P and K are stable nutrients that stay in place from year to year, so they built up in the soil if they are not consumed by crops. P and K and can be maintained over the long term bases on crop yields.
Nitrogen is more volatile and must be managed to only supply short term crop needs. Nitrogen can be lost even if it is not consumed by crops. Nitorgen level need to be tested more often and in more locations than P and K level.
There is a soil nitrogen cycle or N-cycle. Organic matter and nitrogen fertilizers applied in Ammonium forms (NH4+) are stable and stay in place in the soil. Soil micro-organizms break down Ammoniums (NH4+) into Nitrite (NO2) and then into Nitrate (NO3) that plant roots can take up. Nitate (NO3)is water soluable and will leach down below the root zone. Soil micro-organizms also continue to break down Nitate (No3) into Nitrogen gas (N2) that evaporates into the air. High soil temperatures, and water saturated soil depleat nitrogen levels more quickly regardless of crop yeilds. So... nitrogen levels must be managed to only supply short term crop needs before it is lost.
Others know more about this than I do and can explain it better than I can.
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