Substrate and Fertilization Introduction
By Robert Paul H

Plants need a balance of macro nutrients, (those they use the most of), and minor or trace nutrients, (which they use to a lesser degree).

The Substrate is the growing medium that the plants are rooted in. It is important to add a source of these nutrients to the substrate, particularly in a new aquarium that has no mulm or fish waste settled in it yet.

It is also advisable to use a medium such as porous gravel that will provide a good CEC and not compact together. CEC, (cation exchange capacity) is the ability of the medium to absorb cation ions, (minerals from fertilizers) and hold them making them accessible to the plants when the plants need them. Sand and coated gravel do not provide a good CEC. Good CEC mediums include porous gravel, clay litter, and clay soils.

Macro nutrients include nitrogen, oxygen, CO2, potassium, phosphorus, calcium. Minor elements include iron, Boron, zinc, manganese, and other trace minerals. Iron is an important element for many plants and is often added to the substrate with other minerals.

Laterite is a sediment soil that is formed in nature by decaying rocks which are high in iron and aluminum. There are a few aquarium products made of laterite, such as Duplarit, and First Layer.  Other sources of iron are soils, clay litter, red pottery clay, and sphagnum peat.

Various substrate methods

• Layered substrate with "sub" soil (soil low in organics), sphagnum peat, gravel, and trace element mix, as outlined by Steve Pushak

• Pottery clay balls enhanced with fritted trace elements, as outlined by Steve Pushak, or commercial additives made for the aquarium

• Clay gravel

• Granular laterite, sphagnum peat, and gravel, my mix of choice.

Fail-safe beginner substrates:

• Commercial products, laterite, clay gravel, clay based additives made for the aquarium

NPK What are those three numbers?

 Nitrogen supplied by the fish, phosphates by the water supply and uneaten food, and potassium to a lesser degree in the water supply. NPK fertilizers should only be added if you have low or unreadable levels already.  RO, distilled, and some bottled spring water will be low in NPK and mineral elements. Most tap water will have sufficient levels of P. Even some aquarium products contain NPK. An NPK fertilizer high in potassium, but low or 0 in nitrate and phosphate is has the least affect on algae. Check the numbers. Single digits are low, double digits are high. In a heavily planted tank with fast growing plants, and a small number of fish, it is possible to have consistent 0 readings of nitrate and phosphate creating a nitrogen and phosphate deficiency for the plants.

Target nutrient ranges

Types of fertilizers

Sources of trace elements
 

• Soils: Iron (Fe) other trace elements High CEC

• Pottery clay: Iron, (Fe) High CEC

• Clay litter: Iron, (Fe) High CEC

• Vermiculite: trace amounts Iron, Potassium, Magnesium Very High CEC

Drawbacks of Soil substrates

When plants are removed, replanted, the soil mixture can come up with the plants and pollute the water. You are better off using a clay gravel if you anticipate moving and transplanting plants often.

The Following information is taken from "Something to Grow on", Cornell University. It is not written specifically for aquariums, but the information is very useful. I particularly like the information on CEC.
 

Ions Cation exchange capacity
(CEC) Quantifies the ability of media to provide a nutrient reserve for plant uptake. It is the sum of exchangeable cations, or positively charged ions, media can adsorb per unit weight or volume. It is usually measured in milligram equivalents per 100 g or 100 cm3 (meq/100 g or meq/100 cm3, respectively). A high CEC value characterizes media with a high nutrient-holding capacity that can retain nutrients for plant uptake between applications of fertilizer. Media characterized by a high CEC retains nutrients from leaching during irrigation. In addition, a high CEC provides a buffer from abrupt fluctuations in media salinity and pH. Important cations in the cation exchange complex in order of adsorption strength include calcium (Ca2+) > magnesium (Mg2+) > potassium (K+) > ammonium (NH4+), and sodium (Na+). Micronutrients which also are adsorbed to media particles include iron (Fe2+ and Fe3+), manganese (Mn2+), zinc (Zn2+), and copper (Cu2+). The cations bind loosely to negatively charged sites on media particles until they are released into the liquid phase of the media. Once they are released into the media solution, cations are absorbed by plant roots or exchanged for other cations held on the media particles. Anion exchange capacity Some media retains small quantities of anions, negatively charged ions, in addition to cations. However, anion exchange capacities are usually negligible, allowing anions such as nitrate (NO3-), chloride (Cl-), sulphate (SO4-), and phosphate (H2PO4-) to leach from the media.

Cation Exchange Capacities for various growing media amendments and selected media.
 

Sphagnum peat moss

Sphagnum peat moss, derived from the genus Sphagnum, contains at least 90% organic matter on a dry weight basis. In addition, this peat moss contains a minimum of 75% Sphagnum fiber, consisting of recognizable cells of leaves and stems. Approximately 25 species of Sphagnum exist in Alberta, Canada and 335 species are present throughout the world. Sphagnum fuscum is an important species bearing many desirable traits. Sphagnum grows in northern cool regions and is also located in peat bogs found in Washington, Maine, Minnesota, and Michigan. Many pores are present in the leaves of sphagnum; when used as growing media, as much as 93% of the water occupying this internal pore space is available for plant uptake (Peck, 1984). After draining, sphagnum peat can hold 59% water and 25% air by volume. Sphagnum is usually characterized by an acidic pH, low soluble salts content, structural integrity, and the ability to serve as a nutrient reserve (Landis, 1990). Although peat mosses are classified into four different groups, variation may exist within any one type of peat moss. Peats of the same classification often differ notably in quality, and even peats from the same bog taken from separate layers can possess different chemical and physical properties.

Sphagnum peat moss is classified as light or dark peat, based on its color. Light peats are characterized by a large amount of internal pore space, 15-40% of the pore space comprises aeration porosity Dark sphagnum peat does not display the elasticity of light peat and is usually not as long lasting.. Dark sphagnum peat moss maintains twice the cation exchange capacity of light peats, yet does not possess as much total or aeration porosity. An associated table lists general characteristics of sphagnum peat moss.

Inorganic media

Materials such as vermiculite, perlite, and sand represent the inorganic fraction often used in container media formulations. These materials generally increase the aeration porosity and drainage yet decrease the water-holding porosity of media. Inorganic components are usually inert materials characterized by a low cation exchange capacity.

Vermiculite

Vermiculite is a commonly used inorganic media component which is mined in the U.S. and Africa. This mineral, comprised of an aluminum/iron/magnesium/silicate mixture, is excavated as a material composed of thin layers. Processing includes heating the vermiculite to temperatures upwards of 1000ƒC, which converts water trapped between the layers of the material into steam. The production of steam results in a pressure that expands the material, increasing the volume of the pieces 15 to 20 times their original size. Vermiculite is sterile because of these high heating temperatures used during processing. Vermiculite is characterized by a high water-holding capacity as a result of its large surface area: volume ratio, a low bulk density, nearly neutral pH, and a high cation exchange capacity attributed to its platy structure. Because it compacts readily when combined with heavier materials, vermiculite is sometimes recommended more for propagating material than container media.

Vermiculite gradually releases nutrients for plant absorption; on average it contains 5-8% available potassium and 9-12% magnesium. This inorganic media component can adsorb phosphate--some of which remains in an available form for plant uptake--but cannot adsorb nitrate, chloride, or sulfate. Vermiculite can fix ammonium into a form that is not readily available for plant absorption. This fixed nitrogen is gradually transformed to nitrate by microorganisms, making it available for plant uptake.

Vermiculite is manufactured in four different grades, differentiated by particle size. Insulation grade vermiculite and that which is marketed for poultry litter (which has not been treated with water repellents) has been used with some success. Vermiculite which has been treated with water repellent, such as block fill should not be used as growing media. Because vermiculite tends to compact over time, it should be incorporated with other materials such as peat or perlite to maintain sufficient porosity. It should not be used in conjunction with sand or as the sole media component, because as the internal structure of vermiculite deteriorates, air porosity and drainage decreases (Landis, 1990).

The particle size of vermiculite influences the water-holding and aeration porosity of the material. Although grade classification is based upon particle size, each grade is represented by a range of particle sizes. Note that grades consisting of larger particle sizes have a higher aeration porosity and lower water-holding porosity than grades consisting of a smaller range of particle sizes. Properties of the four vermiculite grades are shown in an associated table.

Perlite

A mineral of volcanic derivation, perlite is a second inorganic component which may be used in formulating container mixes. This chemically inert material is extracted in New Zealand, the U.S., and other countries and is usually mined by scraping the earth's surface. The processing method includes a grinding and heat treatment (up to 1000‰C) which results in very lightweight, white sterile fragments. As the ore is heated, internal water escapes as steam, resulting in the expansion of the material.

Perlite has a very low cation exchange capacity, low water-holding capacity (19%), and neutral pH. The closed-cell composition of perlite contributes to its compaction resistance, enhances media drainage, and heightens the aeration porosity of peat-based media (Bilderback 1982). Because perlite contains only minute amounts of plant nutrients, liquid feeding is a practical mode of fertilization. Be aware of possible aluminum toxicity in acidic media (pH < 5).

The very low levels of fluoride perlite contains is not likely to pose plant health problems. Any soluble fluoride present in a media characterized by 6.0 < pH < 6.5 will precipitate out of the media with excess calcium from sources such as gypsum, limestone, or calcium nitrate.

Although perlite has several positive attributes, it also has drawbacks. Perlite consists of many fine fragments which, when dry, can lead to lung or eye irritation. In addition, because water clings to the surface of perlite, it may tend to float in the presence of water (Landis, 1990).

Perlite contains, on average, 47.5% oxygen, 33.8% silicon, 7.2% aluminum, 3.5% potassium, 3.4% sodium, 3.0% bound water, 0.6% iron and calcium, and 0.2% magnesium and trace elements (Perlite Institute, 1983). Although a uniform categorization of perlite does not exist, individual producers of this inorganic component assign grade levels. Perlite classifications for horticultural use are listed in an associated table. This inorganic media amendment is sometimes recommended for use only in propagation media because of its low bulk density and tendency to compact.

In comparison with sand, polystyrene, or pumice, perlite has the greatest inner total porosity. Coarse perlite is characterized by approximately 70% total porosity, 60% of which is aeration porosity. Perlite can retain two to four times its dry weight in water, which is much greater than that of sand and polystyrene, yet much less than the water-holding capacity of peat and vermiculite (Moore, 1987).

Sand

Sand has been used as an inorganic media component to add ballast to containers. Some sands contain calcium carbonate which may raise media pH undesirably. A rise in pH may lead to nutrient deficiencies, particularly of minor elements such as iron and boron. A few drops of dilute hydrochloric acid or strong vinegar may be added to sand to test for carbonates; if bubbling and fizzing result, carbonate is present as a result of carbon dioxide production. Sand used for container media should have a 6 < pH < 7. Sand maintains good drainage, a low water-holding capacity, and a high bulk density when used independently of other materials. Because of its shape and size, sand can obstruct pore spaces, decreasing drainage and aeration, instead of improving porosity. Various sand particle sizes have been recommended for container media use, including ranges of 2-3 mm or 0.05 - 0.5 mm (fine sand) in size (Landis, 1990). In addition, another recommendation suggests that 60% of the particles be within 0.25-1.0 mm range, and 97% be greater than 0.1 mm and less than 2 mm (Swanson, 1989). Uniformity coefficients assigned to sand mixtures signify the amount of sand which is within a certain size range; a coefficient < 4 is evidence of a homogeneous sand mixture (Swanson, 1989). If the correct grade of sand is used, the wet ability of the media is enhanced.

Calcined clays

When fired at high temperatures, some clays, fuel ash, and shales form stable compounds that possess low bulk densities and internal porosities of 40-50%. Though calcined clays alter the physical attributes of media in a positive way, they also decrease the level of water-soluble phosphorus in the mix. Because calcined clays are characterized by a high cation exchange capacity, fertilizer application rates may need to be modified if calcined aggregates are incorporated into the media mixes (Bunt, 1988).

Pumice

Pumice is produced as volcanic lava cools; escaping steam and gas contribute to its porous nature. This alumino-silicate material contains potassium, sodium, magnesium, calcium, and slight amounts of iron. Pumice can absorb K, Mg, P, and Ca from the soil solution and render it available for plant absorption later (Bunt, 1988).