Test Kit Issue

[BenB]

I posted this in my journal, but I thought it was worth posting elsewhere for people who might not see it there.
My Salifert NO3 (expiration May 2026) test kit kept giving me a low reading despite adding what I thought was plenty of nitrate.
I decided to buy fresh refills for my old LaMotte test kit.
Salifert: 1ppm NO3
Lamotte: 6ppm NO3-N = 26.4ppm NO3
No idea what's up with the Salifert. Maybe too much exposure to the humidity here in the south degraded the reagents.
I double checked that I was doing the test properly.

Salifert


Lamotte

 

[Art]

In the past, I dealt with bad batches of Salifert test kits. It happens. I don't know if they go bad or if it's quality control.

I tend to prefer staying with scientific brands like Lamotte or Hanna for test kits. I have no reason to other than thinking that because most scientific labs rely on them, they may be more careful with their production.

Having said that, I want to share my thoughts on testing. They are primitive and only my own so feel free to ignore or criticize.

Best method:

1. Start with pure RODI water and add a controlled amount of nitrate so that I know my starting point.
2. Test endpoint nitrate using standard reference solution method described below using Lamotte or Hanna kits.

Almost best method:

1. Test using standard reference solution method using Lamotte or Hanny kits.

Standard reference solution method:

1. Using RODI or distilled water, add 5 ppm nitrate to a known water volume.
2. Test using your test kit.
3. Take a picture of the test kit color.
4. Do the same but adding 10 ppm nitrate.

Now you have a picture to compare your future test kits to. Of course, this is not terribly accurate but that doesn't matter. What you want to know if more or less that you have enough nitrate for your plants to consume. Then let your plants guide you.