Soil Texture

For the entire detailed protocol, see the full downloadable manual

Texture refers to the relative proportion of sand, silt, and clay-sized particles in a soil. Texture is sometimes described by differentiating heavy (high-clay) and light (sandy or loamy) soils. Soil texture is a major determinant of the workability, water retention, and drainage of soils, along with the level of organic matter (see the POXC and particulate organic matter tests) and level of aggregation (see the aggregate stability test). Clay and loam retain more water for crops than sandy soils; sandy soils drain easily, while clay soils can have issues with draining quickly especially if they become compacted or lack organic matter. In addition, the level of clay in soil can markedly affect its fertility characteristics since crop nutrients such as potassium, calcium, and magnesium are held on the particles of many types of clay. Higher clay content soils can also generally store more organic matter over time if they are receiving organic matter inputs like manure or crop residues.

Because sand, silt, and clay are three components in soil that sum to 100%, the texture of soils can be categorized on a textural triangle seen below.

Above, the soil textural triangle. Different texture types like “loam” or “sandy loam” are described by their proportions of sand, silt, and clay. For example, a clay loam has proportions of about 35% silt, 30% sand, and 35% clay (the point in the diagram just above the ‘O’ of clay loam) with a range of values that define the light green “clay loam” zone. To avoid confusion in using the diagram, pay attention to the direction of the percentage labels around the outside, which align with the lines you should use to assess a point on the diagram, and also their relation to the 100% types of clay, silt, and sand.

Materials you will need:

  • Water (any type will do, stream water is acceptable)
  • For the FAO test, some people find it helpful to roll out the “sausage”, “pencil”, and other shapes on a flat surface like a board, table, or sheet of plastic on the ground.
  • A 2 mm sieve may be helpful to prepare a sieved sample before testing, but pebbles and coarse fragements can also be simply removed by hand.

Method step by step:

Option I. USDA feel method:

The USDA (United States Department of Agriculture) method is a relatively precise method once you practice it a few times. The final step in assessing a wet pinch of soil will come more easily once you have assessed a few different soils and can note the difference between gritty and smooth feel of a soil. However imprecision at this step does not usually create much of a difference in the final classification, since for example a clay loam type soil is immediately adjacent to a loam soil in the soil texture diagram (see above). We have noticed that first-time users often find the FAO method (Option II below) easy to use as well. The two methods can be used to check each other when performed on a soil, since both are relatively rapid and should give similar answers.

Refer to the chart below for the procedure, also available for download here, and also the manual (see the link at the top of the page)

Option II. United Nations Food and Agriculture Organization (FAO) feel method:

Refer to the images and method steps below:

  1. In the same way as the USDA feel method, start by forming a ball of diameter ~ 3cm, like a soil putty with water, without stones that can interfere with the test (using 2mm sieved soil is optimal). The putty-like ball must have just the amount of water to be moldable without sticking too much to the hand, and it is worth kneading with patience until mixing all the dry soil with water. We’ve noticed that often beginners end up adding too much water so that the putty smears rather than rolling into a sausage in the next step. If so, add a little bit more soil. Regardless, if this ball cannot be formed, it is classified as sand (see figure above)
  2. If a ball can be formed, next you should try rolling the ball into a sausage, about 6-7 cm long. If the “sausage” falls apart as it is rolled, it is classified as a loamy sand
  3. If a 6-7 mm sausage can be formed, try to roll the sausage further into a “pencil” about 15-16 cm long. if the pencil cannot be formed but falls apart, the soil is a sandy loam.
  4. If the pencil can be formed, try to bend it into a half circle. If the half-circle cannot be formed or falls apart, the soil is a simple loam.
  5. If the half circle can be formed without breaking, try to continue bending the “pencil” into a complete circular ring with an approximate diameter of 5 cm.
  6. If this ring cannot be formed without breaking, the soil can be classified as a silt loam or a silt soil.
  7. If the ring can be formed but some cracks appear as it is bent, the soil may be a number of types that tend to be clayey without having enough clay to be formally called clays, such as a clay loam, a silty clay or sandy clay. These are all the types that border the “clay” type in the textural triangle (figure at top of this page) as well as the sandy clay loam type. By following the same strategy of feeling a wet pinch of soil in step 5 of the USDA feel method (Fig. 6), these types may also be distinguished.
  8. Finally if the ring can be formed with very few cracks, and tends to look more like potter’s clay rather than a soil, it is likely the clay type of soil.