Phosphorus plays a vital role in energy transfer and is an important "ingredient" of genetic compounds and enzymes. Healthy root development and flower formation also depend on phosphorus availability within the plant. Phosphorus is taken up by plants through the roots in the form of phosphate (P04 '), which is present in the substrate at much higher levels than in the water. This is because phosphate will react with metal oxides - notably iron oxide - more frequently in open water, creating insoluble forms, such as iron phosphate, that cannot be used by plants. In open water, there is greater movement and mixing, hence the increased likelihood of contact between phosphates and metal oxides. This contact does not occur as often in the substrate, where phosphate remains in a usable form. In some cases, C02 produced by the roots during respiration can break down the bonds within insoluble phosphate compounds and allow phosphates to become available for plant uptake.
Phosphate is often present in fish food, so levels are rarely deficient in the aquarium. In an average aquarium phosphate levels are frequently 1-3 mg/liter, while in natural conditions levels are normally only about 0.005-0.02 mg/liter. Low phosphate levels are not normally a concern, but high levels can encourage algae to bloom. To grow strongly, algae require phosphate levels above 0.03 mg/liter; since these levels are usually exceeded in the aquarium, algae blooms are often the result. Under normal conditions, most phosphate is "locked away" in the substrate and unavailable to algae. There should be no need to add phosphate to a planted aquarium, although it may be present in some nutrient-rich and soil-based substrates.
Oxygen is taken up by plants in its gaseous form (02), as water (H20) and as carbon dioxide (C02). Oxygen is • a vital structural component of cells and used during photosynthesis, although it is also a waste product of photosynthesis. Plants obtain the majority of their oxygen through their roots and from respiration. (It is also released from the roots.) Aquatic plants have large internal "channels" that make up a high proportion of their structure. These are used for transporting oxygen around the plant, most notably to the roots. Once oxygen is transported to, and released by, the roots, it combines with carbon and/or organic elements within the substrate, creating C02, which is taken up for photosynthesis. Releasing oxygen around the roots also helps to prevent localized anaerobic conditions, which can damage roots.
Despite this high usage and waste of oxygen, plants do not do well in high-oxygen conditions and require only a small dissolved oxygen (D.O.) content. This is because when dissolved oxygen levels are high, a number of nutrients, especially iron (Fe), bind with oxygen and become too large to be assimilated by plants. High oxygen levels prevent plants from obtaining other vital nutrients in sufficient quantities. During the day, plants photosynthesize and produce waste oxygen, so there are never oxygen-deficiency problems at this time. The only time when oxygen becomes low is at night, when plants do not photosynthesize but continue to use up oxygen through respiration. In a heavily planted aquarium with little water movement or a large number of floating plants, the air/water gas exchange is reduced and oxygen levels may drop severely. However, levels rarely drop too low for plants, although they may drop below the levels needed by fish. In most cases, it is not necessary to provide oxygen and/or aeration in planted aquariums.
Adding oxygen through aeration is not normally needed in a planted tank.
Phosphorus is mainly absorbed through the roots, here of Crinum thaianum
Floating plants, such as this Salvinia natans, are ideally placed to receive light and nutrients.
Anubias grows best attached to rocks or bogwood, where its roots can absorb nutrients such as sulphates directly from the water.
Potassium is a very important plant nutrient that should not be ignored in a planted aquarium. It is a key component of a plant's biological systems and used in protein synthesis, the opening and closing of stomata (pores), seed development, root production, disease resistance, and photosynthesis. A potassium deficiency creates an overall weakness in a plant's development and appearance and also impedes Iphotosynthesis:
Plants absorb potassium as an ion (K+) from the water, rather than from the substrate, despite the fact that both in nature and in the aquarium, most potassium leaches from the soil or substrate. We do not entirely understand why this should be so, but allowing potassium to remain in the substrate may increase the availability of ammonium to plant roots. As tap water contains very small quantities of potassium, it is important to introduce it artificially to the aquarium, either by means of a liquid fertilizer or, more commonly, as one of the ingredients contained in nutrient-rich or soil-based substrates. Potash and/or granite dust are often mixed with nutrient-rich substrates to provide potassium.
Sulphur is used in the production of amino acids, proteins, and chlorophyll, and is normally present in adequate quantities in tap water. Plants absorb sulphur in the form of sulphate (S042~), and many aquatic soil-based substrates contain sulphate in quantities that are quite adequate for most plants. Some fertilizers also contain forms of sulphate. Rainwater also contains relatively high levels of sulphate, but these levels vary considerably. One of the main reasons why you should be cautious about using rainwater in the aquarium is the high level of sulphur that can be present during the first few minutes of rainfall. Sulphur in its raw form is a dangerous chemical and should not be introduced into the aquarium in large quantities.
Floating plants, such as this Salvinia natans, are ideally placed to receive light and nutrients.
These nutrients are required only in extremely small quantities and are often described as "trace elements." They play important roles in the biochemical processes that sustain life and are vital for good plant health.
Boron is absorbed by plants In the form of borate (B033~) and Is required for cellular membrane function, root growth, carbohydrate transport, metabolic regulation, and flower production. Aquatic fertilizers often contain boron in the form of borate 'or borax (sodium borate) and it is also present in most tap water sources. Normally, boron deficiency is unlikely to occur in the aquarium and the nutrient is not considered a vital additional source of fertilization.
Chlorine is absorbed by plants in the form of chloride (Cl~) ions and is used for osmosis, ionic balance, and also in photosynthesis. Chloride is normally present in sufficient quantities in tap water (even after the use of dechlorinators), so should not present any nutritional problems for plants.
Aquatic plants use nickel as an ion (Ni2+) in extremely tiny amounts in the production of the enzyme urease (which breaks down the nitrogenous compound urea into ammonia). It is normally present in tap water and soil or in nutrient-based substrates. Nickel deficiencies or excesses should not occur under the vast majority of aquarium conditions.
Iron is an important micronutrient used in respiration, enzyme production, and chlorophyll synthesis. Plants will absorb iron both through their roots and leaves. As a nutrient, iron is most useful to plants in the ion form Fealthough in the presence of oxygen it converts into Fe3+, which is difficult for plants to assimilate. This problem can be overcome by using chelates or chelated iron. (Chelates are dissolved organic substances that bind to metals and prevent them from forming larger molecules through oxidation.) The most common chelate used in fertilizers is EDTA, which is often used to supply chelated iron (FeEDTA). Fe2+ is then slowly released by the chelated iron and becomes available to plants.
Although iron - or, more precisely, chelated iron - is heavily promoted as a vital plant fertilizer, there is a plentiful supply in soil-based or nutrient-rich substrates. Iron and natural organic chelates, combined with a low-oxygen substrate, ensure that iron is always present in an available form. Most aquatic fertilizers contain iron and chelates and should always be used in aquariums without soil-based substrates.
Tiny quantities of micronutrients are needed by aquarium plants, such as this Echinodorus 'Rubin' narrow leaves.
Plants absorb copper as an ion (Cu2+), both from the water and the substrate, although humic acids and organics in the substrate often bind with copper and other metals, making them unavailable for plant uptake. Copper is a key part of enzymes that facilitate respiration, but is only needed in tiny amounts by plants. Additional fertilization with copper is not required. Indeed, in most cases, tap water contains far more copper than plants require, but since they have little or no control over how much they absorb, they simply take up whatever is available. If they absorb too much, the result is metal toxicity, typically resulting in brown spots and tissue breakup. Copper is occasionally used in treatments to control parasites or algae, so use these with care.
The maximum safe level of copper in water is much higher for humans: 1.3 ppm (parts per million, equivalent to mg/liter) than for fish: 0.02 ppm. For this reason, plants offer themselves as a vital resource for reducing copper concentrations in the aquarium, which may be at dangerous levels for fish if the water is sourced from tap water. Plants are sometimes used exclusively for the purpose of removing copper and other dangerous metals.
Manganese is absorbed as an ion (Mn2+) through the roots and leaves of aquatic plants. It activates enzymes used in chlorophyll production and photosynthesis. Plants require relatively low levels of manganese, but it is still an important micronutrient In most cases, manganese is present in tap water in sufficient quantities, but fertilizing with a liquid fertilizer will ensure that plants do not suffer a manganese deficiency.
Molybdenum is an important nutrient for aquatic plants. It is a component of an enzyme used by plants to reduce nitrates (N03~) into ammonium (NH4+) for protein synthesis, and is especially important in hardwater conditions, where little or no ammonium is; available as a source of nitrogen. Plants absorb molybdenum in the form of molybdate (Mo042~). This is available in tap water, but additional fertilization with substrate, tablet, or liquids will ensure sufficient levels in the aquarium.
Zinc, an important nutrient for overall plant health, is a component of many enzymes and is involved in chlorophyll formation. Zinc is taken up in its nutrient form (Zn2+) through the leaves and roots. At high concentrations it is toxic to both fish and plants, although sensible use of liquid fertilizers should ensure that these do not occur. Traces of zinc at levels high enough for aquatic plants are present in tap water, liquid fertilizers, and nutrient-rich substrates.
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