Pumice and the Oregon nursery industry

By Gabriela Buamscha and James Altland

Despite the widespread use of pumice in Oregon nursery production, we know very little about its properties and how they influence nursery crop growth. This article describes the geological, chemical and physical properties of pumice. We also discuss the implication of these properties on the production of nursery crops. All of this research is being generously funded by the Oregon Association of Nurseries.

What is pumice?

Pumice is a porous and lightweight rock similar in appearance to perlite. The word pumice is derived from the Latin pumex, which means “foam.”

Pumice is a type of igneous rock, which is formed from molten or partially molten material. Some igneous rocks form from the cooling of molten materials below the earth surface and are classified as plutonic. Plutonic igneous rocks have large and easily identifiable mineral deposits because of their slow cooling process (granite is an example). The size and source of the mineral deposits in plutonic rocks are used to classify them.

Other igneous rocks, including pumice, are formed when molten rock is forced above the earth’s surface (now called lava) and cooled quickly. These types of igneous rocks are classified as volcanic. Because of the relatively rapid cooling process, mineral deposits in the rock are trace or nonexistent, and thus not useful in classifying the rock. Instead, these rocks are classified by texture and color.

Volcanic rock texture is either glassy or vesicular. Vesicles are small holes or pores formed when gas and steam expand as the rock cools. Pumice contains many small vesicles, which is the primary reason it is so light in weight.

Color is a result of chemical composition. Chemical composition of volcanic rocks refers primarily to silicon (Si) and oxygen (O) content, reported as SiO₂ (Table 1). The higher the SiO₂ content, the lighter or whiter the color.

Pumice is most often classified as rhyolite, white in color, with vesicular texture (many vesicles). Its vesicles cause it to be lightweight, so much so that it floats on water.

Pumice is commonly defined as an inert material with neutral pH, low salt content and the inability to contribute much to plant nutrition. However, some pumices can release small amounts of sodium, potassium, calcium, magnesium and phosphorous. Not all pumice is the same. Pumice deposits from different parts of the world have different physical and chemical properties.

Materials similar to pumice

Scoria is another volcanic igneous rock common to the Pacific Northwest. Scoria, often called cinders, has larger and more easily identifiable vesicles. It is a basalt igneous rock with 50 percent or less SiO₂. It is characterized by marked vesicularity, dark color and greater density than pumice. It is often used as decorative mulch in landscapes. Scoria crushed into very small particles is sometimes applied to road surfaces to increase vehicular traction.

Pumice and scoria are mined raw materials that are graded to a specified particle size; whereas, perlite and vermiculite are mined and processed materials.

Perlite is a glassy alumino-silicate mineral of volcanic origin. The raw material is crushed and exposed to high temperatures (1,600 °F), which results in expanded, white, lightweight particles. During the heating process, perlite is expanded from four to 20 times its original volume. Perlite does not compress and consequently promotes good porosity and drainage. Because of local availability of pumice in the Northwest, it has replaced perlite in most commercial mixes.

Vermiculite is a silicate mineral containing aluminum, iron and manganese. It is a type of clay and contains a series of thin, parallel plates. The raw material is subjected to intense heat (up to 1,832 °F), which expands the vermiculite particles and gives them an accordion-like structure. Vermiculite increases the water-holding capacity of container substrates. It also has cation exchange capacity and slowly releases potassium and magnesium for plant uptake.

Where does pumice come from?

Pumice is mined throughout the world in regions with volcanic activity and pumice deposits. Oregon is the major pumice producer in the United States. Two operations are currently active in the state. One mines the Bend deposit in Deschutes County, and the other the Mazama deposit in Klamath County.

The Bend deposits are the result of the Bend Highland Eruption that occurred about 400,000 years ago. A ribbon-like layer of pumice approximately 20 feet thick was deposited around the present-day city of Bend. On top of the pumice layer is a 20- to 100-foot deep deposit of finer volcanic ash called Tumalo tuff. Geological studies indicate that the Bend pumice and the Tumalo tuff were formed during the same eruptive event. The Tumalo tuff layer seems to have protected the pumice from weathering and alteration of its original properties. The Bend deposits have been mined for more than 50 years.

The Mazama deposit was formed when Mount Mazama collapsed around 6,850 years ago, forming the caldera now occupied in part by Crater Lake. This eruption generated a pumice and ash deposit over an enormous area of western North America. The Mazama ash bed is identifiable throughout the northwestern U.S. and in three Canadian provinces. These deposits also occur in ribbons, although they are closer to the surface with only a thin layer of volcanic ash and topsoil covering it. Mining of these deposits for use in the nursery industry has only started in the last three years.

Pumice deposits from Bend and Mazama occur in tightly packed ribbons. These ribbons of pumice crumble into small particles, generally less than 1 inch in diameter, upon being excavated from the ground. Pumice is sorted into piles of differing particle size by using large automated screening machines.

Horticultural uses

In the western United States, pumice is mined locally and used as an inorganic component in growing media. Pumice is usually added to bark or peat moss to increase aeration, porosity and drainage. Container nurseries in Oregon incorporate pumice to their mixes up to 33 percent of total volume. This material is also used as the sole component for sticking cuttings in propagation flats and in-ground propagation beds.

Chemical properties

Chemical properties of Douglas fir bark and pumice reported in Table 2 were measured using the saturated media extraction method. This is a standard procedure used for testing the nutrient capacity of greenhouse and container nursery substrates.

Most nursery containers are composed primarily of bark, with a relatively small amount of pumice. It is tempting to compare the Bend and Mazama pumice to decide which is better for use in container media; however, the four pumice samples analyzed have substantially lower levels of all nutrients compared to Douglas fir bark. Based on the relatively small amount of pumice used in containers compared to bark, it is safe to say that the miniscule levels of available nutrients found in pumice have no impact on nutrient availability in nursery substrates.

All pumice samples, especially those from the Bend deposits, have a significantly higher pH than bark. It is hard to predict what (if any) impact pumice has on container pH when mixed at typical rates. The effect is probably negligible, but nonetheless, we will evaluate this in the future.

The most noticeable difference between Bend and Mazama pumice is color. Bend pumice is whiter compared to the Mazama pumice, which has a yellow tint. Recall that pumice in the Bend deposits was sealed under 20 to 100 feet of the Tumalo tuff. The Tumalo tuff protected the Bend pumice from weathering, erosion, leaching of organic materials from the surface and other factors that might influence color.

Differences in color are also a result of chemical composition. Pumice from the Bend deposit is classified as a rhyodacite (between rhyolite and dacite, Table 1), while pumice from Mazama is classified as dacite. Mazama pumice has higher concentrations of minerals, most of which are feldspars, pyroxenes and hornblende (minerals rich in iron, magnesium and/or sodium). Based on the chemical analysis in Table 2, it seems minerals present in Mazama pumice are likely iron-rich types.

While Mazama pumice is inherently more yellow due to its mineral composition (Table 1), some of the coloration can also be explained by staining. Mazama pumice is mined in ribbons closer to the Earth’s surface, and organic matter leaching from the surface can stain the pumice. Differences in pumice color most likely have no effect on nutrient availability in container substrates.

Physical properties

There are differences in pumice density. Bulk density is a measure of how much a material weighs for a given volume. This is important for selecting container substrates, because bulk density will influence the final weight of the container and thus shipping costs.

Particle size of the pumice sample greatly affects its bulk density, in that smaller particles pack more closely together, resulting in higher bulk densities. Because all four products are from different screen sizes and therefore have different particle size distribution, it’s impossible to compare apples with apples. Nonetheless, bulk density readings of the raw materials provide some insight as to the impact of each material on container weight.

Total porosity is the percent of container volume composed of pore space (see Digger, September 2003). Pores in a media can be filled with either water or air. After a container is completely saturated and allowed to drain, the fraction of the container filled with water is called water-holding capacity, and the fraction filled with air is called air space. Total porosity of each pumice sample was similar. Differences in water-holding capacity and air space were attributed to the differences in particle size of each sample.

It is impossible to predict how pumice of different sources and screen sizes will impact physical properties of container substrates based solely on the values listed in Table 2.

Summary

This article provides only basic information on pumice as a raw material. Studies this summer at OSU’s North Willamette Research and Extension Center will examine the impact of pumice on container properties. Does pumice affect substrate pH? We assume it doesn’t. Does pumice increase container drainage or decrease container bulk density? We assume it does. We will check those assumptions and let you know what we find.

• Anonymous. 2005. Basic facts about perlite. www.perlite.net/redco/basic.htm
• Geitgey, R.P. 1992. Pumice in Oregon. Oregon Department of Geology and Mineral Industries Spec., Paper 25.
• Gunnlaugsson, B., and S. Adalsteinsson. 1995. Acta Horticulturae 401:131-136.
• Jessey, D., and D. Tarman. 2005. Igneous rock identification.
• Landis, T.D. 1990. The Container Tree Nursery Manual. USDA, Forest Service.
• U.S. Department of the Interior. 2001. Crater Lake. www.nps.gov/crla/pumice.htm.

Dr. James Altland is a nursery crop extension specialist at Oregon State University’s North Willamette Research and Extension Center in Aurora, Ore. He can be reached by e-mail at james.altland@oregonstate.edu or at (503) 678-1264 ext. 46. Find more information on this and other nursery-related topics at his Web site, http://oregonstate.edu/dept/nursery-weeds/. Gabriela Buamscha began her graduate studies at Oregon State University in January 2005. For the next several years she will be evaluating the chemical and physical properties of Douglas fir bark and pumice, as well as the influence of water quality on growth of container crops.

Digger magazine is produced by the Oregon Association of Nurseries Publications Department. publications@oan.org