understanding soils - wk 7

Today's follow up to the understanding soils unit is hugely interesting to me. Up to this point in my life, the truth of what's underneath the ground I'm standing on and indeed the exact type of soil in my own garden has been a complete mystery to me! But as Marcus points out this is the most important aspect to understand in Horticulture. It makes sense that you have to completely understand the soil you are working with to correctly design a new garden, recommend plants or solving deficiencies in existing plants. Everything comes back to the soil.

Today's focus is to touch base again with the different types of rocks (Igneous and Sedimentary) and their corresponding soils and then take a look at the range of tests one can do to ascertain the characteristics of a soil.

Marcus with our different soil types and tests (Jarrod looks really impressed here!)

The first test is a Texture test (or ribbon test). This is a test to establish the 'Texture Class' of a soil and the proportions of sand, silt and clay. This ranges from pure sand down to heavy clay and all the combinations in between.

I found this Triangle description in an old CSIRO booklet in the Govt. House crib room.

Adding a little water to a handful of the soil you want to test - firstly notice if the water soaks in readily or just sits on the top. This can tell you right here if the soil is hydrophobic or not. Then work the water in until you can firmly compress the soil into a roll.

Working the test soil into a a roll for the ribbon test.

Observe it's feel (this is a feel test after all). The grittiness (sand), silkiness (clay) or sponginess (silt) will all correspond to a place on the texture scale. Finally to the ribbon part. Press the roll flat into a ribbon shape squeezing it out the front of your hand. How long the soil stays together before it breaks off and falls will be able to be measured and also correspond to a place on the Texture Class list.

The roughly 25mm peds from the classroom test soil.

With this new found knowledge - I decide to apply it in action on my Rotation Day at Government House the following Thursday. The task of the day was to measure and design an irrigation plan for a bed near the Japanese Pond. I suggest that before we decide on it's watering requirements we should map and document the existing plants (mostly to be able to position the watering system on the design but also to determine the plants water needs) and perform a texture test to find out what soil we are working with. After a short pause and a raised eyebrow - Mick (who is looking after me that day) humors me and and with a grin becomes entirely enthusiastic about the idea. 

Performing a texture test on the Japanese Pond bed soil at Government House.

From first look the soil seems to quite a bit of organic matter (probably due to the bed's mature trees regularly supplying a thick carpet of leaf litter) but is not very well aggregated and appears to have a very fine texture. It is very dry and when we add the water it beads on top and just rolls off the surface without soaking in. Mick is really surprised and a bit shocked to see that this means the soil has become hydrophobic. I ask if we should add some organic matter to it and Mick agrees a few trips to the compost heap will be in order now before we lay any irrigation system down. So already the soil test has become useful!

Next I work the water into the soil to form a roll. I can feel some very fine sand particles and it also feels quite silky and workable. I'm surprised how long the ribbons get before they break, as I was certain it would be more crumbly from looking at the dry soil. I estimate the majority of ribbons measure around 25 mm with some longer pieces. I add a bit more water and it gets silkier and the ribbons longer. I feel the organic matter is responsible for the smaller breaks.

The average ribbon length is approx 25 - 30mm  (sink hole is a reference)

We decide to drop a ped into a glass of rain water and see what happens. Initially the glass clouds orange a little and the ped softens at the edges but doesn't break apart. We decide to come back at the end of the day to check it out.

Just after dropping the soil ped into water.

So - not being entirely sure of the myself yet of course (but if I had to guess though) I would classify the soil as either a Silty Loam (Ribbons to 25mm. Coherent but will crumble. Smooth and silky feel.) or a Sandy Clay Loam (Ribbon 25-38mm long. A strongly coherent cast. Sand grains can be felt.) I'm leaning toward the Sandy Clay Loam but not sure as the bed sits at the bottom of a sandstone quarry basin.

A few hours later.... mostly broken down and still a touch cloudy.

After a couple of hours in the water the ped has dispersed more and spread out on the bottom of the glass and the cloudiness is still there although I can't see evidence of a halo around the soil yet) I'm even more confident of the Sandy Clay Loam diagnosis now as I'm pretty sure (if I was paying attention in class!) that the dispersing of the soil and cloudiness means more of a clay content.  However I've forced Mick to raise his other eyebrow now, so will have to check with Marcus.....stay tuned :)