Fundamentals of Soil Science (1+1)

Introduction

Definition

• Density of any substance is its weight per unit volume. It is expressed in gram per cubic centimeter (g cm-3) or mega gram per cubic meter (Mg m-3).
• Two density measurements- particle density (absolute specific gravity) and bulk density (apparent density) are common for soils.

Particle Density or absolute specific gravity (p_s):

• Particle density of soil is a measure of the mass per unit volume of the soil solids only. It is expressed in terms of Mg m-3.
  
• Texture and structure do not affect particle density. However, organic matter, which is a soil solid, readily influences particle density. Organic matter weighs much less per unit volume than soil minerals. Soils high in organic matter have lower particle densities than soils similar in texture that are low in organic matter. Particle density varies with the type of soil minerals present as well as the amount of organic matter.
• The particle density of most mineral soils is in the range of 2.60 to 2.75 Mg m-3. When unknown, particle density of mineral soils is assumed to be 2.65 Mg m-3. Generally quartz, feldspars, and colloidal silicates dominate the mineral fraction of soils. The particle density of these minerals averages about 2.65 Mg m-3.
• When large amounts of heavy minerals, such as hornblende or magnetite, are present, the soil particle density is greater than 2.65 Mg m-3. Soils formed in volcanic parent materials, such as pumice or ash, generally have particle densities less than 2.65 Mg m-3.

Bulk Density (p_b)

• The mass of a unit volume of dry soil (both solids and pores) is known as bulk density. It is also expressed in terms of Mg m-3.
• Soils that are loose, porous, or well-aggregated will have lower bulk densities than soils that are compacted or non-aggregated. This is because pore space weighs less than solid space (soil particles). Sandy soils have less total pore than clayey soils, so generally they have higher bulk densities. Bulk densities of sandy soils vary between 1.2 to 1.8 Mg m-3. Fine-textured soils, such as clays, silty clays, or clay loams, have bulk densities between 1.0 and 1.6 Mg m-3.