As the subject states, I am looking for a way to test for the presence and quantity of a compound important for plants. This compound should preferably be so crucial that difference in quantity (of smaller soil samples) should be grounds enough to suggest a relationship between plant-soil compatibility and the quantity of the compound.
So far, I have thought about the compounds Nitrate, ammonium and phosphorus, but I have no clue how to test for them, in soil.
Any ideas?