For plants, nutrients are the inorganic and organic compounds from which they can extract the elements that make up their bodies. These elements themselves are often referred to as nutrients.
Depending on the location of the plant (terrestrial or aquatic ), the nutrients are extracted from the air , water and soil . These are mostly simple inorganic compounds such as water (H 2 O) and carbon dioxide (CO 2 ) as well as ions such as nitrate (NO 3 - ), phosphate (PO 4 3− ) and potassium (K + ).
The availability of nutrients varies. It depends on the chemical behavior of the nutrient and the site conditions. Since the nutrient elements are required in a certain proportion, the availability of an element usually limits the growth of the plants. If you add this element, the growth increases. This process is called fertilization .
17 chemical elements are required for the growth of green plants : carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), potassium (K), magnesium (Mg), calcium (Ca) , Sulfur (S), manganese (Mn), zinc (Zn), iron (Fe), copper (Cu), boron (B), chlorine (Cl), molybdenum (Mo) and nickel (Ni). C, H and O are absorbed via air (CO 2 ) and in the form of water (H 2 O), the remaining elements in terrestrial plants via the soil. Due to the different quantities required, a distinction is made between bulk elements (N, P, K, Mg, Ca, and S) and trace elements (Mn, Zn, Fe, Cu, B, Cl, Mo, and Ni). The metals among the elements are absorbed by the plant in the form of metal ions, while the non-metals are usually absorbed in the form of chemical compounds - chlorine is an exception (absorption as chloride).
Classification of nutrients
A classification of nutrients is possible in different ways depending on the question. In addition to the classification according to non-mineral , mineral or organic , a grouping is also made according to availability, mobility, necessity or the required amount of the nutrient. One can distinguish core nutrients from main nutrients and micronutrients.
An important classification of nutrients is based on their need:
- necessary, essential nutrients, for example potassium; In addition to the core elements of organic matter (C, O, H, N and P), K, S, Ca, Mg, Mo, Cu, Zn, Fe, B, Mn, Cl in higher plants, Co, Ni;
- alternatively required, substitutable nutrients. It is primarily about different forms of bonding of a core element, e.g. B. nitrogen as nitrate , ammonium or amino acid .
- useful nutrients: Na + as a partial functional substitute for K + ;
- dispensable nutrients - about 70 elements that occur naturally; For example, iodine, which is essential for animals and humans, is unnecessary for plant nutrition.
In addition to the core nutritional elements carbon , hydrogen , oxygen , nitrogen , phosphorus and other main nutritional elements such as potassium , sulfur , calcium and magnesium, there are a number of micronutrient elements whose optimum effect is often very narrow, i.e. H. only small differences in the amount of these trace nutrients or micronutrients cause deficiency symptoms or over-fertilization.
Since hydrogen and oxygen are absorbed from the air as water and the carbon as carbon dioxide, they are often not counted among the nutrients. Nevertheless, a lack of water is just as harmful to land plants as a lack of carbon dioxide is to submerged aquatic plants and algae.
Since typical biomass has an average composition of the core elements of
- C 106 H 180 O 45 N 16 P 1
these must also be available in the corresponding proportions. This availability is realized differently in terrestrial biotopes than in aquatic ones.
For example, there is practically always a carbon supply on land due to the carbon dioxide content of the air, while the corresponding supply in the water can be used up. Then many specialized aquatic plants can alternatively cover their carbon needs from hydrogen carbonate. A replenishment of carbon dioxide through the water surface from the air is slow and only leads to low concentrations (0.5 to 1 mg / l). Most of the carbon dioxide content in water comes from the respiration of organisms.
The nitrogen demand on land is usually met by the nitrate and ammonium content of the soil and the groundwater. Specialized land plants can form a symbiosis with nodule bacteria, which are able to bind nitrogen gas (N2) into a biologically usable form. In aquatic biotopes, blue-green algae ( Cyanobacteria ) are able to fix nitrogen. Only their N-containing metabolites and decay products then make the increased N supply available to the ecosystem.
Phosphorus is required in the comparatively smallest amount, but its availability is usually very limited because of its tendency to form poorly soluble compounds, so that it often represents the minimum factor. In aquatic systems, P is the limiting factor in principle, unless it is intentionally fertilized with phosphate , as in carp ponds . Otherwise, phosphorus is the cause of the eutrophication of lakes and rivers.
The mechanisms of nutrient uptake and the usability of the nutrients for the plants depends on biological processes, physical and chemical soil properties or the physical and chemical water quality; important influencing variables on land are the available soil volume - the nature of the rhizosphere , the soil moisture , the soil pH in the soil solution , the sorption of nutrients, the mobility or water solubility of the nutrient. The course of temperature and humidity determine the mineralization of organic matter by soil organisms .
When determining the nutrient requirements in terrestrial biotopes, the pH value of the substrate and the effect of the nutrient compound used on the soil reaction must therefore be taken into account ; Nitrogen can be used, for example, as a basic nitrate ion NO 3 - , as an acidic ammonium NH 4 + or as a basic calcium cyanamide CaCN 2 . Lime ammonium nitrate supplies the nitrogen in a neutralized but acidic form.
The existing buffer capacity of the substrate is important for avoiding too high a salt content in the "nutrient solution", i.e. the pore water of the soil. In addition to the osmotic harmful effect of nutrient salts that are too concentrated, toxic reactions - especially of micronutrients - occur even at low concentrations. The relative toxicity of borates , for example, is 1000 times higher than that of sodium sulphate, which may cause purely osmotic damage.
The nutrient dynamics in the substrate represent a constantly changing dynamic equilibrium. Water-soluble, mobile nutrients can easily be absorbed by the plant roots, but can also be easily washed out. Immobilization creates easily mobilizable reserves that can be converted into immobile reserves through fixation processes. If the equilibrium conditions change, these reserves are used to deliver (defix) and finally mobilize the nutrients.
A substrate has ideal nutrient dynamics, which stores many nutrients in an easily mobilizable manner - and thus protects them from being washed out - holds surplus in buffer systems without fixing, but supplies sufficient nutrients when withdrawn.
Sprengel's law of the minimum
Every plant needs the nutrient elements in a certain proportion, as shown above based on the typical composition of the biomass. The law of the minimum by Carl Sprengel , published in 1828, popularized by Justus von Liebig in 1855 , states: The element that is available in the minimum amount in comparison with the required proportion determines the maximum possible growth of the plant. No nutrient element can be replaced by another. Therefore, the excess of one element does not compensate for the undersupply with another nutrient element.
A well-known comparison image is often an open barrel made of staves of different lengths ( minimum barrel ) that is filled with water. The staves represent the amount of one nutrient available. The barrel can only be filled with water up to the level of the shortest stave.
The law of the minimum when fertilizing is of great importance . Here the attempt is made to provide the nutrients as precisely as possible in proportion to their needs. Therefore, soil analyzes carried out beforehand must show which elements should be topped up by how much.
- Arnold Finck : Plant nutrition in brief . 5th edition. Hirt / Borntraeger, Berlin 1991, ISBN 3-443-03100-5
- Günther Schilling : Plant nutrition and fertilization . 1st edition. Part I plant nutrition; Part II fertilization. VEB Deutscher Landwirtschaftsverlag, Berlin 1987 (Part II: ISBN 3-331-00014-0 )
- Dietrich Uhlmann: Hydrobiology . 1st edition. Fischer, Stuttgart 1975, ISBN 3-437-30212-4
- ↑ Mongi Zekri, Tom Obreza: Calcium (Ca) and Sulfur (S) for Citrus Trees. (pdf) Department of Soil and Water Sciences, University of Florida / Institute of Food and Agricultural Sciences, July 2013, accessed on August 24, 2019 .
- ↑ green24 , Frank (gardener).
- ^ Environment: Biology 7-10 , Ernst Klett, p. 61.