Weakly weekly..?

[Jasonmc89]

I have always been confused by the very popular "weakly weekly" method of fertilising.

The reason is because I don't know what the fertiliser is to begin with. Some fertilises have NPK number of 20-20-20, while some, such as Orchid Focus Grow have the numbers 2.2–1.2–2.1.

So is it the case that the Orchid Focus fert is already about 10 times weaker than the typical balanced 20-20-20 one?

It seems to me that an orchid grower using the weakly weekly method with a balanced 20 fert will be feeding their plants a stronger feed than I would by using the Orchid Focus fert and full strength or even higher!

Is this correct? Or does it not really work that way?

The reason I'm considering trying his method is because I think a few of mine are showing signs of over-fertilising, even though I stick to the Orchid Focus's recommended doses.

Help me.

Thanks.

[Ray]

You're correct about the formulas. A 20-20-20 is ten times the concentration of a 2-2-2. If using the stronger one to match the weaker one, you simply use 1/10th the amount.

However, "weakly" doesn't tell you squat. Don't forget that fertilizer manufacturers know how to make fertilizer, not how to feed orchids. (And they DO want to sell more fertilizer.). Professionals control their applications by parts-per-million nitrogen (ppm N), with their chosen formula.

How much to use depends upon many factors, including the types of orchids, their potting medium, and probably most importantly, how frequently you feed and water them.

For the most part, orchids are not particularly "heavy" feeders, especially compared to terrestrial plants, and fertilizer also falls very low on the hierarchy of orchid cultural needs.

I prefer to feed a tiny bit of fertilizer at each and every watering, flooding them every time (which flushes the pot as I do). I shoot for about 25 ppm N. if I watered once a week, I might go for 100. Don't be scared by the units. If you divide 2 by the %N on the label of any fertilizer, the result is teaspoons per gallon for 25 ppm N.

For the 2-2-2, 2/2=1 tsp/gal for 25 ppm N, while the 20-20-20 would be 2/20=1/10. For weekly feeding, it's 4 times that, so divide 8 by the %N.

[Jasonmc89]

Thanks for the information!

I get that to use a general purpose fert on orchids, you would definitely have to dilute it quite a bit, but surely the ones specifically formulated for use with orchids are supposed to be mixed up as per the products instructions?

I get confused because even though I use a fert made for orchids, people still say that I should be doing the whole weakly weekly thing and dilute it twice or even four times what the label says..

[PaphMadMan]

Even fertilizer made for orchids tends to be heavy if you follow manufacturers instructions. Remember they are in the business of selling you more fertilizer. The recommendation might be fine for vigorously growing young plants for a few months in an ideal greenhouse environment to get them to selling size, but not necessarily right for long term health of a diverse collection in your home environment.

The instructions on the product do take the strength of the formulation into account, so weakly means no more than 1/2 recommended strength, and in most cases I would cut that to 1/10 if you are applying weekly or at nearly every watering. Be assured that as long as you are using a fertilizer with micronutrients at 1/10 of the manufacturers recommendation it will never hurt your plants and will almost certainly never be the limiting factor on your success growing them.

Too much fertilizer causes many problems. Too little fertilizer, as long as there is some, is very unlikely. More fertilizer never makes up for imperfect conditions or care, and might make things worse. And no one's conditions are perfect.

[orchid60]

I am no expert but if using say fish emulsion or one that colours the water I put just enough to change the colour of the water and it seems to work ok, as for the others I just put a very small amount in the bucket, then you won't over fertilize

[bil]

Go to Ray's page and read all he has on fertiliser.

As he says, fertiliser is the least of a plant's worries, and won't make up for any lack in light, water or air to the roots.
Of course fertilise that is too strong will burn the roots very quickly, doing a lot of harm.

Remember, the fertiliser they are evolved to use comes in tiny amounts every time it rains.