Now that I’ve gotten some soil samples, it’s time to start prepping them for analysis! We’re doing this once a week at the Morton Arboretum soil lab, since it’s better equipped for a large number of samples than the Botanic Garden.
We decided that for our soil protocol, we’re going to analyze two 15 cm cores from each plot, rather than compositing every five plots in a transect into one sample as originally planned. This will give us data that’s a lot more specific, but it also means that we have 10 samples per transect, and with usually 2 transects per site that leaves 20 samples per site with 3 to 4 sites a week, so 60 to 80 samples to analyze a week! Luckily Bob’s a trooper and has been helping me out an enormous amount 😀
Step 1: Sieve the samples
This breaks up the samples so they’re much easier to work with, and produce more accurate results when processed. With the super clay-ey soils this takes a very. Long. Time.
Step 2: Clean the vials
To make sure that the vials are clean, we bake them in the oven at 100 degrees Celsius for about 40 minutes, then put them in a desiccator (below). The desiccator makes sure that water doesn’t condense on the surface of the vials as they cool, which would change their initial masses.
Step 3: Weigh the empty vial
This way we can just weigh the samples in their vials and subtract the weight of the vial to get the soil mass. It was surprising how much the mass of the vials varied!
Step 4: Put samples in vials and cups for analysis
To determine gravimetric soil moisture and soil organic matter, we’re going to use samples in the vials. First we burn the samples at 100 degrees Celsius for several days, and the change in mass from the original “wet” sample reflects the gravimetric soil moisture, since it’s the mass of the soil that has been removed in baking. We then take the same samples and burn them to ash at 300 degrees Celsius, leaving behind only the mineral content of the soil. The change in mass from the baked sample to the ashed one reflects the soil organic matter, since all organic matter has been burned away.
You can really since the difference in texture between these two samples! One was from a sandy area and one from a more clay-ey one.
We also put a scoop-full of each sample in a Dixie cup. All of these samples will air-dry in the lab until the end of sampling in August, at which point we will mix them 1:1 with deionized water, then test them for pH and conductivity, which shows the ions present in the soil.
Step 4: Burn! (100 degrees)
Here’s what the samples look like after drying for a few days (on the second shelf from the top).
One thing’s for sure: I always have dirt under my nails these days!