Physical Properties of Soil
Soil Density
Particle and bulk density are measurable soil properties used to express soil density. Particle density is the average density of the soil particles (i.e., mineral composition), and bulk density is the mass per unit volume of dry (moisture-free) soil in its natural state, including both the particles and pore space. The density of soil particles is a result of the chemical composition and structure of the minerals in the soil (Figure 4.8).
Particle Density
Particle density is a physical property of soil, defined as the mass per unit volume of soil solids, reported in grams per cubic centimeter (g/cm3). Particle density is essentially the same as the specific gravity of a solid substance. Particle density depends upon the chemical composition and crystal structure of solid mineral particles and is not affected by pore space. As such, particle density is independent of the size of particles and their arrangement in the soil
Importance of Particle Density
Particle density data are used to better understand the physical and chemical properties of the soil. For example, the particle density indicates the relative amounts of organic matter and mineral particles in a soil sample. The chemical composition and structure of minerals in a soil sample can be deduced by comparing the soil’s particle density to the known densities of minerals such as quartz, feldspar, micas, magnetite, garnet, or zircon.
Bulk Density
Related to soil structure, and a very important physical indicator, is bulk density. The bulk density of soil reflects the mass or weight of a certain volume of soil, including air space. Compared to particle density, bulk density takes account of the total pore space in a soil body. Bulk density can provide insight to other soil properties such as porosity, water storage, fluid permeability, and compaction. Bulk density is expressed as the ratio of the weight of the soil to the volume of the soil, and is measured in grams per cubic centimeter (g/cm3).
Soil Bulk Density Affect on Plant Growth
High bulk density is an indicator of soil compaction and low soil porosity. Compaction reduces crop yields and restricts the growth of plant cover that helps to protect the soil from erosion. It affects infiltration, rooting depth, available water capacity, soil porosity and aeration, availability of nutrients for plant use, and activity of soil micro-organisms, all of which influence key soil processes and productivity.
Soil Properties Affect on Bulk Density
The bulk density of a soil is inversely related to porosity. Generally, loose, porous soils and those rich in organic matter have lower bulk density. Sandy soils have relatively high bulk density since total pore space in sands is less than that of silt or clay soils. Finer-textured soils, such as silt and clay loams, that have good structure have higher pore space and lower bulk density compared to sandy soils. Organic matter content of the soil affects bulk density, generally decreasing the bulk density as organic matter content increases, and vice-versa.
Measuring Soil Bulk Density
Numerous field methods have been developed for measuring soil bulk density. These include the core or cylinder method, excavation method, clod method, and radiation method. These methods seem to be labor intensive and time-consuming. Some of these methods have been compared with each other and the results of these comparisons showed high inconsistency under field conditions.
Identifying Compacted Soil
Compacted soil is firm or hard to break up by hand, normally paler in color, has few roots, is dominated by horizontal cracks/fissures and platy structures, and has few visible pores. Roots are often the best indicators of soil compaction. Finer roots will run horizontally across the surface of a compacted layer and taproots similarly become pronged and run laterally if they cannot push through the soil. This compacted layer is often referred to as a plow pan and is often associated with intense tillage (Figure 4.9).
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