Fertilizer PPM: how to determine

[Leucadian]

I'm trying to understand some basics about fertilizing my orchids. I am collecting rainwater, mixing up a concentrate, then diluting it for my weekly watering, believing that the weekly weakly should mimic natural conditions. I bought a cheap dissolved solids/conductivity meter to ensure I didn't make a fatal mistake and burn everything to a crisp.

When I started thinking about it, I realized that none of the fertilizers tell me what the concentration of N is at their recommended rate. All fertilizers are marketed with NPK percentages, but the dilutions are by volume (teaspoons per gallon for instance). I bought some MSU (pure water) 13-3-16 fertilizer from a vendor (blue spheres of different darkness plus dust) who recommended 1t/gal.

I mixed up a gallon at a 16x concentration, then diluted that by 64:1 so my final applied solution is 1/4 recommended concentration. I measured 1t of the dry mix at 4.6 g, so I have 1.15 g in my gallon. A gallon weighs 3.785 kg, so 13% of 1.15g in 3.785kg water gives me 0.0000395 g/g = 39.5 ppm N. Full recommended dilution would be 4x that, 160 ppm after rounding.

I've been feeding the plants with the 1/4 dilution for a couple of months, and they are thriving (spring in San Diego, they should be doing well).

My questions are first did I calculate this correctly? and second what should I be reading on my PPM meter? My rainwater comes in at 60ppm, and the 1/4 strength solution is about 660ppm, about 16 times what I calculate. So how does the meter calculate PPM?

Or is the meter only useful to check consistency between batches, not the actual concentration of N or anything else? (Or maybe the PPM measured is of the full 4.6 g of dry fertilizer, so the total PPM is (1/.13) * 39.5 = 303, or 2 times what I calculated.) Obviously I am missing something here.

[AnonYMouse]

Umm...Read up at First Rays.

Here's how I did it. I went here first. I target for <50 ppm N (low dose every watering)

I mix up a solution per label instructions then dilute it farther by doing some math, use a TDS meter to verify that TDS is acceptable (<200 ppm TDS) then feed/water as needed.

It's not going to be accurate because I use household measuring spoons/cups and settling that's bound to happen with granular/powder fertilizers but this method keeps the levels safe.

[Ray]

TDS meters are notoriously inaccurate. From what I've learned from the pros in the industry, orchid hobbyists are pretty much the only folks who use them. They are cheap electrical conductivity meters with a built-in conversion to ppm, but different chemicals will give different conductivities at equal ppm's, so how can you know if it's correct for your specific chemistry?

Greencare, the manufacturer of the MSU fertilizers, gives the electrical conductivity numbers versus ppm N, as that's how professionals control their dosage. For the MSU RO formula, 0.008mS/cm per ppm N, so if the EC meter is good, measure that of the pure water, subtract that from the EC of the solution, and calculate away.

The best way to use a TDS meter is to measure by mass, mix accordingly and accurately, then measure the TDS. Then, on your next mix, you can use it to see if it's the same.

By the way, the bulk density of the fertilizer can change batch-to-batch, so go with mass, rather than volume, if you want to be accurate.

They also quote 2.91g/gal for 100 ppm N, by the way.

[Leucadian]

OK, 2.93 g per gallon for 100 ppm N. I measured 1 teaspoon as 4.6 g, so their recommended concentration works out to be 156 ppm, almost exactly what I calculated, 160 ppm N.

What I would like to see is the statement 'XXX ppm N at recommended concentration of 1 t / gal' on every fertilizer container, or at least published where we can easily find it.

Thanks for the comments and the numbers from the manufacturer (I am using the pure water GreenCare formulation for the MSU mix).

[Ray]

The trouble is your desire for a recommended concentration.

I feed at every watering, which can be three times per week in mid summer, once a week in winter, so I use about 25-35 ppm N.

If I fed only once a week, I would likely use something in the 75 ppm N, and if every two weeks, 125 ppm N, which is what they did in the MSU study experimenting with the formula.

So what is "the recommended" concentration???

I have several articles discussing this - including ppm calculators - at my informational website, which is pretty accessible.

[Leucadian]

I don't expect the fertilizer vendor to tell me what concentration to mix for my plants and my watering schedule. All I'm asking is for them to tell me what the N concentration is when I mix their recommended solution. Then I can dilute it down to my target. One vendor may say 1t/gal, while another indicates 0.5t/gal. Are they both providing the same concentration of N? They don't say.

Your website has the most information on this issue of any that I have found. Thank you for that.

From your website, I went on to find an article by Peters and Kackley 'Anatomy of a Fertilizer Label' that helped a lot. It shows a label for Jacks Professional 20-3-19 fertilizer, and there at the bottom of the label is '1teaspoon/gallon =188 PPM N'! Exactly what I am asking for on hobbyist grade packaging.

[Ray]

That must be a pretty low density powder! The bulk density of many powders is in the 1-1.2 g/cc, and using 1, a teaspoon of that fertilizer per gallon would give you closer to 265 ppm N.