What constitutes desirable soil structure?

My previous blog discussed changes in Western Canadian cropping systems over the last 40 year period and the benefits that have accrued to soil organic matter, soil structure and water infiltration because of those changes. The question not explored thus far is, why have those cropping system changes had a beneficial impact on soil structure? The answer lies largely in the impact of these system changes to the biological processes that influence soil structure development. The other key issue is that tillage, which is so deleterious to soil structure, has been greatly reduced with the advent of  min-till /z-till systems.

Before discussing soil structure development further it is appropriate to describe what constitutes good soil structure. Fundamentally it involves the development of a relatively stable soil aggregate system that produces many crack/pores/fissures etc which support/enhance the normal vertical movement associated with water infiltration, gas exchange and root growth. In contrast, soil structures with a horizontal orientation produce barriers to the normal vertical processes and hence are considered undesirable.

Figure 1. A soil profile showing very good soil structure.

Figure 2. A soil profile showing undesirable structure in the second horizon.

Plant root systems, particularly dense fibrous root systems, produce beneficial soil structure.  Roots have the ability to enmesh or knit together soil particles into larger desirable soil aggregates. In addition the normal metabolic processes of roots involve the release of various slimes, gums and mucilages. These organic compounds  act as nature's cement, to further stabilize aggregate formation. Development of stable aggregate structure is desirable because well-aggregated soil can more rigorously resist the destructive powers of wind and water erosion. The forces of tillage however are likely to destroy even relatively stable aggregates.

Crops and cropping systems also enhance water infiltration because of the influence of old root channels associated with previous crop growth. These channels provide a rapid pathways for water infiltration
(see figure 3); however, tillage will destroy these channels or at least destroy its continuity. Pore continuity refers to a pore system connectedness. An optimum pore system will consist of cracks, pores, or root channels that support a continuous (uninterrupted) flow of water from the soil surface to subsurface. Intensive tillage tends to disrupt the entire pore system to the depth of tillage. The subsurface pore system is therefore no longer connected with the surface, reducing water infiltration to greater depth.

Figure 3. Microscopic view of an old root channel.                                                                                   

Another important component of water infiltration is preferential flow. Preferential flow describes water movement in soil via preferred pathways. In essence, soil water moves via the path of least resistance rather than moving  through the entire soil matrix. In this context I recognize two types of preferential flow, movement though macro pores and root channels.  This concept is illustrated in an experiment by Dr. Bing Si, College of Agriculture and Bioresources, University of Saskatchewan. Dr Si applied water, amended with a greenish dye, to the surface of a heavy textured soil to assess water infiltration. The irrigated area of the soil profile was later exposed with the use of a small backhoe to examine water movement. The path of the dye clearly shows the importance of the macro pore systems, particularly at depth (figure 5), in enhancing the naturally slow infiltration associated with clay textured soil.

Figure 4. Moisture movement into soil at surface.

Figure 5. Moisture movement in the 9 to 14 inch segment.                                 

In Figure 4, the dye pattern reveals a more general infiltration process through the bulk surface soil with some indication of preferential flow. Deeper in the profile (figure 5) the influence of preferential flow is more clear, water infiltration is more rapid and deeper than expected. Water and associated dye has taken the path of least resistance and has not infiltrated the entire soil profile. This concept is important when considering possible contaminant movement into a heavy textured soil. Extensive crack development supports preferential flow and thus has the potential to move a contaminate more swiftly and to a greater depth than expected based on its natural slow infiltration properties.

In summary, a reduction in number and intensity of tillage operations combined with a continuous cropping system, supports desirable soil structure development and superior water infiltration. Increased organic matter and reduced mechanical disturbance allows the development of stable soil aggregates which produce a vertical pore system that makes possible more rapid entry of water, even in fine textured soils. This improved infiltration is the result of stable soil structure, production and maintenance of root channels, enhanced pore continuity and the potential for preferential flow.