Landscape
And
Nursery
Dialog
Mary Ann Rose
Commercial Landscape
& Nursery Specialist
The Ohio State UniversityJuly, 1996
The Use of Composted Rice Hulls for Nursery Container Media
Keywords: container media, alternative substrates, nutrition, soil amendments, nursery, container production
R ice hulls are a relatively low-cost container media amendment that has been increasing in popularity over the last few years. A fine, uniform product is achieved by first hammer milling particles to about 1-2 mm, then composting 18-24 months. According to Dan Kuczmarski, Forrest Keeling Nursery, Elsberry, MO, the outstanding feature of this product is its size uniformity and stability over time. After a year or more in the container, there is little apparent breakdown of the hulls.
Rice hulls have a pH around 5.7. Compared to peat they have a relatively low water-holding capacity, holding a weight of water that is about 50% of their dry weight. At the 1996 OSU Nursery Short Course, Dr. Ted Bilderback presented a comparison of rice hull physical properties to those of peat and bark. These figures (table 1, below) demonstrate that rice hulls have a high proportion of air-filled pore space.
Kuczmarki's experience with rice hulls at Forrest Keeling Nursery suggested that the material also has a low capacity to store nutrients. Low water-holding or nutrient-storing capacities are not necessarily negative features as long as long as rice hulls are combined with components that provide nutrient and water holding capacity. Many growers in Ohio are using composted municipal sludge in rice hull mixes, and the sludge does a good job of balancing rice hull
chemical and physical properties.
At Forrest Keeling Nursery they have used rice hulls in nursery container media at 40% by volume, using pine bark (40%) and sand (20%) as the other components. This mix appears to me to be a faster-draining mix than most Ohio growers are using. I personally have had good results using rice hulls at about 25% by volume. Thus, without extensive research on this product, an incorporation rate somewhere between 25% and 40% appears to be a good place to start if using rice hulls for the first time
Some growers report dissatisfaction with rice hull mixes because of their tendency to remain dry at the surface of the container, while in fact the bulk of the growing medium may be quite moist. Growers that 'eyeball' their cans for irrigation decisions cannot rely on this method for rice hull mixes! However, there may be benefits from a dryer medium surface. At Forrest Keeling they had a lower incidence of fungus gnats, algae, and moss when rice hulls were used. It's also conceivable that weed seed germination might be reduced (but I seriously doubt it!). Could the dryer surface also effect the release of topdressed fertilizers?...... Maybe. An interesting research project for all my spare time...........
A final note on rice hulls concerns its use as a soil amendment. This product has been used in golf greens to provide aeration in the soil. A few growers of ornamentals may be using it as well in liner or seed bed production. Brian Decker, of Decker Nursery, Groveport, Ohio, reported that he was going to substitute rice hulls for compost in his liner beds this year, having found that composts tend to raise the pH of our Central Ohio soils to unacceptable levels. It will be interesting to see whether rice hulls catch on in liner production. I would hazard a guess that compost is still probably a superior choice for field soils if soil pH is not a concern.
Table 1. Physical Properties of Composted Rice Hulls, Peat and Bark.
Figures from Dr. Ted Bilderback, North Carolina State University.
Total Solids Total Porosity Air space Container
Capacity
Rice Hulls 13% 87% 27% 60%
Peat 10% 90% 18% 72%
Pine Bark 11% 89% 24% 65%
Note: Total Solids plus Total Porosity = 100% of container volume
Air space plus container capacity = Total Porosity
Air space = the % of container volume that is filled with air after a completely saturated medium is allowed to drain. Container capacity = the % of container volume that is filled with water after a completely saturated medium is allowed to drain.
Literature Cited
1994. Dan Kuczmarski. Amending the Cost of Media. American
Nurseryman. Vol. 179(10):47-52.
