21 December, 2002

Processing Soil Samples

Remember those bags of soil we’ve been collecting? Well, now that we are back at McMurdo Station, we begin our work with the samples. We need to weigh our smaller amounts of soil for the various tests. We need to do this for each of the bags of soil we have. First, we stack the samples in a refrigerator to keep the soils as cold as they are in the field naturally. We stack them by grids to help organize our work.

The weighing goes as follows:

a. 100 grams for yeast and bacteria and for testing pH

b. 50 grams to bake at 105 °Celsius for 24 hours

c. 8 grams for the Potassium chloride test

d. 4 grams for testing total carbon and nitrogen

e. 150 grams to look for invertebrates like nematodes

f. 100 grams for DNA extraction from the soils using a kit g. 100 grams to archive for future studies.

After a soil sample is weighed out, Laurie prepares the samples for the oven. Rusty prepares the samples for the yeast and bacteria testing as follows: a. Pour 20 mls of soil into a 50 ml Falcon tube with calibrations on the sides b. Add sterile distilled water to 45 ml mark on tube

c. Hand mix by shaking – 30 seconds

d. Let settle – 30 seconds

e. Hand mix again for 30 seconds

f. Let settle for 30 seconds

g. Put on ice until ready to filter sample

h. Place a black gridded filter on each of the manifolds (top) of the filter system

i. Place a pre-filter on top of the black filters (this is to catch tiny soil parcticles)

j. Attach the glass piece of the system with a clamp

k. Mix the samples to be immediately filtered by shaking 30 seconds l. Let sample settle for 30 seconds

m. Pipet 10 mls of the liquid suspension into the filter system do two for each sample – one for the yeast plate and one for the bacteria plate n. Turn on vacuum pump and putt the water through the filters o. Open system, set aside the pre-filter and place the black filter, grid side up, on the plate with the growth media (we marked the yeast plates with red and the bacteria plates with black. We also labeled each plate with the sample while it was being filtered.)

p. Place growth plates with filters hopefully containing some organisms, in the incubator set for 15° C.

We suspect that the organisms are slow in growing and will take a week or longer before colonies will appear. What do you think will happen?

Pre-filters from the first set of samples Here is what the pre-filters looked like. We discussed whether we should save them. What do you think? Is there a reason to save the pre-filters? Notice the slightly different colors? I also noticed how the thickness of the material caught on the pre-filter varied as well.

Measuring soil by volume Rusty measures out the soil sample for filtering. What kind of tube is he using?

Measuring conductivity Rusty is measuring conductivity of the suspension of water and soil parcticles. These are the same tubes we used to pipet out 10 mls to filter. We make use of as much of the material as we can. Some of the other tests are done in the chemistry lab. The chem lab provides services for the PIs and we will take advantage of their help.