Horray for Roots!
This article was first printed in the Sept/Oct 2009 issue of the Organic Broadcaster, published by the Midwest Organic and Sustainable Education Service.
Have you been pondering roots lately? Most of us haven’t spent a lot of time thinking about the role that the root system plays in a plant’s life. Bud Markhart, Professor of Organic Horticulture at the University of Minnesota, has spent a lot of his time not only understanding roots, but really appreciating their critical importance in the life of plants. His enthusiasm for roots was obvious as he led a workshop session held in February 2009 at the MOSES Organic Farming Conference in La Crosse. Bud helped everyone understand and appreciate the function and value of a plant’s root system. We’d like to share some of his thoughts with you here.
Before zeroing in on roots, Bud starts with a little basic horticulture lesson. All living organisms need to acquire resources. While animals directly consume packets of resources, plants must collect their resources from a dilute environment. The fact that this collection is from dilute resources is a key factor in how plants are designed. The leaves and above-ground part of the plant collect sunlight, carbon dioxide (CO2), and oxygen (O2). The roots take in water, minerals, and oxygen.
Water Collection is Key
Beyond this basic water collection, roots have several other functions. They provide an anchor for the plant in the soil, they adsorb things, they store things, but they also act as a filter to keep things out of the plant. In the process of “doing their job” roots consume resources- in fact, no matter what the root shape or structure, over 45% of the photosynthate created by a plant is used by the roots.
A Little Basic Physiology
Looking at a cross section of a root, one will see the outer epidermis surrounding the cortex, which contains the central, large transport cells. The layer inside these two is the endodermis, which is the tissue that acts as a filter to keep unwanted stuff out of the plant. These endodermal cells have very thick walls, which stop unwanted things from going through. Items are forced through the cell wall membrane which acts like a filter or strainer to keep unwanted things such as bacteria, viruses or extra nutrients out.
Back to Water, and the SAPC
Diffusion is the most important mechanism for getting nutrients into the root. Diffusion is more or less the random movement of elements back and forth- elements will randomly get close enough to the root and be adsorbed. Diffusion works great at short distances, and not at all well over long distances. As long as a root is contacting minerals in the soil, there is a chance of their being adsorbed. There is a zone of adsorption within which diffusion will be able to work and nutrients taken up. These zones can be very small and the mechanism very slow.
To compensate for the inefficiency of the diffusion mechanism, plants have adapted by developing huge amounts of root surface area, or large “root complexes.” These root complexes are made up of branch roots, root hairs and important associations with microbes in the soil. This last mechanism, in which symbiotic relationships are developed with mycorrhizae, bacteria, fungi and other microorganisms in the soil, is incredibly significant and mistakenly overlooked by the chemical farming industry, Bud notes. Microbes help by attaching to the root and providing more surface area for adsorption. They also provide ways to solubilize nutrients and make them more easily adsorbed. However, these microbes do ask a price- they need a place to live and reproduce, and also take energy from the plant’s resources. They use carbohydrates, fertilizers and photosynthates from the plant, and in return help increase nutrient adsorption. Microbes provide a great way for plants to expand the adsorptive surface area.
Healthy Growth is Critical
The root tip is the active zone of elongation. The root hairs, where the adsorption takes place, is right behind the tip. Behind the active adsorption area is an area called the maturation zone. Mature roots do not adsorb water or nutrients. As the root tip grows and moves through the soil, a portion of the root behind matures and stops bringing in resources. As long as the root tip is growing all is well, but if the tip stops growing for some reason, the maturation process does NOT stop. If this happens, the zone of adsorption will be reduced and the plant can die. Things that contribute to the slowing or stopping of root tip growth include not enough water, soil compaction, soil too hot, soil too cold, floods, too much fertilizer and other causes. We need to optimize root growth in order to optimize adsorption of water and nutrients for optimal plant growth. We can keep roots healthy by keeping the soil healthy, with a balance of nutrition, the right water holding capacity and the right aeration. Ideally soil is made up of 50% dirt, 25% water and 25% air. Compaction is a roots’ “worst nightmare” Bud notes, as it takes away air spaces for both air and water storage and makes it harder for roots to push through to access nutrients. The best way to ensure optimum root growth, Bud reminds us, is to have high levels of organic matter to balance the soil particles.
Some root systems are better at doing their work than others, and Bud closes the session with details of interesting work he is doing grafting herbaceous annual plants onto root stock of disease resistant or otherwise superior root stock. Success has been found using this method with tomatoes, and Bud sees the sample technology as being useful for larger scale vegetable growers. For more on this research and more, visit Bud’s website at the University of Minnesota horticulture department, http://horticulture.cfans.umn.edu/Bud_Markhart.html
Jody Padgham has been with MOSES since 2002. She is the organization's Financial Manager, and the editor of the Organic Broadcaster newspaper. Jody raises poultry and sheep organically on a 60-acre farm in west-central Wisconsin.Return to TOP