By Margareth Sekera
Structural deterioration takes place to a greater or lesser degree in almost every field, and its consequences include soil that is too silty or too compacted. To make the steps that take place in this situation clear, the sequence of images in Figure 11 below depict the inflow and draining of the water and the resulting changes sustained by the soil.
The images illustrate the water permeating the soil and the increasing saturation that results. The image in the upper left clearly shows the porous, spongelike structure of crumbled soil. The coarse pores are still filled with air, but the crumbs themselves are already saturated with water. Scattered air bubbles are contained within the interiors of the crumbs.
The image in the upper right shows water flowing out of the saturated crumbs and coating them with a thin film of water. Since the transfer of water from the fine pores into the coarse pores produces volatile changes in its surface tension, this process also causes a physical attack against the soil’s structure. While flowing out, the water tears off pieces of soil from the crumbs, beginning the process of microerosion.
In the image in the lower left, the water saturation has proceeded further. Instead of interconnected air channels, there are now large pockets of air inside of the coarse pores. The fine material that has collected now flows through the soil in the films of water.
The image in the lower right depicts a state of full water saturation is depicted in. Other than some small embedded air bubbles, the entire volume of the cavities is filled with water. In the upper area, the structure has already broken down. Comparing the four images provides a simple demonstration of the destructive effects of water.
Even more blatant structural changes are brought on by the outflow of the water. Air begins to flow into the coarse pores once again. The air bubbles that are embedded in the channels move closer to each other. This takes place via pulsating, fitful movements. It causes some of the channels to widen, while other parts of the cavities fill gradually.
In the remaining channels the water flows in a film-like manner along the sides and carries eroded material along with it. The fitful forward motion of the water-air menisci especially impacts areas where eroded material is stored. The movement of the water in the cavities is not consistent; you can frequently observe water moving along with eroded material on one side while causing the soil to become silty on the other. The effects are no different than when a flowing river carries away material along one of its bends only to deposit it further along its course.
In its final stage, the water is so thoroughly drained that only the walls of the large cavities are still covered with a film and no further water movement is easily perceptible, though we can assume that some is still taking place. The dense compaction of the soil can be clearly seen at this point.
Unfortunately, it isn’t possible to make out the motion of the water and the resultant transfer of the soil material from these photomicrographs. Since the flowing water film transports fine eroded material, the direction of the flow is visible, however it frequently switches, especially when water escapes. It’s common for one part of a channel to be coated with flowing water while the other part is covered by apparently motionless water. The violent movement, especially while water is flowing out, often breaks off whole chunks of soil and washes them away.
It’s not unusual for this process to also cause old channels to fill up and new ones to form. If you can visualize the idea of this happening several times over the course of a growing season, then you’ll understand why the soil can be so compacted at harvest time. As this compaction increases over time, however, the water’s flow rate will become more sluggish, mitigating the microerosive effects.
The less stable the crumb structure, the more extreme these effects will be. The destructive power of the water decreases as the stability of the crumbs increase through biological tillage. In freshly worked or fallow soil, water can attack much more furiously than in soil in which living organisms have formed a solid structure through biological tillage and the formation of a humus lining.
The dangers of erosion
Any slope, even the smallest depression, carries the risk of soil erosion due to downward-flowing water. We differentiate between two different forms of erosion: sheet erosion and furrow erosion. The latter takes place in furrows in the soil, where water collects and carves deep furrows as it flows, making the destructive effect of water obvious to anyone.
Less conspicuous but more common and significant is the damage caused by sheet erosion, in which water washes away fine soil from the surface and deposits it into depressions in the earth. In flat or gently rolling terrain, this causes the formation of the well-known “loam crests,” which always cause problems with working the land and are responsible for erratic crop growth. Sheet erosion is especially perceptible when the sterile subsoil becomes visible. It’s common to find different soil compositions in a small area without ever considering that sneaking sheet erosion is taking place, a constant potential threat to a farmer’s work.
A more in-depth look at the problem tells us the following: the primary cause of erosion is absolutely not the downward-flowing water, but rather the fact that the field is not absorbing the water quickly enough. Friable soil with a structure that hasn’t been broken down by rain and has a gradual transition between the topsoil and the subsoil will certainly absorb rain faster than topsoil that breaks down in the rain and accumulates such a backlog of water that it can only flow away via the surface. The subsoil can absorb water many times more quickly if there’s no layer of compacted topsoil acting as a barrier. This is thus the primary cause of erosion.
It begins with the “microerosion” in the soil, which causes the individual crumbs to lose their water resistance and to dissolve in the rain. “Macroerosion” first sets in when water can no longer be absorbed quickly enough or properly distributed due to structural breakdown. The more fundamental cause of soil erosion is therefore a lack of friability in the soil, and both can be considered maladies of a cultivated field.
Figure 12 below shows a beet plot that has been affected by sheet erosion. The beets are fully exposed and the soil is so crusted that it will have to be plowed over and tilled anew.
With this in mind it’s possible to take a symptomatic approach to fighting erosion (i.e., to remove the appearance of erosion by minimizing how much water drains off of the slope). Plowing across the slope, making use of grass balks, and building terraces are all strategies that can help as they restrict the flow of water and in doing so help ensure that the fine earth is redeposited.
Instead of these methods of defensive warfare against erosion, however, it seems more promising to attack the root of the issue and to eliminate what’s causing the damage — in other words, to take an offensive approach. This can be accomplished by increasing the stability of the tilled soil and above all by making sure that transfer between the topsoil and the subsoil remains possible so that the field can quickly absorb water.
Due to their heavy rains, Americans must make use of every available method of erosion resistance, fighting the erosion both offensively and defensively. In Europe, a prevention-focused approach is possible, and it seems preferable to not just combat the visible effects of erosion but to eliminate the causes as well. Any regimen of soil care must also encompass this task, and with its help it’s possible to master soil erosion.
Source: Healthy Soils, Sick Soils