Crop Comments: Favoring carbon sinks over soil compaction
Ever wonder why it is sometimes possible to pull up mature corn plants with very little effort? In such cases, the majority of roots are in the top three inches. Soil testing – to maximize the efficiency of fertilizer inputs – often reveals soil’s physical limitations when probes hit compacted layers. Soil compaction impairs root depth as well as water and nutrient availability.
Bigger farms and bigger tractors write a prescription for compaction. Pushing shovels (or augers) into topsoil helps us “feel” compaction while letting us examine stunted root growth patterns.
The first layer encountered is the surface, where we notice over-tillage, accompanied by no surface residue and often by destroyed soil structure. These issues abandon vulnerable soil particles to ravaging raindrops. The larger the raindrops – like in cloudbursts – the greater their vertical speed, often exceeding 20 mph. The force with which drops hit the surface varies: droplets falling at 20 mph cause four times the impact force as droplets hitting at 10 mph. These forces, generated by undeflected raindrops, pulverize soil surfaces. With no protective cover – plus broken soil structure from excess tillage – the soil surface becomes a slurry of small particles plugging pores, reducing soils’ ability to absorb moisture, oxygen and carbon dioxide.
In this situation, the top half-inch of soil is destroyed before the growing crop can protect its surface. The most fertile part of the soil is lost first, with stones and subsoil left behind; this disruption hurts yields. When corn is strip-tilled or no-tilled into winter forage stubble, there is very little of this raindrop-caused surface sealing, because stubble dissipates raindrop impact. Abundant hollow stems and accompanying dying roots provide openings to absorb heavy rain, channeling it to the roots of the next crop, indirectly benefiting earthworms. The same happens with autumn-killed sods that are no-tilled in spring.
The next layer down can be felt by probing with a shovel or soil auger. (I prefer the latter.) That layer slows these mechanical instruments (but doesn’t stop them) – but it does stop roots. This blockage is particularly common in fields that were chiseled and/or disked. With my soil auger (if not deflected by rocks), I can bore down to the plow-pan and then through it. Lacking any physical trait comparable to an auger, crop roots stall out at the plow-pan.
Soil scientists generally agree that a plow-pan is a subsurface horizon or soil layer having a high bulk density. It also boasts a lower total porosity than the soil directly above or below it as a result of pressure applied by normal tillage operations, such as plows, disks and other tillage implements. Plow-pans may also be called pressure pans, tillage pans or traffic pans. Plow-pans are not cemented by organic matter or chemicals. They are the result of pressure exerted by humans, whereas hard pans occur naturally.
Hands-on examination of these worked-up fields lets us feel or sense the plow-pan three to four inches down, approximately one-quarter of the diameter of the disk. Also, tandem or offset disks move large particles to the surface, sifting the finer particles down to the bottom of the disk layer. To make matters worse, the soil is often wetter deeper down and disk action smears thin, root-limiting layers at the bottom of the disk’s track. Thus, in many corn fields, stalks can easily be pulled out of the ground, having only rooted three inches down. In this situation, many of the corn roots are actually growing horizontally, flattened and distorted – much less adept at nutrient and moisture transfer.
This abnormal man-made restructuring of soil introduces the concept of carbon sinks or, more accurately, the lack thereof. As I’ve described different ways in which soils are abused, I’ve spelled out what carbon sinks aren’t. Now let’s examine what they are, shining a positive light on the concept.
“A carbon sink, often referred to as a carbon pool, is any system that absorbs more carbon than it releases, effectively removing carbon dioxide (CO2) from the atmosphere and storing it in solid or liquid form,” according to greenly.earth/en-us. “This process, known as carbon sequestration, is critical in mitigating the effects of climate change by reducing the amount of CO2 in the atmosphere.”
Any cropping practice causing soil compaction undermines its natural ability to function as a carbon sink. Huge amounts of crop energy inputs – destined for photosynthesis (and thus yield) – are diverted to force roots through compacted soil. Result: The corn is growing on about one-third the soil volume that it could have, requiring higher fertility to optimize yields. Here, as mentioned earlier, corn culture is limited to the top three inches of soil.
Tillage often hides this top layer destruction. Often primary tillage breaks up large blocks of soil, rendering them down into brick- and softball-sized lumps, commonly smooth and shiny like pottery and very low in organic matter. Often when we shove a spade into soil, we think we hit a stone at different spots, stopping this implement at seven- to eight-inch depth (or less). Turns out there was no stone, but a virtual roadbed of compacted soil completely limiting rooting depth. Chisel plow and moldboard bottoms leave compacted deeper layers. Heavy rains help create a man-made water table on top of the plow-pan, drowning all the corn roots below three inches. Ironically, at the surface, the crop appears unfazed.
In many cases deep compaction injury is multiplied by spreading manure with too few axles for its load and too high tire pressure to effectively support the total weight – for example, 8 tons/axle, 15 psi/tire. Excess tire pressure causes surface compaction; excess axle load causes deep compaction. Drag hose injectors take off much of the weight, but operating when the soil is not dry will re-create compaction, even with injectors.
Many farmers still believe that frost counteracts compaction. Unfortunately, they’re wrong, particularly with today’s bigger tractors. Compacted soils seriously shed oxygen (a small problem) and CO2. The latter is a big greenhouse gas problem, downgrading the carbon sink status of the land in question.
by Paris Reidhead
