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AGR-HORT49-05 · Lesson 6 · 14 clock hours · lab

Fertility and Irrigation Management

Objective. Students calculate fertilizer injection rates using a concentrate stock-tank method, calibrate a proportioner (injector), and apply a fertility program that matches crop nutritional requirements at documented target ppm N.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Fertility and Irrigation Management in Greenhouse Production

Greenhouse crops grow in a controlled environment, but they depend entirely on you to supply nutrients. Soil-grown field crops can draw on mineral reserves; container media cannot. Every nitrogen, phosphorus, and potassium atom your plants use must arrive through the irrigation water — a practice called fertigation. Getting the concentration right protects plant health, conserves inputs, and prevents nutrient runoff into the environment.

Why Parts Per Million (ppm) Matters

Nutrient concentration in fertigation solutions is expressed in parts per million (ppm), which equals milligrams of nutrient per liter of solution (mg/L). Most greenhouse crops tolerate nitrogen in the range of 100–300 ppm N, depending on crop stage and species. Young transplants and cool-season crops often run 100–150 ppm N; vigorous vegetable or bedding-plant crops at peak growth commonly run 150–250 ppm N. Exceeding the target causes salt stress and leaf burn; falling short causes deficiency and stunted growth.

The Stock-Tank (Concentrate) Method

Proportioners, also called injectors, draw a small volume of concentrated stock solution from a tank and blend it into the main irrigation line at a fixed injection ratio — for example, 1:100 (one part concentrate mixed with 99 parts water to yield 100 parts total). Common ratios are 1:100, 1:128, and 1:200.

To hit a target ppm at the diluted (delivery) end, you must make the stock solution concentrated enough that dilution brings it to exactly the right level.

Basic formula:

> Stock solution concentration (ppm) = Target ppm ÷ Injection ratio

For example, at a 1:100 ratio targeting 200 ppm N: stock must contain 200 × 100 = 20,000 ppm N.

Converting that to grams of fertilizer per liter of stock:

> grams per liter = (target ppm in stock ÷ 1,000,000) × 1,000 mL/L ÷ (% nutrient ÷ 100)

Or more directly: grams of fertilizer per liter of stock = (ppm N in stock) ÷ (% N × 10,000)

A 20-20-20 fertilizer contains 20% N. To deliver 20,000 ppm N in the stock: 20,000 ÷ (20 × 10,000) = 0.10 g fertilizer per gram of water — which equals 100 g of fertilizer per liter of stock solution. Always check this value against the solubility limit on the fertilizer label before you try to dissolve it.

Calibrating the Injector with an EC Meter

Electrical conductivity (EC) measures the total dissolved salts in a solution. Because fertilizer salts conduct electricity, EC is a fast proxy for nutrient concentration. EC is expressed in millisiemens per centimeter (mS/cm).

A commonly used approximation: 1 mS/cm ≈ 700 ppm total dissolved solids (this conversion factor varies by fertilizer formulation; your fertilizer label or supplier data sheet will give a more precise factor). For a 20-20-20 solution targeting 200 ppm N, the expected EC of the diluted solution can be estimated from the label's EC factor.

After preparing your stock and running the injector, collect a sample of diluted solution from the irrigation line, measure its EC, convert to approximate ppm, and compare to your target. If actual ppm is more than ±10% from target, adjust the stock concentration or verify the injector ratio mechanically.

Leaching Fraction and Pour-Through Testing

Container media accumulates salts over time. Applying a leaching fraction — typically 10–20% more water than the container holds — flushes excess salts through the drainage holes. Without periodic leaching, root-zone EC climbs and plants suffer.

The pour-through method measures leachate EC without removing roots or media. You irrigate, wait about 30 minutes, then collect the liquid that drains from the container's bottom and measure its EC. Compare the result to published acceptable ranges for your crop and stage. Vegetative growth stages generally tolerate moderate EC; seedling and rooting stages require lower EC.

pH and Water Quality

Always measure solution pH alongside EC. Most greenhouse crops perform best in a root-zone pH of 5.8–6.5. High pH locks out iron and manganese; low pH can cause toxicity of the same elements. If source water is alkaline, acidifying agents may be needed — follow label directions and handle acids with full PPE.

Safety

Personal protective equipment (PPE) is required any time you handle concentrated fertilizer stock or acids. Wear safety glasses and chemical-resistant gloves throughout this lab. Concentrated fertilizer dust irritates eyes and mucous membranes. When working with any pH-adjusting acid, add acid to water — never water to acid — and work in a well-ventilated space. Know the location of the eyewash station before you begin. Label all stock-solution containers clearly with contents and concentration. Dispose of excess solution according to your facility's nutrient-management plan; do not discharge concentrated fertilizer solution to storm drains.

Keeping Records

A complete fertigation log is not optional — it is a professional and regulatory requirement. Your log must include date, crop, fertilizer lot number, stock concentration prepared, injector ratio used, target and measured ppm N, target and measured EC, leachate EC, volume applied, and the initials of the person who performed the work. Accurate records allow you to troubleshoot crop problems and demonstrate compliance.

Key terms8

FertigationThe practice of delivering dissolved fertilizer nutrients to plants through the irrigation water supply.
Parts per million (ppm)A unit of concentration equal to milligrams of a substance per liter of solution (mg/L), used to express nutrient levels in greenhouse fertigation.
Stock solutionA highly concentrated fertilizer solution held in a tank and drawn by a proportioner/injector, then diluted into the irrigation line to reach the target delivery concentration.
Injection ratioThe fixed ratio at which a proportioner blends concentrate with irrigation water; for example, 1:100 means one part stock is added to 99 parts water.
Electrical conductivity (EC)A measure of a solution's ability to conduct electricity, expressed in mS/cm, used as a rapid indicator of total dissolved fertilizer salts in irrigation and leachate solutions.
Leaching fractionThe portion of applied irrigation water that passes through and drains out of the container, flushing accumulated salts from the root zone.
Pour-through methodA technique for sampling root-zone leachate by irrigating a container, waiting briefly, then collecting and testing the drainage water for EC and pH without disturbing the plant.
Guaranteed analysisThe legally required fertilizer label section stating the minimum percentages of each declared nutrient, used to calculate how much product to add for a target ppm.

Worked examples4

1. Calculate the grams of 20-20-20 fertilizer needed to prepare 10 liters of stock solution that will deliver 200 ppm N through an injector set at a 1:100 ratio.
  1. Step 1 – Find the required ppm N in the stock solution. Stock ppm N = Target ppm N × Injection ratio = 200 ppm × 100 = 20,000 ppm N.
  2. Step 2 – Convert ppm N in the stock to grams of fertilizer per liter. The fertilizer is 20% N (from the guaranteed analysis), so each gram of fertilizer contains 0.20 g of N. Grams of fertilizer per liter = (ppm N needed in stock) ÷ (% N × 10,000) = 20,000 ÷ (20 × 10,000) = 20,000 ÷ 200,000 = 0.10 g fertilizer per gram of water. Because 1 liter of water = 1,000 g, this equals 100 g of fertilizer per liter of stock.
  3. Step 3 – Scale up to 10 liters of stock. Total fertilizer = 100 g/L × 10 L = 1,000 g.
  4. Step 4 – Verify against label solubility. A typical 20-20-20 fertilizer has a solubility well above 100 g/L at room temperature, so dissolving 100 g per liter is feasible. Confirm on your specific label before proceeding.
Answer: 1,000 g (1 kg) of 20-20-20 fertilizer dissolved into 10 liters of stock solution.
2. A student collects a sample of diluted fertigation solution from the irrigation line and measures an EC of 1.4 mS/cm. Using a conversion factor of 700 ppm per mS/cm, what is the estimated ppm of total dissolved solids? The target was 200 ppm N. Is the solution within ±10% of the 200 ppm N target, assuming that for this formulation the N fraction of total dissolved solids is approximately 28.6%?
  1. Step 1 – Convert EC to estimated total dissolved solids (TDS). TDS = EC × 700 = 1.4 mS/cm × 700 = 980 ppm TDS.
  2. Step 2 – Estimate ppm N from TDS. ppm N = TDS × N fraction = 980 × 0.286 ≈ 280 ppm N.
  3. Step 3 – Calculate the acceptable range for 200 ppm N target (±10%). Lower bound = 200 × 0.90 = 180 ppm N. Upper bound = 200 × 1.10 = 220 ppm N.
  4. Step 4 – Compare. 280 ppm N is above 220 ppm N, so the solution is outside the ±10% tolerance. The student should reduce the stock concentration or recheck the injector ratio.
Answer: Estimated 280 ppm N — outside the ±10% tolerance (180–220 ppm). Adjustment is required.
3. A student needs to prepare 5 liters of stock solution using a fertilizer with 15% N and an injector ratio of 1:200, targeting 150 ppm N at the diluted end. How many grams of fertilizer are needed?
  1. Step 1 – Find required ppm N in the stock. Stock ppm N = 150 × 200 = 30,000 ppm N.
  2. Step 2 – Find grams of fertilizer per liter of stock. g/L = 30,000 ÷ (15 × 10,000) = 30,000 ÷ 150,000 = 0.20 g per gram of water = 200 g per liter of stock.
  3. Step 3 – Scale to 5 liters. Total fertilizer = 200 g/L × 5 L = 1,000 g.
  4. Step 4 – Note the high concentration (200 g/L) and confirm it does not exceed the fertilizer's stated solubility limit before mixing.
Answer: 1,000 g of the 15% N fertilizer dissolved into 5 liters of stock solution.
4. After irrigating a flat of seedlings to verify leaching fraction, a student collects 250 mL of leachate from containers that received 2,000 mL total. What is the leaching fraction, and is it within the recommended 10–20% range?
  1. Step 1 – Calculate leaching fraction. Leaching fraction = volume drained ÷ volume applied = 250 mL ÷ 2,000 mL = 0.125, or 12.5%.
  2. Step 2 – Compare to recommended range of 10–20%. 12.5% falls within 10–20%.
  3. Step 3 – Conclusion: the leaching fraction is acceptable. The student should still measure leachate EC and confirm it is within the published range for the crop stage.
Answer: Leaching fraction = 12.5%, which is within the recommended 10–20% range.

The math in this lesson2

Learners multiply target ppm N by a 1:100 injector ratio and use 20% N to find grams of fertilizer per liter.
7.RP.3N-Q.1
Brush up in the math halls at newSchoolWorks.com:
In class: Stock solution math: students use a provided fertilizer label (guaranteed analysis, solubility) and an injector ratio to calculate grams of fertilizer needed to prepare a stock solution that delivers a target N concentration (e.g., 200 ppm N) at the diluted rate; calculations are shown step-by-step on a worksheet. · Injector calibration: students verify injector ratio by collecting a sample of diluted solution, measuring EC, and back-calculating ppm N against the expected value using a conversion factor; they record actual vs. target ppm and adjust concentrate if needed. · Crop fertigation application: students apply the calibrated fertilizer solution to a bench of plants, record application volume, check for runoff and drainage from containers (verifying leaching fraction), then collect a pour-through leachate sample and confirm EC is within the acceptable range for the crop stage.
Materials: Water-soluble fertilizer (20-20-20 or similar), mixing containers, gram scale, proportioner/injector, EC meter, pH meter, graduated cylinder, fertigation log sheet, crop EC target range chart, safety glasses, chemical-resistant gloves
Check: Calculated stock solution delivers ppm N within ±10% of target as confirmed by EC measurement; leachate EC is within published acceptable range for the crop stage; fertigation log is complete; pass requires both numerical checks met and log completed.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Greenhouse crop production and fertility management
1. A grower uses a 1:100 injector ratio and wants to deliver 150 ppm N to the crop. What concentration of N (in ppm) must the stock solution contain?
Greenhouse equipment and calibration
2. Which instrument is used in the injector calibration step to verify that the diluted fertilizer solution is at the correct concentration?
Root-zone management and salinity
3. A student measures a leachate EC that is significantly higher than the published acceptable range for the crop stage. What is the most likely cause and the correct corrective action?
Fertilizer label interpretation
4. Why is the guaranteed analysis on a fertilizer label essential for calculating stock solution concentration?
Greenhouse irrigation and leaching management
5. What is the purpose of maintaining a leaching fraction of roughly 10–20% during greenhouse container irrigation?
Safety and PPE in greenhouse chemical handling
6. During stock solution preparation, minimum required PPE includes:
Greenhouse nutrient management and pH
7. A grower's source water has a pH of 8.2. If uncorrected, which pair of micronutrients is most likely to become unavailable to greenhouse crops grown at that pH?
Record-keeping and regulatory compliance
8. A fertigation log is required in greenhouse production primarily because:
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