Pumice in Horticulture

The topics linked below examine the different roles pumice plays in the horticultural and greenscaping spaces to improve the performance of potting, gardening, greenhouse, and landscaping soils.

Pumice vs. Perlite

Let’s begin with a familiar pumice-like mineral well-known to growers: expanded perlite. Perlite is a horticultural mainstay. Widely used. Wonderfully effective. But for less money (in quantity) and more green cred, pumice is a card you’ll want to have in your hand. Pumice is physiochemically analogous with expanded perlite, more durable, and a bit heavier and grippier, staying fully blended in the soil.

Magnified views of the foam-like surface structure and cavity-rich nature of pumice (left) and expanded perlite (right). Research1 quantifies that pumice is the physiochemical equivalent of expanded perlite in terms of performance as a soil amendment.

AMENDMENT ECONOMICS. Perlite is arguably the most recognizable and widely used non-compost soil amendment in horticulture, especially for potting and garden soils. It is valued for its contribution to desirable soil structure and water and nutrient retention. But the process of crushing, grading, and then flash-heating perlite ore to expand (pop) it to its useful state adds to its cost and carbon footprint. The cost may be negligible in a potting mix or bagged garden soil, but for large-scale soil amendment projects and extensive turf management applications, the cost of perlite is typically prohibitive. Pumice, on the other hand, is volcanically calcined (expanded to it foamed-stone state). It is simply dozer-ripped from the shallow deposit, pushed into a stockpile, and, after a preliminary crushing and screening, is amendment-ready.

NO PERFORMANCE DROP. There is no performance drop using pumice for potting soils or to amend poor native soils. A University of Illinois research project1 studied pumice as a perlite substitute for improving growing soils. Specifically, the chemical properties and surface characteristics were compared. They proved analogous, with pumice exhibiting a greater pore size span. The report summary states: “Pumice and perlite were shown to have similar physiochemical properties which subsequently translated into similar behavior in blended soil mixtures. It proved equally, if not even more effective in some ways, than perlite. A subsequent companion plant growth study (not reported herein) further confirmed the suitability of pumice as a soil amendment. Plants grew equally well in pumice and perlite media.”

[1] Evaluation of Pumice as a Perlite Substitute for Container Soil Physical Amendment. Dianne A. Noland, L. Art Spomer & David J. Williams; Department of Horticulture, University of Illinois

STAYS PUT. Pumice holds a durability and bulk density advantage over expanded perlite, and that added density resists hydraulic lift (floating up onto the soil surface) from repeated waterings. Another attribute working to bind pumice within the soil is its rough surface form factor—it is super-grippy, with shape variety and myriad micro-biting edges that bind with other soil particles to stay firmly suspended in the soil matrix.

IDEAL PARTICLE-SIZE RANGE. Functional soil exhibits a particle-size curve that includes a variety of interlocking sizes, creating pore spaces on both a micro and a macro level. For general in-soil use, the most economical pumice grade—1/8 fines MN/SoilRox Amender XF—provides a wide range of particle sizes that blend effectively with any soil type, and in the poorest soils at structural extremes—loose, granular sands to fine, platey clays, this pumice grade actually sweetens the particle-size variety curve. Bu contrast, horticultural perlite grades contain very few of those valuable fines. Of course, our pumice is available in a variety of grades, with progressively fewer fines and larger stones—useful for specialized potting and other engineered soil mixes.

Other Pumice Alikes

Additional products, primarily expanded clays and shales, are on the market and tout many of the use roles and performance metrics of pumice. The dirt under the rug with the furnace-expanded products (including perlite) is the prodigious energy burn to calcine, or foam, these frothed mineral products. A closer look at each is found on the Pumice Vs. X website.

SITE PUMICE VS. X

Potting Soil Component

Common dirt and containerized growing are not a winning combination. Growing thriving plants in pots takes a specialized non-soil—even “garden” soil isn’t good enough (the bagged stuff labeled as garden soil is topsoil blended with amendments). Containerized growing is a struggle against moisture buildup and soil compaction, and a specialized potting soil makes all the difference. The inorganic component of most potting soil mixes is typically either perlite or vermiculite. Pumice is a less well known, but fully functional option.

THE NEED FOR A MINERAL COMPONENT. Like any soil, the organic components in potting soils break down, settle, grow more compact. Pore spaces collapse and air exchange and drainage functions tank. Hence the need for a mineral component: they extend the life of potting soil, and the best ones, like pumice, do so while also making a functional contribution.

GRADE OPTIONS. Pumice is available in a wide variety of grade options—from grades with a lot of moisture-holding fines to those with the fines screened out. These grade options are found in two types: mine grades with some moisture, and dried, more extensively processed plant grades (crushed, screened, and size-consistent and/or blended). These grade options open the possibilities for developing specialized potting soils that meet the direct needs of the plant growing in the pot, the setting (indoor or outdoor), the preferred watering and feeding schedule, production goals, even the local climate. For example, moisture retention: blend more pumice fines, hold more water. (But, unlike vermiculite, pumice never gets soggy and swells). Use a grade with fewer fines and larger particles spread throughout the soil matrix, you get more aeration and rapid drainage, with less moisture retention, as preferred by succulents and cacti.

BULK DENSITY. Potting soils are notoriously lightweight, what with flyweight ingredients like peat moss, ground pine bark, and perlite. That can be an issue with tall plants, heavy fruits, and or windy outside conditions. Using pumice instead of vermiculite or expanded perlite lends a more heft to the container. Pumice also has a grippy surface, allowing it to effectively anchor plant roots and interlock with other soil particles.

NEUTRAL pH. Hess pumice has a neutral pH, so no need to compensate in the acid/alkaline equation.

COLONY SHIPS. Potting soils are (typically and by design) sterile, which is why some kind of soluble plant food is added to provide nutrition. For those that choose to activate the soil by introducing beneficial soil microbes to planted containers, blending a bunch of little foamed glass stones, riven with micro and macro pores and all kinds of nooks and crannies, provides the perfect scaffolding for microbial life to thrive.

PUMICE MULCH. Larger pumice stones also make a great surface mulch—attractive and protective and useful in mitigating swings in soil temperature. Some users claim a pumice mulch helped eliminate the problem of gnats swarming the damp soil.

Growing Succulents and Cacti in Pumice-Amended Soil

Pumice grow media, either used alone or blended with other soilless components, allows the succulent or cacti enthusiast to dial in the ideal drainage rate via particle size selection.

Succulents thrive in a growing media that drains well while retaining enough moisture and nutrients to meet the needs of the plant. A careful balance of grow media type, structure, and watering frequency is the key plump, happy succulents. Pumice, a natural frothy-stone with a nook-and-cranny-riven character, is widely used by succulent enthusiasts because it provides the means to create and maintain that ideal balance.

THE RIGHT FORM FACTOR. Viewed at a micro level, pumice is sponge-like, riven with cavities and hollows of various sizes, angular and edgy and rough. Pumice is also amorphous (non-crystalline) and glassy—it won’t absorb moisture and swell. Pumice holds moisture by virtue of its physical structure—both within the tiny-to-microscopic pores in the mineral foam and on the rough, irregular surface. Nutrient-laden water is also caught and held in the spaces between the tiny stones themselves, making this type of moisture retention ideal for drainage rate manipulation. Simply adjusts the stone size blend to dial up the desired drainage rate. Roots don’t rot yet moisture and nutrients are still available for plant uptake, while the vital aeration spaces needed by roots to expel and absorb gases remain open.

PARTICLE SIZE. Succulent enthusiasts using pumice can adjust the particle size profile to meet the moisture needs of a wide variety of plant-types in a wide variety of climates. The pumice-soil profile can also be blended to serve the watering schedule. As a rule of thumb, pumice soils made up of larger particles drain more quickly that those consisting of smaller, more tightly packed particles, which hold onto moisture longer.

AND MORE. There’s more. Pumice is certainly classified as lightweight (easy on your racks and shelves), but it’s bulky and grippy enough to confidently anchor the plant roots. The foamy nature of pumice makes a great home for colonies of soil microbes...or you can forgo the need for in-soil nutrition by using a quality liquid fertilizer with each watering. For garden-grown succulents, pumice makes a great soil amendment, improving drainage rates. Used as a top dressing, pumice protects the crown of the plant from soggy dirt rot during big-rain events.

Improving the Structural Profile of Lawn and Landscape Soils

This clay soil turf bed is heavily amended with pumice (1/8 fines MN) to provide for a more friable, compaction-resistant, breathable root zone with better drainage and increased moisture retention. The larger pumice particles are readily visible, while the fines are tightly integrated amid the clay soil particles.

Too often, the native soils where a new lawn will be seeded or new flowerbeds, shrubbery, and other landscaping will be installed is of poor quality—both in available nutrients and soil profile. A load of compost can improve both, but the downside of using compost as the ONLY soil amendment is longevity. For long-lived, semi-permanent efforts like lawns and landscape constructs, compost alone isn’t good enough. Pumice is an ideal supplementary amendment.

PUMICE WORKS IN THE SOIL ON TWO LEVELS. Pumice functions as a structural conditioner, physically amending poorly structured soils—like a heavy clay—to be less dense by holding the soil particles apart to create elemental tilth and friability, which enhances root zone breathability and drainage. That structural improvement allows for the development of vibrant, deep-rooted plants.

The second benefit is found in the foamy, sponge-like nature of the little pumice stones themselves. Pumice granules—even the fines—are riven with nooks and crannies, pockets and holes that grab and hold moisture and nutrients, then make them available to the roots over a longer period of time. The sponge-like texture of the tiny pumice stones also provide a great matrix for colonies of beneficial soil microbes.

PUMICE IS AN ENDURING AMENDMENT. Organic composts are wonderful and a great addition to any soil improvement effort, but any structural improvements from compost are lost as, over time, the compost breaks down and is consumed. Without a steady schedule of compost re-application and blending/tilling, the structural advantage provided by compost is lost as the soil returns to its poorly structured native state. For constructs like lawns, parks and landscaping meant to thrive in-place for decades, compost alone simply isn’t good enough. Unlike compost, pumice is a mineral—it will not breakdown in the soil, remaining functional for the long term, resisting compaction, holding moisture, and facilitating drainage.

SOIL AMENDING RECIPES. Determining soil type using the jar method (or similar), provides a reference point from which to correct the performance of a out-of-balance (non-loamy) soil type. These following recipes include using a good compost to jump start the nutrient content in the soil.

AMENDING CLAY SOIL: Spread 4 inches of SoilRox Amender XF and 2 to 3 inches of good compost. Blend into 6 to 8 inches deep into the clay soil.

AMENDING SILTY SOIL: Lay down 2 inches of SoilRox Amender XF and 2 to 3 inches of good compost. Blend into 6 to 8 inches deep in the silty soil.

AMENDING SANDY SOIL: Spread 1 inch of SoilRox Amender XF and 2 to 3 inches of good compost. Blend into 6 to 8 inches deep into the sandy soil.

NOTE: These recipes are, of necessity, generalized. More thorough soil-typing followed by examination and testing of the amended soil profile would be necessary to determine the quantity of added pumice to attain ideal conditioning.

THE GREEN FACTOR. When it comes to green-cred, pumice has it—literally, in spades. Pumice is the product of a volcanic eruption, and, as such, is naturally calcined (super-heated), making its amorphous (non-crystalline), foamed-glass-stone character nature-made. Pumice needs no super heating—it does not need expanded, or popped, to achieve its useful physical state. Other soil amendment products on the soil-amendment market—perlite, vermiculite, expanded clay, and shale products (like Utelite™ or Perma-Till™)—are mined as a hard ore, crushed, then flash-fired under intense heat to acquire their puffed, pumice-like usefulness.

Turf Management

Pumice is a nature-crafted, economical soil-blended or top-dressing amendment, ideal for large-scale turf applications such as golf courses, parks, medians, and lawns.

This new lawn was seeded in pumice-amended clay soil. Hess grade 1/8 fines MN was tilled into the tight, sticky native soil to restructure the composition of the root zone, improve drainage and retain moisture.

SOIL BED PREPARATION. As detailed in the topic above, pumice is extremely functional as a structural amendment to poor native soils—especially those prone to compaction and runoff or sandy soils that drain too fast and hold too little moisture in the root zone. Blending pumice into the root zone before seeding or laying down sod provides an economical fix that improves the native soil profile for the long-term.

TOP DRESSING. Pumice can also be used as a top-dressing for established lawns when combined with mechanical aeration. Aeration is an effective technique to improve the health and profile of compacted turf soils. Holes are punched (using either a core or spike method) into and slightly below the root zone, allowing refreshed access for air, water, and nutrients. Aeration also promotes the development of humus, the key to a healthy topsoil. High-traffic turf growing in clay soils especially benefits from mechanical aeration. Aeration also often proceeds reseeding and fertilization, making both more effective.

Adding Pumice: Over the course of two or more aeration events, a soil-amending grade of pumice is liberally sprinkled over the lawn and raked, working it into the holes created by the aeration equipment. Over time, as the plugs break down and the pumice-charged holes are filled with roots and humus, the pumice is thoroughly integrated into the soil and the root zone, building a long-term foundation for strong, resilient turf—resisting compaction, retaining moisture and nutrients, supporting critical microbial life, expediting root zone gas exchange, and facilitating drainage.

Pumice is pH neutral (7.2) and will not alter the acidity or alkalinity of the soil.

Lightweight Soils for Green Roofs and Planting Structures

Greenroofs and similar elevated planting structures require a grow media that sits light and easy, yet provides a low-maintenance, well-structured, highly-functional foundation for thriving vegetation. That means an engineered soil designed to meet the unique demands of the project. Pumice checks key boxes in terms of lightweight form factor, in-soil endurance, neutral pH, natural purity, and a variety of grades, including the economical mine grades.

X-REF PUMICE AS A COMPONENT OF ENGINEERED SOILS

Soilless Grow Media

High-efficiency grow systems deliver nutrients with the water, and thus rely on on a enduring, stable soilless grow media to provide support for root and plant, hold moisture and nutrients between watering cycles, allow for controlled, systematic drainage, and endure crop after crop.

Aquaponics and Hydroponics Systems

Seedlings growing in Hess pumice stone (grade 5/16C) in an aquaponics flood and drain system.

Soilless pumice grow media provides a combination of benefits in unique, pH neutral, lightweight, naturally foamed stone:

BALANCE. Pumice delivers the ideal balance between drainage, moisture retention, and open gas exchange in the root zone. Depending on crop needs, drainage rates are customizable via media size selection.

STABILITY. Pumice is lightweight, yet substantial enough to support the root system and the plant and fruit above. This happens on two levels: the grippy, interlocking nature of the rough-edged stones, and the relative density of our particular deposit of pumice.

MEDIA LIFE. Pumice does not get soggy, does not break down, and can be cleaned (if necessary) and replanted numerous times.

PURITY. By the particular origins of the Hess pumice deposit, our pumice has been naturally and relentlessly washed of impurities before being deposited on shore of an ancient inland sea. As for mineral makeup of our horticultural grades, they are (typically) 98% pumice stone and 2% other igneous mineral (like obsidian), which are not removed through the mining process.

SUSTAINABLE. Pumice is a naturally occurring stone with a nature-made (volcanically foamed) form factor. It is simply mined, crushed, screened to grade, cleaned, and packaged. No fuel-sucking expansion furnaces or other manufacturing processes are necessary to attain useful form.

PONICS STONE™. Hess markets select pumice grades under the Ponics Stone™ brand, which are available for retail purchase online via the Pumice Store or from the Ponics Stone website. Pallet-sized quantities (in supersacks or stacked bags) are also available.

Bonsai Soils

Bonsai enthusiasts have long relied on pumice and pumice-blended soilless media to get the best results from their efforts. The challenge in the patient art of Bonsai is finding the ideal balance of soil, tree type, container size, nutrient delivery and watering schedule—and then fitting it to developmental stage of the tree.

Take control: pumice grow media can be size (grade) blended for the ideal combination of drainage, moisture retention, and nutrient availability to meet the needs of each tree and its container.

PUMICE STONE MEDIA. The pumice from the Hess deposit in southeast Idaho is extremely pure, the result of centuries of relentless wave action from an ancient inland sea that washed the pumice clean of heavy metals, salts, and other impurities. The Hess grades marketed for Bonsai soils are heated and dried, then screened to grade and sifted to remove fines. Essentially a foamed glass stone, the random particle shapes and sizes provide excellent aeration, will not compact, hold moisture and nutrients on surface nooks and crannies while allowing full control of drainage rate via media size selection and grade blends. The rough-hewn, finely-riven surface of each tiny stone provides an ideal habitat for beneficial bacterial and mycorrhizae as well as grippy purchase for mosses or cover mulches.

RUTSU™ PUMICE GROW MEDIA. Hess markets a selection of pumice grades especially for Bonsai applications under the Rutsu™ brand name and available to purchase online from the Pumice Store. These grades (and others) are also available in wholesale quantities for packaging and or blending by Bonsai supply retailers.

Pumice in Greenhouses: Soil and Soilless Applications

Greenhouse growing, whether in raised beds or in containers, calls for engineered soils, even soilless grow media, depending on the growing process and crop.

GREENHOUSE SOILS. For all the reasons detailed above and on the Pumice as a Component of Engineered Soils page, light, frothy pumice is a highly functional component of engineered soils for horticultural purposes.

SOILLESS MEDIA. Hydroponics and aquaponics systems that source natural light are often contained in greenhouse-type structures. These carefully calibrated and highly-efficient systems rely on soilless grow media. The benefits of pumice as a soilless grow media are covered above.

Pumice as a Mulch

Our larger pumice grades—3/4 x 5/16 or 1 x 3/4—are used as a decorative, functional mulch. Pumice mulches suppress weed growth, retain moisture in the soil, protect soil from erosion, and help regulate temperature fluctuations in soil. And unlike bark mulches, they stay put. Hess pumice is a light grey-white in color.