In a column a couple months ago, I tackled the subject of soil compaction, mostly discussing what goes wrong with over-inflated tractor tires. I also dealt with how soils suffer when they’re too wet, often traumatized by heavy equipment.
Soil compaction occurs when soil particles are pressed together, reducing pore space between them. Heavily compacted soils contain few large pores and less total pore volume, thus a greater density. Compacted soils have a reduced rate of both water infiltration and drainage. This happens because large pores more effectively move water downward through the soil than do smaller pores. Finally, while soil compaction increases soil strength, a compacted soil also means roots must exert greater force to penetrate this denser medium.
A type of compaction that I did not investigate back then was the kind associated with rainfall of varying intensities striking unprotected soils. University of Wisconsin meteorologists addressed the physics pertaining to rainfall. Here’s what they said about raindrops plummeting earthward: A falling object experiences a frictional drag that counters the downward force of gravity. When the gravity and frictional drag are balanced, what occurs is called equilibrium fall speed (commonly known as terminal velocity). Terminal velocity depends on the size, shape and mass of the raindrop and the density of the air. Raindrops, by definition, are at least 0.02 inches in diameter, but not bigger than about 0.25 inch in diameter; larger than that, that mass of moisture breaks apart into smaller drops because of air resistance.
Precipitation drops smaller than 0.02 inches diameter are called drizzle. Even tinier moisture masses are called cloud droplets (about 0.0004 inch in diameter) and fall at a mere approximate 100 feet per hour. Only a very gentle upward movement of air is required to keep them afloat. Raindrops are larger than droplets, coming in different sizes, with the smaller raindrops traveling about 2 mph. A large raindrop, about the size of a house fly, has a terminal velocity of about 20 mph. That kind of speed can cause compaction and erosion of the soil by force of impact.
I believe my discussion of raindrop velocities dovetails well with Tom Kilcer’s February 2023 Crop Soils News, which he titled “Caring for Your Soil’s ‘Skin.’” Kilcer is a retired Extension agronomist and Certified Crop Advisor who managed Cornell’s Valatie Research Farm for over 30 years, during and after his Extension service. I’ll quote from Kilcer’s lead-in paragraph in that issue: “The interface between the soil surface and the atmosphere above it is a critical juncture. Both vital air and water must cross this boundary to supply the root system beneath the soil. Numerous measurements have indicated that 60% of the roots are within four inches of this zone. Raindrops strike this interface with the force of little bombs, exploding the soil surface into tiny particles that then plug the porosity of the interface and stop air and water from crossing. If this wasn’t bad enough, most tillage systems are designed to pulverize the soil surface to kill emerged weeds and to provide a fine seed/soil contact for rapid germination. This makes the soil skin more susceptible to sealing of the surface pores.”
He continued that oxygen at the root surface is critical to enable roots to use plant energy to grow and absorb nutrients. Air moves through the pores in the soil unless they are plugged. If the water is sealed away on top of the topsoil, it cannot be used by the crop to produce yields. Quoting Kilcer again: “Adding injury to insult, the water that cannot penetrate at the rate it is being dropped from the sky will then follow down slope taking the small, most fertile and productive soil particles loosened by the raindrops with it, leaving behind the stones and compacted lumps and subsoil.”
Perhaps the most treacherous, although subtle, crop input loss is that of surface-applied manure. It is easily kidnapped, for want of a better term, in water as floating lumps in bedding pack, or as fine particles in liquid-applied manure. This removes expensive fertilizer that the growers bought when they grew the crops; this loss shorts the crop where the manure was originally applied. The final insult is that the water courses being drained now become polluted with nutrients, intensifying algae blooms while deteriorating water quality. What comes to mind here is the infamous Gulf of Mexico Dead Zone.
Steps taken to protect the soil “skin” also protect one of the most important modifiers of the soil surface – Lumbricus terrestris, commonly called earthworms (or nightcrawlers), which drill air/water holes from the surface down into the soil. Nightcrawlers are one of the most important creatures: they routinely leave holes a quarter of an inch in diameter. According to Kilcer, “We have measured them to continuously go three to four feet deep in New York climate zones. Alfalfa roots were following the holes down in the soil. Not only do they [worms] leave holes, their castings (manure) leave a very stable soil structure on the surface, protecting the hole and keeping it open. In one of our trials of corn planted no-till on fall-killed sod, in a four-foot by 12-foot area, there was a worm hole every six inches. Profitable farmers increasingly no longer leave their soil bare over the winter. A cover crop or winter forage will provide surface protection in the off season, as long as when it is removed or killed, the stubble residue remains on the surface. Thousands of acres are now protected as farmers learned that they can grow a very profitable triticale winter forage that acts like a cover crop on steroids.”
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