Experiment

The log is a graph of resistance and depth. The vertical axis shows depth in centimeters and the horizontal axis shows resistance in thousands of ohms.

The log begins at a depth of 2 cm because at 1 cm the resistance was too high to measure on our ohmmeter. The maximum value it can read is 2,000,000 ohms.

Interpreting the Data

What does our log show? It appears that the resistance is generally higher near the surface. Although the resistance does not always drop from one level to the next, all the readings from 5 cm down show a lower resistance than any of the readings from 4 cm up. What accounts for the difference? We know from the Electrical Resistivity of Materials experiment that dry soil has a high resistance while wet soil tends to have a lower resistance. This is because water with some impurities conducts electricity while most dry soil does not conduct electricity well. It seems likely that the soil is wetter several centimeters below the surface than higher up. When the plant is watered the water flows down and wets the soil throughout the pot. Then it starts to evaporate from the top first. So it is likely that the deeper soil is holding more water. There are other possible factors that could produce this result. There might be a different soil composition at different levels, but this isn't likely since all the potting soil all came from the same bag and was relatively uniform.

There might be a higher salt content in the deeper soil. We know that salt water is a better conductor of electricity that fresh water. But this explanation is weak because potting soil is not salty and we never added salt to the soil. Maybe there’s some oil mixed into the soil near the surface. Oil does not conduct electricity. This is also an unlikely explanation because there’s no reason to believe that oil was introduced into the soil and the soil near the surface does not feel oily.

Our original explanation, that the soil near the surface is drier, is most likely correct.

Redesigning Our Logging Tool

We’re thinking about building a better logging tool. This one has some defects and limitations that we think can be corrected with a better design.

The tool is not very sturdy. It works well for soft soil, but a pencil point is not a very good drilling tool, and the pencil itself could break if more pressure were applied to it.

In the oilfield there is another another approach called "wireline logging" in which the logging tool doesn't do the drilling. Instead, it’s attached to a cable and then lowered into a hole that’s already been drilled. The pencil tool models "logging while drilling" where the logging tool is integrated into the drill pipe.

Maybe we’ll design and build a wireline logging tool next time.

Another problem with our tool is that the wiring is not well protected. With the wires running along the outside of the tool the plastic insulation is subject to wear.

The tool is not sufficiently waterproof. After just one use the electrical tape started to unravel. We think this is because of dampness in the soil.

We’ll work on it. Maybe you can come up with a better design also. If you do, share it with us by sending a description, pictures and a log.