Soil Texture Test. Complete Guide

(Jar Test / Sedimentation Test)

In this article, I’ll try to clearly explain what this test is and how to perform it properly.
It’s no secret that this test is very often assigned to field personnel, and they need a solid guide to avoid mistakes and obtain reliable measurements.

 

Why is this test needed?

After collecting soil samples during soil borings, we need to determine the composition of those samples and classify them.
The main criterion is the soil’s ability to absorb and transmit water.

This directly affects which engineering decisions will be made for sewer and drainage systems on a specific site (SWM, septic, infiltration).

 

Soils usually consist of three different fractions

• Sand
• Silt
• Clay

Depending on the percentage ratio of these three components, we obtain a specific soil type according to the USDA Soil Texture Classification.

 

Physical principle of the test

The test is based on the principle of particle settling in water:

• Large particles (sand) settle quickly
• Smaller particles (silt) settle more slowly
• The finest particles (clay) settle very slowly and may remain suspended for hours

The particles are separated by settling time.

 

Now let’s move directly to performing the test

Sample preparation

Before performing the test, the samples must be prepared.
The samples should be removed from the bags in which they were collected in the field and laid out to dry.

I recommend placing each sample on a shallow plate together with its sample label.
The label is extremely important. Each sample must be accompanied by its label throughout the entire process, with all data recorded on it. This helps prevent mixing up samples and preserves all information.

The size of each sample should be approximately two tablespoons.

Next, each sample must be inspected and cleaned.
Remove all organic material (wood fragments, sticks, grass, roots), as well as large stones greater than 2 mm (≈ 0.08 inches).

From each sample, form a thin patty roughly the diameter of the plate and leave it to dry.
Forming a patty is especially useful if you plan to crush the sample manually later, and it also helps the sample dry faster.

After the sample has fully dried (usually the next day), the crushing stage begins.
You can crush the sample by hand using a spoon until it reaches the consistency of fine sand, or you can use a coffee grinder, which is significantly faster.
Do not leave any clumps. The clay must be completely broken down.

 

Next, we will need the following reagents

Texture Dispersing Agent

This is the key reagent for performing the test. It allows the soil fractions to separate properly. You can purchase it online.

Why is it needed at all?

The dispersing agent:

• breaks down aggregates (clumps);
• prevents clay particles from sticking together;
• allows sand, silt, and clay to settle separately instead of as clumps.

Without it, clay content is almost always underestimated in the results.

In reality, it can be replaced 😊

 

Best substitutes

Calgon (no additives!)

Why it works:

• Essentially the same class of chemical as a laboratory dispersing agent
• The closest available substitute

How to use:

• 1 teaspoon per 1 liter of water (≈ 1 tsp per quart)
• Dissolve in water first, then add to the soil

⚠️ Important:
You need a pure powder, with no fragrances, dyes, fabric softeners, or other additives.

 

 

Dish soap (Dawn, Palmolive, etc.)

How to use:

• 2–3 drops per jar (0.5–1 L / ~1 quart)
• Do not shake until foamy

⚠️ Cons:

• Does not fully break clay aggregates
• Results are approximate, but acceptable for a field test

 

 

Washing soda (Sodium Carbonate, Na₂CO₃)

How to use:

• ½ teaspoon per 1 liter of water (≈ ½ tsp per quart)

⚠️ Cons:

• Worse than Calgon
• May slightly change pH (usually not critical for particle-size separation)

 

 

🚫 What is NOT recommended

• ❌ Salt (NaCl) — increases clay flocculation
• ❌ Vinegar / acids — distort settling behavior
• ❌ Sugar, alcohol — useless
• ❌ Hand soap — often contains thickeners

 

 

The second reagent is water

Tap water can be used, but boiled or distilled water is preferred.

From this point on, we will assume that you are using a Texture Dispersing Agent, since you are a true surveyor 😄

  

Test methods

I know three methods for performing this test.
All of them are similar and differ only in details and speed.

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Method 1 — ONE_JAR (single jar)

This is the most economical method in terms of the number of jars used.
For each sample, you only need one jar.

Procedure

Add approximately 15 units of crushed dry soil into a CLEAN jar.

Add the Texture Dispersing Agent (it usually comes with a dropper bottle).
If you add a bit more than necessary — it’s not a problem.

Add water up to approximately 45 units (my jars go up to 50 units).
Fill the jar to about ⅔ of its height, leaving some space at the top.

Close the jar with a lid and start shaking for 2 minutes.
You need to shake aggressively.

You are not gently swirling it.
You are not rocking it.
You are not “petting” it.

You are shaking it like all your anger in life is concentrated inside this jar.

 

After thorough shaking, place the jar vertically and immediately start a 30-second timer.

Exactly after 30 seconds, observe how much sediment has settled at the bottom of the jar.
This is sand.

If the suspension is very cloudy and the sediment is not clearly visible, you may use a temporary jar.
Carefully pour all the liquid into the temporary jar, read the sediment level in the original jar, and then pour the liquid back.

Exactly after 30 minutes, the second stage begins.
Measure the SILT sediment.

Then, after approximately 24 hours, measure the remaining settled CLAY.

The test is complete.

 

Now we calculate the percentage of each fraction

Let’s introduce the notation:

• “Sand” — Sand @ 30 s
• “Silt” — Sand + Silt @ 30 min
• “Clay” — Total final

Formulas

Let:

• Sₛ = Sand @ 30 s
• Sₛᵢ = Sand + Silt @ 30 min
• T = Total final

Then:

• Sand% = (Sₛ / T) × 100
• Silt% = ((Sₛᵢ − Sₛ) / T) × 100
• Clay% = ((T − Sₛᵢ) / T) × 100

 

Next, the data is written onto cards, and using the standard soil texture chart, you determine the soil type.

And that’s how it’s done for each sample 😊

 

 

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Method 2 — DECANT_3JAR (decanting into multiple jars)

This method allows you to read the remaining fractions more accurately, and in my opinion, it is the best option for beginners.

The idea is simple: after each settling interval, you decant the suspension into a new jar.
As a result, you end up with three jars, each containing a single fraction.

Procedure

Prepare the soil suspension the same way as in the ONE_JAR method.

After 30 seconds:

• Carefully decant the water containing silt and clay
• Sand remains in the first jar

Let the decanted liquid stand for 30 minutes.

Then:

• Decant the clay into a third jar
• Silt remains in the second jar
• Clay ends up in the third jar

I think the diagram explains everything clearly.

 

Formulas

Let:

• S = Sand
• I = Silt
• C = Clay
• T = S + I + C

Then:

• Sand % = (S / T) × 100
• Silt % = (I / T) × 100
• Clay % = (C / T) × 100

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Method 3 — TOTAL_REMAINDER (remainder = clay)

This is not really a separate method, but more of a trick to avoid waiting the full 24 hours for final results.

The idea is simple.
If you started with 15 units of dry soil, then measured 5 units of sand and 5 units of silt, it’s obvious that 5 units of clay remain.

In this approach, you directly measure sand and silt, and calculate clay as the remainder.

 

Procedure

• Add 15 units of dry soil
• Add dispersing agent and water
• Shake
• After 30 seconds, measure sand
• After 30 minutes, measure silt
• Clay is not measured directly

 

Formulas

Let:

• T₀ = Total initial amount (usually 15)
• S = Sand
• I = Silt

Clay (units):

Clay = T₀ − S − I

Percentages:

• Sand % = (S / T₀) × 100
• Silt % = (I / T₀) × 100
• Clay % = (Clay / T₀) × 100

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Control checks (very important)

The following must always be true:

• Sand + Silt ≤ Total
• Sand + Silt + Clay = Total
• Sand % + Silt % + Clay % = 100 %

If this is not the case — there is a measurement error.

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Common mistakes when performing the test

• Clay clumps (soil not fully crushed)
• Insufficient shaking
• Jar not vertical
• Incorrect timing
• Decanting while capturing lower fractions

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That’s it for the test itself

It is also common to determine the Hydrologic Soil Group (A–D) for each soil layer.
What it is and how it is determined will be covered in a separate article.

It’s worth saying that this is a fairly complex process, and without laboratory testing it relies more on experience than on a strict methodology.
I would not recommend beginners to apply it without proper background.

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Hydrologic Soil Group (A–D)

Determined based on texture and clay content:

Group	Characteristics
A	Sandy soils, high infiltration
B	Light loams
C	Heavy loams
D	Clays, poor infiltration

Hydrologic Soil Group may change with depth, so it is assigned at the sample level, not for the entire boring.

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Since I know that humans are, by nature, very lazy animals, and since I happened to have some free time, I wrote a program called SoilTest Logger.
With it, you can calculate all results using all three methods and generate a ready-to-use report with a soil texture chart and a boring log cross-section.

You can download the program from this page:
https://prs516.blogspot.com/2026/02/soil-test-logger.html

If you found this article useful, I would really appreciate it if you shared the link with your colleagues or posted it somewhere on a forum.

 

 

Thanks for your attention!

If you have any questions, feel free to leave them in the comments.