This discussion will focus on the absorption of nutrient elements by the plants, their expression in the composition and on the decisive influences of the nitrogen on the absorption of all other nutrients. The absorption of minerals by the plant differs from the absorption of water by the root, firstly because water is a single entity while there are many and various minerals. We will start by pointing out that it is incorrect to talk of macro-elements and micro-elements, but they should be termed macro-nutrients (N, P, K, Ca, Mg, S,) since they are taken in large quantities but their atomic weight does not exceed 40. The micro-nutrients are found in the plant in minute quantities the size of their atoms is larger. The heaviest atom in the plant, Molybdenum, is the lowest in concentration in the plant tissue (Fig. 4.01).
Testing the elemental composition in the entire maize plant shows that the number of atoms of each element as relative to 1 molybdenum atom varies from 100 for copper to 1 000 000 for nitrogen (Fig. 4.01). Nevertheless, molybdenum is essential for the most important reaction of nitrate reduction in the plant. Nitrate cannot serve as a source of nitrogen without molybdenum.
The central role of nitrogen in research and practice of plant nutrition is a result of its direct and indirect influence on the absorption by plant of all the other elements. The classical way to relate to the concentration of any nutrient element is as a % by weight of the dry matter of the plant. It is usually classified into 5 categories:: deficient, low, adequate , high, and toxic concentration (Table inserted in Fig. 4.02). Expressing the concentration of elements as a % of the dry weight is misleading and makes it harder to understand the system. It is used in agriculture to calculate the amount of nutrients that are exported from a given area by the growing crop.
To understand plant nutrition it is important to know the amount of the electrical charges which each atom brings in or carries with it upon it's entry to the root.
The composition of the salts and ions absorbed by the plant is not the same composition of the solution that surrounds the root cells because of the characteristics of selectivity of the cell membranes. For example: in a particular sea water algae less sodium is absorbed than its proportion in the solution, while potassium is taken-up much more than its concentration in the solution. On the other hand, in a different algae, grown in fresh water low in chloride and sodium, these elements are concentrated in the in the cell, in contrast to their concentration in the external solution (Fig.4.03).
An example of the chelation of calcium is given in Fig. 4.04. The calcium, which is surrounded by 3 rings, loses its character as a cation towards the external solution. Most of the metals that constitute plant enzymes are present in chelation bonds.
Typical examples are the iron links in the Haem Compounds ( Fig. 4.05 ), and the bond of magnesium in chlorophyll ( Fig. 4.06). The stability of the chelate is influenced by the pH and therefore, for example - the magnesium "escapes" from the chlorophyll easily with a reduction in pH (or with the addition of ammonium).
In other words, iron, magnesium and the other elements does not behave as free ions in the cell solution but are linked to an organic complex. In this way, for example, iron is absorbed and moved in the plant as a chelate with citric acid.
Let us examine the electron transport system during photosynthesis in the chlorophyll (Fig. 4.07).
When a photons absorbed by Photo system II (PsII) two electrons are released. The high energy free electron transfers its energy using iron and magnesium containing enzymes to the creation of the energy-rich compound ATP. Oxidation reactions in Fig. 4.07 illustrate the involvement of the metallic elements Fe, Mn, Zn, which serve as acceptors and donors of electrons in the photosynthetic process.
The purpose of photosynthesis is to change inorganic carbon (CO2) into organic (CH4). This is the most widespread chemical reaction in the plant all over the globe.
Another very important reaction is the addition of monophosphate to ADP (adenosine diphosphate). This is the reaction that leads to charging the excess energy in the energy-rich phosphoric link in the ATP (adenosine triphosphate). The enzyme which is responsible for this (ATPase) needs a very specific element , the magnesium (Fig. 4.08).
Why Ca Cannot Substitute for Mg in this Process?
The residence time of the hydration water on the magnesium atom is 10-3 whereas on
calcium is 10-8 seconds. That means that a water molecule is attached to the magnesium atom 10 000 times longer than to the calcium. Since Mg hold the water stronger than the Ca, and a molecule of water must leave the ADP and the phosphate ion before ATP is produced it is the Mg that makes the enzyme operation. Without magnesium there is no photosynthesis and no ATP.
In other words there is no life.
Four groups of elements can be defined in terms of the nature of their absorption by plants:
1. C, O, H, N, S
Taken up by the plant in various forms: as gasses (CO2, O2 and H2O vapor) or as anions or as cations. They are all linked in the processes of oxidation and reduction in the plant. Although some of them are taken in their as oxidized state they are all found in the plant cells only in their reduced stage. The bicarbonate is the regulator of the pH in the system. These elements are taken up by the plant in the following ionic or molecular forms: O2, CO2, HCO3-, H2O, NO3-, NH4+, SO4-2, SO2 .
2. P,B.Si - The Oxi-anions.
These are taken up by the plant as anions or soluble non charged group. The phosphorous H2PO4- (phosphate) as anion . The silicon is taken up both as the soluble molecule Si(OH)4 and as a silicic anion H3SiO4- according to the pH of the soil solution.
The form of the boron uptake, whether as B(OH)3 or as the anion, H2BO3- depends on the pH. The function of the boron is not completely clear. It is clear that when there is a lack of calcium it is easy to obtain boron poisoning in plants. A lack of calcium encourages the excess uptake of boron up to a toxic concentration.
The metabolic function of silica and its role is still not known. There are many data on its occurrence, firstly in cereals, primarily on the margins of the leaves and husks. The silica also appears as serrated crystals on the margins of the leaves of grapes, maize, wheat, cucumbers and melons, where it acts to reduce the damage of insects and other pathogens. Silica-rich grass grinds the teeth of the cattle and sheep that graze on it, which is not the case with legume pasture where the plants contain very low levels of silica.
3. K, Na, Mg, Ca
Are taken up as cations and generally involved in electrical balance of the systems. Ions of these elements always maintain their valence in all the processes and compounds in the plant.
4. Fe, Cu, Zn,Mo
Taken up by the plant through the membranes of the root cells and hairs, as chelates.
Let us return to table 4.01 which indicates the amount of atoms, taken up by maize. It is clear from the table that million nitrogen atoms are present in the plant per one atom of molybdenum. while N to P ratio is 4 . The type and quantity of nitrogen nutrition influences all other processes influenced by plant nutrition.
Nitrate enters the cell cytoplasm and is reduced there by the enzyme nitrate reductase which contain in it one atom of molybdenum. Two electrons are consumed during the reduction of nitrate to nitrite. Nitrite enters the chloroplast where the second reduction occur, with the help of nitrite reductase, in which nitrite accepts a further six electrons and becomes ammonia. The ammonia in the chloroplast trigger with the involvement of ATP and sugar, the first amino acid to be synthesized - glutamine, from which the other amino acids are synthesized. These reactions are very fast, otherwise the accumulation of ammonia in the cytoplasm, in which the pH is 7.5, would result in an immediate ammonia poisoning of the cell.
In Fig. 4.01 the content of nutrients in the plant tissue as expressed as a % of the dry weight. To understand the relative contribution of the nutrients in values of the electrical charge, it is better to express the concentrations in terms of milliequivalents of charge (meq) per kg of plant dry matter. For example the atomic weight of potassium is 39.1 and it always carry one positive charge K+.
Therefore 39.1 mg of K carries on them 1 milli equivalent. If a plant contain 3.91% K and we want to express that concentration in meq/kg units than we have to follow the following reasoning: 3.91% means 3.91 g K per 100 gram of plant dry matter or , 39100 mg K per 1000 g of dry matter. Therefore 39100/39.1= 1000 meq positive charges per 1 kg of Dry matter that are contributed by potassium. Although the nitrogen atom is trivalent, it enters the plant as nitrate bearing only one negative charge in the form of NO3- and therefore the equivalent weight of nitrogen in nitrate is taken as 14. The phosphate is present in the plant as H2PO4- monovalent ion of phosphorous (in spite of the known value in chemistry as P being trivalent.) We shall consider it monovalent as it is found in this form in the plant, and its equivalent weight 31. Sulfur which is taken up as anion SO42- has an equivalent weight of 16. In that way the charge contribution of each nutrient element is presented in Fig. 4.09
In an analysis of maize dry matter it was found (in units of meq/kg dry material the data as presented in Fig. 4.10
The difference between the total sum of cations 1777 and the sum of anions 687 is 1090 meq/kg dry matter. Such a difference is impossible to accept in practice since the electrical balance must be kept constant. In order to balance this difference the plant produce organic anions to balance the excess of inorganic cations over inorganic anions.
This difference stems from the fact that the nitrate enters the plant as an inorganic anion but is reduced in the plant and its charge is transferred to organic anion.
The main chemical equation here is :
NO3- + 8 e- + 8H= NH3 + 2H2O +OH-
Since OH- is toxic in the plant it is being titrated in the cell by CO2 to produce the digestible anion of bicarbonate (HCO3-).
In summary of the nitrate reduction process an organic anion is produced in the plant mainly, malate, oxalate, citrate, pyruvate, and oxaloacetate.
The sulfur ion SO4 2- is also reduced to the form of SH2 in organic amino compounds releasing 2 OH- groups per each sulfur reduction. The balance between cations and anions uptake is therefore balanced by the organic anions in the system.
The balance is presented in that way in the following table (Fig. 4.10 ).
Nitrate and Sulfate anions are responsible for organic acid production in their transformation from inorganic anions into major constituents in the proteins. If 4123 meq anions are taken up during the production of 1 kg dry material abut only 1777 meq cations (Fig. 4.10), then in order to balance the anions the plant must take up H+ from the water and leaves OH- in the external solution.
In other words : when nitrate + other anion uptake is in excess of the major 4 cations uptake the external soil solution looses protons and the pH increases.
The case of ammonium nutrition
If the nitrogen is taken up only as ammonium, the ammonium will appear as a cation in the cation column of the table in Fig. 4.11 In that case the total cations uptake (Cu) is more than the total of anion uptake (Au). In order to balance the excess absorption of cations hydroxyl must be accepted from the practically unlimited amount of this anion in the water molecules - and the soil will become acidic. Another mechanism to explain this external decrease in pH is to assume that when NH4+ is taken up a H+ is released from the root. Whatever the exact uptake mechanism is, the result is that uptake of nitrate -Increase soil pH, and the uptake of ammonium - Decrease the soil solution pH.
The results in Fig. 4.12 , presents what happens with the uptake of nitrogen by tomato from a nutrient solution that contained either only nitrate or both ammonium and nitrate at various ratios. As the ratio of ammonium increased over the total nitrogen it becomes
clear that the nitrogen uptake is not in the same ratio as in the solution, but in favor of the ammonium: Close to 80% of the nitrogen taken up was ammonium while in the solution it did not exceed 40%. As a result of this preference the pH in the solution decreased.
One of the characteristics of the difference between uptake of ammonium and of nitrogen is demonstrated in the next table (Fig. 4.13).
So long as nitrate, or ammonium and nitrate is fed into the solution, there is no major difference in the accumulation of dry matter. When there is only ammonium - there is less production of the dry matter. Another characteristic is that the content of the magnesium in the tissue (and the content of the calcium) is less when high ammonium is present in the solution (table in drawing 4.15 ).
Fig. 4.14 represent the results of Rhodes grass fertilized with ammonium sulfate in a Sandy soil in a Kibbutz Gaash(a) and in clay Soil in Ramat Hakovesh (b). One can see from the graphs in the sandy soil that the ammonium in the soil is higher than the nitrate whereas in the heavy clay soil - their concentrations are very similar.
In testing the grass composition ( Fig. 4.15) it is possible to distinguish a decrease in calcium and magnesium in the plants grown on soil (a) as compared to that of soil (b) as a result of the higher concentration of ammonium present in Soil (a). The potassium was less affected by the competition with the ammonium than the uptake of the magnesium and calcium.
In an experiment with tomatoes in sand, in containers with a circulated solution in various proportions of ammonium to nitrate, with ammonium nitrogen at maximum of 40% of total N, the plants hardly survived but the plants that used only nitrate fertilizer were bearing heavy fruit yield at exactly the same soil and climate conditions (Fig. 4.16 ). The yellowing typical to describe a magnesium shortage was observed in the high proportion of ammonium in the circulated nutrient solution, although the magnesium concentration in the nutrient solution was the same (Fig. 4.17). The explanation - the ammonium uptake resulted in low pH in the root, stalk and petioles The proton compete with Ca and Mg uptake and on the strength of the magnesium chelate bond in the chlorophyll.
It is important to remember that not only the ratio of ammonium to nitrate determines the pH around the root, but in practice the difference between the amount of total uptake of all the cations and that of anion uptake. A large increase in the concentration of nitrate will lead to pH elevation around the root and as a result a disturbance in the uptake of elements whose solubility is reduced with the increase in pH ( phosphorous, iron, manganese).
In the field it is very difficult to regulate the ratio of ammonium to nitrate and to keep in the soil excess ammonium, and therefore, due to the nitrification, high ammonium fertilization will eventually ends up as high nitrate concentration next to the root and an increase in pH as a result, with all the consequences to plant uptake and development.
An example from a maize growth experiment in a running solution shows how the use of nitrate only, already above 4 meq/L of N-NO3 cut off any increase in the plant yield ( Fig. 4.20), while higher yield are expected at 8 and even 10 meq N/L .
The absence of a response due to higher nitrogen concentration is due to induced P deficiency. An examination of the composition of the xylem sap of the plants, shows the phosphorous concentration steadily reduced as the concentration of nitrates in the nutrient solution increases. It appears that uptake of nitrate decreased the uptake of chloride to a very great extent, but almost stopped the uptake of the phosphate at the later stages of plant growth (Fig. 4.21 ). The increase of pH near the root, decreases the concentration of the monovalent phosphorous H2P04- relative to that of HP042-which is not taken up by plants (Fig. 4.22 ).
Not all plants will be influenced by the ratio between the ammonium and the nitrate in a similar way. When a legume (chick peas) and a cereal (sorghum) grow in the same root medium (Fig. 4.23) next to each other , legume acidified it's root zone while the pH increased next to the sorghum roots with the same proportion of nutrients in the root medium . In mixed cropping of cereal and legume, the legumes can assist the cereal in the uptake of phosphorous and iron. The legumes acidify the vicinity of the root even with nitrate nutrition, as it governs the uptake of the nitrate by a feedback mechanism that stops (or reduces) nitrate uptake when the pH increases near the root which stops iron uptake. If iron is not taken up the uptake of nitrate stops, the pH falls , iron is taken up again and so does the nitrate.
In certain plants the roots respond to the lack of phosphorous by creating excess citric acid and as a result they acidify the soil near their roots. The citric acid may extract phosphorous from non soluble P-compounds in the soil more (Fig. 4.24 ).
The use of nitrification inhibitor can turn into a useful auxiliary aid in fertilization. The peanut plant in Fig. 4.25 was grown in a soil containing 95% lime and was fertilized with ammonium sulfate . The plants exhibit strong chlorosis. When nitrification inhibitor ( N-Serve) was added the chlorosis disappeared. Adding a nitrification inhibitor forced the uptake of ammonium and the acidification of the root surface was enough to mobilize sufficient iron to prevent chlorosis even at extremely high calcareous soil (Fig 4.26).