I recently started digging with a 16″ (40 cm) spade. I play in the soil a lot, but this tool has expanded my horizons. (Please keep reading even if that soil pun made you roll your eyes). It’s a very simple tool, but it can reveal a lot about soil quality, root growth, and how cover crops can alleviate some of the problems we create for our soil through tillage and traffic. Researchers in Europe have developed a visual evaluation of soil structure to help farmers evaluate soil quality using just a spade.
Digging in to see soil compaction
This spade allows me to dig in below the plow pan (if there is one, which there usually is), and pull up a whole chunk of soil. There are a lot of things happening in soil that are not visible to the naked eye, but there are also some highly visible phenomena:
Plow pans can persist in soils for years, and even though a surface soil may seem light and fluffy, the subsoil can present an obstacle to root growth, water infiltration, and gas exchange. Not all plow layers are root restrictive, but repeated tillage and traffic tends to compact the subsoil more and more. Researchers in Finland used medical technology (CT scans) to show just how enduring subsoil compaction is, and their technology (a little more advanced than the spade…) shows much more than we can see with our eyes.
Soil compaction occurs not just as a result of tillage, but from other machinery traffic, animal traffic, and even human foot traffic.
Reconnecting subsoil and surface soil with cover crops
Some, but not all, cover crops can reconnect surface and subsoil and counter soil compaction by creating deep root channels, or “biodrilling.” This can increase infiltration and gas exchange, and provide paths of less resistance for subsequent cash crop roots to access soil nutrients and moisture.
Forage radish is particularly good at biodrilling and creating vertical fractures in the soil. Generally, larger taprooted dicotyledonous crops like radish, rape, and lupine are much better at getting though compacted soil than monocotyledonous crops like grasses. In fact, grass roots tend to be very limited at getting through dense soil. Cover crop roots are incredibly laborious to study in a controlled way, but they’re easy to observe with a tool like a spade. This picture may tell the story, but there’s a lot of work invested in the data to back the story up!
Biodrilling is sort of a biological version of subsoiling, but it is a distinctly different mechanism than pulling a subsoiler through the field. One of the differences between subsoiling with steel and subsoiling with a cover crop like radish is the optimal soil moisture required. Subsoiling needs to be done with minimal soil moisture or it can create more structural damage than it relieves. To facilitate biodrilling, however, roots need adequate soil moisture. In fact, one of the ways in which roots penetrate through soil is by shrinking and swelling with water and pushing their way through. Fall is often a good time for cover crop root growth and biodrilling as there is generally adequate soil moisture.
Interest in cover crop biodrilling has been tremendous because it presents a much less energy-intensive way of alleviating soil compaction than subsoiling, and cover crops provide other benefits at the same time! With radish, however, people frequently think that it isn’t doing its job because when they pull the big fat root out, it only goes 6″ into the soil, with another 6″ sticking out above the soil surface. This is a common misconception because the real “business” part of the taproot gets broken off when it is pulled out of the ground. Beyond what can be seen when the plant is pulled out, the small taproot frequently grows well over 4′ and often as deep as 6′ into the soil. This is far below the depth of any subsoiler.
Seeing and measuring the effects of biodrilling cover crops
To measure the impacts of a biodrilling cover crop on soil compaction, a penetrometer (the commonly used tool to measure resistance) can be misleading because root channels are random throughout the soil. We, as humans, may not be very good at finding the channels (areas of less resistance), but cash crop roots are.
Combining cover crop benefits
Having studied radish and other Brassicas extensively in our lab, we run the risk of sounding biased against grasses and other cover crops, even when we don’t intent to, so it’s important to point out that to get the most out of cover crops, the best thing to do might be to plant a mixture of cover crops. This can improve soil quality in more than one way by adding a diverse range of functionality. That said, if soil compaction is one of your main concerns, it’s probably a good idea to include radish or rape in your mix.
Like weed suppression, seeding date is very important for biodrilling by radish. It’s getting to be a bit too late now to get the full benefit, but there’s always next year.
For more on Brassica roots, biodrilling, and soil compaction generally:
Williams, S.M., and R.R. Weil. 2004. Crop Cover Root Channels May Alleviate Soil Compaction Effects on Soybean Crop. Soil Sci. Soc. Am. J. 68:1403-1409.
Chen, G., and R.R. Weil. 2010. Penetration of cover crop roots through compacted soils. Plant and Soil 10.1007/s11104-009-0223-7.
Ray Weil discusses cover crops and soil compaction at a field day: