“Soil porosity” refers to the amount of pores, or open space, between soil particles. Pore spaces may be formed due to the movement of roots, worms, and insects; expanding gases trapped within these spaces by groundwater; and/or the dissolution of the soil parent material. Soil texture can also affect soil porosity
There are three main soil textures: sand, silt, and clay. Sand particles have diameters between .05 and 2.0 mm (visible to the naked eye) and are gritty to the touch. Silt is smooth and slippery to the touch when wet, and individual particles are between .002 and .05 mm in size (much smaller than those of sand). Clay is less than .002 mm in size and is sticky when wet. The differences in the size and shape of sand, silt, and clay influence the way the soil particles fit together, and thus their porosity.
Soil porosity is important for many reasons. A primary reason is that soil pores contain the groundwater that many of us drink. Another important aspect of soil porosity concerns the oxygen found within these pore spaces. All plants need oxygen for respiration, so a well-aerated soil is important for growing crops. Compaction by construction equipment or our feet can decrease soil porosity and negatively impact the ability of soil to provide oxygen and water.
- Four 100ml graduated cylinders per group (or a measuring cup and two clear plastic bottles)
- Fine, playground-style sand and coarse, aquarium-style gravel
- Blank piece of paper and something to write on
- Pencil or pen
- Metal spoon or gardening spade
- Divide into small groups. On a piece of paper, make a data table like the one below for each group.
Soil particle type Volume of Water used (ml) Gravel Sand
With each group taking four graduated cylinders, fill one cylinder with 100ml of sand, one with 100ml of gravel, and two 100ml of water.
Discuss the experiment: Which substance has more pore space: gravel or sand? How did you make this decision?
Have each group fill the cylinder of sand with the water (be sure to not let the water overflow). Record the amount of water used in the data table.
Repeat step 4 with the gravel and the second cylinder of water.
Discuss as a group what happened and why? Was your initial hypothesis accurate?
Before leaving the classroom, though, refill two of the graduated cylinders with 100ml of water. You will also need paper, pens, and pencils to record observations. Draw the data table below for each group.
Survey area Volume of Water used (ml) # 1 # 2
Find a place outside where it is permissible to collect small soil samples and have each group choose a survey area.
Record observations of this survey area. Look at the types of plants growing in the soil, evidence of wildlife, etc. Is the soil in the shade or in direct sunlight? Sketch what you see.
Once survey area observations have been made, obtain a small sample of soil to determine its texture. Is the soil wet or dry? If it’s wet, does it feel gritty (sand), smooth and slippery (silt), or sticky (clay)? Can you see and measure individual particles? Record all of your texture observations.
Now have each group fill its empty graduated cylinder with 50ml of soil. Pour water from one graduated cylinder into the soil until water just covers the top. Record the volume of water used in the data table next to Survey Area #1.
Pick a new survey area (if possible, with different vegetation). Repeat steps 3 through 5, and record the volume of water used in the data table next to Survey Area #2.
Return to the classroom and discuss your results: Was there a difference in soil porosity? Were there similarities? For the soil samples with similar porosities, did they have the same soil textures? Do you think these soils provide adequate water and air for plants? What types of plants live in these soils? Do factors such as sunlight or soil texture seem to affect the porosity of the soil?
For more, visit the NPS.
Soil and Water Relationships
Soil moisture limits forage production potential the most in semiarid regions. Estimated water use efficiency for irrigated and dry-land crop production systems is 50 percent, and available soil water has a large impact on management decisions producers make throughout the year. Soil moisture available for plant growth makes up approximately 0.01 percent of the world’s stored water.
By understanding a little about the soil’s physical properties and its relationship to soil moisture, you can make better soil-management decisions. Soil texture and structure greatly influence water infiltration, permeability, and water-holding capacity.
Soil texture refers to the composition of the soil in terms of the proportion of small, medium, and large particles (clay, silt, and sand, respectively) in a specific soil mass. For example, a coarse soil is a sand or loamy sand, a medium soil is a loam, silt loam, or silt, and a fine soil is a sandy clay, silty clay, or clay.
Soil structure refers to the arrangement of soil particles (sand, silt, and clay) into stable units called aggregates, which give soil its structure. Aggregates can be loose and friable, or they can form distinct, uniform patterns. For example, granular structure is loose and friable, blocky structure is six-sided and can have angled or rounded sides, and platelike structure is layered and may indicate compaction problems.
Soil porosity refers to the space between soil particles, which consists of various amounts of water and air. Porosity depends on both soil texture and structure. For example, a fine soil has smaller but more numerous pores than a coarse soil. A coarse soil has bigger particles than a fine soil, but it has less porosity, or overall pore space. Water can be held tighter in small pores than in large ones, so fine soils can hold more water than coarse soils.
Water infiltration is the movement of water from the soil surface into the soil profile. Soil texture, soil structure, and slope have the largest impact on infiltration rate. Water moves by gravity into the open pore spaces in the soil, and the size of the soil particles and their spacing determines how much water can flow in. Wide pore spacing at the soil surface increases the rate of water infiltration, so coarse soils have a higher infiltration rate than fine soils.
Permeability refers to the movement of air and water through the soil, which is important because it affects the supply of root-zone air, moisture, and nutrients available for plant uptake. A soil’s permeability is determined by the relative rate of moisture and air movement through the most restrictive layer within the upper 40 inches of the effective root zone. Water and air rapidly permeate coarse soils with granular subsoils, which tend to be loose when moist and don’t restrict water or air movement. Slow permeability is characteristic of a moderately fine subsoil with angular to subangular blocky structure. It is firm when moist and hard when dry.
Water-holding capacity is controlled primarily by soil texture and organic matter. Soils with smaller particles (silt and clay) have a larger surface area than those with larger sand particles, and a large surface area allows a soil to hold more water. In other words, a soil with a high percentage of silt and clay particles, which describes fine soil, has a higher water-holding capacity. The table illustrates water-holding-capacity differences as influenced by texture. Organic matter percentage also influences water-holding capacity. As the percentage increases, the water-holding capacity increases because of the affinity organic matter has for water.
Water availability is illustrated in the figure by water levels in three different soil types. Excess or gravitational water drains quickly from the soil after a heavy rain because of gravitational forces (saturation point to field capacity). Plants may use small amounts of this water before it moves out of the root zone. Available water is retained in the soil after the excess has drained (field capacity to wilting point). This water is the most important for crop or forage production. Plants can use approximately 50 percent of it without exhibiting stress, but if less than 50 percent is available, drought stress can result. Unavailable water is soil moisture that is held so tightly by the soil that it cannot be extracted by the plant. Water remains in the soil even below plants’ wilting point.
One can see from the table that soil texture greatly influences water availability. The sandy soil can quickly be recharged with soil moisture but is unable to hold as much water as the soils with heavier textures. As texture becomes heavier, the wilting point increases because fine soils with narrow pore spacing hold water more tightly than soils with wide pore spacing.
Soil is a valuable resource that supports plant life, and water is an essential component of this system. Management decisions concerning types of crops to plant, plant populations, irrigation scheduling, and the amount of nitrogen fertilizer to apply depend on the amount of moisture that is available to the crop throughout the growing season. By understanding some physical characteristics of the soil, you can better define the strengths and weaknesses of different soil types.
The table and figures were originally published by the Institute of Agriculture and Natural Resources at the University of Nebraska – Lincoln.
Soil Structure and Macropores
Home > Indicators > Soil Structure and Macropores
What it is: Sand, silt and clay particles are the primary mineral building blocks of soil. Soil structure is the combination or arrangement of primary soil particles into aggregates. Using aggregate size, shape and distinctness as the basis for classes, types and grades, respectively, soil structure
describes the manner in which soil particles are aggregated. Soil structure affects water and air movement through soil, greatly influencing soil’s ability to sustain life and perform other vital soil functions.
Soil pores exist between and within aggregates and are occupied by water and air. Macropores are large soil pores, usually between aggregates, that are generally greater than 0.08 mm in diameter. Macropores drain freely by gravity and allow easy movement of water and air. They provide habitat for soil organisms and plant roots can grow into them. With diameters less than 0.08 mm, micropores are small soil pores usually found within structural aggregates. Suction is required to remove water from micropores.
Photo: Crumbly structure of surface soil is associated with adequate organic matter content.
Why it is important: Important soil functions related to soil structure are: sustaining biological productivity, regulating and partitioning water and solute flow, and cycling and storing nutrients. Soil structure and macropores are vital to each of these functions based on their influence on water and air exchange, plant root exploration and habitat for soil organisms. Granular structure is typically associated with surface soils, particularly those with high organic matter. Granular structure is characterized by loosely packed, crumbly soil aggregates and an interconnected network of macropores that allow rapid infiltration and promote biological productivity. Structure and pore space of subsurface layers affects drainage, aeration, and root penetration. Platy structure is often indicative of compaction.
Specific problems that might be caused by poor function: Clay soils with poor structure and reduced infiltration may experience runoff, erosion, and surface crusting. On-site impacts include erosion-induced nutrient and soil loss and poor germination and seedling emergence due to crusted soil. Off-site impacts include reduced quality of receiving waters due to turbidity, sedimentation and nutrient enrichment. Water entry into a sandy soil can be rapid, but subsurface drainage of sandy soils with poor structure can also be rapid such that the soil cannot hold water needed for plant growth or biological habitat.
Practices that lead to poor soil structure include:
- Disturbance that exposes soil to the adverse effects of higher than normal soil drying, raindrop and rill erosion, and wind erosion
- Conventional tillage and soil disturbance that accelerates organic matter decomposition
- Residue harvest, burning or other removal methods that prevent accumulation of soil organic matter
- Overgrazing that weakens range and forage plants and leads to declining root systems, poor growth and bare soil
- Equipment or livestock traffic on wet soils
- Production and irrigation methods that lead to salt or sodium accumulation in surface soils
What you can do: Practices that provide soil cover, protect or result in accumulation of organic matter, maintain healthy plants, and avoid compaction improve soil structure and increase macropores.
Practices resulting in improved soil structure and greater occurrence of macropores favorable to soil function include:
- Cover Crop
- Conservation Crop Rotation
- Irrigation Water Management
- Prescribed Grazing
- Residue and Tillage Management
- Salinity and Sodic Soil Management
For more information go to Soil Management Practices.
Photo: High residue and cover crops contribute organic matter to soil, while no-till management helps protect organic matter and allow accumulation. Organic matter provides food for earthworms and other soil biota. All play a role in developing or protecting soil structure and macropores to help soil function at a high level. Inset shows relationship of macro- and micropores to soil aggregates.
Evaluating soil structure and macropores:
Soil structure is described in the Soil Quality Test Kit Guide, Section I, Chapter 11, pp. 23 – 27. See Section II, Chapter 10, p. 76 for interpretation of observations.
Soil Particles, Water, and Air
Moisture, warmth, and aeration; soil texture; soil fitness; soil organisms; its tillage, drainage, and irrigation; all these are quite as important factors in the makeup and maintenance of the fertility of the soil as are manures, fertilizers, and soil amendments.
—J.L. HILLS, C.H. JONES, AND C. CUTLER, 1908
The physical condition of a soil has a lot to do with its ability to produce crops. A degraded soil usually has reduced water infiltration and percolation (drainage into the subsoil), aeration, and root growth. These conditions reduce the ability of the soil to supply nutrients, render harmless many hazardous compounds (such as pesticides), and maintain a wide diversity of soil organisms. Small changes in a soil’s physical conditions can have a large impact on these essential processes. Creating a good physical environment, which is a critical part of building and maintaining healthy soils, requires attention and care.
Let’s first consider the physical nature of a typical mineral soil. It usually contains about 50% solid particles and 50% pores on a volume basis (figure 5.1). We discussed earlier how organic matter is only a small, but a very important, component of the soil. The rest of a soil’s particles are a mixture of variously sized minerals that define its texture. A soil’s textural class—clay, clay loam, loam, sandy loam, or sand—is perhaps its most fundamental inherent characteristic, as it affects many of the important physical, biological, and chemical processes in a soil and changes little over time.
The textural class (figure 5.2) is defined by the relative amounts of sand (0.05 to 2 mm particle size), silt (0.002 to 0.05 mm), and clay (less than 0.002 mm). Particles that are larger than 2 mm are rock fragments (pebbles, cobbles, stones, and boulders), which are not considered in the textural class because they are relatively inert.
Soil particles are the building blocks of the soil skeleton. But the spaces (pores) between the particles and between aggregates are just as important as the sizes of the particles themselves. The total amount of pore space and the relative quantity of variously sized pores—large, medium, small, and very small—govern the important processes of water and air movement. Soil organisms live and function in pores, which is also where plant roots grow. Most pores in clay are small (generally less than 0.002 mm), whereas most pores in sandy soil are large (but generally still smaller than 2 mm).
The pore sizes are affected not only by the relative amounts of sand, silt, and clay in a soil, but also by the amount of aggregation. On the one extreme, we see that beach sands have large particles (in relative terms, at least—they’re visible) and no aggregation due to a lack of organic matter or clay to help bind the sand grains. A good loam or clay soil, on the other hand, has smaller particles, but they tend to be aggregated into crumbs that have larger pores between them and small pores within. Although soil texture doesn’t change over time, the total amount of pore space and the relative amount of variously sized pores are strongly affected by management practices—aggregation and structure may be destroyed or improved.
Chapter 4: Summary and Sources | Top | Water and Aeration
Soil Colour | Soil Particles | Bonding and Aggregation | Porosity | Changing Soil Structure | Soil Strength
Porosity is the pore space in soil between mineral particles (and solid organic matter) filled with either air or water. The pore space both contains and controls most of the functions of soil. It is not just the total amount of pore space that is important, but the size distribution of the pores, and the continuity between them which determines function and behaviour of soil.
Size distribution of pores
Pores range in diameter from a few millimetres right down to just a fraction of a micron (i.e. one thousandth of a millimeter). The following table gives some detail of this range.
Pore sizes in soil (Rowell 1994)
|Pore diameter (µm)||Nature of the pore|
|A 20 mm crack|
|4000||An earthworm channel (4 mm)|
|300||The diameter of a grass root|
|60-30||The smallest pore that will be air filled at field capacity|
|10||A fungal hypha|
|2||The size of a bacterial cell. The largest clay particle. The smallest pore from which a plant can readily obtain water|
|0.2||The smallest pore that will give up water to the suction exerted by a plant root. The pore size corresponding to the permanent wilting point|
|0.003||The largest pore filled with water in an “air dry” soil. The pore size is approximately ten times the diameter of a water molecule|
This table indicates that different pores act in quite different ways. It is convenient to group them in terms of behaviour.
Transmission pores are the large pores which enable root growth, air movement and water movement. They are visible to the naked eye, indeed even with a x5 hand lens, and range between 30 to 60 µm. They are often called macropores. The volume of a soil occupied by transmission pores should be >10 % if plant roots are to get adequate oxygen. Coarse textured, sandy soils, and well structured soils with a lot of biological activity, have a large proportion of pores in this size class.
Storage pores retain water (ie. they do not drain under the force of gravity) which is then available for use by plant roots and soil organisms. The proportion of these pores in a soil controls the plant available water capacity. They (along with even smaller pores) are termed micropores. They have diameters between 0.2 and 60 µm. The volume of a soil occupied by them might range from <10% in a loamy sand to >20% in a good loam.
These hold water so tightly that it cannot be extracted by roots or soil organisms – they are less than 0.2 µm in diameter. Fine textured or clayey soils have the larger proportion of their pores in this class. A heavy clay might well have 25 % of its volume as residual pores.
In visual terms macropores and micropores can be represented as follows:
The soil acts as a reservoir for water needed by plants but about half the water in the saturated soil moves down through large pores past the root zone and is lost for plant ant use. That lost is called gravitational water. When all of the pore space of a soil is filled with water, it is said to be saturated. The point where drainage ceases and the maximum amount of water is retained in the soil is known as the “field capacity.”
Approximately one-half of the water that remains in the soil (one-fourth of the total water) is available to plant roots and is known as capillary water. It is free to enter plants and be used for their growth.
The remaining one-half of the water that remains in the soil is held so tightly to soil particles that it is unavailable to plant roots. The total amount of unavailable soil moisture identifies the “permanent wilting point” of a given soil.
Figure 2-1 illustrates those soil-moisture concepts.
Since irrigation is to provide water to the soil for plants to use, you should examine the soil. A typical soil can be looked upon as a three-phase system with approximately half the space occupied by solid material, one-fourth by gas, and one-fourth by liquid.
The solid phase consists largely of inorganic materials known as sand, silt, and clay that range from 2 mm to less than 0.002 mm in diameter. Sand particles are the largest (2.00 mm to 0.05 mm) and consist mainly of quartz. Sand has a gritty feeling when rubbed in the hand. Silt particles (0.05 mm to 0.002 mm) have a velvet-like feeling, while clay (less than 0.002 mm), the smallest size fraction, is sticky when moistened in the hand.
Most soils also have a small portion, 0.1 to 10 percent, of organic material which is extremely important because it increases water-holding capacity, improves structure, and provides plant nutrients as it decomposes.
Figure 2-2 illustrates a Soil Texture Triangle used in the United States. The triangle has been divided into regions such as “clay” or “sandy loam” depending upon the relative amounts of various sizes of soil particles present. To read the chart, the clay lines go horizontally to the right from the clay side of the triangle, the silt lines go down at a 45° slope from the silt side of the triangle, and the sand lines go up to the left at a 45° angle from the bottom (sand) side of the triangle.
The various size fractions of the soil are mixed into different combinations called soil textures. For example, a soil texture such as loam consists of 28 to 50 percent silt, 25-52 percent sand, and 7.5 to 27.4 percent clay. Soil containing equal amounts of the three separates is a clay loam.
The various textures indicated on the textural triangle (Figure 2-2) are generally grouped into three categories. Sands, loamy sands, and sandy loams are usually referred to as coarse-textured soils. Loams and silt loams are medium textured soils; while clay loams, silty-clay loams, and clays are known as finetextured soil. The fine-textured or clay soils are known as heavy soils, while coarse-textured soils are called light soils.
The terms heavy and light soils originated from the ease that tillage implements can be drawn through them. Thus, clays are difficult soils to draw implements through in contrast to sands whose tillage requires less power.
The relative proportions of soil particles by size can be shown easily by letting a sample of soil settle in water. Fill a fruit jar about two-thirds full of water. Pour in soil until the jar is almost full. Replace the cover or put one hand tightly over the top of the jar and shake it vigorously. Then put the jar on the table and let the soil settle (Figure 2-3). Allow plenty of time because the very small particles settle slowly.
Figure 2-2. The soil texture triangle (from Handbook No. 436 U.S. Department of Agriculture, Washington, D.C., 1975)
Figure 2-3. Determining approximate quantities of different sizes of soil particles
Then hold a card or heavy piece of paper against the side of the jar and draw a diagram showing the different layers. Label each layer (clay, silt, sand). Do this with several soils taken from different places and compare the diagrams or compare the jars directly. Soil particles vary greatly in size. The largest (sand) particles settle to the bottom first. The fine (clay) particles settle slowly; some, in fact, are suspended indefinitely.
Soil texture has a large influence on the amount of water it can store for plant use and the rate at which water moves through them (Tables 2-1 and 2-2. The coarse-textured soil can hold for plant use only 20 to 120 millimeters of water per meter depth of soil. At the other extreme, the fine-textured soil will hold from 140 to 200 millimeters per meter of soil. Familiarity with soil textures makes the irrigation design and operation easier and more efficient.
The root zone of plants depends upon the size of the plants. Most major field crops root about 1 to 1¼ meters deep. Most of the roots are nearer the surface, as shown in Figure 2-4. Most water below about 1 meter is essentially unavailable to plants.
Wilting points of most soils are from one-third to two-thirds of field capacity. Table 2-1 shows some typical ranges of available soil moisture to be expected with various soil textures. Irrigation in a typical soil would be required immediately when soil moisture declines to 50 percent of field capacity.
Table 2-1. Range of readily available soil moisture for different soil types
Percent of moisture based on dry weight of soil
Permanent wilting point
Available water per unit depth of soil, mm/m
Adapted from: Booker, L. J., Surface Irrigation. FAO Agricultural Development Paper, No. 95, Rome, 1974.
Table 2-2. Long term infiltration rates for indicated soil types
Infiltration rate in mm per hour
Figure 2-4. Moisture use in relation to root zone and available moisture (S.C.S. Inf. Bull No. 199)
Most crops in deep, uniform soils use moisture more slowly from the lower root zone than from the upper soil. The top quarter is the first to be exhausted of available moisture. The plant then has to draw its moisture from the lower three-quarters of root depth. That stresses the plant because adequate moisture to sustain rapid growth cannot be extracted by the roots.
Soil structure is a term used to describe the arrangement of the soil particles Structure refers to a compounding or aggregation of soil particles into various forms such as platy, blocky, prismatic, etc. The moisture and air relationships of a soil are influenced by structure. An example of this is when soils containing some clay are trampled by animals while it is wet and the clay fills the pore space, thus creating a denser soil. Upon drying, such soil forms a massive structure and is said to be puddled when dry.
Soil density and pore space
The density of soils is generally measured by two methods, “bulk density” and “particle density.” Bulk density refers to the specific gravity of a volume of soil including its pore space. Thus, a cubic meter of soil including its pore space weighing 1,150 kg, divided by the weight of a cubic meter of water, 1,000 kg, has a bulk density of 1.15.
Particle density is a term used to measure the specific gravity of a soil excluding pore space. In other words, we are concerned with measuring the density of the soil particles without considering the air space between particles. Thus, if we assume that the soil just mentioned has a pore space of 48 percent, we can determine the particle density by dividing the weight of the soil particles (1,150 kg) by the weight of an equal volume of water (52 percent of 1,000 kg), 520 kg. The particle density will be 2.22 in this example. For any given soil, particle density will always be greater than bulk density. For most mineral soils, particle density averages about 2.65. Large portions of organic matter will tend to lower this figure. Bulk densities of soils will vary according to the texture and pore space. Bulk densities of coarse-textured soils usually range from 1.3 to 1.8, while fine-textured soils usually range from 1.0 to 1.3. Organic soils, depending upon the content of organic matter, usually range from 0.2 to 0.6.
To eliminate moisture contents as a variable, all calculations involved in determining particle density and bulk density are based on oven-dry weights of soil. Bulk density and particle density of a soil, if known, can be used to determine the “pore space” of a soil with this formula:
PS = 100 – (B.D./P.D. x 100)
PS = pore space in percent
BD = bulk density
PD = particle density
Substituting the previously determined figures for bulk density and particle density, gives us a pore space of:
PS = 100 – (1.15/2.22 x 100)
PS = 100 – 51.8
PS = 48.2 percent
The percentage of pore space in a soil is important because it determines the amount of water soil can hold when saturated.
Water retention in soils
Water is generally considered to be the universal solvent. The water molecule is dipolar, which means that one end of the molecule is negative in charge and the other is positive. Since opposite charges attract each other, this property permits the molecule to adjust to most materials.
Another property of water is relatively high surface tension. Surface tension is a measure of cohesion or attraction of water molecules to other water molecules. In contrast to cohesive forces, there are the adhesive forces. Adhesion is the attraction of water molecules to other molecules such as glass or soil.
Those soil properties, combined with various soil particle sizes, profoundly influence the movement, storage capacity, and retention of water in soil. Coarse-textured soils consisting of large particles tend to have fewer, but individually larger, diameter pores than fine-textured soils with small particles such as clay. A clay soil will have more total pore space, lower bulk density, and smaller diameter individual pores than a coarse-textured soil such as loamy sand.
Having a relatively high adhesive force, water is retained more in the fine-textured soils than in coarsertextured soils. This can be visualized by comparing the ease with which water can be poured through a two-inch diameter pipe to the difficulty of moving water through a capillary tube.
The relationship of water adhesion to soil particles can also be considered from the viewpoint of specific surface. Specific surface refers to the amount of surface area exposed to air or water. For example’ a solid one-inch cube has a specific surface of six square inches. If the same cube is sliced in three places giving eight one-nalf inch cubes, the total surface will be twelve square inches, so a soil such as clay has a high specific surface and, consequently, a high percentage of the water adhering to the particles.
Water infiltration in soils.
The rate that water will move into and through soils is important. Infiltration the rate surface water moves into soil. Percolation is the rate water above field capacity moves downward by gravity.
Infiltration is important because under natural rainfall, infiltration indicates how rapidly water will move into the soil. Low infiltration rates lead to more runoff and, frequently, to soil erosion.
Percolation rates must be high to sustain high infiltration rates. In many soils, layers of various-textured soils influence infiltration rates over long periods. For example, a clay layer beneath a sandy loam area results in a decreased infiltration rate when the sandy loam area becomes saturated.
In some cases, high infiltration rates are not desired. For example, ponds used to store irrigation water should have low infiltration rates. Rice paddies, where permanent flooding is desired, require less irrigation water when infiltration rates are low.
Fine-texture soils generally have lower infiltration rates than coarse-textured soils. The long-term infiltration rate refers to the millimeters of water that enters the soil per hour after a constant infiltration rate has been established. Some values are shown in Table 2-2.
The infiltration rate of a soil, an important factor when designing an irrigation system, may be measured in several ways. Unless large amounts of water are available, the methods are only approximate because if a small surface area is wetted, water will move vertically and laterally. Hence use as large an area as possible when measuring infiltration to minimize the effects of lateral movement, which might not occur during normal rainfall or irrigation conditions.
A measurement technique is to place a wall or dam around a level area to be tested, a 2 to 4 meter square area. Fill the small pond with water and keep the surface of the ground covered. After a length of time, one to a few hours, when ground appears to be well saturated near the surface, determine the rate of infiltration by measuring, usually with a bucket, the rate at which water must be added to keep the surface covered.
For one square meter of area, an infiltration rate of 1 liter per hour equals 1 millimeter (mm) per hour. If a stake is placed in the pond, the rate at which water goes down on the stake could be measured but the meniscus (the curvature of the water surface near a wall) makes this difficult to read.
As an example, a fence around an area is made of lumber enclosing an area of 4 m². After a constant infiltration rate is reached, 50 liters of water must be added to maintain the water level. The infiltration rate is then
50 liters/4 m² = 12.5 liters/hr m² or 12.5 mm/hr.