Soil Jar Test Calculator

What you need

• A tall, straight-sided jar with a tight lid (a quart mason jar works)
• Dry soil from the spot you want to test
• Water
• Powdered, non-foaming dishwasher detergent
• A ruler and a marker

Straight sides matter: when the jar’s width doesn’t change, each layer’s height is proportional to its volume.

Step by step

1 minute → sand · 2 hours → silt · 1–3 days → clay (sometimes weeks)^[1].

Why the layers separate

Particles settle out of the water by size^[1]. Sand grains, up to 2 mm across, reach the bottom first; clay particles, under 0.002 mm, settle last^[2]. Soil labs measure the same thing more precisely with a hydrometer, from the different settling rates that Stokes’ law describes^[3]. Our guide to sand, silt and clay explains that lab method.

Bits floating on the water are organic matter, which is good for your soil^[4]. Leave them out of the measurement: the test reads only the settled sand, silt and clay, which is why the steps start by picking out roots and debris^[1].

Worked example

Colorado State University Extension’s worksheet example: sand 3.0 in, silt 0.5 in, clay 1.5 in^[5]. The total deposit is 5.0 in, so sand is 3.0 ÷ 5.0 = 60%, silt is 10% and clay is 30%. On the triangle that’s sandy clay loam^[2]. Measured as jar marks instead (3.0, 3.5 and 5.0 in from the bottom), you get the same result.

Got your percentages? Learn how to read the triangle by hand, or print a worksheet for the classroom.

Example readings and their classes

Each row is classified with the Soil Survey Manual’s class definitions^[2].

Getting a reliable result

• Dry the soil and crush the clumps before it goes in the jar.
• Shake the full 10 to 15 minutes; the shaking breaks the soil into single particles^[1].
• Don’t move the jar while it settles.
• Wait until the water is clear before marking the clay. That’s usually 1 to 3 days, and some soils take weeks^[1].

Jar test, ribbon test or lab?

The jar test gives numbers, but Colorado State presents it as a way to estimate texture^[1]. The ribbon test is quicker and gives a class without numbers.

For exact figures, a soil lab runs a particle-size test that reports the sand, silt and clay and the USDA texture class; Penn State’s Agricultural Analytical Services Laboratory is one that offers it^[6].

Ways people use the jar test

• Before planting a new bed: know whether you’re starting from sand, loam or clay.
• Checking a load of topsoil before you spread it.
• A school science project: the layers are visible within hours, and the calculator turns them into a real USDA class.
• Comparing your garden with USDA’s mapped soil for your address.

How we calculate this

In layer mode, each percentage is that layer’s thickness divided by the sum of all three. In jar-mark mode, sand is the first mark, silt is the second minus the first, clay is the third minus the second, and the total is the third mark. Percentages are rounded to one decimal and kept summing to 100, then classified with the Soil Survey Manual’s class definitions^[2], the same rules as the texture triangle.

Frequently asked questions