Sciencemadness Discussion Board

Any soil scientists out there?

Twospoons - 15-11-2005 at 14:11

I love gardening. I love growing vegetables without nasty pesticides. I want happy healthy plants. So naturally I want to be able to analyse the nutrient content of my soil. Nitrogen, potassium phosphorus and pH I can do with a kit from the garden store (though the pH colour chart is appallingly hard to read).

So the question is how do I test (quantitatively) for S, Ca, Mg, Cu, Zn, Mo, B, Fe , given standard lab glassware?

I know the soil labs use fancy hardware like IR and mass spectrometers ( and I suspect thats the only answer) but if anyone has any bright ideas I'd like to know.

chemoleo - 15-11-2005 at 14:45

Well there are numerous inorganic reactions to do this with a high degree of sensitivity, but qualitatively. It is very hard to determine the amount of something when it's in the microgram range (such as Mo).
Really, what you need is a good spectrometer, and a variety of organic dyes, that specifically react with this ion or that. The amounts are estimated by the absorption, relating this to the molar absorption constant.

But if you want qualitivative assays, I am sure most here a capable of helping on this :)

Invest in a spectrophotometer

Hermes_Trismegistus - 15-11-2005 at 14:48

This might sound like a trite answer to your question but believe me it's not.

The spectrophotometer has been around (sans electrics) for a LONG time and is the undisputed workhorse of analytic chemistry.

I personally beleive that after a vacuum pump, a spectrophotometer is the next major piece of equipment that an amateur should purchase.

I am currently wondering myself if I should start saving for one now, or maybe take up B+W photography. Tough choice:( but with about 4-500 bucks for a used spectrophotometer, I might just take up photography.

The only good news is that they are REALLY common on the used market, because both chemistry and biology use them extensively. LabX, ebay etc.

spectrophotometry basics

Twospoons - 15-11-2005 at 18:05

"The devil farts in my face again ..." *sigh*

As you say, a quick look at Ebay reveals a bewildering range of used gear. Any pointers on what to look for, should I decide to invest?

denatured - 21-11-2005 at 00:16

Until you buy a spectrometer ... don't worry about nutrient contents in your garden...

What is its type ? have you tried growing something and nutrient deficiency signs appeared?

Tacho - 21-11-2005 at 02:46

Why not built one? Seems pretty straightforward to me. A cheap digital multimeter, a photocell, photodiode or both, some high power LEDs of different collors...

Twospoons - 21-11-2005 at 13:19

I had a look at the principle of the atomic absorbtion spectrometer, and it seems reasonably straightforward. The key to isolating specific elements appears to be in the light source which is a hollow cathode lamp containing the element of interest. So it only emits light on the wavelengths which will be absorbed by the element you're looking for. Broader sources would seriously compromise the sensitivity, as other compounds in the sample would affect the total absorbtion.
The elements I'm looking for would be in the 10-100 ppm range.

Still, the lamps can be had from ebay, for less than the whole machine, and the rest doesn't seem too hard to build.

On the other hand I can get a soil sample tested for $60 by a professional lab so unless I find another compelling reason to get my own analyser, $60 is pretty cheap really.

Tacho - 22-11-2005 at 02:07

When I think of a spectrophotometer analisys I think of adding a few drops of a specific reagent to the solution to be tested and measure the change in light absorbtion compared to reference samples. Am I wrong?

Can you do that without a specific reagent?

DrP - 22-11-2005 at 03:35


When I think of a spectrophotometer analisys I think of adding a few drops of a specific reagent to the solution to be tested and measure the change in light absorbtion compared to reference samples. Am I wrong?

The light is absorbed at distinct wavelengths which correspond to the frequencies of vibration of the bonds in the sample being tested. (Only light vibrating at the same frequancy as the atomic or molecular bonds is absorbed) Thus, the absorption specrum is an 'Atomic/molecular fingerprint' which can be used to identify what types of bonds are in the sample and thus tell you what your unknown sample is (with experience in the interpretation of spectra)

The idea of a reference sample is usually to compare the spectrum from the solvent to that of your sample, so you can ignore the peaks which come from the solvent when trying to determine your unknown.

Sorry if you know this already Tacho, but it sounded like you may not from your last post - I may have misread it.


[Edited on 22-11-2005 by DrP]

Tacho - 22-11-2005 at 04:05

Let me see...

1- spectrometer - outlines the absorbance spectrum of a sample in a range of electromagnetic frequencies (not necessarily light).

2- spectrophotometer - says how much light was absorbed in a given frequency.

I always saw # 2 as a way to compare the result of a light-absorbance-changing reaction with a set of known samples to test how much reactant was there in the first place.

Hermes link seems to confirm my beliefs. There is another thread on spectrometers (not spectrophotometers) where I just posted an idea, by the way. I would like to hear your opinions there.

DrP - 25-11-2005 at 07:47

Hi Tacho,

Sorry for the delay in the reply - been busy. Im pretty sure the names are fairly interchangable. Last year I bought an old second hand Perkin Elmer 841 Infrared spectrophotometer - It gives absorbance spectrum of a sample across the IR range. It too could be used to follow the change in absorbance of a mixture as a reaction takes place amounst other things.

To get concentrations out, as you said, you need to have a reference. Atomic absorption spectroscopy is usually used for soil analysis - you have to know what frequency your target metal atoms will absorb at and beam in light with the same.

I'm not to sure about building one for myself - your idea sounds sound as long as you know the frequencies of the light source and that the detectors are sensitive enough. I would imagine you could use a prism on rotatable disk to scan.

One of our Uni lecturers built a Raman spectrometer in a large black box using different high powered lasers as the radiation source. Fidley to set up I think but not my expertise.


Niels Bohr - 2-12-2005 at 09:09

Why wouldn't you use chromatography ? You could find the particles you are looking for according to solubility, Size,Charge, and binding affinity.

just a wild idea ?

DrP - 21-12-2005 at 08:35

You couldn't use chromatography because the elements he's looking for will be tied up in complexes.