A ternary plot is a useful chart for vizualing three variables which (almost always) sum to a constant. This constant is usually 1.0 or 100%.

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First, let's add some labels to our axes! Suppose we have a soil sample that consists of three soil types: sand, silt, and clay.

These three together form 100% of our soil sample.

There are three axes, one for each component. In each "corner" of the triangle we're at 100% of one of the soil types. On the axis opposite to the corner, the soil type is at 0% — ie there's none of it in the sample.

On this line, soil type silt is 0%.
Then our soil sample consists *only* of
clay and
sand.
Likewise, on this line
clay is 0%.
And on this line sand is 0%

Now suppose a soil sample consists of
20% sand and
80% silt, and consequently;
0% clay., because it has to sum to
100%.
The point must be located on the axis opposite to clay,
because there is *no clay in this sample*.

This is the 80% silt line and this is the 20% sand line. The point is at the intersection of these lines.

Let's do one more, this time with all three soil types! This sample consists of 30% clay, 40% silt and 30% sand.

This is the 30% clay line, this is the 40% silt line, and this here is the 30% sand line.

Once again the point follows from the intersection of the lines. Because the three proportions cannot vary independently, really only two lines are necessary to find the point.

I hope you understand how a ternary plot works now! Now go ahead and make a bunch of ternary plots and let your colleagues/friends know about this site.

Other helpful pages about ternary diagrams:

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