Sometimes it’s possible to pull up mature corn plants with little or no effort. In this situation the majority of roots are in the top three inches. Autumn soil testing – so as to maximize the efficiency of fertilizer inputs – often reveals soil limitations as the probe hits compacted layers. Soil compaction adversely impacts root depth and water and nutrient availability. Bigger farms and bigger tractors combine with haste to become a recipe for compaction.
We can use a shovel to “feel” the compaction and to look at roots’ growth patterns. The first layer we see is the surface. Over-tillage, no surface residue and destroyed structure leave surface particles vulnerable to raindrop impact. The larger the raindrops (as in cloudbursts), the greater the vertical speed of the drops (over 20 mph). The force with which drops hit surface varies as the square of the speed (simple physics). 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 and broken soil structure from excessive tillage, the soil surface soon becomes a soup of small particles plugging pores, reducing ability to absorb moisture and nutrients (particularly oxygen).
This happens because the top half-inch of soil is destroyed before the growing crop protects the surface. The most fertile part of the soil goes first, with stones and subsoil left behind – trauma clearly affecting yields. When corn is strip-tilled or no-tilled into winter triticale forage stubble, there is very little of this raindrop-based surface sealing. Stubble dissipates the raindrops’ impact. Abundant hollow stems accompanying the dying roots provide openings to absorb heavy rain, channeling it to the roots of the next crop, indirectly benefitting earthworms. The same happens with fall-killed sods that are no-tilled in spring. The next layer down can be “felt” with a shovel or soil auger. That soil layer slows the shovel or auger, not stopping them – but it does stop roots. This blockage is especially prevalent on fields that were chiseled and disked.
According to Certified Crop Advisor Tom Kilcer, we can “feel” the plowpan at three to four inches down: “This measure is one-quarter the diameter of the disk. Also, tandem or offset disks move large particles to the surface and sift the finer down to the bottom of the disk layer. Adding more insult to injury, the soil is often wetter as we go deeper and the disk’s action smears a thin, root-limiting layer at the bottom of the disk. That is why in many corn fields the stalk pulls out of the ground at three inches. There were few if any roots deeper than that. Many of the corn roots are actually growing horizontal and are flattened and distorted.”
He stressed that a huge amount of plants’ inputs, destined for photosynthesis and yield, are diverted to the task of trying to force roots through compacted soil. The result is that the corn is growing on one-third the volume that it could have and so it needs even higher fertility to optimize yields. Corn culture may be limited to the top three inches of soil.
Tillage often hides this top layer destruction. Many times, primary tillage has broken up large blocks of soil, rendering them down into brick- and softball-sized lumps, commonly smooth and shiny like pottery.
Kilcer writes, “Finally, we get to why we were digging in the field in the first place – rooting depth. Many a field we put the shovel in and thought we hit a stone in multiple spots that stopped at the same seven- to eight-inch depth. There was no stone, but a roadbed of compacted soil that completely limits rooting depth. The bottom of the chisel plow or the bottom of the moldboard plow leave a compacted deeper layer. I clearly saw this in a chisel plow tillage study on silty clay soil. Heavy rains in August had produced a perched water table on top of this layer and drowned/killed all the corn roots below three inches. (Ironically,) the crop looked great from the surface.”
He pointed out that in many cases, deep compaction injury was multiplied by spreading manure with a spreader that has too few axles for the load and too high a tire pressure to effectively support the weight (eight tons/axle; 15 PSI/tire). Excess tire pressure causes surface compaction; excess axle load causes deep compaction. Drag hose injectors have taken off a lot of the compaction weight, but then operating when the soil is not dry will put it right back in again, even with injectors.
A common misconception among many farmers is that frost counteracts compaction. It does not! Nor does a perennial sod crop. Kilcer mentioned a 15-year alfalfa timothy hay field that clearly showed compaction from the moldboard when it was originally plowed for seeding. The individual plow share marks could be seen, showing that no roots went below seven inches. This was plowed with a smaller tractor when the soil was still very supple at the plow shear layer. The worst were in soils the farmers thought would not compact. Kilcer has seen these root limiting layers in sand, gravel, silt and clay!
A few months ago, I discussed compaction in a column. I examined how higher tire pressure compacts soil on North Carolina’s Outer Banks. In the involved experiment, a four-wheel drive pickup truck with 60 PSI tires gets badly stuck in a gently sloping sand bank, unable to even back away from the mess. The operator then reduces tire pressure to 15 PSI. Then the truck easily backed out of the ruts, effortlessly negotiating the sandy slope. The truck’s tire track was 40% wider with the 15 PSI tires than with the 60 PSI tires. The operator explained that the much greater contact area between tire and sand markedly lowered soil compaction as well as how far the wheels burrowed in.