Confusion over MSU / RainMix dilution & frequency

[powderherface]

Hello, I bought some RainMix (similar fertiliser to MSU, easily available in Europe), the label says "use at every watering". However, I spray my orchids daily, and I imagine this shouldn't be administered daily. Anyone more knowledgeable know what frequency is implied by this dilution?

[Roberta]

The best watering for orchids is to do it thoroughly, let it run through the pot. This flushes out any bad stuff, and also pulls air into the root zone. Then, let it drain well. The "when to water next" decision depends on the medium, the humidity - all related to the drying rate. You can estimate that by watering well, weigh the pot on a kitchen scale. Do it the next day, when the rate of weight loss slows down time to water again. For fertilizing, once a week should be plenty, jjust water the rest of the time. (Orchids are light feeders) Since orchids don't absorb water through their leaves (just roots) the "deep watering" is more useful than spraying.

[powderherface]

Originally Posted by Roberta

The best watering for orchids is to do it thoroughly, let it run through the pot. This flushes out any bad stuff, and also pulls air into the root zone. Then, let it drain well. The "when to water next" decision depends on the medium, the humidity - all related to the drying rate. You can estimate that by watering well, weigh the pot on a kitchen scale. Do it the next day, when the rate of weight loss slows down time to water again. For fertilizing, once a week should be plenty, jjust water the rest of the time. (Orchids are light feeders) Since orchids don't absorb water through their leaves (just roots) the "deep watering" is more useful than spraying.

Sorry I wasn't clear, the orchids are all mounted & live in a greenhouse environment, this is why I say "sprayed" (and hence daily).

My confusion is, they recommend "every watering", but simultaneously I have seen "¼ strength once a week" i.e. weakly weekly. So it is not clear to me whether the RainMix dilution is already "weakly weekly" or not, for instance.

[Roberta]

Ahhh... Different scenario. That's nice and dilute. You'd probably be fine with that, maybe every 3 or 4 times use plain water so it doesn't build up.

[Ray]

For daily. - even several times a day spraying, I'd use no more than 20-25 ppm N.

Looking at the more accurate mass measurement, you just divide the desired ppm N by the nitrogen percentage of the fertilizer, in decimal form, 20/0.118=169.5 mg/L (ppm)and 25/0.118=211.9 mg/L.

A simple way to estimate volumetrically is to divide 2.08 or 2.6 by the %N in the formula; the result is ml/L for 20 or 25 ppm N, respectively.

2.08/11.8=0.176 ml/L
2.6/11.8=0.22 ml/L

[powderherface]

Originally Posted by Ray

For daily. - even several times a day spraying, I'd use no more than 20-25 ppm N.

Looking at the more accurate mass measurement, you'll want 0.17g/L for 20 ppm N or 0.21g/L for 25.

A simple way to estimate volumetrically is to divide 2.08 or 2.6 by the %N in the formula; the result is ml/L for 20 or 25 ppm N, respectively.

2.08/11.8=0.176 ml/L
2.6/11.8=0.22 ml/L

Thank you Ray that's really helpful. So 0.21g/L comes out as a little under ½ their recommended "at every watering" concentration (1g/2L i.e. 0.50g/L). I feel a bit nervous about going for ½ dilution daily, but maybe I'm missing something about this and it's fine? If so, would love to hear your perspective.

[Ray]

Originally Posted by powderherface

Thank you Ray that's really helpful. So 0.21g/L comes out as a little under ½ their recommended "at every watering" concentration (1g/2L i.e. 0.50g/L). I feel a bit nervous about going for ½ dilution daily, but maybe I'm missing something about this and it's fine? If so, would love to hear your perspective.

What is the frequency they intend with "at every watering"? I sure don't know.

The mass of nitrogen provided can be thought of as being analagous to the calories we take in - lots of them in one, giant meal per day, or few in several, small meals. As long as the total is what is needed, we're good (although multiple, small meals are supposed to be healthier).

Before I retired and downsized to a home with no greenhouse, my watering was automated, "raining" a 25 ppm N solution on the plants daily in summer and every few days in winter.

Recognize that for your mounted plants, the mass of nitrogen they take up is limited by the volume of the velamen on their roots., and that is minimal. Once that's saturated, they're absorbing nothing more until they are wetted again.