Lecture 5

Chapter 3

Soil Physical Properties--Solid State

The physical properties of soil are often the most important properties for evaluating the suitability of a soil for a particular purpose. Physical properties are properties one can describe with physical measurements such as length, mass, and temperature. Can the soil support trees, aquatic plants, desert shrubs? Or is the soil too cold, or too anaerobic, or too prone to drought. Can the soil withstand vehicle traffic, or will it fail under stress? A complete answer to these questions requires a thorough knowledge of the physical properties of the soil.

1. Physical properties are characteristics described by physical measurements. These often are the most important properties for determining the limitations and practical uses for a unit of land. These properties are texture, structure, bulk density and porosity. Aeration, color and temperature are also physical properties, and are discussed in the next lecture. Soil mechanics is an engineering field based on soil physics, and is discussed in Lecture 7.

2. Texture refers to the relative proportions of sand, silt and clay. See Table 3-1 in the textbook to learn about what the USDA calls "soil separates". Basically, the part to remember is that sand is from 2.0 mm diameter to 0.05 mm. Silt is from 0.05 mm to 0.002 mm. Clay is mineral material smaller than 0.002 mm diameter. Organizations other than USDA use slightly different criteria. Sand is primarily quartz mineral, but can include feldspars and other small rock and mineral particles. Silts are also mostly quartz, and are often more angular than round. Clay-sized minerals are usually secondary clay minerals. Clay minerals do not have to be clay-sized according to the USDA soil separates criterion. Do not count organics or minerals > 2 mm diameter in determining soil particle sizes. The texture triangle (see Figure 3-1 in the textbook) is used to determine a texture class, based on soil separates. A texture class is the name of a region within the texture triangle, such as "silt loam". One can determine texture of a soil crudely by the feel of the soil, or more exactly by various lab techniques, such as sieving, or by measuring the rate at which suspendable particles fall through water. This is accomplished using Stokes Law, which says that the rate of fall through a fluid is a function of particle diameter.

3. Surface area is closely related to texture. Small particles have more surface per weight or per volume than do large particles. Break a pencil in two. Go ahead, do it. Now observe that the two broken pieces weigh the same and occupy the same volume as did the original whole pencil. However, the surface area has increased because the break is exposed. One could now break the pencil many more times, each time creating more surface area, but no more mass, weight, or volume. This illustrates the general rule that the smaller the particles, the more surface area per unit mass or per unit volume. In addition to this surface area affect, smaller particles arrange themselves in such a way that soil pores are smaller. Both surface and pore size phenomena are important because water adheres to mineral surfaces and has an affinity for small pores. Therefore clay (lots of surface, small pores) and sand (little surface area, large pores) differ markedly in water-holding properties. In common language, we call a clay soil a "heavy" soil because it is normally wetter than other soils. Soil texture affects plants in various ways, including water delivery, aeration, physical support, and fertility.

4. Structure is the arrangement of soil particles into aggregates. Peds are natural structural units; clods are artificial. Peds form by organic cementing agents, cation bridges, and in response to pressure from traffic or shrinking and swelling. Peds are described by their type (shape), class (size), and grade (strength or distinctness). The structure types are: platy (thin, horizontal), prismatic (long vertical faces), columnar (like prismatic but rounded tops), blocky (polyhedrals, angular or sub-angular), granular (small spheres or polyhedral units, with faces that are not casts of adjoining peds), and structureless (massive or single grain). Structure affects root penetration, infiltration, and aeration. The loss of organic matter or the presence of too much sodium cause structural deterioration and fewer pores.

5. Bulk density is the weight of a volume of bulk soil. Water is the standard by which other densities are compared. For water:
1 g/cc = 1 Mg/m3 = 1 kg/L

Soil is about 50% pore space by volume, so this 50% of volume does not contribute to the weight of a dry soil, i.e., it dilutes the weight of solids over a larger bulk volume. Typical soil minerals have a particle density of 2.65 g/cc. Therefore a typical soil density is about half of particle density, or 1.3 g/cc but ranges from 0.9 to 1.8 g/cc. To determine bulk density, one determines the weight and volume of dry soil, as follows:

Can full of dry soil: diameter = 6 cm
height = 2.5 cm
volume = p (6/2)2 (2.5) = 70.7 cm3
weight = 99g
bulk density = 99 g/70.7 cm3 = 1.4 g/cm3

One might wish to know, how much does a field of soil weigh? From bulk density, one can answer this question:

1 hectare = 10,000 m2
15 cm = 0.15 m
volume = 1500 m3
if bulk density = 1.4 g/cm3 = 1.4 Mg/m3
(1500 m3) (1.4 Mg/m3) = 2100 Mg = 2,100,000 kg
» 2 million kg/ha-15 cm
» 2 million lb/acre-6"
» 1000 tons/acre-6"

6. Porosity is a measure of pore space. Pore space influences how much water and air a soil can hold. On a volume basis:

% pore space + % solid space = 100%
% pore space = 100% - % solid space
but, % solid space = (bulk density/particle density) x 100%
or, (particle w/bulk volume)/(particle w/particle volume)
% pore space = porosity = 100% - (bulk density/particle density) X 100%

Pore size is also important as mentioned above, mainly because smaller pores have greater affinity for water than do larger pores.

Students are encouraged to look up the following vocabulary words in the textbook glossary and to browse the following web sites.

Vocabulary

 bulk density particle density ped porosity texture clay loam sand silt soil separate

Web sites

From a Michigan Extension publication, a layman's practical description of soil textures.
URL: www.msue.msu.edu/msue/imp/modop/00001810.html

A terrific collection of soil photos, including photos of soil structure.