How do plants absorb minerals
The plants can absorb the nutrients through the roots (soil fertilization) as well as through the leaves (foliar fertilization).
Nutrient absorption through the root
The roots can only absorb nutrient salts as ions dissolved in the water. For this reason, the soil should be sufficiently moist to allow the absorption of mineral fertilizers and compounds mineralized by soil organisms. The water in the soil splits the fertilizer salts into ions. Calcium nitrate (Ca (NO3)2) for example into the positively charged calcium ion (Ca2+) and into two negatively charged nitrate ions (NO3-).
The nutrient salt ions can be found in the soil in three forms:
- as freely moving ions dissolved in the water (very easily absorbed)
- exchangeably bound to negatively charged clay and humus particles (relatively easy to absorb)
- as reserve nutrients in the crystal lattice of minerals or in organic matter that is difficult to decompose (difficult to obtain)
The nutrients can be absorbed in different ways. One recording option is the Root suction. The negative pressure in the root area, which results from the water evaporation of the leaves, causes the plant to suck the soil water to the root hairs. The ions dissolved in the water are also sucked in. The root suction extends a few centimeters and only captures freely moving ions. The amount absorbed depends on the concentration of ions in the soil water and the water consumption of the plant. The soil solution in the immediate root area is depleted by the ion uptake of the plant.
This results in the so-called diffusion. Since the ion concentration is much higher further away from the root area, the ion distribution is balanced out as the ions or molecules migrate towards the roots. This balances out the concentrations. Water is used as a transport and solvent. For this reason, symptoms of nutrient deficiency are common in drought.
That too Root growth can promote nutrient uptake. In this way, root tips that come into direct contact with exchangers can literally "graze" them. The root grows specifically towards the exchangers. The growth rhythm, which is different for each type of plant, means that the roots are not always equally active. If one knows about the root activity of a plant, this knowledge can be used for the optimal choice of fertilization date. In times of high root activity, the nutrient uptake is particularly good. This saves fertilization costs and reduces the risk of leaching.
Processes in the root
The prerequisite for water absorption by the roots is that the salt concentration in the root cells is higher than in the soil water. The nutrient salts must therefore migrate to an area of high salt concentration when they are absorbed. Since this is not possible, the ions have to be brought into the root hair cells while consuming energy against a concentration gradient. Since the cell membrane is impenetrable for the ions, they are transported through special carriers and thus get inside the cell. The energy for these processes is generated through root breathing. This is also the reason why the plants often show symptoms of nutrient deficiency when they are waterlogged. A loosening of the soil helps against this, as this promotes gas exchange and thus leads to increased root breathing and increased nutrient uptake. The roots should also be adequately supplied with reserve substances.
To a limited extent, the plant can exclude nutrient salt ions from the uptake. This is due to the fact that certain carriers are available for certain ions, which are based on the electrical charge and the ion diameter. The potassium and ammonium ions have the same charge and approximately the same diameter. Because of this, they compete for the same carrier. If there is an oversupply of one of the two ions, the other is absorbed in smaller quantities. This phenomenon can lead to a lack of nutrients and is called antagonism. This phenomenon also occurs, for example, in soils or substrates that are well supplied with lime. Because of their high concentration in the soil solution, the calcium ions collect there at the roots and inhibit the uptake of other ions, such as magnesium and potassium ions.
Anions, such as SO42- and NO3-, can usually be found freely in the soil solution. Cations, on the other hand, are bound to clay and humus particles. They can only be absorbed by the plant if the plant releases them from H.+-Ions displaced by the exchangers. The H+-Ions are a waste product of root respiration. With the help of the H+-Ions can be released by the plant in exchange-bound nutrients and, to a limited extent, reserve nutrients.
Also the delivery of the HCO3--Ions has a reason. In order to maintain the predominant electrical charge in the plant cells even when the ions are absorbed, there must be an HCO for each anion3-Ion and an H for each cation+-Ion to be delivered.
Plants also secrete other compounds with which nutrient salts can be made available. These include, for example, the so-called chelates, which liberate fixed ions and release them to the plant roots.
Changes in the soil reaction due to the uptake of nutrient salt
If the cation of a mineral fertilizer salt is absorbed by the plant faster than the anion, it releases more H to balance the charge+-Ions off. This lowers the pH of the soil solution. In this case it is a physiologically acidic fertilizer. This reaction usually takes place when the cation is monovalent and the accompanying anion is divalent. Typical examples are potassium sulfate (K2SO4) and sulfuric acid ammonia ((NH4)2SO4). In the case of sulfuric acid ammonia, there is also the fact that the ammonium ions (NH4+) in the soil to nitrate (NO3-) being transformed. Here again H+Ions released.
A preferred uptake of anions, however, leads to an increased release of HCO3--Ions. This increases the pH value. In this case, therefore, one speaks of physiologically alkaline fertilizers. Sodium nitrate (NaNO3) and calcium nitrate (Ca (NO3)2).
Every mineral fertilizer has a certain influence on the soil reaction. When fertilizing with ammonia sulfuric acid, the released H.+-Ions a loss of lime. This side effect is undesirable and must later be compensated for by appropriate liming. Nonetheless, this surge of acid also has the advantage that trace elements become more readily available.
Nutrient absorption through the leaf
The leaves of the plants can absorb nutrients directly in an aqueous solution. In general, it would even be possible to feed exclusively through the leaves. Plants receive a certain amount of fertilization from the rain, but this is only minimal. An exclusive plant nutrition through the leaf is usually too costly. Nevertheless, foliar fertilization plays an important role in some areas.
It is surprising that the stomata of the leaves play no role in nutrient uptake. If you take a closer look at the structure of the wax layer, you will find wax platelets that are embedded in a base fabric. Normally they lie together like scales without gaps and thus hinder the passage of water or evaporation. When the air humidity is high, the base fabric swells, creating the finest pores between the wax platelets. Through this, the nutrient solution can penetrate the leaf and finally reaches the cell membranes of the uppermost cell layer. From here on, nutrient uptake is exactly the same as that through the roots. The other above-ground organs, such as stems or fruits, are also able to absorb nutrients. However, since the leaves have the largest surface area and a lively metabolism, they play the most important role.
Martin Degen, Karl Schrader (2002):Basic knowledge for gardeners. Ulmer Verlag. Stuttgart. ISBN 3800111888
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