Jar test: Silt vs Clay

Hi
how do I tell, in a jar test whether the soil sample has silt but not clay, or it has clay but not silt (it may have sand or not)?

The determination is by particle size.
Larger particles are going to settle out first and as they get smaller, they settle out nearer the top.
Silt would be found just underneath clay, sand would be found underneath silt.
Usually it takes at least 24 hours to perform a decent jar test, I usually give them three days of settling time before I check for the layers, that gives the clay particles time to settle out.

Redhawk

Hi Bryant
thanks for your reply. I think I didn't explained myself clearly enough.
My doubt is what if there is no clay, for example? how do I know that the second layer after the sand is silt or clay? Same thing in case there is no silt, how do I recognize the clay?
I say this because in the jar the particles of silt and clay may look very similar (to me at least) since I see an homogeneous layer right after the sand.
So if one of the  two is missing how can I tell which one is?

The easy way is to do the "tooth" test, you use a Q tip and gather a small sample from the jar then rub that on one of your front teeth, you will notice the gritty feel or the lack of a gritty feel.
The sure fire method is to put a smear on a slide and check the particle size under a microscope.

Clay makes a smooth looking layer, silt makes a layer that looks slightly gritty, sand looks like a beach or a piece of sand paper.

Redhawk

Bryant,      Doesn't silt (which is very fine sand) separate out rather quickly 5-10 min. whereas clay is much finer and separates out hours or even days later? If so, then wouldn't it be safe to say that if a soil sample stratifies out (when the water becomes clear) in less than 10 minutes its a good indicator that you have no clay?

yes that could be thought of being a safe assumption, however there is a caveat to this assumption.

Precisely defined, silt is sand that has been ground (weathered) to a fineness beyond sand but not yet fine enough to be clay.
Silt usually takes about an hour to settle into a defined layer if the sample being tested has all the components of "classic soil", this is because the silt particles are just fine enough to be temporarily suspended by the clay particles.
(The minimum size for sand is 0.05mm, from there, smaller particles are silt (0.049 down to 69um and smaller than 69um is clay.)

A very simple test is taste it.. if it feels gritty on the tongue it's silt if it feels smooth it's clay.

Oh didn't see that had already been said.. sorry! There was a joke at uni on how to tell a real scientist from a geologist.. only geologists lick their specimens.

Thanks all for your feedback. I tasted some tiny bit of the sample and indeed it feels somewhat gritty. There is some clay still floating around but very little since the water is quite clear and so it was pretty much straight away after shaking.

Antonio,  Good work, but don't let your observations stop there. Go around to different places and collect some soil samples and see how they differ. So much can be learned by doing such a simple test. Then if your real ambitious, go to List of Dr. RedHawk's Epic Soil Series Threads and begin to go down the rabbit hole even further.

Antonio Scotti wrote:Thanks all for your feedback. I tasted some tiny bit of the sample and indeed it feels somewhat gritty. There is some clay still floating around but very little since the water is quite clear and so it was pretty much straight away after shaking.

It sounds like your soil type would be classified as Sandy, Silty loam. This is a good soil for growing in when it has good quantities of organic material as one of the components.
It is also one of the harder soil types to get organic materials to stay in since they tend to be used up fairly quickly.

Redhawk

Thanks Brent.
Why organic materials tend to be used up fairly quickly in this type of soil?

hau Antonio,

Sandy, silty soils have such good drainage that all the leachate we want to stick around, can't unless there are things to hold onto it (organic materials).
One of the best ways to get this type of soil to hold onto both organic matter and water is to add small amounts of clay, this however isn't always practical but you can substitute char for clay in such soils.
Char is a rather remarkable material it gives microorganisms places to live, it holds water and it prevents other organic matter from being used up as fast is can be used up in sandy type soils which have no char in them.

To make char is fairly simple, it is charcoal that has been heated so that only the carbon base remains, it isn't really necessary to use high heat in an oxygen deprived environment to make decent char either.

I have used simple burn barrels that have just a few small holes in the bottom and a lid with the small bung stopper removed, this limits air intake enough when the barrel is packed with dried out wood.
I start mine in the center at the bottom with some waxed cardboard lit and dropped in, then I put in the last 3 or 4 sticks of wood and drop the top on once I see some of the wood burning.
It takes a while to burn through but it gives a pretty good batch of char once it has burned out.(flames might come out the top hole you have for exhaust gasses, or they may not)

From that point you just break up the charred wood into small pieces and spread over the soil. I use a broad fork to open the soil so the char can trickle down into it with out turning the soil over.
The microorganisms in your soil will be attracted to this char and over a year the char will become "biochar" and your soil will behave as if it is a sponge for water and organic matter placed on the surface will slowly sink into the soil and remain there.
This is how we can duplicate Terra Preta which took the Amazon dwellers quite a while to install in their farm lands.
We are doing it on a smaller scale and we aren't burning trash and relying on the thickness of the burn pile to limit oxygen, that is our edge today, we know what parts of their burns do the best at what we want for our soil.

Redhawk

By checking the actual percentages after the jar test, I see that there is some 25% sand, 2,5% clay and 72,5% silt (more or less).
Since the objective is building a cob oven this soil, as is, is not good: too little clay.
Still, if I add up enough clay to it could this become usable? After all, as was mentioned in an earlier post, silt is just very fine sand, so I could consider that what I actually have is 97,5 % sand and 2,5 clay soil.
if I'd add the correct proportion of clay, I could be done!..or not? Or is it better not to use a soil that has so much silt instead.
Any thoughts on this idea?

I think that adding enough clay would work fine with that soil profile.