8 Soil Density
The flow of liquids and through soil is essential for the existence of plants and other soil organisms. Those flows are dependent on soil porosity and pore connectivity. Soil porosity is also dependent on, and inversely related to, soil density. If a soil is compacted, the soil solids are packed into a smaller volume, and the particles get packed closer together. This results in less total pore volume. Thus as soil density increases, soil porosity decreases. Soil density is relatively simple and cheap to measure. This laboratory entails a demonstration of the use of a soil density sampler, and a problem set that is focused on common soil density calculations.
Learning Objectives
- Measure soil bulk density and calculate pore volume relationships.
Materials
- Soil bulk density sampler
- Soil Density Problem Set
Recommended Reading & Viewing
- Soil Bulk Density Overview (CropWatch – Youth, 2013a)
- Soil Bulk Density Test (CropWatch – Youth, 2013b)
- Bulk Density (USDA NRCS, 2008)
- Soil Bulk Density/Moisture/Aeration (USDA NRCS, 2019)
Prelab Assignment
Using the recommended reading and viewing resources and the introduction to this lab, answer the questions listed below. These definitions/questions will provide a concise summary of the major concepts addressed in the lab. They are also useful as study notes for exams.
- Define and explain the differences among particle density, bulk density, and specific gravity.
- Describe the relationship between 1) bulk density and porosity and 2) particle density and porosity. Write a mathematical expression for these relationships.
- What is the typical range of bulk density values for mineral soils? What is the range of bulk density values for an organic soil?
- What are the sizes of macropores and micropores?
- Briefly describe the processes of soil structure (aggregate) formation.
- Describe several examples of soil management practices that increase or decrease soil bulk density.
Introduction
Soil physical properties control the mechanical behavior of soils and will strongly influence land use and management. Several important physical properties have been discussed in other labs: texture, structure, color, and consistency. In this laboratory, additional physical properties will be measured, and their implications for land management will be discussed.
Bulk Density, Particle Density, and Pore Space
Soil represents a unique arrangement of solids and voids. The voids, or pore space, are important for air and water movement and storage. The total pore space consists of the voids between sand, silt, and clay particles and voids between soil aggregates. Therefore, texture and structure govern the amount of soil pore space. Organic matter affects the solids portion of the soil but also influences porosity indirectly through its effect on structure.
Density refers to a mass per unit volume. Bulk density of a soil refers to the mass of a volume of dry soil. The volume includes both solids and pores.