New Fertiliser - confused about dilution rates

[RosieC]

great method... but won't work for a liquid fertiliser which is what I'm getting (and what I use now).

I don't use any somewhere arround every 30 days, but it varies, because it's more a case of when I remember it needs a flush without any.

[Ray]

To be truly scientific about it, I think the smart methodology is to think in terms of the mass of nutrients applied over time, and back-calculate to the dosage, as large, commercial nurseries selling foliage plants do.

Poinsettia growers know it takes 0.5g nitrogen (plus the other elements, as provided by their preferred fertilizer formula) to raise a plant from cutting to blooming for Christmas sales. By knowing the volume of medium in the pot, its % retention, the feeding frequency, and the time to harvest, they calculate the concentration that will deliver that half a gram over that time period.

I don't have that much solid information, but I can follow the general protocol:

I start with the understanding that a dilute fertilizer is a safe fertilizer for most plants. 125 ppm N seems to be fine for phrags, so I use that as my "baseline".

This time of year, I water three times a week, and assume the watering volumes to be about the same each time. I use that 125 ppm N solution of the MSU RO formula fertilizer, so we might say I give 375 ppm per week. (In reality, it's 375 ppm x the volume x the percent retained, but who knows that they are?)

If I wanted to maintain that feeding rate, but only feed 3 out of 4 waterings, I would need to increase my dosage rate to 4/3 x 125 = 167 ppm N. If I wanted to feed every other time, 250.

I have a more detailed discussion of that here: Feeding by Mass

[RosieC]

Thanks Ray. That actually makes a lot of sence... of cours knowing the retention rate is pretty hard so like you I will stick with the 125 ppm N as a baseline.

I had not thought of the fact that the number of times you water with/without fertiliser makes such a difference (apart from my monthly flush to clear them out again). I also know my flush doesn't clear ALL of it out, so that's another unknown.

Anyway, many thanks for the advise and I will read that more detailed 'Feeding by Mass' when I get a momment at home.

[Ray]

Originally Posted by RosieC

I had not thought of the fact that the number of times you water with/without fertiliser makes such a difference (apart from my monthly flush to clear them out again). I also know my flush doesn't clear ALL of it out, so that's another unknown.

Just think of feeding as an analogy of eating. Let's say you consume 2000 calories a day to maintain your health. If you only consumed 2000 calories every other day, you'd eventually become a lot less healthy.