How to measure the amount of Osmocote x Lt (or Ounces) in a Pot

[Ray]

I think you’re looking for a very simple calculation to a not-so simple situation.

The scenarios I described were using the water retention as the controlling factor, trying to think of it as a liquid fertilizer would work, and I was working with applying a 100 ppm N solution applied once a week.

If your container only holds a small amount of liquid, it’s going to provide a small amount of nutrition when you feed. In such a situation, chances are, you’ll be feeding and watering more frequently than the once a week @ 100 ppm N that was my basis.

The factor you’re missing is exposure.

Unlike terrestrial plants, whose hairy roots grow to completely fill a container - and can therefore extract almost all applied nutrients - orchid roots only occupy a relatively small volume and the root hairs are microscopic. In order for the plant to get water and nutrients, the solutions must come into direct contact with them.

In order for the plant to get a sufficient mass of nutrition, the concentration, volume, and frequency of application all come into play. Think of it like calories for our own diets. One can eat several, low calorie meals per day (analogous to low ppm, high frequency fertilization) or gobble up one large meal (higher ppm, less frequently).

Then there’s the root structure. A plant with a few, thick roots has a relatively small volume of velamen to absorb nutrients, so will need more frequent, or more concentrated fertilizer application than will a plant with an extensive array of thinner roots. Also, the more roots in the pot, the greater the likelihood some of the fertilizer solution will actually come into contact with them.

So now translate that to pellets scattered in the medium. Using a nutrient solution wets the entire root system for a short period of time. Using pellets only releases it in pockets. In both scenarios, drying cuts off the supply.

You can always “ratio” the liquid volumes and frequency to guesstimate the amount of pellets needed. If “x” works for a container that holds 150g of water once a week, then if the same pot volume of another medium only holds 75g, you can either double the pellet quantity or double the watering frequency. In the situation where the retained water is only 15 grams, using 10x the amount seems likely to be damaging to the roots, but increasing the addition a smaller magnitude AND a the frequency of watering might be a better balance.

It is for all these complexities that lead to imprecision that prevents me from using slow release fertilizers. I prefer to know exactly what I’m applying

[Cach26]

Originally Posted by Ray

I think you’re looking for a very simple calculation to a not-so simple situation.

The scenarios I described were using the water retention as the controlling factor, trying to think of it as a liquid fertilizer would work, and I was working with applying a 100 ppm N solution applied once a week.

If your container only holds a small amount of liquid, it’s going to provide a small amount of nutrition when you feed. In such a situation, chances are, you’ll be feeding and watering more frequently than the once a week @ 100 ppm N that was my basis.

The factor you’re missing is exposure.

Unlike terrestrial plants, whose hairy roots grow to completely fill a container - and can therefore extract almost all applied nutrients - orchid roots only occupy a relatively small volume and the root hairs are microscopic. In order for the plant to get water and nutrients, the solutions must come into direct contact with them.

In order for the plant to get a sufficient mass of nutrition, the concentration, volume, and frequency of application all come into play. Think of it like calories for our own diets. One can eat several, low calorie meals per day (analogous to low ppm, high frequency fertilization) or gobble up one large meal (higher ppm, less frequently).

Then there’s the root structure. A plant with a few, thick roots has a relatively small volume of velamen to absorb nutrients, so will need more frequent, or more concentrated fertilizer application than will a plant with an extensive array of thinner roots. Also, the more roots in the pot, the greater the likelihood some of the fertilizer solution will actually come into contact with them.

So now translate that to pellets scattered in the medium. Using a nutrient solution wets the entire root system for a short period of time. Using pellets only releases it in pockets. In both scenarios, drying cuts off the supply.

You can always “ratio” the liquid volumes and frequency to guesstimate the amount of pellets needed. If “x” works for a container that holds 150g of water once a week, then if the same pot volume of another medium only holds 75g, you can either double the pellet quantity or double the watering frequency. In the situation where the retained water is only 15 grams, using 10x the amount seems likely to be damaging to the roots, but increasing the addition a smaller magnitude AND a the frequency of watering might be a better balance.

It is for all these complexities that lead to imprecision that prevents me from using slow release fertilizers. I prefer to know exactly what I’m applying

Okay, I understand the point Mr. Ray, thanks for the explanation.