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Quiz: Growing Media and Crop Management

Test your understanding of growing media properties, crop selection, transplanting techniques, succession planting, and post-harvest handling with these questions.

1. What mandatory preparation step is required before using new rockwool in a hydroponic system, and why?

  1. Dry the rockwool in an oven to kill pathogen spores before first use
  2. Soak in pH 5.5 nutrient solution for 24 hours to neutralize its alkaline buffering capacity
  3. Rinse with distilled water three times to remove manufacturing residues
  4. No preparation is required — rockwool is inert and ready to use from the package
Show Answer

The correct answer is B. Freshly manufactured rockwool has a pH of 7.0–8.0 due to its mineral fiber composition. Using it unprepared raises the pH of nutrient solution passing through it, causing iron and micronutrient deficiencies. Soaking in pH 5.5 nutrient solution for 24 hours pre-acidifies the fibers, neutralizing the alkaline buffering capacity and bringing the medium into the optimal pH range before planting. This step is non-optional in rockwool use.

Concept Tested: Rockwool pH Preparation

2. What is the key trade-off between water retention and air porosity in growing media, and which medium best balances both properties?

  1. There is no trade-off — media can have both high water retention and high air porosity simultaneously
  2. High water retention leaves less pore space for air; coconut coir provides the best balance of both
  3. High air porosity is always preferred; perlite is the best choice for all crops
  4. Water retention and air porosity are both determined solely by irrigation frequency, not medium properties
Show Answer

The correct answer is B. Water retention and air porosity are opposing properties — a medium that holds lots of water fills pore spaces that could otherwise hold air. Coconut coir achieves a relatively good balance: it holds approximately 50–60% of its volume as water while maintaining moderate-to-high air porosity due to its fibrous structure. This makes it suitable for fruiting crops on drip systems. Perlite is maximum air porosity but near-zero water retention, requiring very frequent irrigation.

Concept Tested: Media Air Porosity

3. Why must raw (unbuffered) coconut coir be pre-buffered with calcium and magnesium solution before planting?

  1. Raw coir has a high pH that will alkalize the nutrient solution
  2. Raw coir has a high cation exchange capacity that will absorb Ca²⁺ and Mg²⁺ from the nutrient solution, causing deficiencies
  3. Raw coir contains salt residues from coconut processing that need to be flushed before use
  4. Raw coir compacts over time without calcium to maintain its fiber structure
Show Answer

The correct answer is B. Coconut coir has a high cation exchange capacity — it will preferentially bind calcium and magnesium ions from the nutrient solution, removing them from availability for the plant. Pre-buffering by soaking new coir in a calcium-magnesium solution before planting satisfies this binding capacity, so the coir no longer strips these essential nutrients from the nutrient solution during the growing cycle. Many commercial "buffered coco" products have this step done at the factory.

Concept Tested: Coconut Coir Properties

4. A grower wants to sterilize expanded clay hydroton pellets between crop cycles. Which method is most appropriate, and why?

  1. Bleach soak (150–250 ppm free chlorine) or hydrogen peroxide soak followed by thorough rinsing
  2. Baking in a conventional oven at 200°F for one hour
  3. Rinsing with plain tap water three times is sufficient for expanded clay
  4. Replace expanded clay every cycle — it cannot be adequately sterilized
Show Answer

The correct answer is A. Expanded clay is one of the most reusable media and can be cleaned and sterilized effectively with either bleach (3–5% solution, 30–60 minute soak) or hydrogen peroxide (3% solution). Bleach is more effective against Pythium and bacteria; hydrogen peroxide breaks down to water and oxygen with no residual concern. Both require thorough rinsing. Oven baking does not reach temperatures needed to kill Pythium oospores. Simple water rinsing does not eliminate pathogen spores.

Concept Tested: Media Sterilization Methods

  1. Lettuce in NFT — 5 liters per plant for optimal head development
  2. Basil in DWC — 8 liters per plant due to dense root development
  3. Tomatoes in DWC or drip — 10–15 liters per plant for the large fruiting root system
  4. Microgreens in shallow trays — 2 liters per tray minimum
Show Answer

The correct answer is C. Tomatoes develop massive root systems during a 90–180+ day fruiting cycle and require the most root zone volume of any commonly grown hydroponic crop — 10–15 liters per plant is the guideline. This is why large fruiting crops are grown in 5-gallon (19L) DWC buckets or in large rockwool or coir slabs. Lettuce needs only 2–4 liters; basil 2–4 liters; microgreens grown in shallow trays require minimal root depth.

Concept Tested: Crop Selection by System Type

6. What is the correct approach to supporting indeterminate tomato plants in a hydroponic system?

  1. Allow tomatoes to grow naturally without support — they are self-supporting once established
  2. Train to a single stem with clip-and-twine or trellising, removing suckers to concentrate energy
  3. Prune to three main stems and allow free horizontal spreading on a mesh trellis
  4. Top the plant at 60 cm height to prevent it from exceeding grow light coverage
Show Answer

The correct answer is B. Indeterminate tomatoes grow continuously upward and require active management. The standard technique is training to a single main stem using clip-and-twine or Dutch high-wire systems, removing "suckers" (lateral shoots growing from the crotch between the main stem and leaf petioles) to concentrate plant energy into the single fruiting stem. Without training and pruning, indeterminate tomatoes produce a sprawling plant with lower yield per unit area.

Concept Tested: Tomatoes in Hydroponics

7. What is the key harvest timing indicator that signals a lettuce plant is about to bolt and must be harvested immediately?

  1. The outer leaves begin to turn yellow while inner leaves remain green
  2. The center of the rosette elongates upward with early bolting and the leaves start to taste bitter
  3. Root color changes from white to tan, indicating the plant has reached peak nutrient uptake
  4. The plant reaches exactly 100 grams fresh weight regardless of visual appearance
Show Answer

The correct answer is B. Bolting is the transition from vegetative growth to reproductive growth — the plant elongates a flower stalk, and simultaneously leaf texture becomes bitter and quality declines rapidly. The visual signal is the center of the lettuce rosette beginning to elongate upward (rather than expanding outward) and the appearance of early flower bud formation. Harvest immediately when this starts — waiting even a day or two after bolting begins significantly diminishes quality.

Concept Tested: Harvest Timing Indicators

8. Immediately after harvesting hydroponic lettuce, what post-harvest step has the greatest impact on extending shelf life from 3–5 days to 10–14 days?

  1. Wrapping in paper towels to absorb excess moisture before refrigeration
  2. Immediate cold-water dip at 10–12°C to rapidly remove field heat, followed by refrigeration at 2–4°C
  3. Treating cut surfaces with a dilute citric acid spray to prevent browning
  4. Placing lettuce near ripening tomatoes in the refrigerator to slow metabolism
Show Answer

The correct answer is B. Rapidly cooling harvested lettuce by immersion in cold water (10–12°C) for 15–30 minutes immediately after harvest removes the "grow room heat" stored in the leaves. Heat accelerates respiration, wilting, and bacterial growth. Refrigeration at 2–4°C with 95–98% humidity then maintains quality. Placing near ripening fruit (tomatoes) is actively harmful — ethylene from ripening fruit causes rapid yellowing and senescence in leafy greens. Citric acid treatment provides minimal benefit.

Concept Tested: Post-Harvest Handling

9. Microgreens are harvested at what stage of plant development, and what is the typical time from seeding to harvest?

  1. At the full-leaf vegetative stage, approximately 30–45 days after seeding
  2. At the cotyledon or first true leaf stage, approximately 7–14 days after seeding
  3. Immediately after germination when the radicle emerges, approximately 2–3 days after seeding
  4. When the first flower bud appears, indicating peak nutritional content
Show Answer

The correct answer is B. Microgreens are harvested when the cotyledons are fully open and colored, and the first true leaves may be just emerging — the vegetable seedling stage, not mature plant stage. This occurs 7–14 days after seeding depending on the species. Fast microgreens (radish, sunflower, brassicas) are ready at 7–10 days; slower types (wheatgrass, beets) take 10–14 days. They are cut just above the growing medium surface with scissors.

Concept Tested: Microgreens and Sprouts

10. What is the difference between reusable and single-use growing media, and which material best exemplifies each category?

  1. Reusable media dissolves in nutrient solution; single-use media does not dissolve
  2. Expanded clay (reusable indefinitely after sterilization) vs. perlite (single-use, collapses after one cycle)
  3. Rockwool is reusable indefinitely; coconut coir is single-use only
  4. All growing media is single-use; the classification only refers to the sterilization chemicals
Show Answer

The correct answer is B. Expanded clay pellets (Hydroton/LECA) are the best example of reusable media — they can be washed, sterilized, and reused indefinitely because the fired clay structure does not degrade. Perlite is the best example of single-use media — it is expanded volcanic glass that collapses and loses its porous structure after one crop cycle, making reuse impractical. Rockwool is technically reusable but difficult to sterilize completely and is typically discarded after one season in commercial operations.

Concept Tested: Reusable vs Single-Use Media