soil excursion - results - wk 18

Soil testing day!

Today we are testing our soil samples collected on last weeks excursion. We lay them out in a row on the table, each with an example rock and three levels of the profile tests (apart from Alluvial which is only one sample) and the differences are great to compare. Laurie runs through each profile type briefly and I decide to put my hand up to test the sandstone profile, as this one sounds most like my soil type at home.

Mudstone - Sedimentary

Dolerite - Intruded Igneous

Basalt - Extruded Igneous

Alluvial - Mix of all of the above!

First up Josie and I start with a ribbon test for each of the three sandstone samples. The soil section I begin with is the deepest profile. When added to water it feels very sticky and clay like and the ribbons I make are extremely long before they break. I can almost mould the whole piece without a break so we decide on the classification 'Medium Clay'. Josie does the top profile that is a dark black/brown colour with quite a bit of organic matter in it. This Josie feels should be classified as a 'Sandy Loam' with the gritty large sand particles clearly felt. I do the middle section of the profile which is much paler and very sandy and hardly stays together at all. The soil is also quite water repellent and takes alot to work it in. The ribbons are barely 1-2cm long so decide on the classification of 'Loamy Sand' after referring to our list. Laurie explains that sandstone soil typically has these 3 distinct layers to it. The top is a very sandy 'hungry' soil that will darken in colour when organic matter is added to it but as it is so free draining the nutrients will quickly be depleted and need repeat applications. The middle layer which is very sandy and poor, acts as a sort of filter or sieve where the nutrients quickly pass through and are washed down and retained in the richer clay base.

The difference between a sandstone (L) and dolerite (R) soil type.

I know which one I'd rather have!

This is exactly like my soil at home.- hungry on top and constantly requiring the addition of organic matter that never seems to last very long. When we dig a hole there is an almost chalky white compressed layer of pure sand just beneath the top layer, that looks very poor and whitish, before you strike orange sandy clay about a metre down.

Phillipa and Josie examine our pH colour metric results.

Next job is to test the pH of the samples. We start with a colour i-metric test and then follow up with a meter test, which is 20gm of soil mixed into 200mls of distilled or rain water. The colour i-metrics test show up at 5.5 (top), 5.5 (middle) and 4.5pH (bottom). The meter test for the 4.5pH sample comes out at 6.0pH so quite a difference between the two!

The final number for the sandstone meter test is 6.0 pH

The final test to do is a salinity test. Laurie has a meter probe that works in the same water mixture as the pH meter so that is great. Measuring parts per million (ppm) the sandstone profile comes out at 39ppm. All of our samples fit roughly within normal parameters (33 - 60 ppm) so there are no salinity problems as we might expect here in Tas. All in all it's been a great day of learning and I think it is all starting to settle down and bed  into my grey matter nicely :)


The results of our hard day of testing.