BioCycle June 2004, Vol. 45, No. 6, p. 43
University of Florida trials evaluate substrate characteristics and impact on containerized plants using varied percentages of partially composted seaweed and yard trimmings.
Kimberly K. Moore

THE ORNAMENTAL horticulture industry has a continuing need for organic materials as a component of the growing substrate for container grown plants. Composts from feedstocks such as biosolids, yard trimmings as well as MSW have been shown to be viable in bedding plant substrates, but plant responses can vary with different compost products. Therefore, it is important to identify physical and chemical substrate characteristics associated with optimum plant growth.
The University of Florida investigated use of a 1:1 by volume mixture of partially composted seaweed and partially composted yard trimmings (SW) obtained from Terranova Industries in Miami, Florida as a component of the growing substrate for angelonia ‘Pink’, shooting star, coreopsis, scutellaria ‘Purple Fountains’, achillea and coleus.
There was no difference in shoot dry weight of angelonia, coreopsis, scutellaria, or coleus plants among the four different percentages of SW compost. Plants grown in control substrates (0 percent compost) were similar to plants grown in 30, 60 and 100 percent SW. However, the shoot dry weight of shooting star plants decreased as the percentage of SW increased from 0 to 100 percent. Shoot dry weights of achillea plants were greatest in substrates without compost (zero percent), but there was no difference in shoot dry weight of achillea plants in 30, 60, or 100 percent compost.
It appears that the SW compost used in these experiments was a suitable component of the growing substrate for angelonia, coreopsis, scutellaria, and coleus plants. However, it did not appear to enhance growth beyond that achieved in control substrates. Furthermore, SW compost did not appear to be a suitable component to the growing substrate for shooting star or achillea plants.
Concerns with using compost made from seaweed include lower nitrogen concentrations and high salt levels. However, compost made from seaweed can have high water-holding capacities. Analysis of the seaweed compost used in these experiments showed that substrates containing 30, 60 or 100 percent SW compost had water-holding capacities similar to the control substrate (zero percent) but higher soluble salt concentrations and lower nitrogen concentrations than the control substrate. However, all of these values were within acceptable guidelines or standards for container substrates used for bedding plant production.
Compost quality will vary with the type of feedstocks used as well as with any pre or postprocessing of the compost product, and the length of time the materials are allowed to compost. All of these factors influence a compost products ability to serve as a suitable substrate to grow bedding plants. In general, it is recommended to use only 30 to 50 percent compost as part of any bedding plant substrate. Few people recommend growing bedding plants in 100 percent compost.
Furthermore, any use of compost in the growing substrate for containerized bedding plant production should be based on plant response as well as substrate characteristics, such as pH, soluble salt concentrations, water-holding capacity, and air-filled porosity. Be aware that the pH, soluble salt concentration, water-holding capacity, and air-filled porosity that will produce optimum plant growth will vary from one bedding plant species to another but there is a range for each of these values that will produce acceptable and marketable plant growth.
Plant response in other compost products may vary from these results. Furthermore, plant response with a different seaweed compost product may vary from these results. Each compost product has unique physical and chemical properties that may change when combined with other substrate components. It is always best to test various substrate combinations before adapting them to an entire production system.
Kimberly K. Moore is an Associate Professor in the Environmental Horticulture Department at the University of Florida based at the Fort Lauderdale Research and Education Center.