MONITORING TOOLS

You may want to have a few monitoring tools on hand. Most of these devices are reasonably affordable and readily available from hydroponic growing suppliers.

• Thermometer
• EC (electroconductivity) meter
• Light meter
• pH strips or test kit

TEMPERATURE

Temperatures that are too high or too low will result in poor germination and increase the risk of disease. The optimum temperature range will vary by crop and lifecycle stage; follow individual growing instructions for each crop you are growing.

Once established, plants need an approximately 10°F (12°C) drop between daytime and nighttime temperatures to grow properly. For example, an ideal temperature range for many crops is 75°F (24°C) in the daytime and 60–65°F (16–18°C) at night. Most crops (other than tropical varieties) cannot efficiently photosynthesize at temperatures exceeding 85–90°F (29–32°C).

Supplemental lighting in the protected culture setting can significantly increase the ambient temperature. Also note that to avoid root damage, temperature of the nutrient solution should be maintained no warmer than ambient temperatures.

pH

Plain water normally has a pH of 7.0–8.2. Most nutrient solutions are acidic, and once added to the water source, will decrease the pH.

The pH of the nutrient solution can significantly affect the plant's ability to take up nutrients. For most crops and hydroponic systems, the ideal pH range is 5.8–6.2 (slightly higher for organic and aquaponic systems).

Ways of measuring pH include using:

• pH meter
• litmus paper test strips
• indicator solution

To understand how pH fluctuates in your specific growing environment, you may want to take routine pH measurements of the following:

• water source
• water/nutrient solution
• growing medium with nutrient solution added

If the pH is off, use a commercially available pH adjustment solution (called "pH Up" or "pH Down") to adjust your pH accordingly.

If you monitor pH regularly, you will see that the pH slowly rises as the plants take up nutrients. When you replenish the nutrient solution, you will see the pH fall again. More dramatic changes in pH can be indicative of disease; for example, root rot can cause the pH to drop to 3.0–5.0, while algal growth can raise pH levels above optimal levels.

LIGHT

Temperatures that are too high or too low will result in poor germination and increase the risk of disease. The optimum temperature range will vary by crop and lifecycle stage; follow individual growing instructions for each crop you are growing.

DAILY LIGHT INTEGRAL

Growers, particularly in the protected-culture setting, find it useful to quantify light levels in units referred to as the daily light integral (DLI). The DLI is the amount of photosynthetically active radiation (photons) that plants receive each day. Just as you can use a rain gauge to measure rainfall, you can use a light meter to measure DLI, typically expressed as moles of light (mol) per square meter (m²) per day.

RECOMMENDED LIGHT LEVELS

Most vegetables need 14 hours of sunlight per day and at least 12 mol per m² per day. Strawberries grown hydroponically prefer 15–25 mols of DLI with a minimum of 12, measured at the canopy level, in the greenhouse. Plants in greenhouses typically experience a 25%–50% reduction in DLI below that of outdoor levels due to glazing and shading from the structure. Depending upon the crop and a host of variables that influence natural light levels, supplemental lighting may thus be needed at certain times of the year or year-round.

LIGHT COLOR

If you are using supplemental lighting, light color is an additional factor to consider. Plants use light within the visible wavelength range of 400–700 nanometers (nm). Herbs and leafy greens fare best with lights emitting a higher proportion at the blue end of the spectrum (450–496 nm), which encourages vegetative growth. Crops like tomatoes prefer lights that emit a higher proportion at the red end of the spectrum (620–750 nm), which encourages flowering and fruiting. Ultraviolet light (UV) aids development of fruit color in crops such as strawberries. LED lights are the most energy-efficient option, and some are adjustable for color to suit crop needs.

HYDROPONIC NUTRIENT SOLUTIONS & CONCENTRATIONS

The home gardener or beginning grower will want to start with one of the many preformulated solutions on the market. Each comes with its own instructions for mixing and application.

To monitor nutrient concentrations, measure the electroconductivity (EC) of the solution in your system over time. Use an electrical conductivity meter to detect the level of total dissolved nutrients in the hydroponic solution expressed on a scale of milliSiemens per centimeter (mS/cm). The package directions will indicate the desired EC levels. Note, however, that EC is not a reliable indicator if you are using organic fertilizer solutions.

Record the EC when you mix up your solution and then monitor it daily. You will see the EC drop as plants take up the nutrients. Once it falls below an acceptable range, you will want to add more nutrient solution; determine the amount to add based on the percentage drop you have seen in the EC.

You may want to take measurements both from the nutrient solution itself and from sample points in your growing medium; this will allow you to get a sense for whether your growing medium is accumulating a build-up of nutrients.

Temperatures that are too high or too low will result in poor germination and increase the risk of disease. The optimum temperature range will vary by crop and lifecycle stage; follow individual growing instructions for each crop you are growing.

RECOMMENDED NUTRIENT LEVELS

Optimum EC levels vary by crop. Plants will require lower EC in warmer months and higher EC in cooler months and when fruiting. A suggested range is provided in Table 2; however, you will need to experiment to find the optimal range for your crop, season, and growing system.

REPLACING THE NUTRIENT SOLUTION

A shortcoming of EC is that it does not reveal the specific chemical makeup of nutrients in the water, only the overall nutrient levels. Plants take up individual nutrients at different rates, and thus it is possible for the solution to become imbalanced over time, even if the EC is still within an acceptable range.

For this reason, the nutrient solution should be completely changed on a regular basis. Recommendations on how frequently to change the solution vary; start by replacing your solution every 3–4 weeks, or whenever you see symptoms of deficiency or toxicity in your plants.

ORGANIC FERTILIZERS IN HYDROPONICS

Using organic fertilizers can present more of a challenge than using synthetic ones.

• The composition of organic fertilizer mixtures is less precise than that of their synthetic counterparts, and nutrient deficiencies can be encountered more readily as a result.
• Organic fertilizers can also contain high levels of carbon, which in excess can contribute to fungal and bacterial growth.
• Finally, EC is not a reliable indicator of actual nutrient concentrations, which can also make it harder to monitor and ensure adequate levels with organic fertilizers.

If you elect to use organic fertilizers, we recommend choosing a product specifically designed for hydroponic systems unless you have the capacity to run trials.

WATER

Testing your water source will help you to understand how naturally occurring elements in the water may affect plant growth. If you have hard water (ie, high concentrations of calcium and magnesium in the water), you may want to use a nutrient solution designed for hard water. Water treated with sodium or other water-softening chemicals can be detrimental to plants. High levels of salt in the water can limit calcium uptake and lead to disease (research suggests that water with salt levels of 3000ppm can reduce yields by 10–25%).

CARBON DIOXIDE

Plants need adequate levels of carbon dioxide (CO₂) to photosynthesize; low CO₂ levels reduce growth and can cause flower and fruit drop, reducing overall yields.

Indoor growing environments can be prone to becoming CO₂ deficient. This is most likely to happen in a closed greenhouse system on sunny, cold winter mornings when ventilation fans are not running and plants are actively photosynthesizing, using up available CO₂. Plants can deplete available CO₂ in as little as just 1 hour in a closed greenhouse.

You can ensure adequate CO₂ with appropriate ventilation.

AIR CIRCULATION

Good air circulation helps reduce disease pressure, dissipate pockets of air that are too high or low in temperature, and as discussed above, ensure plants receive adequate CO₂. Air movement can also help seedlings develop a thicker stem, producing a shorter, stockier, less leggy plant.

OXYGEN LEVELS

Oxygen is imperative for plant growth. Overwatering and compaction of the growing medium can both limit oxygen to the roots, leading to root death. Using a medium that supports good aeration is important for maintaining healthy oxygen levels.

Oxygen levels in the nutrient solution are a function of temperature; when the nutrient solution is too warm, the plants' access to oxygen is compromised. Some growers use an air pump to aerate the nutrient solution and a water chiller to cool the nutrient solution down to an optimal temperature.

Johnny's Hydroponic Growing Resources

• Downloadable, printable Tech Sheet/PDF version of this web article
• Introduction to Hydroponic Growing • Types of Systems; How Hydroponics Differs from Growing in Soil; Scheduling Plantings; Specialty Techniques; How to Choose Varieties for Hydroponic Production Article
• Hydroponic Growing Media • Common Options & Selecting a Medium, Using Plugs vs Soilless Medium, Monitoring Salt Build-up & Maintaining Medium Hygiene Tech Sheet (PDF)
• Hydroponics Information Guide 4-pp Brochure (PDF)
• Hydroponics Variety Listing Insert (PDF)
• Pests & Diseases of Greenhouses & Hydroponic Systems Tech Sheet (PDF)

General Hydroponic Growing Resources

• Hydroponic Society of America.
• Hydroponics. University of Florida: IFAS Extension.
• Cornell Controlled Environmental Agriculture.

Plant Nutrition

• Guide to Symptoms of Plant Nutrient Deficiencies. University of Arizona Extension.

References

1. Bartok, J.W. 2015. The Importance of DLI. Greenhouse Management.
2. DeBoer, B. 2015. Electrical Conductivity and Monitoring Plant Nutrition. Maximum Yield.
3. Helweg, R. 2014. How to Grow Fruits, Vegetables & Houseplants Without Soil. Ocala, FL: Atlantic Publishing Group, Inc.
4. Hopper, E. 2015. Dissolved Oxygen: The Hidden Necessity. Maximum Yield.
5. How to Grow Hydroponics: Temperature & Humidity. Hydroponics — Simplified.
6. Jones, B.J. 2016. Hydroponics: A Practical Guide for the Soilless Grower. CRC Press.
7. Mansfield, M. 2016. The Power of pH. Maximum Yield.
8. Morgan, L. 2014. The How-To of Organic Hydroponics. Maximum Yield.
9. Peterson, D. 2018. Techniques, varieties fuel success with hydroponic strawberries. Vegetable Growers News.
10. pH/TDS/PPM Levels for Vegetables. Home Hydro Systems.
11. Resh, H.M. 2013. Hydroponic Food Production: A Definitive Guidebook for the Advanced Home Gardener and the Commercial Hydroponic Grower. 7th ed. New York: CRC Press.
12. Resh, H.M. 1993. Hydroponic Tomatoes for the Home Gardener. Santa Barbara: Woodbridge Press Publishing Company.
13. Rorabaugh, P.A. 2015. Introduction to Hydroponics and Controlled Environment Agriculture. UACEAC.
14. Stewart, K. 1992. Hydroponics for the Home Gardener. Ontario: Key Porter Books.
15. Torres, A.P. & Lopez, R.G. Commercial Greenhouse Production: Measuring Daily Light Integral in a Greenhouse. Purdue University Extension.