PhotosynQ measures more than just plants: A history of our forays into measuring soils

Over the past several years there has been quite a bit of interest in measuring soil properties, which makes sense given that most of the plants we care about grow in soil. In response to that interest, we have developed numerous PhotosynQ prototypes, protocols and macros to measure soils over the past two years.

We started by using the MultispeQ to measure soil active C using potassium permanganate. This method used a cuvette to measure a color change in solution. The problem with using colorimetry, however, is that it requires users to do wet chemistry in the field. We would rather avoid that.

Another approach was to measure in situ C mineralization as an indicator of soil health. We have built multiple iterations of in situ soil C chambers (below). In general, the results from these chambers were positive, but there were a few drawbacks. One is that the results were highly dependent on soil moisture content and temperature. Therefore, we would need to collect a lot of data at different moisture and temperature conditions to account for these variations, much like we need to collect photosynthesis data at multiple light intensities to account for the effect of light on Phi2, PhiNPQ and PhiNO. The second problem is that the prototypes were quite clunky, and generated a lot of funny looks around campus when they were half-buried with random wires hanging out. If enough people were interested in using the in situ chambers, we could make a few mechanical changes to make them look less like an IED.

Soil chambers

Going back to the drawing board, we brainstormed different ideas to simply assess soil health without having to take a lot of measurements or have multiple devices. This led us to develop a simple tool for measuring soil C mineralized from a sealed container. Using a “24-hour C mineralization burst” we can control for different temperature and moisture conditions by first air-drying, and then rewetting soil samples in quart jars. Then we use a syringe to sample headspace in the jar and inject it into a pass-through CO2 sensor.

C min

The technology is pretty simple, just a CO2 sensor connected to a microcontroller, loaded with PhotosynQ firmware. To demonstrate our new SoilspeQ, we worked with a professor at MSU and took soil samples from a field where she was testing the effect of cover cropping on soil quality and maize productivity.  We took many samples from different areas including soil that had mixed cover crops, soils without cover crops and some soil from the bare ground bordering the field plots. We also collected the soil from 1-5 cm and 5-10 cm deep, so we could see if there were differences between them. Check out the results here.

Our final approach to measuring soils is still in its early stages, but we are looking forward to see where it goes. We have teamed up with a professor at Colorado State University to develop microfluidics cards. The goal is to use reagent embedded cards to reduce in field wet chemistry and accurately measure key soil properties. We then use the MultispeQ or a version of the CoralspeQ to measure the color change. Our initial test, using Al3+ in solution at different concentrations is shown below.  

Al Fig

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