Rhizometer: A Technique for Observing and Measuring Root Growth and Their Influence on Substrate Physical Properties In Situ

Root growth of greenhouse crops is both influenced by, and influences, the substrate in containers. Current methods of measuring root growth of crops during/after production do not include undisturbed assessment of whole root systems, root growth over time, or the influence of root growth on substrate physical properties.  The Rhizometer is a new technique developed to observe root growth of small plant material (e.g. seeds and plugs) and also measure substrate physical properties and the effect of root growth (in situ) on those properties over time. Rhizometers were constructed from clear cylindrical plexiglass tubes (7.6 cm tall x 7.6 cm inside diameter) which allowed for visible observations of root systems.  Rhizometers were purposefully constructed to fit the base plates of the North Carolina State University (NCSU) Porometer, a method of determining substrate physical properties including total porosity (TP), air space (AS), and container capacity (CC).  The objective of this study was to use the Rhizometer apparatus to measure changes in substrate physical properties over time as influenced by two plant species with differing root fineness/structure. Rhizometers were filled with a moistened 60:20:20 (peat:perlite:vermiculite) substrate and tapped five times to achieve similar bulk densities prior to planting.  Rudbeckia hirta ‘Becky Yellow’ was selected for its fine (small) roots and Begonia x hybrida ‘Dragon Wing Red’ was selected for its larger/tuberous roots. For each species, 40 Rhizometers were planted. Every seven days after planting 10 Rhizometers of each species were harvested.  Of those 10, five were used in the NCSU Porometer method, and the remaining five were washed for root dry weight determination (the substrate was rinsed from the root balls).  The study was conducted for four weeks, resulting in a total of 80 Rhizomters used. Begonia had a linear decrease in substrate TP, AS and CC over four weeks. There was a linear increase in root dry weight (mass) observed during that time.  Substrates containing Rudbeckia plants had a linear increase in root dry weight over time; however, there was no change in TP, AS or CC as a result of the fine nature of the root system.  These data show how substrate physical properties in a container can be influenced differently depending on the crop being grown.