Laterites, baked and unbaked.

You hear a lot of questions and some controversy about laterite now. Just what is "laterite," anyway? It's a highly-weathered tropical clay, or at any rate one that has formed in climates that were tropical/subtropical during the geological past. Laterite clay bakes hard as a brick ("later" is Latin for a fired brick or tile) in the tropical sun. Laterite derives from a wide variety of rocks weathering over millions of years in a well-drained but seasonally humid climate with fluctuating soil water levels, where water has even leached out silica. All organic substances have been weathered out too: reddish tropical lateritic soils, in the Amazon basin for example, are notoriously unproductive, once the forest cover has been burned off. What remain behind in laterites are oxides and hydroxides, mostly of aluminum, silica, iron, plus small amounts of calcium, magnesium, titanium, potassium. Laterite's flat, plated crystalline clay structures have many negatively-charged molecular sites. Such sites are hard to come by in the aquarium,aside from laterite and decomposed organic matter (such as "humus") and organic solutes (such as "humic acids").

Clay from backyard subsoil or even potter's clay would also have negative charge sites, but many of those sites will already be occupied with positive ions of calcium (Ca⁺⁺) and magnesium (Mg⁺⁺), which will adversely affect the hardness and pH. Laterite pure and simple has very little Ca or Mg remaining in it. Laterite binds ammonium and other positively-charged ions like Fe⁺⁺, K⁺, etc. and holds them, until a root hair exchanges another positive ion for the ammonium, which the plant uses to build amino acids and proteins, or until nitrifying bacteria scavenge it. The cations that cluster around attract other anions like phosphate (PO₄¯). In this way laterite, like other sources of colloidal clay in your substrate, can help you maintain lowered levels of phosphate in the water, which in turn will discourage algae. Phosphate is also readily adsorbed onto the kind of iron oxides that make Flourite so reddish, Diana Walstad points out. Plant roots can tap into these precipitates by respiring CO₂, which gives a highly localized acidic reaction that dissolves the precipitates. Not the kind of substrate microzones you'd want to disturb with a gravel vacuum.

Thus, though laterite itself is depleted of nutrients (save the micronutrient iron), it binds to itself many plant nutrients. Laterite's high CEC ("cation exchange capacity"). the capacity to adsorb and hold positively charged ions until a plant's root hair takes them up, is often casually mis-imagined as making laterite itself "nutrient-rich."

Some links. For a good easily-understood description of the chemistry of all this, see George Booth's "AquaticConcepts."

Stephen Pushak's carefully-written Sept 1995 post about laterite, CEC and the chelation of metal ions is like a brief article embedded in the long, somewhat diffuse, highly informative thread "Everything you want to know about laterite," archived at theKrib.com. You should read the whole thread, which was sparked by the early advocacy of iron, laterite and substrate heating in The Optimal Aquarium.

Jamie Johnson's 2000 article "Substrates for the planted aquarium" provides analysis and overview of physical and chemical properties of laterites and other substrate components. He defines the types of lateritic clay. He requested substrate clay samples through the Aquatic-Plants Digest and wound up running tests on 25 commercial products, local clays and homemade blends. His tables provide some figures for use in discussions to back up your opinions about laterites, but his sections on the purposes, installation, lifecycles of various substrate elements are more widely applicable.


Red or gray? Laterite's porous and clay-like structure may be rich in insoluble iron oxides. Typical exposed surfaces of iron-rich laterite are blackish brown to reddish and tend to have a slaggy, lavalike appearance. The ferric oxides give it a rich look: however, acidic and anoxic conditions in the substrate are needed in order to leach any of its iron. Other perfectly acceptable laterites are poor in iron and gray in color. Laterites are inert: that means they won't affect the water's chemistry, neither stabilizing nor raising pH for example, nor leaching ions that would increase the water's total dissolved solids.

Cat litter is one market for laterite. The kind of unbaked laterite used for cat litter is bentonite, which is low in iron, giving it a dull buff to gray color rather than rusty reds. In their cat litter role, laterite products can reach the consumer market adulterated with perfumes, clump-formers and colorants that could slowly leach a low-grade toxicity if you were to use them in an aquarium. That could result in general malaise and mysterious deaths in aquarium systems that use some cat litter laterite in the substrate. Laterite can also be marketed for sweeping compound at your automotive outlet. This grade isn't safe for aquaria either; though laterite packaged by automotive companies as cleaning compound is unlikely to contain perfumed additives, it may have other kinds of contaminants. I've read angry forum posts from aquarists who were disappointed to find that laterite marketed at auto shops contained low-level toxins. They were warning us stay away from unbaked laterite altogether.

I suggest instead that you buy your cheap laterite in the form of "hypoallergenic" cat litter. It will still be the cheapest laterite you can find. I warn you that some very sensible people would disagree with me. Chris "Nomad" Cow, for one, has had negative experience, even with non-automotive laterite. He posted at AC, 21 March 2000,

"In my case, I observed classic low-level toxicity problems rearing their ugly head about two months after I filled the tank. It slowly killed about half the fish in the tank before I figured it out; the rest were listless and ignoring food, etc. When they were removed into other tanks, they recovered almost immediately (within an hour at most), and went back to their old voracious selves (and most have since bred for me). The substrate was laboriously hauled out and dumped in the back yard. Since refilling with gravel and the same rocks, decor, etc., there have been no problems in the 8 months following 'the incident'."

Cat litter laterite is unbaked. It has no advantages over the lightly-baked laterite products like Schultz Soil Conditioner or the naturally baked, iron-bearing Flourite beyond its price (it is cheaper) and its dense silty texture, which is supposed to give root hairs a "grip." Two national brands that are widely available and often recommended are Hartz pH5 and Lasting Pride. Their tested contents are tabled in Jamie Johnson's article mentioned above. When I use cat-litter laterite, I lay it down dry at the very bottom of the tank. If you're even considering pre-washing it, because you don't want that fine silt, cat litter is not for you, believe me. Remember that another use of bentonite is as slurry-- drilling mud for the oilfields. A thread characterizing bentonite and the virtually identical laterite montmorillonite is archived at www.koivet.com.

I should add, however, that energetic Red-Finned Blue Botia in one tank that has cat litter in the underlayer managed for many months to keep enough lateritic silt constantly stirred up to achieve a naturalistic "white water" effect, which you couldn't call truly decorative.

Various brands of cat litter laterite in substrate experiments were reported in 1997 at www.ugcs.caltech.edu/~boingy/litter.html None of these were the hypo-allergenic types.

Flourite. This Flourite I've been mentioning is Seachem Laboratories' tradename for their handsome dark, natural-looking red lateritic clay, rich in ferric iron (the insoluble ion) and containing other minerals, like manganese. Geological pressure and heat have lightly baked the native clay until it is partially fused, in other words, it is "sintered." Chin See Ming posted to the Aquatic-Plants Digest, 5 July 2001, a capsule interpretation of the geology, which sounds accurate. The clay is then broken up ("fracted" they tell us) into a fine gravel, and screened. It holds its hard gravelly texture much better than cat litter, and retains its porosity. I see Flourite as a complement to unbaked laterite, not a rival. Some aquarists have a quite fanatic allegiance. Seachem maintains a good honest informative website: www.seachem.com

On balance, I'd recommend unwashed Flourite or Profile/Schultz Clay Soil Conditioner as fine substitutes for cat litter in the under layer of a substrate, simply because they hold their consistency better in water. Only the Flourite or Schultz or Turface destined for the upper layer gets washed first, just enough so as not to cloud the water.

On the "Putting It All Together" page you'll find my not-flawless technique for laying down unwashed substrate materials without clouding the water.

Caveat: many aquarists, when faced with Flourite, are mistakenly tempted to keep washing it until what they have left resembles aquarium gravel; part of Flourite's value to plant roots, however, is in the fine stuff it contains. Don't wash it any more than you feel you have to. As for washing cat litter: it's like panning for gold: if you have any left at all, it's because you haven't washed it enough!

Profile/Schultz Clay Soil Conditioner (Schultz assumed the production of the Profile product in 1999) is available cheaply in 40-lb. bags at your garden center. According to a company spokesperson, the product is the same as their Aquatic Soil Conditioner, a natural inorganic arcillite clay composed of silico-aluminate with a little (5%) iron, not as much iron as Flourish, with a dun color between gray and tan. Unlike Flourite, the clay has been kiln-fired to give it a ceramic-like structure that is three-quarters pore spaces, chemically inert and high in CEC ("cation-exchange capacity"). The broken angular surfaces resist compaction, and it seems reasonable to expect that there may eventually be some anaerobic bacterial de-nitrification within the pore water.

As with Laterite, too-enthusiastic washing can remove the dust and small grit particles that plant rootlets need. For the lower substrate layer, one swish in a bucket is really enough washing.

Turface is a similar fired-clay product from Profile/Schultz, more reddish in color and coarser in texture, widely used for amending soil on athletic turf and a good amendment to aquarium substrates too. It's also available by the bag. There are useful posts on Clay Soil Conditioner and Turface at thekrib.com.

Duplarit is a red, iron-rich laterite mined in Sri Lanka and packaged for the Dupla Corporation. It is described, with evocative photos, by Kaspar Horst, one of Dupla's founders and a co-author of The Optimum Aquarium, at the Dupla website,along with a certain amount of mystification that might give you an inflated impression of the importance of iron in growing aquatic plants, and even that the reddish color of some tropical waters is owing to ferric iron (rather than humic acids), or that laterites not packaged for Dupla are somehow false imitations.