Individual elements and their importance for plant organisms.
The average Nitrogen content in plants is about 3%. This element is the basis for creating plant proteins necessary for their growth and reproduction. During the decomposition of 
organic substances, which plants cannot assimilate in their initial state, amino acids are formed, but their assimilation by plants is also very difficult. Then bacteria living in the bottom layer of water and mainly in the substrate come into play. They process organic substances into amines, nitrites, and nitrates. Plants easily extract these compounds from the water and substrate and assimilate them. Nitrogen deficiency leads to a decrease in the chlorophyll content in leaves, primarily in older ones, and to a decrease in plant size. In an aquarium populated by fish, nitrogen starvation of plants is practically non-existent. An excess of nitrogen compounds is more frequently observed.
However, since the protein decomposition process is long, at a certain stage, especially in a new aquarium, plants may starve until the bottom microflora copes with processing the accumulating organics, and plants are supplied with the required amount of nitrogen. In an old aquarium with rich vegetation, as well as in a new aquarium, signs of nitrogen starvation can be observed: premature death of old leaves, yellowing of leaf edges and tips gradually spreading to the entire leaf blade, and slowed growth. This is due to the fact that the bottom microflora cannot cope with the processing of organic compounds and does not have time to convert them into a form suitable for plant assimilation. In this case, small additions of nitrogen fertilizers can be applied to the aquarium.
Which of these compounds are best to use? The choice depends on the active reaction of the environment, i.e., the pH value. It is better to add nitrates to an aquarium with a strongly acidic environment — pH below 6.5. Experiments have shown that in acidic water they are better assimilated by plants than ammonium salts. On the contrary, in neutral and slightly alkaline water, ammonium nitrogen is assimilated much better. In this case, it is preferable to use urea, or carbamide. When signs of nitrogen starvation appear in an old aquarium, urea can also be added to the water. Urea should be added to a new aquarium only after signs of macrophyte growth appear. The uniform addition of small amounts of nitrogen fertilizers will not affect aquarium inhabitants, as plants will manage to assimilate all nitro compounds.
In terms of consumption, Phosphorus comes next after nitrogen. This element takes a most active part in the processes of energy storage and expenditure, as well as in the processes of 
plant respiration and nutrition. Phosphorus accumulates in the largest amounts in young plant shoots. Phosphates serve as the primary source of phosphorus for plants. Phosphorus enters the aquarium with fresh water and fish food. Remnants of organic substances enter the substrate, where they are transformed into inorganic phosphate ions and assimilated by plants in this form. With a lack of phosphorus, the red pigment anthocyanin accumulates in the leaves, the leaves become smaller and narrower. Signs of phosphorus starvation include darkening of young leaves, curling of leaves and shoots, and the appearance of brown and reddish-brown spots on old leaves. Superphosphate is most widely used. When signs of mineral deficiency appear, complex fertilizers containing phosphorus are added to the water.
The importance of the biogenic element Potassium for plants is diverse. It does not induce the development of 
algae. It promotes the normal course of photosynthesis and participates in the formation of nutrients. This element participates in carbohydrate synthesis and accumulates mainly in young plant tissues. Potassium takes part in most enzymatic processes occurring in plant tissues. The highest concentration of potassium ions is found in plant leaves. A deficiency of this element disrupts nitrogen metabolism and leads to tissue death. Potassium deficiency usually manifests as brown and yellow spots on the leaf margins. Because an aquarium is a relatively isolated system, the amount of potassium in the water may be insufficient for plant development. Potassium is conveniently added to the aquarium as part of complex fertilizers, and monobasic potassium phosphate can also be used.
Sulfur is found in substances necessary for carrying out various redox reactions during photosynthesis. A deficiency of this element delays plant growth and reproduction.
Calcium plays an important role in the selective permeability of cell membranes. A deficiency of this element leads to insufficient membrane “density.” If young plants lack calcium, they turn pale and malformed. The amount of calcium in water determines its hardness, and therefore, only in very soft water—and not just soft, but having a hardness close to zero—can plants experience calcium starvation. In aquarium conditions, calcium deficiency is usually not observed, as it is always present in tap water in sufficient quantities.
Magnesium is of great importance in the life of plants. This element plays a significant role in metabolism, especially in young plant organs. However, hardness in aquariums is often increased by adding only calcium salts to the water. With a deficiency of this element, leaves turn yellow due to chlorophyll deficiency. In this case, plants may experience magnesium starvation, which manifests as white spots between the leaf veins followed by the breakdown of leaf blade tissues. Its deficiency in water is much more common than calcium deficiency. Therefore, when artificially increasing water hardness, a combination of magnesium and calcium salts must be used.
Silicon is also often classified among macronutrients. It is part of the skeleton of many terrestrial plants, providing strength to their stems. Aquatic plants, being in suspension, absorb silicon in significantly smaller quantities, since skeletal strength does not play such an important role for them. In aquarium conditions, silicon starvation is practically never observed.
