Extension Ag Update
May/June 2006
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Assessing Soil Quality: Evaluating Soil Infiltration

Ellen Phillips, Extension Educator - Crop Systems, Countryside Extension Center
708-352-0109, ephillps@uiuc.edu

The infiltration rate is a measure of how fast water enters the soil.  Measuring the infiltration rate can give an indication of soil structure and the presence of soil compaction.  Running an infiltration test is quite simple and uses tools that most of us already have. The materials needed are a six-inch diameter ring about six inches deep, plastic wrap, distilled water and a watch or stopwatch.

When to measure infiltration?  The tests should be done when the soil is at field capacity or between 12 and 48 hours following a saturating rain or irrigation. 

Where to sample?  Make sure the sampling area is free of residue and weeds.  Any vegetation present should be trimmed to the soil surface before inserting the ring.

The first step is to gently press the ring three inches into the soil.  Then firm the soil surface only around the inside edges of the ring to prevent extra seepage. Minimize disturbance to the rest of the soil surface inside the ring.  To prepare the ring, place a sheet of plastic wrap completely over the soil and ring.  This will prevent the soil surface from being disturbed when the water is added.  Gently pour one inch of distilled water into the plastic in the ring.  Very gently pull the plastic wrap out, leaving the water in the ring.  Record the time.  Stop recording time when the soil surface is just glistening.   If the soil surface is rough, stop timing when half of the surface is exposed.

Soil Permeability Classes

Infiltration Rate
(minutes per inch)

Infiltration rate
(inches per hour)

Infiltration Class

<3

>20

Very rapid

3 to 10

6 to 20

Rapid

10 to 30

2 to 6

Moderately Rapid

30 to 100

0.6 to 2

Moderate

100 to 300

0.2 to 0.6

Moderately slow

300 to 1,000

0.06 to 0.2

Slow

1,000 to 40,000

0.0015 to 0.06

Very Slow

>40,000

< 0.0015

Impermeable

One more time: For the most accurate data, repeat all of these steps.  Because the first inch of water may be wetting the soil as it penetrates, the second infiltration test gives a more accurate infiltration rate.

Understanding Infiltration: How fast water enters a soil depends on the soil type, soil structure and the soil water content. The infiltration rate on dry soil is faster than wet soils since water will be pulled into the soil.  This is why the initial soil moisture content should be similar at all sites you are taking an infiltration rate. It will make comparisons between cropping systems and soils more accurate.

Soil texture affects the infiltration rate.  Sandy soils normally have the most rapid infiltration rates.   Soil texture interacts with structure, influencing the movement of water into the soil. The infiltration rate of heavier soils is influenced by the degree of structure and the size of pores at the surface. Large continuous pores at the surface can cause rapid infiltration.  Infiltration decreases when the size or amount of pore space decreases due to structure breakdown, clogging of pores by particles, or saturation of pore when water deeper in the profile reaches dense subsoil layers. 

Soil use and management influence soil infiltration rates. Tilling wet soils can create compaction layers deeper in the soil.  Compacted soils have less pore space, resulting in lower infiltration rates below and a wetter surface soil.    Wet spots that were tilled to dry them out faster, often end up showing the most moisture stress later in the season, due to reduced infiltration and increased  subsurface compaction. 

Multiple tillage trips can quickly reduce the moisture stored in a soil.  In a silt loam soil, one to two inches of soil moisture can potentially be lost with each tillage pass.  This can dry the seed zone enough to negatively impact germination and emergence of the seed, despite having adequate moisture below the seed zone.

Surface crusts seal the soil surface, rapidly decreasing the infiltration rate. Tillage that breaks up surface crusts or compaction zones will increase infiltration.  However, excessive tillage can break apart aggregates, increasing compaction and creating conditions for even greater surface crusting to occur by disrupting pore continuity. 

Cropping systems that increase plant roots or earthworms may have a higher infiltration rate due to the number and larger size of pores left behind at and near the surface.   Continuous no-till will, over time, improve soil structure resulting in better infiltration and  reduce runoff.  Increased rates of stable organic matter from additions of residues or use of cover crops can increase the aggregate stability and result in increased infiltration.  Residues also absorb the impact of raindrops, slowing runoff and allowing more time for infiltration.  As a result, there is usually less erosion and crusting.