FIGURE 3-15 d.7.0 1 d..0.1 7 d.-5.0 d. 25 d.-3.0 0.05 0-20 Sizes of particles of indicated diameters() in millimeters 2 a-d: Using the size particles above and the photos below. Estimate the average diameter of each sample in millimeters. You will need to measure the diameter of a quarter in millimeters and then estimate as best as you can. Soil Texture and Soil Structure Soil texture refers to the mixture of the sizes of its individual particles and the proportion of the different sizes of soil separates. The figure below illustrates the particle size comparisons and the table that follows indicates the soil texture grades. Particles smaller than gravel are considered part of the soil, while gravel, pebbles and cobbles are not Silt: 0.05 -0.002 millimeter Clay: