Step by step instructions for a soil infiltration test
I’ve been fortunate over the past few months to visit some of New Zealand’s most panoramic farms, assessing soil health and identifying areas which are putting a drag on farm performance and quality.
Many farms around the country are in serious water deficit, so it could be easy to assume that water on these parched soils would rapidly soak in. Not necessarily. If biological function has been poor, pastures are overstocked/overgrazed, the soil has been left bare or mineral balances are out, water (and air) movement becomes restricted. Infiltration rates are influenced by soil types, however even sandy soils can become compacted.
A well structured soil can quickly drink in any rainfall. While a poorly structured soil will tend to slump (slake) blocking soil pores, leading to ponding and run-off. This surface crusting leads to the germination of poorer ground covers, including moss, dock and thistles. If you want to maximise quality and yield then efficient water capture is key.
We have done infiltration tests this month which were taking over 30 mins for the soil to soak in an inch of water (less than 1mm/minute). This means that when your rainfall finally does arrive, it is mostly ineffective, as much of the rain that falls is going to run-off or evaporate. A costly exercise when so many of us need every mm right now.
The aim of any profitable regenerative ag business is to improve soil structure; maximising infiltration and minimising run-off.
The Infiltration test is an economical tool to compare areas on your farm, and track the affects of your management programme. Compare areas around your farm; include at least one which has been receiving optimal management, or is less prone to overgrazing and sprays – roadsides can be very telling.
Identifying how well your soil can handle a drink can be a most sobering experience.
You will need:
- 200mm long plastic downpipe about 150mm diameter (cut on an angle to make it easier to insert). Mark the inside of the tin at 100mm then 25mm marks up the side of the tin.
- 100mm long knife
- 1 litre of rain or distilled water
- Hand sledge and wood board or block
- Stop watch, ruler, note book and pen.
Step one: Clear the sampling area of surface residue, etc. If the site is covered with vegetation, trim it as close to the soil surface as possible. Make sure the site is dry.
Step two: Press the sharpened end of the pipe into the soil until it is 100mm deep into the soil or lay a wooden board over the top of the can. Strike the board with the mallet until the pipe is driven into the ground to the 100mm mark.
If the soil contains rock fragments and the can cannot be inserted to the appropriate depth, gently push the can into the soil until it hits a rock fragment.
If the soil is very dry and compacted, then use the knife to cut a slit into the soil for the cylinder, whilst disturbing the soil as little as possible.
Step three: Insert a ruler and add 25mm water at a time. Ideally if you can measure the amount of water needed to fill your pipe to 25mm this will improve the accuracy of the water added. Volume of Cylinder = π r2 · h . So for the 150mm wide pipe at 25mm depth= 70ml
Step four: Start the timer as you begin pouring the water into the pipe. When the last bit of water disappears check the time again and see how many seconds/ minutes it took for 25mm of water to soak in.
Repeat Infiltration Test:
Using the same pipe, perform Steps 3 and 4 with a second and then third 25mm of water. Record the number of minutes elapsed for the second infiltration measurement.
All of the tests should be conducted consecutively.
If you conduct multiple tests and they produce the same result, this result is most likely an accurate estimate of the saturated infiltration rate.
If the soil is saturated, the infiltration test will not work, so wait a few more days for the soil to dry out.
If the soil surface is uneven inside the ring, count the time until half of the surface is exposed and just glistening:
The moisture content of the soil will affect the rate of infiltration; therefore, two or three infiltration tests are usually performed (if soil is dry). The first 25mm of water wets the soil, and the second 25mm gives a better estimate of the infiltration rate of the soil.