Field Infiltration Rate

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Field Infiltration Test:

 

Purpose:  To determine how field management practices have affected the rate at which rainfall will soak into the soil.  Three factors will affect surface infiltration rate (if soil is the same texture, slope, time of year, etc.)  1) presence of a compacted layer, 2) bulk density, or general level of compactness, and 3) surface porosity, which is affected by tillage, the presence of crop residue, earthworms, root and other biological activity, and of most interest, water stable aggregates, formed in healthy soils by the action of beneficial fungi.

 

Tools:  Assemble these and take to the field with you.

 

       a sturdy metal ring, 6 inch diameter irrigation pipe, cut into a 5 inch section (length), with one edge sharpened on a grinder.

       2 inch x 4 inch board, about 2 feet long, placed on the ring before pounding

       small sledge hammer

       small tape measure or ruler to measure that the ring is 2 inches out of the ground

       pint sized plastic container with a line drawn on the outside for 1 7/8 cup level

       a couple of gallon jugs to carry water to the field (will need about 1 pint per test, or 8 tests per gallon of water)

       kitchen grade thin plastic sheet or food wrap

       stopwatch or watch or clock with a second hand

 

Procedure:

 

1)    Find five representative spots in the field to test.  Avoid field edges, traffic ways, tire tracks, large cracks in the ground, etc.  Mark these five spots with a small flag, stick, or other temporary marker.  Note on your field map approximately where these 5 spots are. 

 

2)    Place a ring onto the soil at spot #1, taking care to remove residue  from the exact spot that the metal ring touches the ground, so that the residue does not make it harder to pound in smoothly.   In general though, leave residue undisturbed as much as possible both inside and outside the ring.

 

3)    Put the 2 inch x 4 inch board across the ring, while holding the board steady with one hand,  pound gently on the center of the board with the small sledge hammer, forcing the ring slowly into the ground.  Pound the ring in 3 inches (should stick out of the ground 2 inches).   Try to keep the ring level, and avoid any sideways movement of the ring that would create a water channel. (see Figure XX  below).

 

4)    Line the inside of the ring with a medium sized square sheet of plastic.  Slowly pour the 1 7/8 cup of water onto the plastic sheet.  It should all stay there, and not leak over the sides.  If you are doing the timing, set your stopwatch to zero, or look at your watch to determine a zero point.  This test is easier if there are two people, and one is watching the time, and the other is watching the water.

 

4)    Gently pull the sheet of plastic out from underneath the water.  This should take about one or two seconds to accomplish.  Donít jerk the sheet, since water may spill, but donít take too long either.  As the water hits the soil, start the stop-watch.

 

5)    Stop the stopwatch when the freestanding water disappears, but the soil has some glisten.  Record this number on your data sheet.  This is the amount of time that it would take for one inch of rainfall to soak into your soil if it fell very quickly.  This test may take anywhere from a few seconds, to several hours to complete.  If there is a crack in the soil, and only one of the five spots you sample goes quickly, you might disregard that data point, and repeat in another spot.  In my experience, generally four of the five rings are pretty close, and one ring might either be much quicker, or much slower.  Similarly, if four of the rings take 20 to 30 minutes, and one takes five hours, you may also disregard this data point.  It was probably due to a localized, compacted layer.  However, you may want to go back and see how extensive the compaction is, how deep it is, and whether there are any management practices that can alleviate the compaction.

 

6)    With the ring in the same spot, put the plastic back in, and repeat steps 4 and 5.  This will be the time required for a second inch of water to soak in.  The second inch often takes longer than the first one, but is an even better indicator of soil quality, since the rate at which the first inch soaks in is partly due to how dry the soil was when you started.  The second inch is a better indicator of porosity, and presence of water stable aggregates, which increase the porosity, and the infiltration rate of soil.

 

Figure XX.  Illustration of an Infiltrometer.

 

 

 

 Interpretation of Field Infiltration Rate:  Basically, the faster water soaks in the better, but there are no absolutes.  Here are some general guidelines, based on several field seasons of experience with this test.

 

Time for:

4

3

2

1

1st inch of water

0-30 seconds.

30 seconds to 5 minutes.

5 - 20 minutes.

20 minutes or more.

2nd inch of water

0-3 minutes.

3-10 minutes.

10-60 minutes.

60 minutes or more.