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Soils - Part 2: Physical Properties of Soil and Soil Water

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Soil may look simple; however, it is an extremely complex system. It is most often described by its physical, chemical and biological properties and processes. Soil is organic or inorganic; inert or active; living or non-living. Soil contains many organisms: bacteria, nematodes, fungi, earthworms, and small animals. From a physical perspective the soil constitutes the building blocks upon which we walk, construct buildings, grow crops and filter natural and manmade compounds. Soil’s physical, chemical and biological properties and processes interact to enhance its value as a natural resource.

[This lesson, as well as the other nine lessons in the Soils series, is taken from the "Soils Home Study Course," published in 1999 by the University of Nebraska Cooperative Extension.]

Introduction

As complex as it is, soil can be described simply. It consists of four major components: air, water, organic matter, and mineral matter (Fig. 2.1). In an ideal soil, air and water fill the pore space and compose about 50 percent of the volume; organic matter accounts for about 1-5 percent of the soil volume; and mineral matter accounts for the remaining 45-49 percent. The partitioning of these four components vary considerably. For example, an organic soil in Michigan may be 45 percent organic, while a desert soil from Arizona may be 60 percent mineral.

The mineral and organic matter fractions of the soil are the solids and serve as the storehouse and exchange sites for plant nutrients and other chemicals. They are important from a fertility and environmental standpoint. It is these fractions, along with cultural practices, that influence other physical properties and processes.

Figure 2.1. Major components of soils.

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This project was supported in part by the National Research Initiative Competitive Grants CAP project 2011-68002-30029 from the USDA National Institute of Food and Agriculture, adminstered by the University of California-Davis and by the National Science Foundation (NSF), Division of Undergraduate Education, National SMETE Digital Library Program, Award #0938034, administered by the University of Nebraska. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the USDA or NSF.