Substrate additives for plant nutrition.
Continued…
Bentonite. This is a clay-based cat litter filler. Judging by its composition, 60-70% of which is montmorillonite, it is a clay with high absorption capacity and, accordingly, CEC. It has been used very successfully for many years as a bottom layer when growing aquatic plants, but it has the same disadvantages as regular clay – it turns into slurry and makes the water cloudy during plant replanting, quickly becoming anaerobic. It is used in a mixture with peat, sand, and activated carbon. Since it completely dissolves in water, the granule size does not play a role when purchasing.
Some fillers may have a pH of 7.5-8.5, making them suitable for growing only certain Cryptocoryne species. Pay attention to what is written on the package, and certainly do not use fillers with fragrances and other additives for this purpose! Nevertheless, it is significantly better to use a kitty litter brand proven in the aquarium hobby than to put clay gathered from a local quarry into the substrate. An order of magnitude better than bentonite is zeolite/sepiolite, which is much less commonly used as a cat litter filler.
Vermiculite (Vermiculite/Yolo loam). Vermiculite is a natural, environmentally friendly, non-toxic material produced in special kilns by firing. The color of vermiculite varies from golden yellow to yellowish-brown. The main and most valuable property of vermiculite is its ability to dramatically and unusually increase its volume by 10-20 times when calcined. A mixture of sphagnum peat with vermiculite has a very high CEC, but vermiculite has a number of drawbacks. If you use clay, you may not need vermiculite, as clay has similar properties.
It is a buoyant material that cannot be used without mixing it with a viscous material, such as clay or perlite, which has its own drawbacks. You can mix vermiculite with gravel and other fillers by kneading the mixture with your palms. Vermiculite slowly releases nutrients for plant consumption. This inorganic material can absorb phosphate – some of it remains in a form accessible for plant consumption, but it cannot absorb nitrate, chloride, and sulfate.
Vermiculite can fix ammonium in a form unavailable for plant nutrition. This fixed nitrogen is slowly transformed by microorganisms into nitrate and becomes available for plant consumption. Vermiculite that has been treated with water-repellent substances cannot be used as an aquarium substrate.
Laterite – from the Latin word later (building brick) – is a porous rock formed by the weathering of aluminosilicates in a hot and seasonally humid climate, having a high iron content, which gives it a reddish color. This soil formation process leaches all nutrients from it except iron. The terms laterite and clay are often confused, but they are not the same thing. Clays usually have a high silicate content and may contain other minerals besides iron.
Laterite is primarily useful for deep-rooting plants. Laterites poor in iron hydroxide and rich in aluminum hydroxides are called bauxites. Laterite is never used as an independent substrate, but only as an additive with gravel or sand. Usually, laterite is used as the bottom layer of the substrate because it is rich in iron. It is good to add a thin layer of sphagnum peat placed on top of the laterite. The peat will provide organic acid, which helps iron transition into a form more accessible for plant use.
In fishkeeping, laterite is used as a fertilizer; in a newly set up aquarium, it provides root nutrition to plants for the first year of life. Then enough fish waste accumulates in the substrate, so when laterite is depleted, the plants already have enough food under their roots. But at the beginning, when we plant them in clean, washed gravel, additional fertilization is required. One box of this Duplas substrate is enough for a 250-liter aquarium.
Granulated Laterite and Duplarite G mixed with a small amount of peat are significantly better than natural laterites, but there are also many complaints about them. In principle, they can be used as a mixture base instead of lava rock, but they lack such porosity and are very expensive. It is very important that it is completely covered with substrate to prevent it from entering the water. If the substrate layer in the foreground is quite low, about 2 cm, it is better not to put laterite under it. If the substrate layer in the back is about 10 cm, pour a layer of laterite in the middle.
Laterite is best used if you plan to grow aquarium plants with a well-developed root system that send roots deep down, such as, for example, Cryptocoryne and Echinodorus species. Most stem plants and mosses get nutrients from the water, so this substrate is not a significant growth helper for them.
Peat is a mixture of partially decomposed organic fossil materials. It is very rich in organics and humic acids. Peat releases less organics into the water than compost or soil. Peat is used as a substrate additive. The humic acids released by peat prevent [Fe++] oxidation and precipitation by attaching to the dissolved iron ion (natural chelator). Humic acids (as well as substrate heating) noticeably lower the redox potential of the substrate. A low redox potential significantly increases the solubility of minerals that can easily change their state, such as iron [Fe++] and magnesium [Mg++] ions.
This greatly increases the concentration of dissolved iron [Fe++] and other micro- and macroelements in it, increasing their availability to plants. It also helps increase the concentration of ammonium [NH4+] formed in the substrate from nitrates, which enhances the availability of nitrogen [N] and phosphorus [P] for plants. An excessive amount of peat in the substrate will lower the pH too much, and with it the CEC. To prevent peat from clouding the water, use a layer of gravel or sand on top of it. Peat has the ability to soften water, reducing its hardness. Due to increased acidity, peat helps create an elevated concentration of plant-available iron in the substrate.
All the materials listed above, when mixed with peat and activated carbon, create a good nutrient base with a high CEC, but the substrate rather quickly (within a year or earlier) becomes completely anaerobic, and plant roots rot. Anyone who has used such mixtures is familiar with the situation of black roots and a hydrogen sulfide odor just a few months after setting up the aquarium.
Calcined clay. Of all substrates with high CEC for top substrate layers, clays that can be thermally processed to produce calcined clay called argillite are the most promising. Soil additives made from argillite are produced in very large quantities, are cheap, and widely available. They have excellent CEC, 74% porosity, and are physically completely stable (this is 2-5 mm gravel) – meaning they have an unlimited service life.
Since calcined clay is not produced in Russia and Ukraine, and imported options are unavailable for sale, you can try to find it at newly constructed or renovated sports facilities – this material simply has no alternative when constructing sports fields. If the calcined clays listed above are not available to you, you can make the bottom layer out of ordinary gravel by adding 10-20% zeolite. This will not spoil the appearance of the main top substrate layer.
Another example of the successful application of porous materials similar to lava rock in the lower substrate layers is Gravelite (Big River Industries, Inc. USA). This is high-temperature expanded clay (not to be confused with expanded clay aggregate/haydite, gravelite is completely different!). It is widely used as building insulation. It is produced in various granule sizes – from 38 mm down to sand for concrete additives. Gravelite is very light, but sinks in water. It is completely inert and does not change pH or water hardness. Since it is made of clay, some CEC may be present. Gravelite is used just like lava rock – a 2 cm thick bottom layer with 8-12 mm granules with the addition of peat. On top – 2-5 mm pebbles.
Methods of laying substrate in an aquarium. Undergravel filter.
Substrate additives for plant nutrition. Clays. 
