- 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.
Chloride and nickel aren’t included in most recipes, as they’re available in sufficient quantities as impurities.
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
- vegetative growth: high in nitrogen, calcium and magnesium
- flowering: high in potassium and phosphorus
- nitrogen restriction inhibits vegetative growth and helps induce flowering.
Symptoms of nutrient deficiencies
- Nice table: https://www.hydroponics.net/learn/nutrient_deficiencies.php
- Description: https://www.hydroponics.net/learn/deficiency_by_element.php
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)
- 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)