Plant nutrition
If we take a piece of fresh plant material and dry it on a stove at 105ºC for 24 hours we will obtain that the resulting dry residue will be approximately 12-15% of the initial weight; almost 95% of this dry waste will correspond to the elements carbon, hydrogen and oxygen (which are obtained from water and air) and the rest is constituted by what we call the mineral content of the plant and is obtained from its support medium (soil or substrate).
Conceptos de Ingeniería Agrónoma
This rest of the dry material consists of:
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Primary macronutrients; nitrogen, phosphorus and potassium.
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Secondary macronutrients; calcium, magnesium, sodium and sulfur.
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Microelements; molybdenum, copper, zinc, manganese, iron, boron, chlorine, silicon, cobalt and vanadium.
The difference between a macronutrient and a micronutrient is that the former are required in greater quantity than the latter.
Another classification divides these 17 elements into metals (K, Ca, Mg, Na, Fe, Zn, Mn, Cu, Mo, Co and V) and nonmetals (N, P, S, B, CL and Si).
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ELEMENT
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DEFICIENCY
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SUFFICIENCY
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TOXICITY
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NITROGEN
(%) |
<2,0 | 2,0-5,0 | – |
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PHOSPHORUS
(%) |
<0,2 | 0,2-0,5 | – |
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POTASSIUM
(%) |
<1,0 | 1,0-5,0 | – |
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CALCIUM
(%) |
<0,1 | 0,1-1,0 | – |
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MAGNESIUM
(%) |
<0,1 | 0,1-0,4 | – |
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SULFUR
(%) |
<0,1 | 0,1-0,3 | – |
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CHLORINE
(%) |
<0,2 | 0,2-2,0 | >2,0 |
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SILICON
(%) |
<0,2 | 0,2-2,0 | – |
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SODIUM
(%) |
<1,0 | 1,0-10,0 | – |
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IRON
(ppm) |
<50 | 50-250 | – |
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ZINC
(ppm) |
15-20 | 20-100 | >400 |
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MANGANESE
(ppm) |
10-20 | 20-300 | >300 |
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COPPER
(ppm) |
3-5 | 5-20 | >20 |
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BORO
(ppm) |
<10 | 10-100 | >100 |
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MOLYBDENUM
(ppm) |
<0,1 | 0,1-0,5 | >0,5 |
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COBALT
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<0,2 | 0,2-0,5 | >0,5 |
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VANADIUM
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<0,2 | 0,2-0,5 | >1,0 |
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ESTIMATION OF NUTRIENT NIVES IN THE DRY MATTER COMPOSITION OF PLANTS.
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Apart from these elements, there is the presence of a large number of chemical elements that are not essential for plant life, but their presence is due to the fact that the absorption of nutrients is not a selective process by the plant.
An element is considered essential when:
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In the absence of a certain element the plant cannot complete its normal biological cycle.
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The element cannot be replaced in its functions by any other element.
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This element must either be a constituent of a plant metabolite or be a catalyst for a chemical reaction of plant metabolism.
Plants do not absorb nutrients in the form of salts but in ionic form (there are positively charged ions called cations, and negatively charged ions called anions).
The main ionic forms of nutrient absorption are as follows:
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Nitrogen. It is absorbed by plants in the form of both nitrate anion (NO3-) and ammonium cation (NH4+). The preference for one form or another depends on the species, temperature and pH. It can also be absorbed in the form of low molecular weight organic substances.
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Phosphorus. It is absorbed either as a monovalent phosphate anion (H2PO4-) or as a divalent phosphate anion (HPO4-2). The way this anion is in the soil depends on the pH of the substrate; in substrate in which the pH of the solution is <7.2 the predominant form will be the monovalent while with a pH> 7.2 the predominant form will be the divalent.
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Potassium. It is absorbed as a K+ cation.
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Calcium. It is absorbed in the form of a Ca+2 cation.
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Magnesium. It is absorbed as a Mg+2 cation.
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Sulfur. It is absorbed in the form of anion sulfate SO4-2.
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Sodium. It is absorbed in the form of a Na+ cation.
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Iron. Its ionic form of absorption is Fe+3 and Fe+2.
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Molybdenum. Its ionic form of absorption is that of the MoO4-2 anion.
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Covered. It is absorbed in the form of cation Cu+ and Cu+2.
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Zinc. It is absorbed as a Zn+2 cation.
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Manganese. Its ionic form of absorption is that of Cation Mn+2.
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Boron. It is absorbed in the form of a cation BO3-3 and BO4O7-2.
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Chlorine. It is absorbed in the form of a Cl-anion.
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Silicon. It is absorbed in the form of a molecule S1(OH)4.
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Vanadium. It is absorbed in the form of V+4.
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Cobalt. Its ionic form of absorption is Co+2.
There are other elements such as Selenium, Aluminum and Nickel that are supposed to have some role in plant metabolism, but it is not yet sufficiently demonstrated.
FUNCTION OF MINERAL NUTRIENTS IN THE PLANT
1. NITROGEN.
It exercises a multitude of functions in plants; it is a constituent of amino acids, proteins, nucleic acids, nitrogenous bases, nucleotides and enzymes. It is also part of the chlorophyll molecule, as well as the cell wall. It is a very mobile element and is directed from the most mature parts of the plant to the youngest.
Deficiency; except for a marked water stress, there is no other deficiency that is as pronounced by vegetables as the lack of nitrogen. Due to the high mobility mentioned above, its lack mobilizes it and transports it quickly from old tissues to actively growing tissues, hence a characteristic symptom of nitrogen deficiency is chlorosis in the old leaves of the plant. If not corrected, chlorosis ends up affecting all the leaves.
2. PHOSPHORUS.
It is another of the ubiquitous elements in the plant; its best-known function is to store and transfer energy in the form of ADP and ATP. It is also part of DNA and RNA, being involved in processes of transfer of genetic information.
Deficiency; the first symptom of the lack of this element is the appearance of a bluish-green coloration in the leaves, forming anthocyanin pigments in them which ends up turning them purple. Being an element with high mobility, symptoms appear first in mature leaves, as well as in nitrogen. If it is not corrected, it causes a reduction in plant growth.
3. POTASSIUM.
Its main function is to regulate the osmotic potential of the plant and maintain its turgor. It also acts on the opening and closing mechanism of the stoma which helps regulate the internal balance of the water. Potassium is not part of any metabolite of the plant, but it intervenes in the flow of sage elaborated through the phloem, contributing to direct the hydrocarbon substances elaborated by the plants towards fruiting organs (storage and reserve organs).
Deficiency. Like nitrogen and phosphorus it is a mobile element in the plant so the symptoms are detected earlier in mature leaves; a mottling of chlorotic spots appears followed by a necrosamiento of the edges and apexes of the leaves.
4. CALCIUM.
It is part of the walls of plant cells providing structural stability and permeability.
Deficiency. Being a little mobile element, its deficiency is first appreciated in the growing areas of roots, stems and leaves; first they present a chlorosis and end up necrosing.
5. MAGNESIUM.
It is a fundamental part of the chlorophyll molecule and acts as a cofactor of enzymes in reactions of plant metabolism.
Deficiency. Internervial chlorosis (yellowish leaves with green nerves; the famous fish scrape) in mature leaves, which indicates that like nitrogen, phosphorus and potassium and unlike calcium, it is a mobile element in the plant.
6. SULFUR.
It is a constituent of the amino acids cysteine, cystine and methionine and is therefore essential in the formation of proteins, vitamins and hormones.
Deficiency. The symptoms are very similar to a lack of nitrogen, except that due to its low mobility chlorosis begins in young leaves. If it is not corrected, it degenerates into an anthocyanin coloration (purple color).
7. SODIUM.
Like potassium, it is involved in the regulation of the plant’s osmotic potential (turgor and stomatal opening). It can replace potassium in some of its functions.
Deficiency. Its deficiency is usually rare since, like chlorine, it is required by plants in very small quantities and most waters have it in more than sufficient quantities.
8. IRON.
Although it is a micronutrient, it is fundamental in the development of plants and is required in large quantities, unlike the rest of micronutrients. Its absorption depends on the pH of the substrate. It is essential in the formation of the chlorophyll molecule; approximately 75% of the iron present in the plant is associated with chloroplasts. It is a constituent of several enzymes and pigments.
Deficiency. It presents as an internervial chlorosis in young leaves. The main cause of this soil deficiency is a high substrate pH, as well as an excess of water.
9. MOLYBDENUM.
Its main function is to be part of an enzyme that intervenes in the reaction of reduction of NO3- to NH4 + once absorbed by the plant, so that it can be incorporated into cellular structures.
Deficiency: it presents as a chlorosis in the leaves of the middle third of the plant and taking the edges of the leaves brown colorations and in the leaflet pinkish and orange pigmentations. This deficiency is more pronounced when the source of nitrogen provided to the plant is in nitric form.
10. COPPER.
Like many other micronutrients, it is involved in oxidation reactions. It intervenes in the formation and composition of the cell wall.
Deficiency. Given their poor mobility, the symptoms appear on the new leaves, rolling up and a chlorosis appears.
11. ZINC.
It is part of the metabolism of auxins, hormones involved in plant growth. It is also related to the formation of the chlorophyll molecule.
Deficiency. Being little mobile, a chlorosis appears on the young leaves, which can often be interventional, with necrotic spots on the tips of the leaves.
12. MANGANESE.
It is involved in the release of oxygen during photosynthesis. It is also part of several enzyme systems, but to a lesser extent than other micronutrients. If an excess occurs, it can cause iron deficiencies due to the competition between these two elements. Deficiency: it is a little mobile nutrient and its lack appears first in the young leaves as an intervenal chlorosis and / or necrotic spots.
Deficiency occurs mainly in calcareous soils, soils with high pH, soils with high organic matter content, an excess of iron and in soils with poor aeration.
13. BORON.
it acts on the metabolism of sugars, as well as on the translocation of these through cell membranes.
Deficiency: its deficiency is very significant, delaying plant development and growth. Dicotyledons require 3 to 4 times more boron than monocotyledons.
14. CHLORINE.
It is involved, along with potassium, in the regulation of the osmotic pressure of the plant and therefore in the regulation of the stomatal opening.
Deficiency: It is very difficult to find chlorine deficiency under normal conditions. Both severe deficiency and deficiency manifest themselves with a tan color on the leaves, progressing to interventional necrotic spots.
15. SILICON.
Silicon is the second most abundant chemical element in the Earth’s crust after oxygen. It is part of enzyme complexes responsible for regulating photosynthesis. It plays an important role in the rigidity of the cell wall and can help give plants greater resistance to pests and diseases (powdery mildew) and greater tolerance to drought. Like boron, it is captured by the plant in the form of a molecule Si (OH)4.
Deficiency: most plants will grow normally without silicon, although in some you could see some type of malformation in flowers or fruits.
NUTRIENT ABSORPTION MECHANISMS.
In plants, the semipermeable barrier that separates the «outside» from the «inside» is the cell membrane and is the one that determines the absorption processes.
Apoplastic pathway: it is the route of entry of most of the water and a part of the nutrients and is produced through the intercellular spaces until it reaches the endodermis. In the endodermis it is located at the caspary band that prevents the passage to the vascular cylinder so that water must enter the cells of the endodermis by osmosis until reaching the vascular bundles of the xylem. Once this barrier is overcome, the water reaches the xylem.
Symplastic pathway: it is the route of entry of a part of water and most of the nutrients. They circulate through the cytoplasm of cells through communications that cells have called plasmodesmos.
Let’s define at this point the raw sap as the mixture of water and nutrients that enters through the root and is distributed throughout the plant through the xylem.
The main mechanisms of nutrient absorption by the plant are:
Passive transport.
They are basically physico-chemical processes that do not require metabolic energy expenditure by the plant.
Diffusion: is a spontaneous process by which a substance moves from one site to another in favor of its chemical potential; they are random thermal movements of the molecules that form a solution. Osmosis is a special case of diffusion through a semipermeable membrane.
Ion exchange: the surface of cells, they exchange ions with the solution that surrounds them. Since negative charges predominate over positive ones in the cell wall, the exchange is cationic.
Mass flow: it is due to the transpiratory flow that circulates through the atmosphere-leaf-xylem-root-soil system.
Active transport.
They are processes against gradient of electrochemical potential that require metabolic energy expenditure by plants.
Ion pumps: these are proteins integrated into the cell membrane and act as channels for ions (aquaporins).
Ion channels: these are hydrophilic pores that cross the membrane. In short, the flow of ions is mainly via symplastic and that of water via apoplastic. Once the ions in the xylem, the upward transport is passive and due to the flow of perspiration. At night once the stomata are closed, it is the positive pressure exerted by the root that recomposes some cavitation problems (formation of cavities filled with steam or gas within a moving liquid) that may have arisen during the day.
INTERACTION BETWEEN ELEMENTS.
We have seen before the symptoms of nutrient deficiency in plants, but the lack of a nutrient sometimes does not mean that it is not present in our substrate; many times the deficiency is due to a poor balance or balance of nutrients in the substrate that is transmitted to the plant. The synergy or antagonism between nutrients depends, among other factors, on the ionic radius and the charge of the ion.
The nitric form of nitrogen requires active work by the plant for its absorption; once inside it can be reduced at the root, which will require a destruction of carbonated substances to obtain the energy necessary for its transformation or it can be sent to the leaves by the xylem taking advantage of solar energy for transformation. In the latter case, energy expenditure is much lower.
The ammoniacal form of nitrogen requires the arrangement by the plant of carbon structures for immediate incorporation because its accumulation can be phytotoxic and cause a potassium deficiency. Therefore, the NH4+/Ntotal ratio should range between 5-10% in substrate cultivation and between 20-25% in soil cultivation. There is also an antagonistic interaction between Cl- and NO3- anions so that an excess of Cl-, very common in saline and/or sodium waters, can reduce the absorption of NO3- by the plant.
The Ntotal/K ratio has a decisive influence on whether a plant is in a vegetative (active growth) or generative (flowering/fruiting) state. A Ntotal/K ratio of 2 indicates a balanced state, that is, it is a relationship in which most plants can develop their full cycle without interference. A ratio of 3 will indicate a clear state of vegetative growth; an Ntotal/K ratio of 1.2 corresponds to a generative state.
As for phosphorus, the contribution of nitrogen in ammoniacal form helps the absorption of phosphorus: the absorption of NH4+ causes the excretion of H+ ions to the environment by the plant; these H+ cause a slight acidification of the root environment that can favor the solubility of some phosphorus salts that would otherwise be blocked or in insoluble form.
On the other hand, phosphorus is antagonistic with respect to some micronutrients, either by the formation of insoluble precipitates, as well as metabolic processes that happen in the plant and that do not allow the translocation of these through the xylem.
Another relationship between cations to take into account is the K/(Ca+Mg) ratio;
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In waters of electrical conductivity below 2 this ratio finds its optimum for plants between 0.35-0.5. Low values will encourage a vegetative state and high values a generative state.
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In waters of electrical conductivity >2 the ratio K/((Ca+Mg)/2) should be framed in values between 0.5 and 1.1; as with waters of ce<2, low values will promote a vegetative state and high values a generative state.
In the particular case of Ca and Mg, an optimal ratio for our plants is always ≥2.
Sodium is also the cause that, although in our substrate we have the adequate amounts of calcium and magnesium, the plant can not absorb these by an excess of sodium.
| ANTAGONISMOS | SINERGISMOS | ||
| POTASIO | BORO | NITRÓGENO | MANGANESO |
| MAGNESIO | POTASIO | MAGNESIO | FÓSFORO |
| MOLIBDENO | COBRE | MOLIBDENO | NITROGENO |
| COBRE |
MANGANESO HIERRO |
POTASIO |
MANGANESO HIERRO |
| FÓSFORO |
ZINC POTASIO COBRE CALCIO HIERRO |
AZUFRE |
NITRÓGENO MAGNESIO MANGANESO |
| ZINC | HIERRO | FÓSFORO | MOLIBDENO |
| BORO | POTASIO | ||
| HIERRO | FÓSFORO | ||
| AZUFRE |
POTASIO COBRE BORO |
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| CALCIO |
POTASIO MAGNESIO MANGANESO ZINC BORO |
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