Infiltration

Infiltration is the downward entry of water into the soil.

The velocity at which water enters the soil is infiltration

rate. Infiltration rate is typically expressed in inches per

hour. Water from rainfall or irrigation must first enter the

soil for it to be of value.

Factors Affecting

Inherent - Infiltration rate is dependent on soil texture

(percentage of sand, silt, and clay) and clay mineralogy.

Water moves more quickly through the large pore spaces in

a sandy soil than it does through the small pores of a

clayey soil, especially if the clay is compacted and has

little or no structure or aggregation (see Table 1).

Depending on the amount and type of clay minerals, many

clayey soils develop shrinkage cracks as they dry, creating

a direct conduit for water to enter the soil. These clay soils

have high infiltration capacities as water moves into the

shrinkage cracks, although at other times, when cracks are

not present, their infiltration rate is characteristically slow.

Dynamic - A reflection of climate and landscape

position, as well as management practices and crop

demand, existing soil water content affects the ability of

the soil to pull additional water into it. Pores and cracks

are generally open in a dry soil. Many of them are filled in

by water or swelled shut as the soil becomes wet, so

infiltration rate is generally highest when the soil is dry. As

the soil becomes wet, the infiltration rate slows to the rate

at which water moves through the most restrictive layer,

such as a compacted layer or a layer of dense clay.

Infiltration is affected by crop and land management

practices that affect surface crusting, compaction, and soil

organic matter. Without the protective benefits of

vegetative or residue cover, bare soil is subjected to the

direct impact and erosive forces of raindrops that dislodge

soil particles. Dislodged soil particles fill in and block

surface pores, contributing to the development of surface

crusts that restrict water movement into the soil.

Compaction results from livestock and equipment traffic,

especially on wet soils, and continuous plowing to the

same depth, e.g. the creation of a plow pan below the

tillage depth. Compacted or impervious soil layers have

reduced pore space and restricted water movement through

the soil profile.

Soil organic matter affects infiltration through its positive

affect on the development of stable soil aggregates, or

crumbs. Highly aggregated soil has increased pore space

and infiltration. Soils high in organic matter also provide

good habitat for soil biota, such as earthworms, that

through their burrowing activities, increase pore space and

create continuous pores linking surface to subsurface soil

layers.

Management that reduces soil cover, disrupts continuous

poor space, compacts soil, or reduces soil organic matter

negatively impacts infiltration. Since tillage negatively

affects all of these properties, it plays an important role in

a soil’s infiltration rate.

Soil Quality Indicators

USDA Natural Resources Conservation Service

Soil Type

Steady Infiltration Rate

(in/hr)

Sands > 0.8

Loams 0.2 - 0.4

Clays 0.04 - 0.2

Table 1. Steady infiltration rates for general soil texture groups in

very deeply wetted soil. (Hillel, D. 1982. Introduction to soil physics.

Academic Press, San Diego, CA)

A one inch layer of water is added to a six inch diameter ring to

measure infiltration rate.

Indicator

P

Test

F

Function

W

Helping People Help the Land...
Relationship to Soil Function

Infiltration is an indicator of the soil’s ability to allow

water movement into and through the soil profile. Soil

temporarily stores water, making it available for root

uptake, plant growth and habitat for soil organisms.

Problems with Poor Function

When water is supplied at a rate that exceeds the soil’s

infiltration capacity, it moves downslope as runoff on

sloping land or ponds on the surface of level land. When

runoff occurs on bare or poorly vegetated soil, erosion

takes place. Runoff carries nutrients, chemicals, and soil

with it, resulting in decreased soil productivity, off-site

sedimentation of water bodies and diminished water

quality. Sedimentation decreases storage capacity of

reservoirs and streams and can lead to flooding.

Restricted infiltration and ponding of water on the soil

surface results in poor soil aeration, which leads to poor

root function and plant growth, as well as reduced nutrient

availability and cycling by soil organisms. Ponding and

soil saturation decreases soil strength, destroys soil

structure, increases detachment of soil particles, and makes

soil more erodible. On the soil surface rather than in the

soil profile, ponded water is subject to increased

evaporation, which leads to decreased water available for

plant growth.

A high infiltration rate is generally desirable for plant

growth and the environment. In some cases, soils that have

unrestricted water movement through their profile can

contribute to environmental concerns if misapplied

nutrients and chemicals reach groundwater and surface

water resources via subsurface flow.

Conservation practices that lead to poor infiltration

include:

• Incorporating, burning, or harvesting crop residues

leaving soil bare and susceptible to erosion,

• Tillage methods and soil disturbance activities that

disrupt surface connected pores and prevent

accumulation of soil organic matter, and

• Equipment and livestock traffic, especially on wet

soils, that cause compaction and reduced porosity.

Improving Infiltration

Several conservation practices help maintain or improve

water infiltration into soil by increasing vegetative cover,

managing crop residues, and increasing soil organic matter.

Generally, these practices minimize soil disturbance and

compaction, protect soil from erosion, and encourage the

development of good soil structure and continuous pore

space. As a short-term solution to poor infiltration, surface

crusts can be disrupted with a rotary hoe or row cultivator

and plow plans or other compacted layers can be broken

using deep tillage.

Long-term solutions for maintaining or improving

infiltration include practices that increase soil organic

matter and aggregation, and reduce soil disturbance and

compaction. High residue crops, such as corn and small

grains, perennial sod, and cover crops protect the soil

surface from erosion and increase soil organic matter when

reduced tillage methods that maintain surface cover are

used to plant the following crop. Application of animal

manure also helps to increase soil organic matter. Increased

organic matter results in increased aggregation and

improved soil structure leading to improved infiltration

rates. Conservation tillage, reduced soil disturbance, and

reducing the number of trips across a field necessary to

produce a crop help leave continuous pore spaces intact

and minimize the opportunity for soil compaction.

Conservation practices resulting in infiltration rates

favorable to soil function include:

• Conservation Crop Rotation

• Cover Crop

• Prescribed Grazing

• Residue and Tillage Management

• Waste Utilization

Measuring Infiltration

The Single Ring (Flooded/Ponded) Infiltrometer Method is

described in the Soil Quality Test Kit Guide, Section I,

Chapter 3, pp. 7 - 8. See Section II, Chapter 2, pp. 55 - 56

for interpretation of results.

Reference: Lowery B, Hickey WJ, Arshad MA, and Lal R.

1996. Soil water parameters and soil quality. In: Doran JW,

Jones AJ, editors. Methods for assessing soil quality.

Madison, WI. p 143-55.

Specialized equipment, shortcuts, tips:

To accurately assess infiltration and compare rates for

different soils, the soils should be at similar moisture

content when taking the measurement. It is recommended

that measurements be taken at field capacity, defined as the

water content of the soil root zone at which drainage (by

gravity) becomes negligible. If the soil is already saturated,

infiltration will not occur; wait for one or two days to

allow for drying to measure infiltration rate.

Time needed: 60 minutes or more depending on soil

conditions

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