Digging In

Understand the health of your soil through infiltration testing

Understand the health of your soil through infiltration testing

By Lisa Kubik on Thursday, 16 July, 2020

Understanding the health of a particular soil may also help explain other things that are happening in that field.  In our area, a rainfall event of three inches or more at some point throughout the summer is not uncommon.  But what’s more important than the rain itself is what happens to the rain once it reaches the soil.

Taking a drive to look at your fields will tell part of the story. Did that rainwater runoff the surface, stay on the surface to create a puddle or did it soak into the soil pores?  This ability for soil to take up water and allow movement within the soil profile is how the National Resource Conservation Service (NRCS) defines infiltration.

The infiltration of your soil can be affected by many factors and how water infiltrates your soil can be even more complicated. What if there was a way to understand where water goes and how healthy your soil is by using a basic infiltration test? There are kits available to get highly accurate tests that can translate across many fields, but for our purposes, let’s use supplies you probably have on your farm.

A shovel is the number one tool that I use when looking at soil health. Just digging up 6 inches of soil can tell you about the history, structure, and drainage ability of that soil. Does the soil crumble between your fingers or does hold together?  Are there any hardpan layers that may have been caused by tillage?  We can learn so much by digging into the soil.

That structure within the soil directly affects how water is either absorbed or runs off during rainfall events.  We are able to compare different soils and how they are affected by rainfall with an infiltration test.

Supplies:

  • Shovel
  • Rubber mallet
  • 2 PVC or Steel rings with 6 inch diameter
  • 2 water containers filled with 444 mL of water
  • Plastic wrap
  • Timer

To get started, figure out what your goals are and which soils you want to compare.  Start on one soil type and pound the first ring in with the mallet halfway into the soil.  It works best to pre-measure the rings and put a mark on the halfway point to signal how far the ring will go into the soil for the test.

  1. Start by pounding your ring in with the rubber mallet.  If the ground is on the harder side, a board across the top might help.  Pound the ring halfway until you get to your mark.
  2. Place plastic wrap inside of the ring.  This will help prevent splash effects that could happen when pouring in your water.
  3. Take your pre-measured 444 mL of water and pour into the plastic wrap inside the ring.  This represents an inch of rainwater.
  4. Start your timer as you slowly pull the plastic wrap off of the ring to let the water soak into the soil on the inside of the ring.
  5. Keep an eye on your ring and stop the timer once the water has completely soaked into the soil.  There will be a shimmer on top of the soil.  Record the number of minutes it takes for 1” of rainfall.  Repeat steps 2-5 a second time to represent a second inch of rainwater.
  6. Repeat this process for your secondary location.  This might be a different field, soil type, part of the field that was managed differently.  Be consistent in the process and record 1st and 2nd infiltration numbers to compare.
  7. Dig up your rings and compare the soil at the bottom of the ring.  Did the moisture reach the bottom?

Once you’ve finished your tests, what differences can you pick out between your soils?  Is it the difference in tillage practices?  Was one of the fields harvested too wet in the fall?  Does one currently use cover crops? 

A soil that has slower infiltration rates is more likely low in organic matter, more easily has runoff in large rain events, and can hold less water in a drought situation, so what management changes might you make based on your in-field infiltration testing?

Don’t worry about having exactly a 6-inch ring or 444 mL of water for your own in-field testing.  At the end of the day, this test is meant to be a learning experience and a great way to compare one field to the next.  Make sure you are consistent and continue to use the same tools and you should learn a great deal about your soils!