I see two possibilities:
- You get poor soil
- You transform your soil
You can walk around your place, and you will see some places with poor soil. This probably is also poor on nitrogen. Try a dry, stony place, with non-black (and not to dark soil), or sandy soil. Nitrogen (in soil, and humus) is made from plants, so you should look where there will be not much deposit.
An other good place, it is deep soil (just a feet down (30 cm)) should be enough to have a less dark soil, so (usually) less rich. Note: near rivers, it is possible that you need to dig too much to find poor soil.
To transform your soil. Usually adding sand should be enough (50% sand, rest normal dirt).
Note: the soil is not characterized only by nitrogen, so other factors could miss-up your experiment (acidity, other nutrients, ...)