Why is it 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 into the topsoil helps us “feel” compaction plus examine stunted root growth patterns.
The first layer encountered is the surface, where we notice over-tillage, no surface residue and destroyed 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 as the square of the speed: droplets falling at 20 mph cause four times the impact force as droplets hitting at 10 mph. These forces pulverize soil surfaces. With no protective cover – plus broken soil structure from excess tillage – soil surface becomes a slurry of small particles plugging pores, reducing soils’ ability to absorb moisture, oxygen and carbon dioxide.
Here, 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 stress 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, 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 fall-killed sods that are no-tilled in spring.
The next layer down can be felt by probing with a shovel or soil auger. That layer slows these mechanical instruments (but doesn’t stop them) but it does stop roots. This blockage is particularly common on fields that were chiseled and/or disked.
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.
Quoting from the greenly.earth website (translated from French), “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. 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 practices causing soil compaction undermine its natural ability to function as a carbon sink. Huge amount of crop 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 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. 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 in August help create a man-made water table on top of the plow-pan, drowning all the corn roots below three inches. Ironically, the crop appears unfazed at the surface.
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 per 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 again, even with injectors. Many farmers still believe that frost counteracts compaction. 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.
Crop Comments Correction
We note the following correction to the close of the “Crop Comments” column printed in the Sept. 4 edition of Country Folks:
Plant breeders at the Land Institute in Kansas have selectively bred Thinopyrum intermedium (intermediate wheatgrass) to develop a variety called Kernza®. The other perennial grain species mentioned was Sorghum halepense, whose developing traits remain a work in progress in the skillful hands of plant geneticists – also at the Land Institute.
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