growing:)]

I think the issue is more about how much fertilizer or how little fertilizer should be used for a certain group of orchids.

Some orchids such as Vandas, can tolerate higher concentrations of fertilization.

Others like Pleurothallids, and Disas can only tolerate rather weak concentrations of fertilization.

Most orchids fall somewhere in the middle.

[King_of_orchid_growing:)]

BTW I would think that because RO/DI water has been purified and many dissolved organics removed, that RO/DI water would be the one that needed a stronger fertilizer concentration to make up for the loss, should this really be an issue at all.

[camille1585]

I don't think it's a question of more or less fertilizer in RO compared to tap water. Your orchids will need the same amount of food whether the water is RO or not. What you do need is a fertilizer made for pure water. RO water has essentially nothing in it, so you will need a fertilizer that will supply everything. The average fertilizer for tapwater provides the basic N,P,K, (and sometimes a few other things) because tapwater already contains many of the trace elements and various (low) quantites of macro elements.

[Ray]

All of the responses above are great.

From a practical, plant nutrition perspective, the only things tap-or well water may have that a pure water source (RO, DI, distilled, rainwater) does not are calcium, magnesium, and iron - usually as dissolved carbonates.

So if you normally use a 10-10-10 fertilizer in tap water, you would still need a 10-10-10 , but with added calcium, magnesium, and iron for use with RO.

The advice you have been given about needing different concentrations is wrong, and may have to do with managing the pH - a fertilizer made for tap water, if used in pure water, will likely result in a very low pH, so the advice is sometimes to reduce the added concentration. Concentration, however, does not play THAT big of a role in pH.