• Topics: Soil and Water Science  | Soil and Water Basics  | Brown, Randall B 

Soil as a Porous Medium: Basics of Soil-Water Relationships - Part I¹

PREFACE

This fact sheet is the first of a series which describe some basic principles of water retention and movement in soils. These principles are relevant for many practical applications of soil physics, such as development of irrigation and fertilizer management schemes or waste disposal practices. These practices will be addressed more specifically in subsequent fact sheets. The reader should be aware that the concepts presented in these fact sheets are based on simplifications with the emphasis being placed on obtaining a better understanding of the basic principles involved, rather than on describing specific management practices. It is assumed that the reader is familiar with current Soil Survey reports in which many basic soil properties are defined.

INTRODUCTION

Soil structure is defined here as the physical constitution of a soil material as expressed by the size, shape and arrangement of the soil particles and the associated voids.

Soils are composed of mineral and organic particles with pores (voids) in between. Mineral particles have a wide range of sizes varying from sand (2 millimeter, or 8 hundredths of an inch, to 0.05 millimeter, or 2thousandths of an inch, in diameter) to silt (0.05 to 0.002 millimeter) and clay (smaller than 0.002 millimeter). Organic particles are found also, particularly in soil near the surface where roots grow and plant residues accumulate. In this fact sheet, different types of soil structure will be examined.

VISIBLE SOIL STRUCTURE

In sands, the mineral grains are assembled together leaving "packing" pores between the grains ( Fig. 1A ). The pictures shown in Fig. 1 were obtained by impregnating natural soils with a liquid plastic which hardens, thus preserving the original soil structure. Cutting the hardened sample into paper-thin sections allows microscopic study of the soil structure. In loamy sands and sandy loams, silt and clay contents are higher than in sands; the finer particles are found at the points of contact of the sand grains, leaving smaller pores in between ( Fig. 1B ). In soils with even higher clay contents and less sand, the "packing" pores between the individual grains are much smaller. In this case, natural aggregates (peds) may form, which are separated from adjoining peds by natural, larger pores such as cracks, channels or other macropores which are significantly larger than the "packing" pores ( Fig. 1C ). Root and worm channels are prominent macropores which can also occur in all types of soils.

Figure 1. 

Soil structure in soil survey reports is described in terms of the size, shape and degree of development of the peds. This, in turn, provides information on the macropores between the peds. How can this morphological information contained in soil survey reports be used? It can be helpful for understanding the differences among soils in terms of retention and movement of water, because these phenomena are governed to a large extent by pore size and shape distributions. Large pores can conduct more water, more rapidly than fine pores. Suction is a measure of the energy required to remove water from a given pore.Therefore, it is easier to remove water from a large pore than from a fine pore. These properties are discussed in more detail in Parts II and III of this series of fact sheets. Using the basic relationships between pore-size distributions, flow rates, and suctions, hydraulic properties of soils can be calculated based on observed physical properties of soils. However, experimental measurements of water content and water movements in soils are often easier to make than measurements of soil particle size and structure.

CHARACTERIZATION OF SOIL STRUCTURE

Soil physical procedures are widely used to characterize soil structure. Methods are available to measure total porosity, which is the percentage of the total soil volume occupied by pores. Sandy soils have porosities between 30% and 40%. Clayey soils tend to have porosities in the range of 40% to 60%.

Porosities (P) are calculated as: P=[1 -BD/PD] where BD is bulk density (the dry mass of the natural, undisturbed soil per unit volume) and PD is particle density (mass per unit volume of the solid particles, without the voids). Bulk-density values are listed in soil survey reports and may range from 1.2 g/cm ³ (grams per cubic centimeter) for clay soils to 1.7 g/cm ³ for sandy soils. Particle density values average 2.65 g/cm ³ , but may be lower in soils with organic matter and higher in soils that contain heavy minerals such as iron oxides.

To illustrate the differences among bulk density, porosity, and particle density, consider a core filled with moist, undisturbed soil whose volume is 100 cm ³ . Wet weight of the soil in the core in 160 g. Dry weight following oven drying of the soil is 130 g. Bulk density (BD) is therefore 130 g/100 cm ³ or 1.3 g/cm ³ . Assuming a particle density (PD) of 2.65 g/cm ³ , porosity (P) is 1- 1.3/2.65, or 0.51.

Most of the pores in clay soils are much smaller than those in sands. It is therefore important to determine not only the total porosity, but also the pore-size distribution. Direct observation in thin sections ( Fig. 1 ) is one possible method to obtain this distribution, but equivalent pore-size distributions may also be derived using physical techniques. These are based on the principle that each pore size is associated with a particular suction at which the pore can be emptied. This relationship ( Fig. 2 ) is valid only for cylindrical, capillary tubes, but it serves as a good approximation of moisture retention behavior in soils. As stated earlier, larger pores can be emptied more easily than finer pores. The volume of water that is released by a soil sample between two different suctions is a measure of the volume of the pores which have sizes corresponding with these two suctions. For example, Fig. 2 shows that a pore diameter of 100 microns (one tenth millimeter, or 4 thousandths of an inch) corresponds with a suction of 30 centimeters (pressure head of -30 centimeters, or -12 inches) while a pore diameter of 70 microns corresponds with a suction of 40 centimeters. We conclude that pores in the size class of 100 to 70 microns occupy a volume corresponding to the volume of water released by the soil between suctions of 30 and 40 centimeters. These volumes can be determined from soil-water characteristic curves and are useful in estimating pore-sizes of soil samples. This aspect will be discussed in more detail in the second fact sheet.

Figure 2. 

Soil structure, then, is very important because it governs the retention and movement of water in the soil as well as the transport of dissolved components (fertilizers, pesticides, etc.) in the water. Basic processes of water retention and movement are discussed in Part II and Part III of this series of fact sheets.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.

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