[hamil]

The pH of soil is usually determined by adding a soil sample to water and then measuring the pH of the liquid. However, it seems that the pH of the liquid is determined by the ratio of soil to the amount of liquid, yes? If I put a small amount of soil in a large amount of water the pH would be small. If I put a large amoun of sopil into a small amount of liquid then the pH would be larger. So, how is the pH of the soil measured? One needs to know the poh to know when to add fertilizer.

[Borek]

1. If you always follow the same procedure (the same amount of soil and the same amount of water) results are perfectly comparable.

2. As long as the deviation from the standard procedure is not large, pH measurement is reasonably accurate, as soil contains plenty of buffers which keep the pH constant despite dilution.

[hamil]

Borek, let me clarify my question. I want to prepare soil for tomatoes. I understand that a pH between 6 and 7 is good, slightly acidic. I have a electrical pH meter. So how much soil must I put into how much water so that my measured pH is indicative of the pH of the soil.

It seems to me that there must be some sort of standard like put 20 grams of soil into 100 ml of water and then measure the pH.

Does this make sense?

[Borek]

http://www.ph-meter.info/pH-measurements-other-procedures

I wouldn't use more than about twice as much water as there were soil. Say - start with 20 g of soil and 20 mL of water, if there is not enough supernatant over the solid to take the measurement, add 10 mL (or even 20 mL if necessary) of water, but not more. Actually if you take the measurement after adding 10 mL and then after adding another 10 mL the difference should be negligible.

Note: you should use DI or ROD water, not a tap water.

And don't forget to calibrate the pH meter.

Compare http://www.cfs.nrcan.gc.ca/bookstore_pdfs/19004.pdf

[hamil]

My soil test kit stated to use one capful of soil (2 ml) per 10 ml of water. I made a larger amount using the same ratio and my pH meter read 8.5 so I guess my soil is alkaline.

Seems like there ought to be a site that explains the chemistry behind the nitrogen, potassium, and phosphorus measurements somewhere. I wonder what the mystery chemicals are.