Can Rice Hulls be Used in Root-zone Media?

I have been following a lengthy debate on the use of rice hulls in root-zone media as a substitute for perlite.This debate is posted on the Fresh Air Forum sponsored by Greenhouse Grower and Today's Garden Center magazines. While the debate has been heated at some points, the one issue consistent in the discussions has been, what is the status of the research on the use of rice hulls in root-zone media.

From 1985 through most of 1994, I was on the faculty at Mississippi State University. Undoubtedly, rice is a major crop for Mississippi farmers and waste from the milling industry has always been an issue. Use of any agricultural waste by-product in root-zone media has always been an opportunity for discussion. Therefore, much of my research at Mississippi State was devoted to alternative media substrates ranging from pine bark to crumb rubber.

In 1994, one of my graduate students, Shelly Dueitt, conducted a fairly in depth evaluation of rice hulls, fresh and aged. I am choosing the term aged and not composted for a significant reason. Most rice milling operations did not actively compost their rice hulls at the time, but just created large piles. These rice hulls used in Shelly's study were uniform in age, just not composted.

Shelly blended six sphagnum peat moss-based media modified with rice hulls. They were evaluated during the greenhouse production of marigolds (Tagetes erecta) and statice (Limonium suworowii) as cut flowers. Seedlings were transplanted into media containing aged or fresh rice hulls at rates of 0 to 50%. Quality plants for each species were grown in all media compared to the control. Pore space for both the aged and fresh rice hulls was satisfactory for good water holding capacity. The bulk density of all media containing rice hulls was greater than the control. The addition of aged and fresh rice hulls increased the pH of the media. Rice hulls, fresh or aged, can be used successfully as a sphagnum peat moss substitute for the production of selected short-term cut flowers.

You can see a copy of her thesis at:

RICE HULLS AS A SUBSTITUTE FOR SPHAGNUM PEAT MOSS IN A SPHAGNUM PEAT-BASED GREENHOUSE MEDIA

Rice hulls as an amendment to root-zone media is not new and was not new back when Shelly conducted her research. Here is an excerpt from Shelly's literature review:

The use of parboiled rice hulls in potting media as a soil or sand substitute in comparison to a peat-sand-soil mix was used to grow Lilium longiflorum Thunb. 'Ace' (Einert, 1972). Three-component media consisting of parboiled rice hulls, sand, peat, or soil was compared to a standard media of peat:sand:soil (1:1:1, by volume) and peat:sand (1:1, by volume) to produce 'Hibernia' and 'Paul Richter' tulips (Einert and Baker, 1973). The media containing parboiled rice hulls increased root growth (Einert and Baker, 1973). Einert and Guidry (1975) stated that composted rice hulls might serve as a soil substitute for container grown plants.

Rooted Juniperus chinensis 'pfitzeriana' liners were planted in greenhouse potting soil and cracked rice hulls and rice hull compost and cracked rice hulls at 20 to 80% of rice hulls (by volume) in each series of media (Einert and Guidry, 1975). A mixture of 1:1:1 (by volume) of sand, field soil, and peat moss were the components before rice hulls were added (Einert and Guidry, 1975).

Laiche (1989) stated that the use of composted rice hulls as an organic amendment for container media compared favorably with media consisting of pine bark alone when growing container-grown woody landscape plants. The composted rice hull medium produced the largest plants and highest rating for shoot quality and highest fresh weight for Ilex crenata 'Compacta' and Ilex vomitoria Straughn's selection (Laiche, 1989) These results indicated composted rice hulls as a single-component growing medium can be used successfully (Laiche, 1989).

Foster and Gilliam (1989) also stated that rice hull compost may be an alternative to peat moss for at least one growing season. Other studies conducted by Laiche and Nash (1990) indicated that a good replacement for pine bark may be composted rice hulls in container media. Media consisting of both 50% and 100% composted rice hulls produced plants with growth that compared favorably with 100% pine bark (Laiche and Nash, 1990). Acceptable growth of three azalea cultivars were obtained by amending media containing pine bark with sand or compos ted rice hulls in comparison to peat moss (Laiche, 1990).

Selected literature cited here:

• Einert, A.E. 1972. Performance of Rice Hull Media for Pot Easter Lilies Under Three Forcing Systems. HortScience. 7(1) :60.
• Einert, A.E. and E.C. Baker. 1973. Rice Hulls as a Growing Medium Component for Cut Tulips. Journal of the American Society for Horticultural Science. 24(3) :2.
• Einert, A.E. and G. Guidry. 1975. Media for Container-Grown Junipers. Arkansas Farm Research. 24 (3) : 2.
• Foster, W.J. and C.H. Gilliam. 1989. Container Media: Rice Hull Compost. Volume 34. Southern Nurserymen Association Research Conference - 34:93-94.
• Laiche, A.J., Jr. 1990. Effects of Media Blends of Pine Bark with Peat Moss, Fresh Pine Wood Shavings, Composted Rice Hulls or Sand on the Growth of Azaleas. Southern Nurserymen Association Research Conference - Volume 35. 35:119-122.

By the way, The Southern Nurserymen's Association Research Conference Procedings are online from 1974 to present. There is lots of information here that you will not find anywhere else.

Shelly's work concluded that, quality plants for each plant species were grown in all media compared to the control. The dry weight of marigolds and statice were similar to the control. Pore space for both the aged and fresh rice hulls was significant enough to have good water holding capacity. The bulk density of all media containing rice hulls was greater than the controls. The addition of aged and fresh rice hulls increased the pH of the media. Soluble salt levels of both rice hull media were not different from the control initially, but after plant growth the soluble salts were less in media with rice hulls. Rice hulls, fresh and aged, can be used successfully as a sphagnum peat moss substitute in a sphagnum peat-based media for short term selected cut flowers. The amount used as the media substitute will depend on the crop to be grown.

The bottom line is, to use rice hulls as a component of a root-zone media, it must be aged or composted. Fresh rice hulls will result in poor plant growth, volulnteer rice seedlings, and hungry rodents.