Soil testing: How to measure infiltration rate effectively

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When the rains come, have your tubes at the ready, because a day or two after is the perfect time to test your infiltration rate. Measuring the infiltration rate in Winter or early Spring can be challenging because the ground is saturated with seasonal rainfall, so it can take quite a long time! The main thing to consider when doing this measurement is you want to take the reading at about the same time/and in approx the same conditions each year to be able to compare year on year and see how it’s changing.

Awareness of how well water infiltrates down into your soil is at the core of knowing your soil health and structure. A good infiltration rate indicates that the top soil has a ‘crumb structure’ and it is well aggregated. Essentially this means that each clump of soil is stuck together with glues and slimes from soil organisms and they are not broken down by water. Therefore the clumps (or aggregates) retain their structure when the water flows around them, also allowing water to quickly flow down into the soil depths. At the same time the clumps provide lots of nooks and crannies for droplets of the water to be stored in. So the water percolates easily, and some of it is stored along the way. This is what we want!

The infiltration rate is the speed at which water enters the soil, and is measured by monitoring the time it takes for a set amount of water to ‘infiltrate’ into the ground. Read the details in our infiltration rate guide here. Understanding how land works with water is highly beneficial for a farmer or any type of grower. It gives an idea of how much rainfall is soaking deep into the ground and how much could be running off and taking the soil with it. Soil washing off the land is like throwing money out of the window, our prime resource going down the drain. The image below of the UK clearly shows the seas brown with soil runoff after a heavy period of rain. Leaking away resources like this does not contribute to a profitable farming plan or an ecological farming system.

The extent of soil erosion in the UK is visible from space. Credit: NEODAAS/University of Dundee

To get a good sample of infiltration rate, you need to measure it at the same time each year, for each field. Do at least two tests per field, maybe one in the ‘best part’ and one in the ‘worst part’. Compare rates between your different fields. Why is the infiltration rate much quicker in one field than the next? Do you manage things differently in one field than the other? Where your infiltration rate is slower, could you look to the Soil Health Principles (cover soil, minimise disturbance, diversity in rotation or plantings, minimal chemical usage, living root in the ground as often as possible) to guide you in a new direction for your management strategy?

The equipment for this test is key to getting a reliable result, finding the right tube is essential! If it’s too narrow it will compact the soil inside it as you drive it into the ground and heavily impact your result (unfortunately we have found that baked bean tins don’t work). When the soil is compacted the infiltration rate will be a lot slower, as it’s harder for water to enter the ground. This will not give you a true reading for your infiltration rate. To avoid this happening we recommend finding some 150 mm diameter (6 inch) tubing or pipe; a flue pipe can work quite well. Cut it to about 15cm depth and make one of the circular edges sharp, so it’s easily pushed into the ground. Read more here for the full instructions.

There are impressive stats to show that even when we think the ground is saturated, there could be even more capacity for the ground to hold water. Healthy soil can hold up to twenty times its own weight in water and increasing soil organic matter by 1% increases the soil’s water holding capacity by 3.7%. So it’s worthwhile putting some elbow grease into improving your soil crumb structure and soil organic matter, because in times of heavy rain you’ll reduce flooding and soil erosion and in times of drought there will be more water available to your thirsty crops. Keeping an eye on your infiltration rate helps to understand how well you’re doing at this. Good luck!

For more information on how to do this test head over to our soil testing page. And if you want to easily record your infiltration rates and other soil tests as you go then our app is your perfect helping hand you can buy the app here or get in touch with us.