Inherent Factors Affecting Soil Infiltration

Soil infiltration refers to the soil’s ability to allow water movement into and through the soil profile. It allowsthe soil to temporarily store water, making it available for uptake by plants and soil organisms. Infiltrationrates are a measure of how fast water enters the soil and are typically expressed in inches per hour. For initialin-field assessments, however, it is more practical to express infiltration time in the number of minutes it takessoil to absorb each inch of water applied to the soil surface. Water entering too slowly may lead to ponding onlevel fields, erosion from surface runoff on sloping fields, or inadequate moisture for crop production. Anadequate amount of water must infiltrate the soil profile for optimum crop production. Porous soils allowwater to infiltrate and recharge ground-water aquifers and sustain base flow in streams. An infiltration ratethat is too high can lead to nitrate-nitrogen or pesticide leaching, if they are not managed correctly.Management measures, such as residue management, cover crops can improve infiltration.Inherent Factors Affecting Soil InfiltrationInherent factors affecting soil infiltration, such assoil texture, cannot be changed. Soil texture(percentage of sand, silt, and clay) is the majorinherent factor affecting infiltration. Water movesmore quickly through large pores of sandy soil thanit does through small pores of clayey soil, especiallyif clay is compacted and has little or no structure oraggregation.Depending on the amount and type of clayminerals, some clayey soils develop shrinkagecracks as they dry. The cracks are direct conduitsfor water entering the soil, causing clayey soils tohave high infiltration rates under dry conditions.Where cracks do not occur, clayey soils have slowinfiltration rates.Infiltration ManagementManagement practices (such as providing groundcover and managing equipment traffic to avoidcompaction) impact infiltration by affecting surfacecrusting, compaction, and soil organic matter.Without a protective vegetative or residue cover,bare soil is subject to direct impact and erosiveforces of raindrops that dislodge soil particles.Dislodged soil particles fill in and block surfacepores, contributing to the development of surfacecrusts which restrict water movement into the soil.Compaction can result from equipment traffic,especially on wet soils, and tillage pans. CompactedPage 1or impervious soil layers have less pore space andrestrict water movement through the soil profile.As soil moisture levels increase, infiltration ratesdecrease. Soil moisture is impacted by wateruptake by plants, residue and vegetative cover,irrigation practices, and drainage measures. Drysoils tend to have pores and cracks that allowwater to enter faster than wet soils. As soilsbecome wet, infiltration rate slows to a steady ratebased on how fast water can move through themost restrictive layer, such as a compacted layer,or a layer of dense clay.Guides for Educators

Soil Infiltration – Soil Quality KitUSDA-NRCSSoil organic matter binds soil particles together instable aggregates, increasing porosity andinfiltration. Soils with a high content of organicmatter also provide good habitat for soil biota,such as earthworms. Soil biota increase pore spaceand create continuous pores linking surface soillayers to subsurface soil layers. Using designated field roads or rows forequipment trafficLong-term solutions for maintaining or improvingsoil infiltration include practices that increaseorganic matter content and aggregation andminimize runoff, soil disturbance, and compaction.Increases in organic matter content result inincreased aggregation and improved soil structure,which lead to improved infiltration rates. Adding solid manure or other organic materialsMeasures that can improve infiltration rates: Avoiding soil disturbanceoperation when soils are wetandequipment Reducing the number of trips across the field Subsoiling to break up existing compacted layers Using continuous no-till Using rotations with high-residue crops, such ascorn and small grain, and perennial crops, such asgrass or alfalfa Planting cover crops and green manure crops Farming on the contour Establishing terraces or other runoff- anderosion-control structuresProblems Related to Infiltration and Relationship of Infiltration to Soil FunctionWhen rainfall occurs at a rate that exceeds thesoil’s infiltration capacity, runoff moves downslopeor ponds on the surface of level land. When runoffoccurs on bare or poorly vegetated soil, erosionoccurs. Runoff carries away nutrients, chemicals,and soil, resulting in decreased soil productivity,offsite sedimentation of water bodies, anddiminished water quality. Tables 1 and 2 show, forperiods of 1 year to 100 years, rainfall frequencies,duration of rainfall events, and inches of rainfall.Table 1. Rainfall intensity and duration patterns for Mead, NE.*FrequencyDuration of Rainfall Event(inches of rain)30 minutes1 hour2 hours1 year1.21.11.82 years1.31.71.95 years1.72.12.410 years2.02.52.8100 years2.83.74.2* D.M. Herschfield, Rainfall Frequency Atlas of the U.S., U.S. Weather Bureau, 1961.Page 2Guides for Educators

Soil Infiltration – Soil Quality KitUSDA-NRCSTable 2. Rainfall intensity and duration patterns for North Platte, NE.*FrequencyDuration of Rainfall Event(inches of rain)30 minutes1 hour2 hours1 year0.91.11.22 years1.11.41.55 years1.51.92.110 years1.82.22.5100 years2.63.43.7* D.M. Herschfield, Rainfall Frequency Atlas of the U.S., U.S. Weather Bureau, 1961.Restricted infiltration and ponding result in poorsoil aeration. This leads to poor root function, poorplant growth, reduced availability of nutrients toplants, and reduced cycling of nutrients by soilorganisms.Infiltration rate is most affected by conditions nearthe soil surface and can change drasticallyaccording to management. Infiltration is rapidthrough large continuous pores at the soil surfaceand slows as pores become smaller. Steady-stateinfiltration rates typically occur when soil is nearlysaturated and are listed for varying textural classesin Table 3. These are average values and should notbe generalized for all soil types.Table 3. Steady infiltration rates for general soiltexture groups in a very deeply wetted soil (Hillel,1982). (Only use these values to compareinfiltration rates to the second inch of water.)Soil TypeSteady infiltration rate(inches per hour)Sands 0.8Sandy and silty soils0.4 - 0.8Loams0.2 - 0.4Clayey soils0.04 - 0.2Sodic clayey soils 0.04What measures are being used that impact infiltration rates?Do they increase or decrease infiltration rates? (Why or why not?)Page 3Guides for Educators

Soil Infiltration – Soil Quality KitUSDA-NRCSMeasuring InfiltrationMaterials Needed to Measure InfiltrationGently firm the soil around the inside of thering to avoid any gaps.3-inch-diameter ringPlastic wrap2. Line the ring with plastic wrap so that it coversthe inside of ring and drapes over the side.Plastic bottle marked at 107 mL (for 1 inch ofwater) or graduated cylinder3. Pour 107 mL of distilled water or rainwater intoplastic-lined ring (Figure 1).Distilled water or rainwaterStopwatch or timerConsiderations – Select a representative testlocation. For comparison, select a location underdifferent management. For example, select a wheeltraffic row and a row without wheel traffic. Ideally,bulk density of each location is known.A single ring measurement is only an estimate, so itis recommended that multiple be tested.The test should not be conducted when the surfacesoil is unusually dry. If necessary, add water andallow enough time to soak in prior to conductingthe test. You can also choose to conduct the testafter rain or irrigation.Figure 1. Water is poured into plastic-linedring.4. Gently pull plastic wrap away. Record the timeit takes for water to infiltrate soil. Stop time iswhen the soil is “glistening.”5. Repeat steps 2, 3, and 4 with another inch ofwater to estimate steady-state infiltration.Infiltration Test6. Record the results in Table 4.1. Clear all residue from the soil surface. Drive thering to a depth of 3 inches using a small sledgeand plastic impact driver or block of wood.7. The ring can be removed with soil intact for useindoors in the respiration test and/or bulkdensity test.InterpretationsRecord first inch, and second inch (steady-state)infiltration rates and associated information bycompleting Table 4 and answering discussionquestions. Infiltration rate reflects the maximumrate before runoff or ponding will occur. ComparePage 4infiltration rates for different fields, soil types, andmanagement systems. In some cases it may takethree or more inches to reach a steady stateinfiltration rate.Guides for Educators

Soil Infiltration – Soil Quality KitUSDA-NRCSTable 4. Infiltration data lty clayloamSilty clayloamFirst Inch ofWaterStartEndTime TimeFirstInfiltrationTime(minutes)Infiltration Rate(inches/hr)Second Inch ofWaterStartEndTime TimeDate: May 1, s/hr)0.300.5Notes:Did the rate change from the first to the second inch? Why or why not? Would you expect a steady-stateinfiltration rate if a third inch of water is added?How does your infiltration time compare to the expected frequency and duration of a 1-inch rainfall inTables 1 and 2 or rainfall events you might expect in your area? Do you expect runoff to occur?Page 5Guides for Educators

Soil Infiltration – Soil Quality KitUSDA-NRCSHow do your infiltration rates compare to the steady-state infiltration rates in Table 3? Are rates higher,lower, or similar to the same soil type? Why or why not?GlossaryInfiltration Rate – A measure of how fast waterenters the soil, typically expressed in inches perhour but recorded in minutes for each inch ofwater applied to the soil surface.Soil Porosity – Amount of pore space in the soil.Soils with higher porosity have more pore spaceand higher infiltration rates than those with lowerporosity.Restrictive Layers – Compacted layers and layers ofdense clay, bedrock, or other restrictive featuresthan limit infiltration below the surface of the soil.Steady-State Infiltration – The infiltration rate issteady and does not increase or decrease as morewater is added. It typically occurs when the soil isnearly saturated.Soil Aggregates – Soil particles held together byorganic matter and related substances. Wellaggregated soils have higher infiltration rates andare less prone to erosion.USDA is an equal opportunity provider and employer.Page 6Guides for Educators

Page 1 Guides for Educators Soil infiltration refers to the soil’s ability to allow water movement into and through the soil profile. allows It the soil to temporarily store water, making it available for uptake by plants and soil organisms. nfiltration I