Research: Nutrients

General

  • Required composition of minerals differs from that for soils
  • Most important: composition in water can change very rapidly eg. because of selective uptake by plants
  • Concentration should be between 1000 and 2500 ppm

About the water

Cornell strongly recommends to start with water analysis.

  • Alkalinity, reported as units of Calcium Carbonate equivalents CaCO3. 0 to 300 ppm is common. More important value than pH.

    • “Depending on your alkalinity, you may need to choose a fertilizer formulation with a greater proportion of acidic nitrogen forms (ammonium or urea) or add acid to neutralize the alkalinity and counter the pH rise.”

    • “Often linked with your water alkalinity are considerable levels of Ca, Mg and S in the water”, also Sodium and chloride and compensate with fertilizer.

  • Electrical conductivity (EC), should ideally beless than 0.25 mS/cm for closed systems.

Needed nutrients

Macronutrients

  • nitrogen
  • phosphorus
  • potassium
  • sulfur
  • calcium
  • magnesium

Micronutrients

  • iron
  • manganese
  • zinc
  • boron
  • copper
  • molybdenum
  • chloride
  • nickel

Chloride and nickel aren’t included in most recipes, as they’re available in sufficient quantities as impurities.

Plant types

For vegetative crops, most nutrient-solution recipes don’t adjust the ratio of nutrients while they grow; whereas, in fruiting crops the ratio may be adjusted to alter the shift between vegetative and reproductive growth.

  • 150ppm N works well for head and leaf lettuce during the main production stage;
  • 175 to 200ppm N is more appropriate for kale, Swiss chard and mustard greens, which tend to be slightly heavier feeders.
  • During seedling propagation a some-what lower fertilizer target of 125ppm N works well for both
  • tomato and other fruiting crops have higher nutrient demands than leafy greens: higher nitrogen, potassium, calcium and magnesium

Plant stages

  • vegetative growth: high in nitrogen, calcium and magnesium
  • flowering: high in potassium and phosphorus
  • nitrogen restriction inhibits vegetative growth and helps induce flowering.

Recipies

Symptoms of nutrient deficiencies

Improving nutrient uptake

To improve nutrient uptake:

  • chelating agents can be added
    • humic substances (Note: science seems to know very little about humic substances. They are complex molecules apparently created during dead plant breakdown, which help with plant growth “substantially” according to some studies)

Mixing

  • Use stock tanks to avoid a nasty precipitate or sludge that will occur when specific nutrients are mixed in the concentrated form (e.g. calcium can combine with phosphates and sulfates to form insoluble precipitates)

Sources