In a previous post I have addressed the difference between zone sampling and grid sampling. From that post one could gather that I am not a big proponent of zone sampling. The reason why is because when establishing the zones to be sampled the method used to establish the zones, e.g., by soil type or Veris data, is not necessarily related to the level of nutrient being tested. As a consequence, variability of nutrient levels within the zone can be just as high as it is within the entire field. If this is the case, why would I want to reduce the number of samples I pull from a field? Am I not getting a less accurate result?
I think one should also ask himself if this method is acceptable for other applications outside of soil sampling.
I think a good analogy would be in the pharmaceutical industry and medicinal drug dosing.
Let’s suppose for a moment that I am a researcher for a leading pharmaceutical company and I have discovered a drug that prevents cancer. I know through my research that all persons can benefit from this drug but dosing needs to be established based on each individuals cancer risk. Those with a high risk should take 40 mg of the drug, 30 mg for an elevated risk, 20 mg for a slight risk, and no treatment for a near zero risk.
However, a problem arises with determining risk.
I know that screening every person for risk is time-consuming and expensive. It’s unreasonable to expect people to get screened because most won’t be able to afford it. However, I know that the benefits of the drug are so great that people need to be taking the drug so I devise a plan to determine the dosing requirements without testing each individual person.
What I decide is to break up all the landmass of the U.S. into “zones” based on common cancer risk. The already determined state boundaries provide an excellent basis for establishment of these “zones” because there is already data collected for each state, thus saving me a lot of time and expense in determining dosage requirements. Based upon my research, I determine the appropriate dosage for the inhabitants of each “zone” as follows:
The residents of states highlighted in red will receive a 40 mg dose, residents of states in orange will receive a 30 mg dose, residents of states in light blue will receive a 20 mg dose, and residents in the dark blue states will not need to take the drug.
Now the important question is if this is an acceptable way to determine individual dosing requirements? I think the obvious answer is NO! In each state we still have a large amount of variability. There are people who would benefit from a larger or smaller dose than the state average calls for, thus, a “zone” approach is vastly deficient. But yet we deem this same approach as acceptable for determining nutrient requirements for our crops. Even though there are areas within each arbitrary “zone” that would benefit from a higher or lower rate of nutrient, we are comfortable with a “zone” average. It doesn’t make a whole lot of sense!
The only good way to determine nutrient levels of the soil is by grid sampling rather than arbitrarily established “zones” which are not related to fertility. Grid sampling allows for an unbiased measurement of a fields fertility, which in turn, gives you the most accurate result. In the end, it will make you more money.