While hard-water (eutrophic) plant species tolerate relatively high concentrations of nutrients, soft-water (oligotrophic/sensitive) species often prefer concentrations that are one order of magnitude lower.

Typical nutrient solutions used for hydroponic cultivation of plants

• Microelements are not listed here, although their use is assumed.
  • Typical average microelements concentrations for full strength Hoagland (µg/ℓ): Fe 1000-5000, Mn 500, B 500, Zn 50, Cu 20, Mo 10.
  • Typical average microelements concentrations for full strength Gaudet (µg/ℓ): Fe 760, Mn 840, Zn 680, B 260, Cu 1, Mo 3.
    • Fe as Fe-EDTA (or FeSO₄), B as H₃BO₃, Mn as MnCl₂, Zn as ZnSO₄, Cu as CuSO₄, Mo as Na₂MoO₄
• The high concentrations of nutrients in hydroponic nutrient solutions (compared to those used in aquariums) are usually explained by the argument that in nature, aquatic plants draw most of their nutrients from sediment, where nutrient concentrations are typically much higher than in the water column. Since sediment is not usually used in experimental (or hydroponic) plant cultivation, the nutrient solution is designed to contain a similar amount of nutrients to what the plants would have available if they also had access to nutrients from the sediment.

Currently, I am testing the following recipes in my experiments, which look promising so far but may still require further adjustments:

Graph of dependence of external nutrient concentration on plant growth rate

• 100% (L.S.) = saturation point, i.e. the concentration at which plants reach their maximum rate of photosynthesis
• 90% = critical concentration, i.e. the concentration at which plants achieve 90% yield
• 50% (½V_max) = half-saturation point, i.e. the concentration at which plants reach 50% of their photosynthetic rate

Relative proportions of elements in plant tissue

The above-mentioned amounts of nutrients [in approximately these proportions] are actually uptaken by plants. This means that for every ppm NO₃, they uptake approximately ppm H₂PO₄, ppm K, ppm Fe, etc. Anything more than that is counterproductive.

• reduced CO₂ availability → limits photosynthesis
  • soft-water macrophytes are often adapted to utilize CO₂ directly from the water column
  • in bicarbonate-rich waters (often alkaline), CO₂ is less available because it shifts to HCO₃⁻ and CO₃²⁻ forms
  • many soft-water species lack the physiological mechanisms (like carbonic anhydrase or HCO₃⁻ transporters) to efficiently use bicarbonate as a carbon source
  • this leads to carbon limitation, impairing photosynthesis and growth
• increased pH → reduces nutrient availability
  • high bicarbonate levels increase alkalinity and buffering capacity, raising the pH of the water
  • see effects of high pH on plants
• ionic imbalance → disrupts nutrient uptake
  • bicarbonate accumulation can cause ionic imbalances in plant tissues
  • this can interfere with electrochemical gradients across membranes, affecting nutrient uptake and cellular homeostasis
  • in soft-water species, which are adapted to low ion concentrations, this can be particularly harmful
• enhanced microelement availability → amplifies toxicity in soft-water species
  • bicarbonate forms bioavailable Fe/Mn-bicarbonate complexes (e.g., FeHCO₃⁺, MnHCO₃⁺) that plants mistake for "easy-to-absorb" forms
  • accelerates oxidation of Fe²⁺ → Fe³⁺, but stabilizes colloidal/cationic intermediates more absorbable by high-affinity transporters
  • displaces chelators (e.g., HCO₃⁻ competes with EDTA/DTPA for Fe³⁺), freeing metals for uptake
  • stimulates root exudates in some species, further enhancing metal mobilization
  result: bicarbonate acts as a "stealth carrier," increasing plant-available Fe/Mn even at low concentrations → higher accumulation in tissues [in soft-water species]

• extremely low or high pH can significantly impair nutrient uptake due to the chemical and physiological effects of excess H⁺ or OH⁻ ions in the water
• high H⁺ concentration (low pH = acidic conditions)
  • displaces nutrient cations (like Ca²⁺ Mg²⁺, K⁺, and NH₄⁺) from root/shoot binding sites
  • increases solubility of toxic metals (like Al³⁺ and Fe²⁺/Mn²⁺)
  • demages root/shoot cells and enzymes
• high OH⁻ concentration (high pH = alkaline conditions)
  • causes precipitation of nutrients (like PO₄³⁻, Fe³⁺, and Mn²⁺)
  • reduces solubility and mobility of key ions (such as Fe, Zn, Cu, and Mn)
  • alters membrane potential and transport mechanisms

• low O₂ (hypoxia/anoxia)
  • impaired respiration
    • Aquatic plants rely on oxygen for aerobic respiration, especially in root tissues.
    • Low O₂ impairs energy production, reducing nutrient uptake and root growth.
  • toxic sediment byproducts
    • Anaerobic conditions in sediments promotes anaerobic microbial activity, leading to the production of toxic compounds like hydrogen sulfide (H₂S) and methane (CH₄). These can damage plant roots and inhibit growth.
  • reduced nutrient availability
    • Anoxic conditions alter redox potential, affecting the solubility and availability of nutrients like iron, manganese, and phosphorus.
• high O₂ (supersaturation)
  • oxidation of nutrients
    • High O₂ levels promote oxidative conditions, which can convert soluble nutrients into insoluble forms, making them less bioavailable.
      • Example: Fe²⁺ (soluble) oxidizes to Fe³⁺ (insoluble), which precipitates as iron hydroxide.
      • Similarly, Mn²⁺ oxidizes to Mn⁴⁺, reducing its availability.
  • phosphorus immobilization
    • In oxygen-rich sediments, iron oxides form and bind phosphate (PO₄³⁻), locking it into the sediment.
    • This reduces phosphorus availability in the water column, which can limit plant growth, especially in oligotrophic systems.
  • increased nitrification rate → increased water acidification rate (pH drop)
    • High O₂ supports nitrifying bacteria, which convert:
      • Ammonium (NH₄⁺) → Nitrite (NO₂⁻) → Nitrate (NO₃⁻)
    • While nitrate is still usable by plants, this transformation:
      • Can reduce ammonium availability, which some aquatic plants prefer.
      • May lead to nitrate leaching or denitrification under fluctuating redox conditions.
  • increased organic matter decomposition → increased nutrient load
    • High oxygen levels accelerate aerobic decomposition of organic matter.
    • This can release nutrients (like nitrogen and phosphorus) from detritus, but also:
      • Increases microbial competition for nutrients.
      • May lead to temporary nutrient immobilization in microbial biomass.
  • risk of gas embolisms
    • Gass bubble disease: Supersaturation can lead to gas embolisms in plant tissues, disrupting internal transport and damaging cells.
  • oxidative stress
    • High O₂ levels can increase the formation of reactive oxygen species (ROS), which damage proteins, lipids, and DNA.
  • altered microbial dynamics
    • High O₂ can shift microbial communities, potentially affecting nutrient cycling and organic matter decomposition.
• …
  
  
• Let's calculate how many milligrams of dissolved oxygen (O₂) are consumed to produce 1 mg of CO₂ during the aerobic decomposition of volatile organic matter in sediment:
  • A simplified version of the aerobic decomposition of organic matter (represented as a carbohydrate like glucose) is:
    • C₆​H₁₂​O₆​ + 6O₂ ​→ 6CO₂​ + 6H₂​O
  • This tells us that:
    • 6 moles of O₂ are consumed to produce 6 moles of CO₂
    • So, 1 mole of O₂ is consumed per 1 mole of CO₂ produced
      • Because 1 mol of O₂ = 32 g, and 1 mol of CO₂ = 44 g, thus for every 44 mg of CO₂ produced, 32 mg of O₂ are consumed.
      • So, 0.73 mg of dissolved oxygen (O₂) is consumed for every 1 mg of CO₂ produced during aearobic decomposition of organic matter in sediment.
      • PS: The minimum dissolved oxygen (DO) concentrations required to support thriving colonies of various freshwater organisms are as follows:
        • 2–3 mg/ℓ for aquatic plants
        • ≥5 mg/ℓ for warmwater fish
        • 4-6 mg/ℓ for invertebrates
        • ≥2 mg/ℓ for aerobic bacteria
  • Now, at 25°C and standard atmospheric pressure (1 atm), the saturation concentration of dissolved oxygen (O₂) in freshwater is approximately 8.26 mg/ℓ.
  • Let's say we don't want the concentration of dissolved oxygen in the water to fall below 5 mg/ℓ (= safe level for fish/invertebrates), so we only have about 3.26 mg/ℓ O₂ (8.26 − 5) left for the decomposition of organic material into CO₂.
  • This amount of O₂ (i.e., 3.26 mg/ℓ) would theoretically be sufficient to produce up to 4.5 mg/ℓ CO₂ [during the decomposition of organic material in the sediment] → x = 3.26 mg O₂ × (44 mg CO₂ / 32 mg O₂). This is therefore the maximum amount of CO₂ that sediment in our aquarium can produce [during the decomposition of organic material] without the concentration of dissolved oxygen in the water falling below the critical level (5 mg/ℓ) and without using artificial CO₂ injection.
  • PS: Fish respiration may produce slightly less CO₂ per mg of O₂ consumed than sediment decomposition (especially if lipids or proteins are the dominant energy source).

• The impact of organic matter (OM) and dissolved organic matter (DOM) on submerged macrophytes, its types/sources, beneficial vs harmful thresholds, sediment toxicity mechanisms, and fulvic adic chelation, as well as algae overgrowth and [bacterial] biofilm dynamics is discussed in a separate article.
  
  

• In progress …