Sciencemadness Discussion Board

An appolegetic request for spoonfeeding - NPK Testing

Sulaiman - 21-2-2023 at 20:37

I have a semi-urgent need to test some incoming fertilizer for my brother-in-law,
he is suspicious of its NPK rating,
not by a little, by a lot eg 15-5-30 could actually be 2-0-5
We will be meeting in a few days anyway so I asked for a sample, just in case I can help.

I'm not optimistic as the only "Chemistry stuff" I have at the moment is pH papers.

I found various NPK test equipment and kits on shopping sites, all for soil testing, not fertilizer testing.
I guess I could make a dilute fertilizer solution?

A quick cheap kit, or a re-usable moderately priced kit are I think, good options,

I found a local lab but they specifically do not test fertilizer. No reason given.

Worst case is me re-united with some of my stuff in late March,
I'm sure I would be able to test by then, maybe with some purchases. research and help.
So
Step 1 : anyone with fertilizer NPK test kit knowledge that can recommend something for a small palm oil plantation.?

Edit : this document prescribes the procedures for a 'propper' analysis
Not realistic for me, I need simpler, cheaper, no need for great precision (+/- 1% is good enough)
Attachment: TestingMethodsForFertilizers2013.pdf (5.4MB)
This file has been downloaded 162 times

[Edited on 22-2-2023 by Sulaiman]

j_sum1 - 22-2-2023 at 00:10

There are aquarium water testers that test for nitrates.
There are others fir ammonia but they are more pricey.
Molybdenum blue is a qualitative test for phosphates.
Potassium... Could be tricky.

I recall random experiments international testing for a range of anions and cations in wood ash. I don't recall if there were any significant ions omitted or if there were any tests that could adapt to quantitative and suited to your applucation. Here us one of the videos.
https://youtu.be/Hnz7X8JzNxA

B(a)P - 22-2-2023 at 01:22

If you have access to a perchlorate salt (other than potassium perchlorate) you could use the poor solubility of potassium perchlorate to do a quantitative test.

Fulmen - 22-2-2023 at 03:21

I did this some 30 years ago (fertilizer plant), a lot of work but doable with limited equipment. Don't remember much detail except for the Kjeldahl / Devarda nitrogen. Phosphorous was gravimetric, producing a bright yellow precipitate and I believe K was titrated using EDTA.

the Kjeldahl is simple enough to do with limited equipment. For NH4-N you add NaOH and distill the ammonia into a precise amount of excess HCl. For NO3-N I think you can use aluminum to replace the devarda's alloy.

Sulaiman - 22-2-2023 at 05:09

The Kjeldahl/Devarda's alloy bit interests me if only for the name and novelty.
I may, after at least a year, set up some sort of agricultural analysis lab - maybe.
For now I'd like something easier even if not very accurate.
(I should be easily able to make an NPK reference sample if required).

Do you think that a soil NPK test kit would give a usable result using a known dilution of fertilizer solution?

B(a)P - 22-2-2023 at 11:00

Quote: Originally posted by Sulaiman  
The Kjeldahl/Devarda's alloy bit interests me if only for the name and novelty.
I may, after at least a year, set up some sort of agricultural analysis lab - maybe.
For now I'd like something easier even if not very accurate.
(I should be easily able to make an NPK reference sample if required).

Do you think that a soil NPK test kit would give a usable result using a known dilution of fertilizer solution?


The only issue with soil kits is finding one that gives you a quantitative result. most that I have seen are semi quantitative (eg high, medium and low), but maybe you already have one in mind that can give results in mg/L or similar. Or if you hunt around you could find some thing. You could test the accuracy of the kit. As j_sum1 said aquarium test kits might be useful as they will give you are quantitative result, but you don't get potassium. Overall though, I think making up a dilute solution in the test kits (soil or aquarium) detection range would be the way to go. I assume you only want accuracy to plus or minus a few percent?

Fulmen - 22-2-2023 at 12:29

I agree, aquarium test kits sounds like a low effort place to start. Can you get a sample of the "good stuff" or a similar quality? it's simpler to interpret data against a known reference. Besides, the simplest analysis is usually a visual inspection.

Sulaiman - 22-2-2023 at 21:46

@ B(a)P & Fulmen... Thanks, off to check now..