Why Do Radishes Push Out of the Ground and Other Mysteries of Soil Compaction by Dale Strickler, Green Cover Seed Agronomist
We get this question a lot, or some version of it, such as the comment “My radish roots hit the hardpan and instead of going through it they pushed out of the ground.” Why does this happen?
The explanation lies in understanding how compaction limits root growth and understanding why radish so often works so well to alleviate compaction. The limiting factor to root penetration is not so much the hardness of soil, but the oxygen content of soil. Roots must have at least 10% oxygen in order to grow. Since oxygen comes from the soil surface and diffuses down, the oxygen content is highest near the surface and decreases with increasing distance from the surface through the pore spaces in the soil. Any layer of soil with limited pore space will act as a barrier to movement of soil oxygen, and thus as a barrier to root growth.
Additionally, it is important to understand the seasonal fluctuations in soil oxygen content. When soil temperatures are high, most of the oxygen in the soil is utilized by soil microbes, not by plant roots. This makes most summer growing annual crops (like corn and soybeans) relatively shallow rooted, since they are fighting the microbes for oxygen. This also means corn and soybeans will hardly ever be able to punch through a plowpan. But when the soil temperatures drop below 50 degrees F in the fall, all of a sudden, the microbes become inactive and the soil becomes relatively oxygenated. But many cool-season plants are able to grow their roots in fall until the soil freezes. This is why it is so important to have cool-season plants in the rotation; it is not that their roots are “stronger” than those of corn or beans, it is that they grow roots when there is more oxygen in the soil.
The observation that radishes often seem to have a lot of their roots sticking out of the ground during much of their growth period often leaves people to believe that they didn’t penetrate the plowpan. And to corroborate this idea, often when the radishes are dug up, sure enough, the roots often stop at the plowpan, thus the idea that the radish roots were “pushed up” when they hit the plowpan. However, when you understand the oxygen dynamic, this becomes less of a concern. When soil temperatures are still warm, the radish roots will stop at a plowpan, because there is too little oxygen yet for it to grow deeper. But good growing conditions are making the plant produce a lot of glucose, and it has to go somewhere, so the roots begin to elevate aboveground. That isn’t all bad, this energy can be used by either grazing livestock or soil microbes (this is important, remember this for later) or, via manure, for both. However, if you watch the radish roots over time, as the soil cools in the fall, once that soil thermometer drops below 50 degrees F in fall those radish roots will stop growing aboveground and start going down. In In the fall of 2018, however, sometimes that did not happen, even after the soil cooled down, because the soil pores were full of water instead of air, thus there was no oxygen in or below the plowpan
But when you begin to wrap your mind around the fact that a plowpan is not a physical barrier, but an oxygen barrier, you begin to see how compaction can be alleviated. The key is to produce open pore spaces in the soil as deeply as you can. Radish roots can do that, at least temporarily, through the physical action of their roots. You can see the big holes left behind when spring arrives, and the roots have rotted away. A subsoiler can also do that temporarily. The problem with a subsoiler, though, is that a subsoiler trip causes as much breakdown of soil organic matter as a pass with a moldboard plow. Also, the aggregates made by a subsoiler trip are not stable, they will break apart rapidly as soon as the soil is wetted, or another the soil is tilled or driven on. Then the soil becomes even more compact than it was before, because unstable soil aggregates are easily compressed into flat platy structures that impede oxygen movement. Organic matter makes aggregates more stable and resistant to compaction, and subsoiling reduces organic matter, hence the folly of subsoiling. It is one step forward, three steps back in the battle against compaction. The real key to fighting compaction, then is to create stable soil aggregates, and this process is performed by soil microbes, particularly soil fungi, and even more particularly by mycorrhizal fungi. And these organisms need fed. Some of these organisms, like saprophytic fungi and bacteria, live off decaying crop residue at the soil surface. Others, called rhizosphere organism, live off root exudates. Others, like mycorrhizal fungi, actually live partially inside plant roots. Mycorrhizal fungi produce the most powerful soil aggregating agent known, called glomalin.
You may already be putting two plus two together and seeing where I am leading with this. Radishes and other deep taprooted cool-season plants may be the first step in fighting compaction by boring big holes through the plowpan, but it is equally important to create stable aggregates by feeding the microbes a continual supply of energy, with a layer of decaying crop residue overlying a living root system for as many days of the year as possible. Because so many acres of cropland have been fallowed for much of the year for over a hundred years, many of the valuable rhizosphere organisms and mycorrhizal fungi are very low in abundance. It may be necessary to reintroduce mycorrhizal fungi by inoculating seed, and it will be necessary to provide a continual supply of living roots to maintain those fungi and the rhizosphere organisms. In addition, if summer grazing crops are included in the crop rotation, the manure can feed dung beetles that can bore holes through plowpans. One summer grazing species, sorghum-sudangrass, is of particular benefit to reducing compaction, not because it has a deep taproot, but because it produces such a huge volume of high energy root exudates. The best compaction alleviation will come from a long duration of growth of a diverse mix of plants from different plant families with diverse root structures that produce root exudates with different nutritional benefits to microbes, and to retain enough of the residue of those plants on the soil surface to provide complete soil coverage. Leave the subsoiler in the shed, or better yet, on the implement dealers lot.