Field Texture

The ribbon test

The most fundamental property of soil is its texture. Soil texture is a description of the relative proportions of the primary particles (sand, silt, and clay) which make up the soil. There is a whole module on this topic in the Soil Structure section which you should work through. Here the aim is to show how you can assess texture in the field by a technique called the ribbon test.

How do you do the ribbon test?

Take a handful of soil. Crush any large, hard aggregates before starting. Then moisten the soil with water and knead to make a ball (or bolus). The aim is to break up all aggregates into primary particles (i.e., sand, silt, and clay). This often takes 1-2 minutes.

Adjust moisture and extrude

Before making the ribbon, it is essential that the soil be at the right moisture content, which is the point at which the bolus just fails to stick to the fingers (the sticky point). Adjust the moisture content by adding soil or water.

Now:

1. Extrude a ribbon of uniform thickness between the thumb and forefinger, and let it hang. Keep extruding until the ribbon breaks under its own weight and then record its length. Repeat a few times and take an average length. Because the coherence of the soil is related to its clay content, the ribbon length gives an approximate indication of the percentage of clay.

Observe bolus properties

Now note how the bolus behaves as it is manipulated

• Coherence indicates some clay.
• Stickiness indicates a high percentage clay, as does resistance to shearing between the thumb and forefinger.
• Grittiness implies coarse sand.
• Fine sand may not be felt but can be heard as crunching when held close to the ear.
• A high silt content is suggested by a bolus that feels smooth and silky, rather than sticky.

Look up the texture grade

Now find the texture grade that best matches your soil sample in terms of ribbon length and behavior of the moist bolus.