Soil Texture and Particle Size Determination
Background Information


Read Plaster (2003), chapter 4, pages 48-54

Important terms for this lab:

  • Soil texture
  • Soil Textural Class
  • Particle size
  • Soil separates
  • Hydrometer
  • Bouyoucos cylinder
  • Stoke's Law
  • sodium hexametaphosphate
  • dispersion
  • soil aggregates

    Lab Study Questions

    Exam Questions from Labs

    Excel Exercise Instructions

    Soil texture refers to the percentage of sand, silt, and clay particles in a soil. Sand, silt, and clay particles are defined by their size. Sizes of soil separates according to the U.S. Department of Agriculture classification system are as follows:
    Very coarse sand2.0-1.0 mm
    Coarse sand1.0-0.5 mm
    Medium sand0.5-0.25 mm
    Fine sand0.25-0.10 mm
    Very fine sand0.10-0.05 mm
    Silt0.05-0.002 mm
    Clay<0.002 mm

    Soil texture has important effects on soil properties. Water-holding capacity, drainage class, consistence, and chemical properties are just a few examples of properties that are affected by soil texture. In general, coarse-textured soils (lots of sand-sized particles) hold relatively little water, drain rapidly, and are low in fertility. Fine-textured soils (lots of clay-sized particles) hold relatively large amounts of water, may be poorly-drained or well-drained, depending on their structure, and can be high or low in fertility, depending on the types of clay particles present.

    Soil texture is determined by hand in the field or by instrument in the lab. The field method (also called the feel method), is easy to learn and is very accurate once you've practiced it and become "calibrated". Taking a moist sample of soil in your hand, you try to mold it into a ball, then a ribbon. By observing its response to these tests, and by "feeling" for sand, silt, and clay effects, the textural class can be quickly determined (refer to lab manual/study guide page 15).

    Soil testing labs use quantitative methods for determining the relative amounts of the soil separates (sand, silt, and clay particles). The method we will use in this exercise, the hydrometer method (or Bouyoucos method ), is based on the fact that larger particles fall more rapidly through a column of water than smaller particles. A soil/water slurry is placed in a graduated cylinder. At different time intervals the density of water is measured to determine how much soil is remaining in suspension. Since sand particles will fall out of suspension first, followed later by silt particles, the relative amounts of sand, silt, and clay can be determined and the textural class calculated.

    Once the relative amount of sand, silt, and clay are known, the soil's textural class can be determined by using a soil textural triangle (see figure below). Each side of the triangle represents the relative content(%) of one of the three soil particle size classes (sand, silt, and clay).

Be sure that you are familiar with the procedure for the Bouyoucos Method of particle size analysis before you come to lab! Your instructor will review the basics only, you (your lab group) will be expected to be prepared to perform the procedure on your own.

Lab Study Questions

Study Questions

1. What is the purpose of the dispersion techniques used for the Bouyoucos method of particle size analysis?

2. How would the results of the particle-size analysis be affected if the sodium hexametaphosphate pre-treatment for dispersion were omitted?

3. Why is a temperature correction required in the hydrometer analysis?

4. What is the difference between soil texture and soil textural class?

5. What soil properties are used to determine textural class by the feel method?

6. What assumption is made by eliminating the oxidative pre-treatment step with the Bouyoucos (hydrometer) method?

7. List potential sources of error with the hydrometer method. The feel method.

8. What is a soil textural triangle? How is it used?

Study Questions Answers

1. The purpose of the dispersion techniques is to separate soil particles that are aggregated by chemical forces and organic matter. This must be done to ensure that what we measure are particles, not aggregates. Otherwise, fine, aggregated particles would fall more rapidly, thereby being counted as larger particles, resulting in an overestimate of the sand content vs clay content of the soil sample.

2. Without the sodium hexametaphosphate, soil particles would flocculate; aggregates would simulate larger particles and settle at a faster rate. The results would then suggest a higher percentage of coarse particles, and a lower percentage of the fine particles than what is truly representative of the soil sample.

3. Water viscosity varies with temperature. Soil particles fall faster in warmer water, slower in cooler water (increase in temperature, decrease in viscosity and vice versa). The hydrometers are calibrated for 20oC.

4. Soil texture is the relative amounts of sand, silt and clay particles in a soil.
Soil textural class is the name given to soils that fall within certain ranges of particle size distribution.

5. Things we can feel, (grittiness, smoothness, stickiness), how well the soil holds its shape, forms a ball, whether it stains our hands, or will form a ribbon.

6. By eliminating the oxidative pre-treatment step with the Bouyoucos (hydrometer) method we assume that there is little or no organic matter in the soil sample.

7. Sources of error with the hydrometer method include: weighing errors, wrong amount of water or amount and % solution of hexametaphosphate, poorly calibrated hydrometer, ...etc.
With the feel method: not enough experience to have "calibrated" our feel, soil too wet or dry, ribbon too thick or thin,...etc.

8. The soil textural triangle is a graphical means of determining textural class, given percentages of sand, silt, and clay particles. The left leg of the equilateral triangle represents % clay, the right leg % silt, and the bottom % sand. Therefore, the further to the left on the triangle, the greater the sand proportion and the sandier the textural class; the further to the top the greater the clay proportion and the clayier the textural class. Loamy textural classes occupy the middle regions on the triangle.

Exam Questions from Labs

Example Exam questions from Lab

1. If a 50g soil sample used in the hydrometer analysis results in a corrected 40 sec reading of 35 g l-1 and a corrected 7-12 hr reading of 25 g l-1, what is the percentage of sand in the sample?

a. cannot determine from the information given
b. 100 X (50-35) / 50
c. (50-35) / 50
d. 35 / 50
e. (100-50) / 50

2. When you are feeling a soil to determine its textural class, silt feels

a. sticky
b. smooth
c. friable
d. stiff
e. gritty

3. Which of the following is a soil textural class?

a. 50% sand, 22% silt, 28% clay
b. loamy sand
c. well drained
d. sticky
e. subangular blocky

Click here for the Answers to these questions.

This document last modified on February 2, 2005

For information about this page contact Kate Butler at: katebutler@psu.edu

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