Improving Plant Health
Understanding the mechanisms responsible for soil and crops health to achieve quantitative and qualitative yield gains: How different nutrients move within the plant, identifying actionable levers, when to intervene to restore nutritional balance. These agronomic basics allow for improved soil-plant system performance.
Principle
The plant is the first pillar of a living soil. A healthy plant is less susceptible to diseases and pests, it promotes higher yields and better seed and fruit quality. It transports more sugars into the rhizosphere, further stimulating microbial life, which contributes to carbon storage and improves soil health.
The health pyramid according to John Kempf
The virtuous circle of the soil-plant system
To develop, a plant needs water, light, oxygen, carbon, and nutrients. It finds these elements in its environment. Optimal nutrition allows for a more advanced functioning of the plant. Like any living being, plant health can improve if environmental conditions improve.
Progressing towards better health restores the natural and biological capacities of the soil-plant system. During this process, the plant shows increasing immunity against soil and aerial pathogens, better resistance to insects, higher lipid production leading to stronger cell membranes and more flavorful fruits with a longer shelf life.
The 4 levels of the pyramid
The plant health pyramid aims to illustrate the 4 major steps to obtain a healthy plant and a more advanced functioning of the plant and soil-plant system.
Levels 1 and 2
- Achievable mainly by working on the plant's nutritional balance, especially when it is possible to use foliar applications of nutritional supplements.
- Speed of action: short term (a few months).
- Farmer's influence mainly on the plant.
Levels 3 and 4
- More difficult to reach because the soil must be healthy and able to provide the plant with most of the nutrients it needs.
- Without the microbial digestive process, plants will never have the excess energy necessary to reach high levels of lipid production and energy storage.
- Speed of action: long term (a few years).
- Farmer's influence on the soil.
Farmer's influence on the plant
Level 1: achieving effective & complete photosynthesis
Removing limiting factors
5 factors are essential and can limit photosynthesis:
Factors on which we have indirect influence
CO2
The plant takes part of the CO2 it needs for photosynthesis from the soil. Implementing practices aimed at increasing soil biological activity raises microbial respiration and thus the amount of CO2 available to crops.
- The first element to consider is the soil structure. In an unstructured soil (compacted, presence of a crust), gases cannot circulate and the plant cannot recover the CO2 it needs.
- The second element is to work towards a living soil approach: reduced tillage, soil cover with presence of living roots, return of organic matter.
Amendments and fertilizers activate and direct the soil microbial flora to provide plants with the nutrients they need.
Water
As farmers, we cannot control precipitation but we can influence the soil's capacity to infiltrate and store water. Conservation agriculture (CA) practices increase the available water capacity, by about +10 to +15% in surface horizons compared to more conventional systems.
In CA systems, there is much greater stability of both density and conductivity, both within a growing season and between years. These practices favor water infiltration capacity and pore network connectivity. Implementing cover crops limits evaporation, promotes infiltration, and reduces runoff.
⚠️ Furthermore, nitrogen nutrition also influences water: the transformation of nitrogen into nitrate form by the plant requires energy and water (on average 4 water molecules per 1 nitrate molecule).
Sun and temperature
A healthy plant produces lipids that protect the leaf from sun rays (for example, this is the principle of "succulent" plants): level 3 of the health pyramid.
Factors on which we have direct influence
Manganese & Iron: minerals essential for photosynthesis:
- Fe : essential to help capture light.
- Mn : essential in water hydrolysis (splitting H₂O into H and O₂).
Other elements:
- N & Mg : contained in chlorophyll molecules.
- Zn : increases leaf width
- P : stores the energy produced by photosynthesis.
Results of complete photosynthesis
- Plants require sufficient levels of magnesium, iron, manganese, nitrogen and phosphorus to reach this health level.
- The rate of photosynthesis increases from 150 to 600%: positive impact on yield.
- The carbohydrate profile of the sap changes (high proportion of complex carbohydrates, low level of non-reducing sugars): better resistance to sucking insects.
- Plants develop resistance to soil pathogenic fungi (Verticillium, Fusarium, Rhizoctonia, etc.).
Level 2: Protein synthesis
Transforming nitrates and ammonium
A healthy plant transforms all nitrates and ammonium into amino acids and then into proteins at the end of each day of growth. The goal is to have a nitrate and ammonium level in the sap close to 0. This is very important for the plant's immunity against piercing-sucking insects and larvae (insects with a simple digestive system) that depend on free amino acids and nitrates in the sap for their nutrition. Ammonium is also an amplifier of infrared, which insects can detect more easily. Nitrate and ammonium levels can be checked through sap analyses. For the plant to transform all nitrates and ammonium into complete proteins, it mainly needs 6 key mineral elements.
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How does the plant transform nitrogen into proteins? -
The 6 key mineral elements to transform all nitrates and ammonium into complete proteins
Influence of agriculture on the soil
Level 3: how are lipids formed?
Lipids are part of the cell membranes composition, so they are always present in the plant. When a plant reaches level 3 of the plant health pyramid by John Kempf (linked to the soil biological activity level), it can store excess energy in the form of lipids:
- Microbial metabolites are amino acids and organic acids (e.g., citric acid), which are chelated with other minerals in the soil solution;
- The plant absorbs most nutrients in the form of microbial metabolites;
- Chelated molecules are more efficient for plant metabolism, resulting in energy savings;
- This energy is stored as lipids.
- Waxes and oils on the leaf surface serve as a protective barrier to prevent pathogen enzymes from functioning;
- Increased resistance to aerial bacterial and fungal pathogens such as downy mildew or rust.
Level 4: formation of secondary metabolites
Secondary metabolism synthesis goes hand in hand with lipid synthesis
Secondary metabolisms are of several types: phenols (e.g., tannins, lignins, flavonoids), terpenes (e.g., sesquiterpenes), etc.
They have several roles in the plant:
- Improving plant resistance via induced systemic resistance (ISR) and systemic acquired resistance (SAR);
- Taste and quality;
- Resistance against insects;
- Messenger molecules (communication with microbial life).
Plant immune responses
Activation of both immune pathways that improve crop resistance to pathogens:
- Induced systemic resistance (ISR) : immune response activated by rhizosphere microbial life or molecules emitted by other plants.
- Systemic acquired resistance (SAR) : generalized immune response induced by a localized infection or attack by a pathogen.
Practical link with the field
I want to improve the photosynthesis of my wheat. What can I implement?
- The first thing to do is to ensure that the essential minerals for photosynthesis are not deficient. Sap analyses are a good agronomic management tool to measure the nutritional status of a crop at a given time. These analyses allow detecting deficiencies and excesses three weeks before the first symptoms appear.
- In wheat, the first analysis is done at tillering, at regrowth, then at the 1 cm ear stage. Based on sap analysis results, foliar applications at key stages (tillering, 1 cm ear, last expanded leaf) are recommended.
- It is also important to have a good soil structure: avoid any kind of compaction (surface and subsoil) as it can reduce gas exchange, water infiltration, and microbial activity.
- Nitrogen nutrition is also a key factor. Excess nitrates in the plant can potentially increase plant water stress and reduce photosynthesis capacity: urea/organic forms are preferred.
Nutrient mobility within the plant
Two distribution pathways within the plant
Xylem
- Transport of raw sap containing water, minerals, and some hormones (e.g., cytokinin).
- Mostly upward flow: direct transport from roots to sinks.
Phloem
- Transport of processed sap containing sugars from photosynthesis, chelated minerals, hormones (e.g., auxins), amino acids, etc.
- Bidirectional flow: from sources to sinks and vice versa.
Elements present according to their mobility
Not all elements are the same when considering their mobility within the plant.
- Mobile elements : easily transported from point A to point B, present in the phloem.
- Immobile elements : present in the xylem. Their mobility increases if the plant absorbs them in chelated form.
- Moderately mobile elements : present in both xylem and phloem. They are immobile when in simple ion form (in xylem) and mobile if chelated in organic form (in phloem).
Why is it important to know this?
- This concept is important to understand when interpreting sap analyses. Contents in old leaves (sources) and young leaves (sinks) give indications on element movements, allowing to anticipate deficiencies before visible symptoms appear.
- If the plant lacks a mobile element (N, P, K, Mg), the content will first decrease in old leaves. If needed, the plant draws mobile elements from old leaves to young leaves. If content is lower in old leaves than in young leaves for mobile elements, a deficiency can be anticipated.
- Conversely, if content is higher in old leaves, it indicates the plant is in excess as it stores it in sinks. Excess mineral nitrogen can make the plant more sensitive to various pests and pathogens.
Main functions of the plant
Each plant has a genetic potential expressed under given conditions, starting as soon as the seed is sown: soil structure, microbial life, water, nutrition, etc. With sap analyses, nutritional stresses impacting yield can be managed.
Four major functions of nutrients within the plant:
- Photosynthesis : 7 essential mineral elements (Zn, N, Mg, K, Fe, S, and P).
- Protein synthesis : OM is essential in nitrate reduction (nitrate reductase) to build protein chains;
- Immunity : silica acts as an element regulator. Calcium and Boron act synergistically to provide rigidity to plant cells;
- Fruiting : Cu, Ca, B, and especially K. Potassium demand arises at later stages. Early potassium fertilization can cause excess. The plant stores it in old leaves and creates antagonisms.
Priorities by stage: the 5 main plant hormones
The link between nutrition and hormones
Hormones are messenger molecules in the plant and act in vital functions. There are 5 main hormones:
- Gibberellins: mainly gibberellic acid, involved in breaking seed dormancy and stimulating cell elongation in stems.
- Cytokinins: stimulate growth of lateral buds lower on the stem, promote cell division and leaf expansion and delay plant aging. Cytokinins are produced in the growing root tips.
- Auxins: mainly indole-3-acetic acid (IAA), promote both cell division and elongation and maintain apical dominance. Auxins induce adventitious root formation and promote fruit growth. They are synthesized in fruits and young shoots and control sugar allocation.
- Ethylene: associated with fruit ripening and leaf fall.
- Abscisic acid: causes winter bud formation, triggers seed dormancy, controls stomatal opening and closing, and induces leaf senescence.
Effect of nutrients on hormones
Stage 1: Germination and establishment
- Abscisic acid content decreases and gibberellins increase.
- The plant begins germination.
- N, P, Ca, and Zn are essential at this first stage.
Stage 2: Vegetative growth
- A high cytokinin/auxin ratio favors shoot development, while a low cytokinin/auxin ratio favors root development.
- Balance between the two is sought to promote both vegetative growth and roots. For this, calcium and boron are crucial at this stage, as they favor auxins.
- After tillering, the goal is to favor: Zn, Fe, Cu, and Mn for flowering preparation and seed development.
Stage 3: Flowering and reproduction
- Boron, calcium, and potassium are crucial for sugar and hormone transport in seeds for cell division (auxins).
- For ethylene and ABA (abscisic acid) formation, copper and Mo are necessary for maturation.
- Nitrogen and Mg remain important as cofactors for hundreds of enzymes.
- Silica plays a role when the plant faces stresses (water, salinity, pathogens, or heavy metals) as it regulates other elements and reduces oxidative stresses (Zn, Mn, Fe, Cu).
Key points to remember
- An effective photosynthesis is the basis of a performant agricultural system.
- Levels 1 and 2 are achievable in the short term by removing limiting factors and achieving nutritional balance.
- Levels 2 and 3 require a broader approach and allow for more resistant crops against pest attacks.
- Micronutrients are crucial for good plant development.
- It is possible to stimulate synthesis of certain hormones by favoring different minerals at specific stages.
