Mineral nutrition of plants

1.1. The Plant as a “Self-Assembling” Structure

The simplest and at the same time the most profound answer is as follows: mineral nutrition is the third, and absolutely essential, component of the process of building a plant organism. Unlike animals, plants do not receive ready‑made organic “building blocks” for their body. They build themselves from three fundamentally different flows of matter.

Imagine this as three resource streams that continuously enter the plant and are assimilated:

1. Stream One: Water.

2. Stream Two: Carbon Dioxide.

3. Stream Three: Mineral Elements.

These three streams – water, carbon dioxide, and mineral salts – are the materials from which the plant, using the energy of sunlight, constructs all its organs, tissues, and cells (Kuznetsov and Dmitrieva, 2006). Without any one of them, plant life and growth become impossible. Let us briefly characterise each to understand the place of mineral nutrition in this trio.

1.2. Soil Is Not “Dirt” but a Reservoir of Ions

Where does the plant obtain these mineral ions? The source is soil. But not soil as a mechanical mixture, but the soil solution – a thin, dynamic system in which ions are dissolved and available to the root.

Soil is not an inert substrate. Its solid phase, consisting of mineral particles (clay, sand) and organic matter (humus), carries an electric charge on its surface, predominantly negative (Hopkins and Hüner, 2009). This negative charge allows soil particles to retain on their surface metal ions – cations such as K⁺, Ca²⁺, Mg²⁺, NH₄⁺ (Connor et al., 2011).

Between ions adsorbed on the surface of soil colloids and ions freely floating in the soil solution, there is a dynamic equilibrium (Hopkins and Hüner, 2009). This is like a huge reservoir that constantly replenishes the solution as the plant removes ions from it. When the root absorbs a potassium ion from the solution, its concentration drops. In response, some of the potassium adsorbed on the soil particle goes into solution, maintaining the availability of the element for the plant. This process is called ion exchange.

Thus, soil is not just a root habitat, but a crucial ionic reservoir that supplies the plant with essential elements. However, the availability of these elements critically depends on many factors: soil pH, its structure, water regime, and, of course, the physiological activity of the root system itself.

1.3. Why Is Mineral Ion Uptake a Complex Task?

It would seem that if ions are present in the soil, all that remains is to absorb them like a sponge absorbs water. But in reality, things are much more complicated. The plant root is not a passive filter but an active organ capable of selectively absorbing and concentrating ions.

Here we encounter the main physiological contradiction. To absorb an ion from the soil, the plant must overcome a fundamental difference in concentration. The concentration of most important ions in the soil solution is extremely low. For example, the concentration of nitrate (NO₃⁻), the main form of nitrogen for most plants, in the soil often does not exceed 1‑5 millimoles, and can be only tens of micromoles (Connor et al., 2011). At the same time, inside the plant cell, in the cytosol, the concentration of the same ions can be orders of magnitude higher. For instance, the potassium content (K⁺) in cell sap can be 1000 times higher than its concentration in soil water (Marschner, 2012).

Imagine: the plant must collect the scattered “crumbs” of mineral salts from the soil and concentrate them inside itself to the level necessary for life. This is an extremely difficult task. Simply “absorbing” them by passive diffusion is impossible – the ion would move faster from the high‑concentration region (inside the cell) to the low‑concentration region (soil) than vice versa.

To extract ions and accumulate them against a concentration gradient, the plant must expend energy. This is not a passive but an active process, closely linked to root respiration and the operation of membrane transport systems (Lambers and Oliveira, 2019; Taiz et al., 2023).

Consequently, mineral nutrition is not just uptake, but active extraction of elements from the environment with energy expenditure and the help of specialised mechanisms. It is this process that will be the subject of our further study.

1.4. What Lies Ahead? Our Questions

Thus, we have identified the problem. The plant is a system of three material streams. The third stream – mineral elements – requires active participation and complex mechanisms. Our course is precisely devoted to understanding how this system works.

Over the following lectures, we will answer the following key questions:

1. How does the root obtain elements? We will study the structure of the root as an uptake organ, its zones, the role of root hairs, and the operation of ion channels and carriers in the plasmalemma. Why does the root not simply “absorb” but selectively “choose” the necessary ions? (Marschner, 2012; Hopkins and Hüner, 2009).

2. How do membranes transport ions? We will delve into the world of membrane transport: what is the difference between passive diffusion, facilitated diffusion, and active transport? How does the proton pump work, and which proteins ensure ion entry into the cell? (Schopfer and Brennicke, 2016; Morot‑Gaudry et al., 2012; White, 2012).

3. How are elements distributed throughout the plant? We will trace the path of an ion from root to leaf: radial transport (apoplast and symplast pathways), xylem loading, upward flow, and phloem recycling (retranslocation). Why are some elements mobile and others not? (Kuznetsov and Dmitrieva, 2006; Lambers and Oliveira, 2019).

4. How does the plant regulate its nutrition? The plant is not a passive consumer. We will examine how it adapts to deficiency or excess of elements: alters root architecture, releases organic acids, regulates the operation of transport proteins. What signals trigger these changes? (Lambers and Oliveira, 2019; Marschner, 2012).

5. How do deficiencies and toxicities affect physiology? We will learn to diagnose nutritional disorders by visual symptoms and understand their biochemical nature. Why does nitrogen deficiency cause yellowing of old leaves, while iron deficiency causes chlorosis of young leaves? (Taiz et al., 2023; Marschner, 2012; Römheld, 2012).

1.5. Why Should an Agronomist Know This? From Theory to Practice

Our course is not just abstract biology. We are training agronomists, and for you this knowledge has enormous practical significance. Understanding the physiology of mineral nutrition allows us to answer the most pressing questions:

• Why do fertilisers not always work? We may apply tonnes of mineral fertilisers to the soil, but the plant cannot use them if its roots are damaged, if there is not enough water in the soil to dissolve the salts, if the soil pH makes the elements unavailable, or if root activity is suppressed. Physiology explains why “being present” in the soil does not equal “being available” to the plant (Connor et al., 2011; Barber, 1995).
• Why do deficiency symptoms appear first on old or on young leaves? This is a key diagnostic feature that directly follows from the mobility of the element. Nitrogen and potassium are mobile and move from old leaves to young ones, so deficiency shows up from the bottom. Calcium and iron are immobile, so deficiency hits the growing points and young leaves at the top. This information is the basis of visual field diagnostics (Taiz et al., 2023; Marschner, 2012).
• Why does drought reduce fertiliser efficiency? We know that ions move to the root by two pathways: mass flow with water and diffusion. During drought, mass flow sharply decreases, and the root becomes “cut off” from reserves of phosphorus, potassium, etc., that cannot reach its surface (Lambers and Oliveira, 2019). This explains why fertilisers are not absorbed when moisture is lacking.
• Why do different crops use the same element differently? Different species have different physiologies: some have powerful root systems, others exude acids or enzymes that mobilise phosphorus, still others form symbiotic associations with mycorrhiza. Knowing these features allows us to select crop rotations, green manures, and cultivation technologies for the most efficient use of mineral resources (Marschner, 2012; Lambers and Oliveira, 2019).

Thus, we are not studying fertilisers themselves, but how a living plant uses them. We are not studying soil chemistry, but the physiology of the process. And this knowledge is the bridge that connects plant science with the practice of efficient and environmentally sound agriculture.

---

2.1. Soil as a Product of Geological Time

Most of the mineral elements required by plants originate from rocks – the parent material. Over geological time, under the influence of climate, water, wind, and living organisms, solid rock is broken down, weathered, and on its surface or from its debris soil begins to form (Connor et al., 2011).

This process is fundamental to understanding what we are dealing with. Soil is not just pulverised rock. It is a complex dynamic system that includes (Connor et al., 2011; Marschner, 2012):

1. Solid phase – mineral particles of various sizes (from sand to clay) and organic matter (humus).

2. Liquid phase – the soil solution, in which salts and organic substances are dissolved.

3. Gaseous phase – soil air containing oxygen, carbon dioxide, and nitrogen.

4. Living organisms – plant roots, bacteria, fungi, soil animals.

During weathering, primary minerals, from which the parent rock is composed, are destroyed. Ions of chemical elements – potassium, calcium, magnesium, iron, and others – are released from their structure. These ions pass into the soil solution or are adsorbed by secondary clay minerals (Connor et al., 2011; Taiz et al., 2023). Thus, an element initially inaccessible to the plant, locked in a crystal lattice, becomes part of the soil system and potentially available to the root.

2.2. Soil Solution and Ion Exchange

Ions that have entered the soil solution are present there as free hydrated particles. It is from this solution that the plant absorbs its mineral elements (Hopkins and Hüner, 2009). However, as we have already mentioned, the concentration of ions in the soil solution is very low, often thousands of times less than in the cell. How does the plant obtain them in sufficient quantity?

The key to understanding this process lies in the properties of soil colloids – the finest particles of the soil solid phase. These are primarily clay and humus particles. They have a huge specific surface area and carry an electric charge, predominantly negative (Hopkins and Hüner, 2009; Marschner, 2012).

It is this negative charge that allows soil particles to adsorb positively charged ions – cations: K⁺, Ca²⁺, Mg²⁺, NH₄⁺. This creates two main forms of mineral elements in the soil:

1. Ions in solution (“intensity”). This is the part of ions that floats freely in soil moisture and can be immediately absorbed by the root.

2. Adsorbed ions (“capacity”). This is a huge reserve of ions held on the surface of soil particles (Hopkins and Hüner, 2009).

Between these two forms there is a dynamic equilibrium (Radin and Lynch, 1994; Connor et al., 2011). As soon as the root absorbs an ion from the soil solution, its concentration drops. This disturbs the equilibrium, and some ions adsorbed on the soil particle immediately go into solution to restore its concentration. This process is called ion exchange (Hopkins and Hüner, 2009; Taiz et al., 2023).

Example with phosphorus:

While studying phosphorus uptake, researchers arrived at a surprising figure. To meet the needs of a growing plant, the phosphorus content in the soil solution must be renewed on average 10 times a day through ion exchange with the solid soil phase (Hopkins and Hüner, 2009; Radin and Lynch, 1994). This is as if you had only a few drops of juice in a glass, but you needed to drink a glassful at a time, and those drops were constantly replenished from a large pitcher. Soil, with its adsorbing capacity, is that “pitcher” that provides long‑term and stable plant nutrition.

2.3. Four Sources of Mineral Elements

Thus, the plant obtains mineral elements from several key sources, which can be represented as four main pathways (Lambers and Oliveira, 2019):

1. Weathering of rocks. This is the primary, geological source. It creates the initial stock of elements in the soil. The rate of this process is enormous on a geological timescale, but very small on the scale of a single plant’s life or a single season (Connor et al., 2011).

2. Atmospheric precipitation and dust. Wind lifts particles of dust and salts into the air, and rain washes them out of the atmosphere. Thus, significant amounts of ions, especially calcium, magnesium, and potassium, reach the soil surface with precipitation. For some ecosystems, especially in coastal areas, the contribution of atmospheric sources can be quite substantial and even comparable to weathering (Lambers and Oliveira, 2019). However, for the short‑term nutrition of a specific crop, this source is not decisive.

3. Recycling and turnover. This is the fastest and, for an ecosystem, the most important source (Lambers and Oliveira, 2019). It includes:

• Decomposition of plant residues (leaf litter, roots, dead parts) by microorganisms. During mineralisation of organic matter, the elements contained in them (nitrogen, phosphorus, potassium) are converted into inorganic ions and become available again to roots.
• Retranslocation – this is an intra‑plant process, which we will discuss in detail later. It is the plant’s ability to move elements from old, senescing organs (leaves) to young, actively growing ones (Lambers and Oliveira, 2019; Kuznetsov and Dmitrieva, 2006).

4. Mineral fertilisers. This is an anthropogenic source that we create to compensate for what we remove with the harvest. In modern agriculture, this is the main way to manage mineral nutrition. However, as we will discuss in subsequent lectures, the efficiency of fertilisers critically depends on plant physiology and soil conditions (Taiz et al., 2023).

2.4. The Task for the Root: Extract an Ion

So, the plant is surrounded by an environment that contains huge reserves of elements. But these elements are either locked in the crystal lattice of minerals, strongly adsorbed on soil particles, or enclosed in complex organic molecules of humus. The task of the root is to extract from this environment exactly the ions it needs at a given moment, and to do so under conditions of extremely low concentration in the soil solution.

This is a highly complex task, and it is solved not passively. The root is not just a “suction organ”. It is an active, evolutionarily adapted tool for extracting mineral elements. That is why we cannot simply “count” elements in the soil. We need to understand: an element may be present in the soil but remain physiologically unavailable to the plant (Connor et al., 2011).

---

3.1. The Concentration Barrier: Inequality of Two Worlds

Imagine that the plant needs to accumulate potassium ions (K⁺) inside itself to a concentration of 100‑150 mmol/L, while in the soil solution there is only 0.1‑1 mmol/L (Marschner, 2012; Hopkins and Hüner, 2009). That is a 100‑ to 1000‑fold difference! The situation is even more dramatic for nitrate (NO₃⁻): its concentration in soil can be tens of micromoles, while in the cell it is several millimoles (Lambers and Oliveira, 2019).

If uptake were a passive process, ions would move according to the laws of physical chemistry – from a region of high concentration to a region of low concentration. That is, from the cell outwards. The plant must do the opposite: collect scattered “crumbs” from the soil and concentrate them inside itself. This contradicts the laws of passive diffusion.

3.2. The Electrochemical Gradient: A Double Obstacle

But the situation is even more complex. Ions are charged particles. Inside the plant cell, there is a negative electrical potential relative to the external environment (usually –100 to –150 mV) (Schopfer and Brennicke, 2016; White, 2012). This potential is created by the operation of proton pumps in the plasmalemma.

For positively charged ions (cations, e.g., K⁺, Ca²⁺), this creates an additional attractive force – they tend to enter the cell under the influence of the electric field. However, as we saw, the concentration gradient works in the opposite direction. As a result, for potassium, equilibrium (where there is no net flux) is reached at a very high internal content, around 100‑150 mM. This means that potassium can enter the cell passively down its electrochemical gradient, despite the high internal concentration (White, 2012; Morot‑Gaudry et al., 2012). But this is true only for potassium and only if the membrane is sufficiently polarised.

For negatively charged ions (anions), such as nitrate (NO₃⁻), phosphate (H₂PO₄⁻), sulphate (SO₄²⁻), the situation is fundamentally different. The negative potential inside the cell repels them. Therefore, for an anion to enter the cell, it must overcome not only the concentration gradient (low outside, high inside) but also the electrical repulsion. This is a double barrier. Such transport is possible only actively, with energy expenditure (Lambers and Oliveira, 2019; Taiz et al., 2023).

3.3. Active Transport: The Cost of the Issue

Active transport is the movement of ions against their electrochemical gradient. It is carried out by specialised membrane proteins – transporters (channels and carriers) – and requires energy. Where does this energy come from? Directly from ATP hydrolysis or, much more often, from the energy stored in the proton gradient (Marschner, 2012; Schopfer and Brennicke, 2016).

At the heart of this is the H⁺‑ATPase (proton pump) in the plasmalemma. This enzyme, using ATP energy, pumps protons (H⁺) from the cytosol outwards, into the apoplast. As a result, two things are created:

1. Electrical potential: outside becomes positive, inside negative (we already mentioned this).

2. Chemical gradient: outside, proton concentration (acidity) becomes higher than inside (apoplast pH about 5.5, cytosol about 7.3).

Together, this is called the proton‑motive force (pmf). It is this force that serves as the universal “battery” of energy for secondary active transport (White, 2012; Schopfer and Brennicke, 2016).

How does this work in practice?

For anions such as nitrate, there are symporter proteins that bind the NO₃⁻ ion together with a proton H⁺ and transport them into the cell (Morot‑Gaudry et al., 2012). The proton moves down its electrochemical gradient (from the region of high concentration and positive charge outside to the region of low concentration and negative charge inside). The energy of this movement is used to “drag” nitrate against its own gradient (Lambers and Oliveira, 2019). This is secondary active transport – it does not require direct ATP hydrolysis but uses the energy stored by the proton pump.

Example with nitrate:

Nitrate uptake is a classic example. Studies using electrodes have shown that adding nitrate to roots causes a rapid depolarisation of the membrane, indicating the entry of positive charges together with the anion. This is direct evidence of H⁺/NO₃⁻ symport (Morot‑Gaudry et al., 2012; Schopfer and Brennicke, 2016).

3.4. Respiration as an Energy Supplier

Obviously, creating and maintaining the proton gradient requires a constant expenditure of ATP. ATP is produced during respiration (oxidative phosphorylation) in mitochondria. Therefore, ion uptake is closely linked to the respiratory activity of the root (Kuznetsov and Dmitrieva, 2006).

Experiments with respiration inhibitors or with absence of oxygen clearly show: as soon as energy supply stops, ion uptake sharply decreases. Moreover, adding salts to the medium often stimulates root respiration – the so‑called “salt respiration” – which is direct evidence of the energetic link (Schopfer and Brennicke, 2016).

The energy costs for ion transport are enormous. According to some estimates, up to 76% of all root respiration energy can be spent on maintaining ion pumps and transporters, especially at low use efficiency (White, 2012). This means that the plant spends a significant part of its assimilates precisely on extracting mineral elements from the soil.

3.5. Summary: Complexity as a Systemic Property

So, why is uptake a complex task? Because the plant must overcome:

1. Concentration barrier: low outside, high inside.

2. Electrical barrier for anions: the negative cell potential repels negative ions.

3. Energy barrier: it is necessary to constantly expend energy on proton pumps and secondary transporters.

4. Competitive barrier: in the soil there are many ions that may compete for the same transport proteins (e.g., K⁺ and NH₄⁺, or NO₃⁻ and Cl⁻) (Marschner, 2012; Lambers and Oliveira, 2019).

This makes the root not a passive filter but a highly complex “mining” system that selectively extracts the necessary elements, expending a significant portion of its energy on this. It is this complexity that is the subject of our study. We must understand how these transport systems are organised, how they are regulated, and how we, as agronomists, can influence their operation to increase fertiliser use efficiency and reduce losses.

---

4.1. How Does the Root Obtain Elements?

The first question that arises is: how does the root, this underground organ, cope with the task of extracting ions from the soil? We will study:

• The structure of the root as an uptake organ. Not all root zones are equally active. The main work of ion uptake occurs in the root hair zone and the elongation zone (Kuznetsov and Dmitrieva, 2006). It is here that the largest number of transport proteins are concentrated, and root hairs greatly increase the absorbing surface, allowing the root to “search” a larger volume of soil (Taiz et al., 2023).
• The role of root hairs and mycorrhiza. We will discuss how root hairs and symbiotic relationships with fungi (mycorrhiza) help the plant overcome the “depletion zone” that forms around the root during intensive uptake of immobile ions such as phosphate (Hopkins and Hüner, 2009; Lambers and Oliveira, 2019).
• Radial transport: apoplast and symplast pathways. Once inside the root, the ion must cross the cortex and reach the central cylinder (stele) to be loaded into the xylem vessels. We will dissect two pathways: movement along cell walls (apoplast) and movement from cell to cell via plasmodesmata (symplast), and learn why the endodermis with its Casparian strips is a critical barrier that forces ions to switch to the symplast pathway (Marschner, 2012; Taiz et al., 2023).

4.2. How Do Membranes Transport Ions?

This is the central question of the entire physiology of nutrition. We will dive into the world of membrane transport and understand which protein machines ensure the selective and energy‑dependent movement of ions across the plasmalemma and tonoplast.

• Passive transport: channels and facilitated diffusion. We will study how ion channels work – proteins that form aqueous pores in the membrane and allow ions to pass down their electrochemical gradient at enormous speeds (up to 10⁸ ions per second) (Schopfer and Brennicke, 2016; White, 2012). We will learn about channel selectivity and how they are regulated (e.g., voltage‑gated channels).
• Active transport: carriers and the proton pump. We will analyse the mechanism of the H⁺‑ATPase – the main pump that creates the proton‑motive force (Schopfer and Brennicke, 2016). We will learn how secondary active transporters (symporters and antiporters) use this energy to absorb anions (NO₃⁻, H₂PO₄⁻, SO₄²⁻) and cations (Morot‑Gaudry et al., 2012; Lambers and Oliveira, 2019). We will understand why for different ions there are high‑ and low‑affinity systems, and how the plant switches between them depending on the ion concentration in the medium.
• Uptake kinetics. We will be introduced to the Michaelis‑Menten model, which describes the dependence of uptake rate on ion concentration, and learn what Kₘ (affinity) and Vₘₐₓ (maximum rate) are (Schopfer and Brennicke, 2016; Marschner, 2012).

4.3. How Are Elements Distributed and Transported Within the Plant?

After an ion is taken up, it must be delivered to its destination – to growing organs, tissues, and cells. We will trace its entire path.

• Xylem loading and upward flow. We will learn how ions are actively secreted into the xylem vessels from pericycle cells, and how root pressure and transpiration create a mass flow of water that lifts ions to the shoots (Kuznetsov and Dmitrieva, 2006). We will understand why the composition of xylem sap differs from that of the soil solution.
• Downward flow via phloem and retranslocation. This is a key point for understanding deficiency diagnostics. Some elements (N, P, K, Mg) can move through the phloem from old leaves to young ones and to reproductive organs. This process is called retranslocation (Lambers and Oliveira, 2019; Kuznetsov and Dmitrieva, 2006). Other elements (Ca, B, Fe) are almost immobile in the phloem. This determines where deficiency symptoms will appear first: on old or on young leaves.
• Symplast and apoplast in the leaf. We will analyse how ions exit the xylem and reach mesophyll cells, where they are incorporated into metabolism.

4.4. How Does the Plant Regulate Its Mineral Nutrition?

The plant is not a passive consumer. It actively regulates its nutritional system depending on external conditions and internal needs. We will study these remarkable adaptation mechanisms.

• Regulation at the root level. How does the plant change root architecture under phosphorus or nitrogen deficiency? How does the feedback system work, where a high internal ion content suppresses the activity of its transporters (Lambers and Oliveira, 2019; Marschner, 2012)?
• Mobilisation of poorly available elements. We will learn about plant “strategies” for mobilising phosphorus and iron. For example, how cluster roots of lupine exude organic acids, dissolving phosphates, or how phytosiderophores of grasses bind ferric iron (Marschner, 2012; Lambers and Oliveira, 2019). These are vivid examples of how physiology solves the problem of “present but unavailable”.
• Influence of external factors. How do temperature, humidity, soil pH, and even light affect the efficiency of ion uptake (Taiz et al., 2023; Kuznetsov and Dmitrieva, 2006)?

4.5. How Do Deficiencies and Toxicities Affect Physiology?

The final but extremely important block is devoted to practical diagnostics. We will learn to “read” the plant to understand which elements it lacks or by which it is poisoned.

• Visual diagnostics. Why does nitrogen deficiency cause general yellowing (chlorosis) of old leaves, while potassium deficiency causes marginal burning (necrosis) of old leaves? Why does iron deficiency manifest as chlorosis of young leaves, and boron deficiency leads to dieback of growing points? We will link these symptoms to the biochemical role of each element (Taiz et al., 2023; Römheld, 2012).
• Diagnostics based on tissue element content. We will learn how plant (leaf) diagnostics are performed and which concentrations are considered sufficient, deficient, or toxic for different crops (Römheld, 2012; Marschner, 2012).
• Element interactions. We will discuss synergy and antagonism in nutrition, where an excess of one element can cause deficiency of another (e.g., K/Ca/Mg or Fe/Mn), and how to take this into account when developing fertilisation programmes (Kuznetsov and Dmitrieva, 2006; Marschner, 2012).

4.6. Why Is This Important for an Agronomist?

At the end of each topic, we will return to the practical question: how does knowledge of physiology help in real work in the field, greenhouse, or farm? We will see that:

• Physiology explains why fertilisers may be ineffective. It is the understanding of the role of the root, membrane transport, and external factors that helps to find the cause, not just “add more” fertiliser.
• Physiology provides tools for managing nutrition. We can influence the availability of elements through pH management, soil moisture, use of green manures and organic acids.
• Physiology underpins breeding for nutrient efficiency. Modern varieties are created not only for yield but also for the ability to better use mineral resources, which is critically important for sustainable agriculture.

Thus, everything we will study has direct practical application. Ahead of us lies a fascinating journey into the world of ions, membranes, and transport proteins, and I hope that by the end of this module you will have formed a coherent system of knowledge about how the plant extracts and uses its mineral “building blocks”.

---

5.1. Why Do Fertilisers Not Always Work?

This is perhaps the most frequent and most painful question in agricultural practice. You apply recommended fertiliser rates, but there is no yield increase, or it is minimal. What is happening?

Physiology provides a clear answer: the presence of an element in the soil does not equal its availability to the plant. There is a whole cascade of barriers that the element must overcome to enter the cell and be used (Connor et al., 2011; Radin and Lynch, 1994).

• Barrier one: mobility in soil. Phosphorus (P) and potassium (K) are poorly mobile ions. They diffuse slowly in the soil solution. If the root system is weak, if the soil is compacted or dry, the ion simply will not reach the root surface. We can apply tonnes of superphosphate, but if the roots cannot “reach” it, it will remain in the soil (Lambers and Oliveira, 2019; Marschner, 2012).
• Barrier two: chemical fixation. Applied phosphorus can quickly bind with calcium ions (in alkaline soils) or with aluminium and iron ions (in acidic soils), forming insoluble compounds (Connor et al., 2011; Taiz et al., 2023). Nitrogen fertilisers can volatilise as ammonia or leach as nitrates. Physiology suggests that to solve this problem we should not just “apply more” but create conditions for mobilisation of these elements – for example, through rhizosphere acidification, use of organic acids, or mycorrhiza.
• Barrier three: root activity. Even if the ion reaches the root, its uptake requires respiratory energy (White, 2012). If the roots are damaged, are in anaerobic conditions (waterlogging), or suffer from carbohydrate deficiency (e.g., due to low light), they will not be able to actively absorb ions, even if they are present in solution. This is why fertiliser efficiency drops in cold or waterlogged weather.

5.2. Why Do Deficiency Symptoms Appear on Different Leaves?

This is a classic question to which physiology gives a simple and elegant answer, based on the understanding of retranslocation (Kuznetsov and Dmitrieva, 2006; Lambers and Oliveira, 2019).

• Mobile elements (N, P, K, Mg): They move easily through the phloem from old, “spent” leaves to young, actively growing tissues. Therefore, when they are scarce in the soil, the plant “sacrifices” old leaves, pumping elements from them to new ones. Deficiency symptoms appear first on the lower, old leaves – yellowing (chlorosis) with nitrogen deficiency, marginal burning (necrosis) with potassium deficiency.
• Immobile elements (Ca, Fe, B, Mn, Cu, Zn): These elements cannot, or can hardly, move through the phloem. They are either fixed in structures (calcium in cell walls) or precipitate in old tissues. Therefore, when they are lacking, young leaves and growing points cannot obtain them from old organs. Deficiency symptoms appear first on the upper, young leaves – interveinal chlorosis with iron deficiency, dieback of growing points with boron deficiency (Taiz et al., 2023; Römheld, 2012).

This knowledge is the basis for rapid and accurate visual field diagnostics. It allows you, as an agronomist, not to guess but to immediately determine what the plant lacks and to take timely measures (e.g., foliar application of iron or boron).

5.3. Why Does Drought Drastically Reduce Fertiliser Efficiency?

Under drought conditions, even applied fertilisers often do not work. The reason is not only that plants close their stomata and slow growth. The key point is the disruption of transport flows in the soil (Lambers and Oliveira, 2019; Connor et al., 2011).

As we remember, ions reach the root by two pathways:

1. Mass flow – movement of ions together with the water that the root absorbs during transpiration. Under drought, transpiration drops sharply, and mass flow almost ceases. For nitrate (NO₃⁻), which is very mobile, this is catastrophic.

2. Diffusion – movement of ions from a region of high concentration to a region of low concentration. In dry soil, diffusion of ions, especially phosphate and potassium, slows down by hundreds and thousands of times, because the water films along which they move become thinner and discontinuous (Marschner, 2012).

Thus, physiology explains why drought makes applied fertilisers physically inaccessible to the root, even if their concentration in the soil is high. Knowledge of this mechanism suggests strategies to the agronomist: use local (band) placement of fertilisers to put elements directly into the active uptake zone of the root, or apply drip irrigation, which maintains moisture and ion mobility in the rhizosphere.

5.4. Why Do Different Crops Use the Same Elements Differently?

Wheat, maize, soybean, sunflower – each crop has its own characteristics in element consumption. Physiology shows that these differences have concrete mechanisms (Marschner, 2012).

• Different root system architecture: Cereals have fibrous root systems that intensively penetrate the upper soil layers, which is good for absorbing immobile phosphorus. Legumes, on the contrary, have a taproot capable of extracting elements from deep horizons.
• Different mobilising ability: Some plants (lupine, buckwheat) can exude organic acids (citric, malic) into the rhizosphere, which dissolve unavailable iron and aluminium phosphates (Lambers and Oliveira, 2019). Others (cereals) use phytosiderophores for iron mobilisation. This makes them more efficient on poor soils.
• Different nutrient use efficiency (NUE): There are varieties that can produce high yields at low soil nitrogen content due to more efficient use and retranslocation (Taiz et al., 2023).

Understanding these differences is the basis for sound crop rotation planning. For example, after lupine or soybean, more available phosphorus and nitrogen remain in the soil for the following crop. This is using physiological features to increase overall fertility.

5.5. What Does This Understanding Give Us?

So, in answering the question “why should an agronomist know this?”, we can highlight several key aspects:

1. Transition from empiricism to logic. You stop acting on the template “applied fertiliser – should get yield”. You begin to understand the reasons why the system may fail and see ways to solve it.

2. Accurate diagnostics. You can quickly and correctly assess plant condition by visual symptoms and tissue test results.

3. Conscious management. You can adjust the fertilisation system taking into account weather conditions, soil type, and crop development stage, increasing the nutrient use efficiency (NUE).

4. Resource savings. By understanding the real needs of plants and soil constraints, you avoid unjustified costs for fertilisers and reduce environmental load.

Remember the main point:

We are not studying fertiliser chemistry. We are studying plant physiology. We are learning to understand how a living organism extracts, assimilates, and uses mineral elements. This knowledge transforms the agronomist from a mere “executor of recommendations” into a true manager capable of making effective decisions in complex and changing conditions.