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Any measure taken to improve a field, be it plowing, fertilization, crop rotation, etc., is employed with the goal of creating a healthy plot that will provide a good yield. But if the soil structure loses its consistency, all of these other measures will be for naught.
Compacted topsoil seriously limits the potential development of plants. The prevailing lack of air is already enough to inhibit their development. The active root area is limited to the portions of the topsoil that are still friable. Plants can no longer utilize any sources of nutrients that lie within the compacted area of the topsoil, and increasing applications of fertilizer are necessary to satisfy their nutritional needs. Most important of all, however, is that the plant is cut off from the water reserves in the subsoil by the emerging barrier layer and left vulnerable to summer droughts. Additionally, each heavy rainfall causes a buildup of water above the compacted topsoil, which causes the topsoil to become waterlogged. The plants thus alternate between too wet and too dry, and this imbalance in their water supply weakens their natural resistances so much that they become especially sensitive to disease or pests.
How strongly a plant reacts to these structural disturbances can be determined from its roots. The plant speaks! Its roots are a reflection of the soil and reveal any problems caused by it. This is important to the practical farmer because the plant roots provide clear evidence of any missteps made during tilling and of the condition of the soil. With cereal crops, surface compaction and depth primarily determine the form of the root network. Friable soil generally leads to a regular system of radicles, crown roots and adventitious roots. The roots can then spread over larger areas, forming a clump-shaped mass. If the soil starts out friable and then later becomes compacted on the surface, then the radicles and crown roots will spread normally, though the adventitious roots will push steeply downward. If the plant’s seedbed is compacted from the start, then all of the roots will extend downward vertically. For cereal crops, the way in which the roots form is very significant. While outward expanding roots also permeate the area between the rows and can make use of that soil’s nutrient stores, steep vertical root systems cannot reach these areas. This leads to both wasted fertilizer and to an intensification of the compaction of the soil, since a considerable portion of the topsoil enters a new growing period without bacteria nutrients, jeopardizing the biological tillage and the formation of new humus.
How do plants with taproot systems react to structural issues?
A particularly demonstrative example is the root system of the sugar beet because both its longitudinal growth and its secondary growth adjust to the structure of the topsoil. Even by the time the sugar beets reach a sugar refinery, they can still reveal the conditions of the supplier’s field. Figure 18 shows the various ways that the sugar beet can develop.
- In undisturbed topsoil, a straight taproot with numerous lateral roots develops.
- If the structure of the topsoil remains undisturbed, then the secondary growth will lead to the formation of a spindleshaped beet body from whose root channels many small lateral roots will emerge in all directions. The area around the tip is especially densely filled with rootlets. This optimal form is a sign of a friable beet field.
- If a thick compacted layer is present in the topsoil, secondary growth will be restricted in that area (shallow beet). The point where a lateral root becomes stronger is usually a very reliable indicator of the depth the blockage begins at.
- If there is a very serious lack of air in the compacted topsoil, then the taproot will eventually die. Its functions will be taken over by some of the lateral roots with which the beet sits on the compacted area. This sort of “rooted beet” can also arise from an initially entirely normal beet setup that then runs into compaction as it develops.
- This young beet root has already been disrupted by compacted topsoil in its early stages; it branches at the point where the obstruction begins.
- Later on, a “rooted” beet will develop from this disrupted root system. The other plants with taproot systems (rapeseed, horse gram, lupine, etc.) react similarly to the structural conditions in the soil and images g, h, and i depict the forms that can occur.
- In healthy soil an elongated taproot forms with lateral roots that gradually taper off as it goes deeper.
- In cases of moderate compaction of the lower topsoil the taproot still remains intact. However, the lateral roots quickly die off and one or two strengthened lateral roots indicate the depth at which the obstruction begins.
- In cases of serious compaction of the lower topsoil the taproot dies and the plant sits on the obstruction with a few strengthened lateral roots.
The images of cross-sections of exposed rapeseed and potato topsoil (Figures 19 and 20) provide further substantiation of the schematic drawings.
In its juvenile stage, the rapeseed root was still able to push downward unhindered, but the topsoil compaction, which happened later, killed off the lateral roots.
The root system of the potato is a very instructive example. The roots and bulbs are limited to the friable upper topsoil. The compacted area of the topsoil does not help in production, instead hindering it by not allowing any water flow.
Any farmer can observe all of these things in his own fields and learn a great deal from them. All you need is a spade to take samples of the soil here and there. A spade turns a farmer into the soil’s personal doctor, since what you can see and learn from a simple spadeful of soil is often more than what you can achieve through complex laboratory experiments. You can see a cross-section of the topsoil and the roots growing within it—and the roots reflect the soil structure.
Almost every farmer finds something worthwhile in this sort of spade sample from his fields, and many have claimed that using this method was the first time they were really able to understand their soil rather than simply being reliant on fate. Figure 21 shows how seriously breakdowns in soil structure must be taken, as it demonstrates the relationship between how friable the soil is and the yield produced at harvest time.
About the Author:
Prof. Dr. Franz Sekera (1899–1955) joined the University of Agricultural Sciences in Vienna’s newly established Institute of Soil Biology in 1939, where he taught plant nutrition and was appointed full professor in 1942. He spent his career focusing on biological problems of soil fertility and tilth. His legacy for the science and practice of agriculture is this widely distributed book, Healthy Soils, Sick Soils (originally published as Gesund und Kranken Boden). His wife, researcher Margareth Sekera (Dipl. Ing.), revised and expanded later editions of this classic work after the death of her husband. She was active in the International Society of Soil Science.