Module 2: Understanding Plant Nutrition

Level: 🟢 Beginner
Prerequisites: Module 1 (Introduction to Hydroponics)
Estimated time: 25-30 minutes
Goal: Understand what nutrients plants need, how to read fertilizer labels, and why pH matters.

What You'll Learn

By the end of this module you will understand the role of macronutrients and micronutrients, how to read NPK ratios on fertilizer labels, the difference between soil and hydroponic nutrients, and why pH control is critical for nutrient availability.

2.1 The Three Macronutrients (NPK)

Plants need three nutrients in large quantities. These are called macronutrients, and they're identified by the letters N-P-K:

Nitrogen (N)

Role: Drives leafy green growth and chlorophyll production. Nitrogen is the building block of proteins and enzymes.

Deficiency symptoms: Older leaves turn yellow (chlorosis) starting from the tips. Growth slows or stops.

In hydroponics: Lettuce and leafy greens need high nitrogen. Fruiting plants (tomatoes, peppers) need less nitrogen once flowering begins, or they'll produce leaves instead of fruit.

Phosphorus (P)

Role: Essential for energy transfer (ATP), root development, flowering, and fruiting. Phosphorus supports strong root systems.

Deficiency symptoms: Leaves develop purple or dark tints, especially on undersides and stems. Root growth is stunted.

In hydroponics: Seedlings need phosphorus for root establishment. Fruiting plants need extra phosphorus during flowering.

Potassium (K)

Role: Regulates water uptake and photosynthesis efficiency. Strengthens cell walls, improves disease resistance, and enhances fruit quality.

Deficiency symptoms: Leaf edges turn brown and curl (necrosis). Older leaves are affected first.

In hydroponics: Potassium is needed throughout the growth cycle. It's especially important during fruiting for flavor and shelf life.

2.2 Essential Micronutrients

Beyond N-P-K, plants need smaller amounts of secondary nutrients and trace elements. These are just as important, but required in smaller quantities.

Secondary Nutrients

Nutrient Role Deficiency Signs
Calcium (Ca) Cell wall structure, enzyme function Blossom end rot in tomatoes, tip burn in lettuce
Magnesium (Mg) Central atom in chlorophyll Yellow leaves with green veins (interveinal chlorosis)
Sulfur (S) Protein synthesis, enzyme activation Yellowing of new growth (unlike nitrogen, which affects old growth first)

Trace Elements (Micronutrients)

Plants need tiny amounts of these, but they're essential:

The good news: Pre-mixed hydroponic nutrients include all of these in the right proportions. You don't need to measure them individually.

2.3 Reading Fertilizer Labels

Every fertilizer bottle has three numbers on the label, like 5-10-5 or 24-8-16. These are the NPK ratio.

What the Numbers Mean

The three numbers represent the percentage by weight of:

For example, 5-10-5 means:

The remaining percentage is inert filler, carrier materials, and trace elements.

Common NPK Ratios for Hydroponics

Growth Stage Ideal NPK Why
Leafy greens (lettuce, spinach, herbs) High N, like 5-1-3 Maximizes leaf production
Vegetative growth (all plants) Balanced, like 5-5-5 Supports overall growth
Flowering/Fruiting (tomatoes, peppers) Lower N, higher P-K, like 2-4-4 Encourages flowers and fruit over foliage

For the lettuce system we're building, look for a fertilizer with higher nitrogen (the first number should be the highest or tied for highest).

2.4 Hydroponic Nutrients vs. Soil Fertilizers

You might be tempted to use soil fertilizer like Miracle-Gro in your hydroponic system. Don't.

Why Soil Fertilizers Don't Work

Incomplete formula: Soil fertilizers assume the soil will provide calcium, magnesium, and many trace elements. Hydroponic systems have no soil, so these nutrients must be in the solution.

Wrong pH buffering: Soil naturally buffers pH. Soil fertilizers aren't designed to keep pH stable in water, and your pH will drift unpredictably.

Insoluble components: Some soil fertilizers contain slow-release particles that don't dissolve fully. These will clog systems and starve plants.

What to Use Instead

Hydroponic-specific nutrients are formulated to:

Popular brands include:

For beginners, a single-bottle "all-in-one" formula is easiest. As you gain experience, you can switch to two or three-part systems that let you adjust the formula for different growth stages.

2.5 EC and TDS: Measuring Nutrient Strength

Once you mix nutrients into water, how do you know the concentration is correct? You can't see nutrients, but you can measure them electrically.

Electrical Conductivity (EC)

EC measures how well the solution conducts electricity. Pure water doesn't conduct electricity, but dissolved minerals do. The more nutrients in the water, the higher the EC reading.

Units: EC is measured in millisiemens per centimeter (mS/cm) or microsiemens per centimeter (μS/cm).
1.0 mS/cm = 1000 μS/cm

Typical ranges:

Total Dissolved Solids (TDS)

TDS measures the concentration of dissolved solids in parts per million (ppm). It's calculated from EC using a conversion factor.

Conversion:
Most TDS meters use one of two scales:

When reading advice online, confirm which scale is being used.

Typical ranges (Scale 500):

Do You Need a Meter?

For a beginner Kratky system, no. Just follow the mixing instructions on your nutrient bottle. An EC or TDS meter is useful for troubleshooting or scaling up, but not mandatory for your first grow.

2.6 pH: The Key to Nutrient Availability

pH measures how acidic or alkaline your solution is, on a scale from 0 (very acidic) to 14 (very alkaline). Pure water is neutral at pH 7.0.

Why pH Matters

Nutrients dissolve in water as charged ions. Plants absorb these ions through their roots. But not all ions are available at all pH levels.

At certain pH levels, some nutrients bind to other molecules and become unavailable to plants. This is called nutrient lockout.

The Goldilocks Zone

For most hydroponic plants, the ideal pH range is 5.5 to 6.5.

Within this range:

Lettuce and leafy greens: pH 5.8-6.5 (optimal 6.0-6.3). Lettuce is pH-tolerant; don't worry about small drifts within this range.
Fruiting plants (tomatoes, peppers): Tolerate slightly lower, 5.8 - 6.2

How pH Drifts

pH naturally drifts over time as plants absorb nutrients. Different nutrients are absorbed at different rates, which changes the solution's pH.

In a Kratky system, pH tends to rise slowly as the plant grows. You'll check pH weekly and adjust as needed (covered in Module 5).

Measuring pH

You can measure pH with:

For your first grow, a liquid test kit is perfect. Digital meters are worth it if you're growing multiple plants or planning to scale up.

2.7 Putting It All Together

Let's summarize what you need to know:

  1. Plants need NPK (macronutrients) plus secondary nutrients and trace elements (micronutrients).
  2. Hydroponic nutrients provide complete nutrition. Don't use soil fertilizers.
  3. NPK ratios vary by plant type. Leafy greens need high nitrogen.
  4. EC/TDS measures nutrient concentration. Follow label instructions for beginners.
  5. pH must stay in the 5.5-6.5 range for nutrients to be available.
  6. Check pH weekly and adjust with pH up/down solutions.

What's Next

In Module 3, you'll compare the six hydroponic systems and learn why the Kratky method is perfect for beginners. You'll also see a cost breakdown and parts list for your build.

Next Steps

Previous: Module 1: Introduction to Hydroponics
Continue to: Module 3: Choosing Your First System

Course Overview:
01. Introduction to Hydroponics
02. Understanding Plant Nutrition (you are here)
03. Choosing Your First System
04. Materials and Setup
05. Planting and Maintenance
06. Troubleshooting and Next Steps