[Jorke]

I am planning on growing a garden this year. I know that i can get a soil test from a lab nearby, but i always wanted to know how I could do it. So, what i am trying to measure is the Ph, phosphorus, potassium and nitrogen levels in the soil.

I know enough to test the Ph myself and i plan on doing so accordingly. They say most plants like a slightly acidic soil. But really it varies from plant to plant. I know i can add lime (calcium carbonate) to reduce the level of acidity which would probably be my problem, due to the lack of rainfall in my area which causes a more acidic soil.

So my real questions are how to measure the amounts of phosphorus, potassium and nitrogen.

Lets start with phosphorus (P):
I looked into it and found an article saying vaguely how they do it. They state they use an acid and color developing reagent to measure the phosphorus level in the soil. They state that the solution turns blue after adding the acid and they measure that using a colormetric method, the intensity of blue shows how much phosphorus is in the soil. Does anyone know of any acid or color developing reagents that would react like this? Normally the phosphorus in soil is phosphate PO4.

Potassium (K):
Literally all they state is that they use a different solution and filter. they then use an atomic absorption spectrophotometer..? what the hell is that? I'm guessing i'm going to need a different kind of method of measurement. So if anyone has any ideas spout them out. thanks. In nature potassium is found in 3 states: unusable by plants (insoluble), partially usable (partially soluble), and readily usable (fully water soluble). Potassium is prevalent as feldspars (KAlSi3O8) and micas but that kind of potassium is not usable by plants. So what i am trying to measure is the amount of readily available potassium. How would one measure this? I don't know the chemical composition of this fully soluble potassium.

Lastly Nitrogen (N):
I learned that Nitrogen can vary from one day to another. So i will be ignoring that measurement. And as precaution i will use a 'green manure' aka 'cover crop' to feed them extra nitrogen before I transplant my vegetables.

P.S. I didn't state the elemental symbols because i thought you dim witted. (haha) But mainly because i'm a novice chemist and i want to remember all of them.

[Borek]

Does anyone know of any acid or color developing reagents that would react like this?

http://en.wikipedia.org/wiki/Phosphate_test

atomic absorption spectrophotometer

http://en.wikipedia.org/wiki/Atomic_absorption_spectroscopy

I'm guessing i'm going to need a different kind of method of measurement.

I am not aware of any simple test for potassium. That is, AAS is simple, but requires a spectrometer.

[AWK]

Good start
http://www.icarda.org/publications/lab_manual/read.htm

[Arkcon]

Garden supply stores do sell soil testing kits for NPK.  They give their results colorimetric result, and see to me, in their description, to be a little ... general.  I don't know if they're very accurate, and I also don't know the exact reactions.  But if you really want to know where your soil is at, you can just buy the kit.  It will be hard to find the real reactions in the kit, because, like Borek: said, AAS is so much simpler.  Granted, and Atomic Absorbtion Spectrometer uses a flame to excite the potassium ions, and a diffraction grating or hollow cathode lamp to generate the proper wavelength, but once you've bought this multi-thousand dollar instrument, you can read potassium concentrations in random solutions all day, just suck up some solution and go.