Density and Porosity SOILS 206 Soil Ecosystem Lab Objectives: 1. Define soil bulk density, particle density, total pore space, and individual pore size. 2. Describe the relationship between soil texture, bulk density, particle density and porosity. 3. Perform calculations involving bulk density, particle density, percent solids and percent pore space. 4. Discuss the factors influencing soil bulk density, particle density and porosity. Introduction Soil is composed of solid material and pores. The solid fraction consists of mineral and organic material. The pores contain variable amounts of water and air. A soil in good condition for plant growth will contain about 50% pore space and 50% solids. While it is sometimes helpful to think of the soil components separately, it should be emphasized that, in a natural soil, the four main soil components are thoroughly mixed. The relationships and interactions between the soil solids and pores are very important and merit much study in soil science. The following exercises are intended to introduce you to the concepts of soil density and soil porosity. Soil Density and Porosity A good understanding of soil density and porosity is essential to soil scientists, engineers, and persons involved in crop and timber production. Soil pores contain air for plant root metabolism and provide a reservoir for water storage. Careful irrigation planning requires that one take into account the porosity as well as the initial moisture content of the soil. Bulk density values provide an extremely useful conversion factor for calculations involving soil mass and soil volume. Bulk density measurements are also used to calculate the total pore volume in a soil as well as the weight of soil to be moved during an excavation. High bulk density values may indicate the presence of compact layers in soil that could restrict root and water penetration. There are two important characteristics of porosity total pore space and the size distribution of the pores. The total pore space of a soil is that portion of the soil volume occupied by air and water. The soil texture, structure, and organic matter content largely determine the amount or total volume of soil pore space. A well-aggregated, fine-textured soil high in organic matter would have high pore space while a massive, coarse-textured or compact soil would have low pore space. Soil pores are generally grouped into two categories: 1. Micropores pores less than 0.06 mm in diameter that are important for water retention and, 2. Macropores pores greater than 0.06 mm in diameter that are important to aeration and saturated water flow. Fall 2005 1
Density Formulas The density of any object is its mass divided by volume it occupies. Soil density is the oven-dried weight per unit volume expressed in g/ cm 3 or Mg/ m 3. We are interested in measuring two different density values: 1. Bulk density (Db), and 2. Particle density (Dp). Bulk Density, Db Bulk density values represent the density of the oven dry soil as a whole; this includes solids and pore space. Db = Ws/ Vt Ws = Oven dry mass of the sample Vt = Total volume of the sample, pore volume + solid volume (cm 3 ). Particle Density, Dp Particle density values represent only the weight of dry soil per unit volume of the soil solids; the pore space is not included in the volume measurement. Dp = Ws/ Vs Ws = Oven dry mass of the sample Vs = Volume of the solids (ONLY) (cm 3 ). Porosity Formula Pore Space, PS The pore space of a soil is that portion of the soil volume occupied by air and water. PS = Vp/ Vt or PS = [1 - (Db/ Dp)] Vp = Volume of the pores * Vt = Total volume of the sample, pore volume + solid volume (cm 3 ). * Vp is difficult to measure, so it is more common to calculate PS from Db and Dp Soil Characteristics of Density and Porosity Fall 2005 2
Bulk density values for mineral soils commonly range from 1.0 to 1.8 Mg/m3. Highly compact subsurface soils usually have high bulk density and low total pore space. In extremely compact soils, where the pore space is near zero, the bulk density value may begin to approach the particle density value. Organic soils, such as Histisols, have very low bulk densities (often less than 1). Bulk density values are affected by soil texture and modified by structure. The large-sized particles in sandy soils tend to pack to fairly defined volumes, which contain few micropores but many macropores. The small aggregated soil particles in a fine-textured soil tend to be separated by many micropores. However, in soils with well-developed structure, large macropores may exist between the peds. In general fine-textured soils (clays and silt loams) exhibit lower bulk densities than coarse-textured soils (sands). However, it is important to note that soil bulk density can vary significantly between soils and within a soil profile and can be influenced by land management. Incorporation of large amounts of organic matter will lower the bulk density while processes that compact the soil will increase bulk density. See p. 36-37 in Gardiner and Miller for a discussion of factors affecting soil bulk density. Tillage operations may have both favorable and unfavorable effects on bulk density and porosity. The short-term effects of tillage are generally beneficial as large clods are broken up and organic matter is returned to the soil. However, intensive cultivation over long periods of time can be detrimental to soil structure. Repeated mixing of the soil hastens the oxidation and loss of organic matter. Numerous passes over a soil with heavy tillage equipment also may break down stable aggregates, increase compaction, and decrease porosity. Furthermore, excessive cultivation significantly increases the potential for soil erosion. Measurements Bulk density is most commonly determined by one of two methods: 1. The core method or 2. The clod method Similarly, particle density may be determined by one of two methods: 1. The displacement technique or 2. The saturation method. Once the bulk and particle density values are known, it is a straightforward calculation to determine pore space. Fall 2005 3
Example 1 A soil core with a height of 6 cm and a diameter of 5 cm weighs 215 g when collected and 188 g when oven-dried. When the core was ground and pored into 100 ml of water, the final volume of the soil-water mixture was 171 ml. Calculate the soil bulk density, particle density, and percent pore space. In this case, use the core method for bulk density and displacement method for particle density. Pore space can be calculated using the bulk and particle density values. Remember that 1 ml of water equals 1 cm 3 of water and that water has a density of 1 Mg/m 3. Bulk density (Db) Ws = oven-dried soil weight = 188 g Vt = πr 2 h = (3.14)(2.5cm) 2 (6cm) = 117.8 cm 3 Dp = Ws/Vt = 188g/ 117.8 cm 3 = 1.6 g/cm 3 Particle density (Dp) Vs = volume of water displaced by ground soil = 71 ml or 71 cm 3 Dp = Ws/Vs = 188 g/ 71 cm 3 = 2.65 g/cm 3 Pore space (PS) PS = 1- (Db/ Dp)*100% = 1- (1.6/ 2.65) * 100 = 40% Example 2 A soil clod has an oven-dried weight of 48 g, a saturated weight of 59 g, and weighs 29 g when suspended in water. Bulk density (Db) Vt = volume of water displaced by coated clod = 26 g = 26 cm 3 Db = Ws/ Vt = 48 g/ 29 cm 3 = 1.66 g/ cm 3 Particle density (Dp) Ws = oven-dried soil weight = 48 g Vp = Vw at saturation = 59 g 48 g = 11 g or 11 cm 3 Vs = Vt - Vp = 29 cm 3-11 cm 3 = 18 cm 3 Dp = Ws/ Vs = 48 g/ 18 cm 3 = 2.66 g/ cm 3 Pore space (PS) PS = Vp/ Vt =11 cm 3 / 29 cm 3 = 0.38 cm 3 / cm 3 or 38% or PS = 1 (Db/ Dp) * 100% = 1 (1.66/ 2.66) * 100% = 0.38 cm 3 / cm 3 or 38% Fall 2005 4
Example 3 Weight to Volume Conversion Bulk density is useful to convert soil weight to volume (and vice versa). What is the volume (in cubic meters) of 300 kg of soil with a bulk density = 1.27 g/cm 3? 300 kg 1000 g 1 cm 3 1 m 3 _ = 0.0236 m 3 1 1 kg 1.27 g (100 cm) 3 In today s lab, you will be using the core method to calculate the bulk density of your assigned soils at three different depths. Procedure: 1. Measure the height and diameter of the soil core to calculate the volume Volume = (area)(depth) area of a circle = πr 2 and the depth is the length of the core 2. Obtain soil moisture cans with oven-dried samples taken from cores. 3. Determine the sample weight and record it in the data table below 4. Calculate the bulk density of each core. Data sheet: Inside Diameter Radius (Radius) 2 Length Soil Volume Sample Depth (cm) Tin Weight Oven-dry Total Dry Soil Wt. Volume of Core (cm 3 ) Bulk Density (g/cm 3 ) Soil Sample Depth (cm) Tin Weight Oven-dry Total Dry Soil Wt. Volume of Core (cm 3 ) Bulk Density (g/cm 3 ) Fall 2005 5