Lots of talk in the Soil Health world lately about all the benefits of generating more living tissue in the upper portion of the soil profile with cover crops, intercropping of companion-type cropping and more. Within all these conversations, written articles and blogs are we forgetting something? I do think so. As a soil scientist who enjoys digging, discovering, observing for some time now I realize soils do have secrets yet to come to light. How we look at the natural fabric of soils compared to looking at how over the years we have altered soils with tillage, applied heavy forces upon the soils, feeling we have to accomplish a tillage pass or planting or harvesting way before the soil has physically drained? A good number of us know about compacting soils, especially clayey soils, wet soil conditions and the consequences. But do we really?
As sure as we know a day is 24 hours in duration, once we as agriculturists manipulate soils wet and often we change dynamics dramatically. That is not even the half of it folks, roots and root architecture take a back seat in a rough riding Jeep of the WWII vintage, not many like that ride and do not want to go again due to the work holding on or in this case – digging. Jake Mowrer, Assistant Professor at Texas A&M wrote in a blog not too long ago (first of February) that plants and plant roots have to exert extra energy to modify soil conditions to improve their chances of survival and obtaining nutrients and water. He said, “The soil is not a very welcoming environment for plant growth. It does not provide everything a plant needs freely and without reservation,” he went on, “In fact, left as is, the soil probably would not produce very many plants at all. Proof of this is found in the tremendous amount of soil modification plants engage in just to improve their chances of survival.”
A lot of what he wrote makes sense. Draw your attention to soils that are wet like those of locales in Central Iowa, or Northern Ohio near Lake Erie, or the Prairie Pothole region of South Dakota and Western Minnesota where surface drainage and internal drainage are problematic to raising consistent good crops of both crops with fibrous root systems and/or tap rooted crops – soybean and corn. Old methods of tilling, planting, cultivation since the first pioneers rode oxen or walked wagons west have not gone away. Growers of foodstuffs and feed crops still till too much or with inversion implements but expecting better results year after year – Albert Einstein made a comment about that which I will let you remember it. Anyway – Dr. Mowrer says in his blog that plants modify the soil; I rather believe plants and plant roots adapt and develop roots to fit the conditions and exert energy to expand to achieve the normal functions of water attainment and accessing nutrients necessary for photosynthesis and reproduction. Maybe that is semantics but I suggest you read his words for he has good points. Carrying this further to my 40 years of looking at roots, I have had all those opportunities to offer folks a view of the their soil profile by encountering over 1700 root pits and enough dirt in my pockets that have nearly clogged my wife’s washing machine once or twice.
Fig. 1: Diagram of roots, Courtesy: Nature
From the crops we grow around the world there are two dominant root types of root systems, the taproot and then fibrous root system. Mowrer writes in his February blog post that tap rooted crops represent one specific strategy to access water and nutrients held deeper in the soil profile than the fibrous rooted crop strategy. Roots of grassy crops emphasize growth of laterals and secondary laterals closer to the surface to extract nutrients and water. A fibrous rooted grass can acquire even small rain events that fall and survive much quicker than a tap rooted plant. So as an example of adapting; with some grassy vegetation in wet soils they have developed means to keep from drowning in long periods of saturation, for instance cattails, sedges and rushes. In the instance of the wetland tree – the species Salix, willow is interesting. Willow’s preference for water means that many of roots will be growing in waterlogged soils or even directly into water. This poses the problem of how to get oxygen in to the root cells. Many plants respond to water-logging by developing air-spaces within the root cortex (the cortex is the parenchyma tissue between the epidermis and the vascular cylinder) which are continuous with the normal air spaces in the shoots above ground. This allows the aerial shoots to supply the roots with air. This spongy, aerated parenchyma tissue is called aerenchyma. I know, big 5 syllable words. These soft tissues for holding oxygen is vital to survival in wet oxygen starved soils for trees and wetland grass-like plants. For maize, the roots do not like wet feet, in fact I have seen maize roots turn back upward when in wet conditions for a period of time, then they stop growing downward and develop more secondary lateral roots above in the more aerated portion of the soil. Another type of adaptation that works well.
In the cross section to the right in a recent article in Nature (Fig. 1) you can see both micropores and macropores inhabited by roots of primary and secondary lateral roots. The purple color surrounding the depiction of the roots is where the soils are being changed or modified as Dr. Mowrer explains in his blog. In all my observations this is “the fitting” of the plants root system and architecture.
The interactions of plant roots and soil structure are two-way, i.e. there is also an effect of plant roots on soil structure. During soil exploration, roots push through the soil and alter physical, chemical and biological properties in their vicinity, the rhizosphere. These alterations may persist after roots are degraded, leaving behind a dense system of connected biopores. These biopores, now become an integral part of soil structure, in turn feedback on root growth providing pathways of low mechanical resistivity with wall properties reflecting former root activity and in part activity of soil fauna (Lucas et. al, 2019 In Nature). In the upper portions of the soil profile that are more aerated the soil biology (both fauna and flora) interact with the roots and rhizosphere to supply nutrients and water, ward off diseases and parasitic invaders and live in harmony with the life root tissues.
I as well as Dr. Mowrer look at the distribution of the root architecture depends upon how the plant interacts with macro and micro channels, voids and aggregates forming from decomposing tissues and root exudates. All of this affect water movement and retention properties of the soil. The voids (including recent and old root channels) hold oxygen and other gases that sustain microbial and fungal life. These voids, old root channels with carbonaceous material take on water and during the winter months (where soils do freeze up) will freeze and the ice crystals will radiate outward expanding the voids modifying the soil even more. All of this aids in soil tilth** and helps roots and plant growth both annual and perennial crops.
**Soil tilth can be described in some general terms; soil physical, chemical and biological properties that promote plant growth, especially that below the soil surface. A soil with proper tilth will be friable (flexible) normally has well developed aggregates and macroporosity and provides the proliferation of roots.
Here at Orthman we see and advise that proper primary tillage management be timed correctly by moisture content, maintaining crop aftermath as much as possible on the soil surface, not rolling or total inversion of the soil. We are much about how Strip Tillage works with our 1tRIPr tool to develop a proper seedbed, place nutrients when desired by the grower, and develop a root zone for a strong and healthy crop outcome.
All of what we communicate via this website is to offer up-to-date information to promote smart stewardship of your soil resources, conserve and protect soils from erosion, improve your farming practices and help you make profit at farming row crops.
References:
Lucas, M., Schlüter, S., Vogel, HJ. et al. Roots compact the surrounding soil depending on the structures they encounter. Sci Rep 9, 16236 (2019). https://doi.org/10.1038/s41598-019-52665-w
Dr. Jake Mowrer’s article is in the hyperlink, highlighted in blue in paragraph 3 above