Hydroponic Nutrient Solution Management

Hydroponics can be defined as the growing of plants in a complete mineral nutrient solution. That is, nutrients are provided to the plant in the form of a liquid containing dissolved fertilizer salts or in the case of organic fertilizer, organic matter that will be converted into inorganic plant nutrients by microorganisms.

Plants may be grown in a substrate (potting mix, peat, perlite, etc.), typically referred to as substrate or aggregate culture, or without a substrate where roots grow directly into the nutrient solution, typically referred to as water or liquid culture.

There are many brands and types of pre-mixed hydroponic fertilizers on the market today. All products have their unique formulation and ratios of nutrients, but if managed properly, one should be able to achieve high quality and high yielding crops with most fertilizers from a reputable company. Amount of fertilizer to add to water can be found on instructions, as this will vary between products.

Most hydroponic fertilizers will be a two-part mix, commonly referred to as an A and B solution (some will be a three-part mix). In order to sell a relatively large amount of fertilizer in a small volume, fertilizer is sold as a very concentrated solution that is intended to be diluted by using relatively small amounts when mixing nutrient solution for plants. At the concentration that fertilizer is sold, calcium and sulfur will precipitate and form gypsum, thus these elements must be separated from each other until diluted to concentrations suitable for crop production. The following article will cover the basic components of a nutrient solution as well as how to measure and manage these components.

ESSENTIAL ELEMENTS OF A NUTRIENT SOLUTION

Elements are found in the nutrient solution as a mixture of fertilizer salts. Salts are absorbed by the plants in a specific ionic form and used by the plant for a specific function.

Macronutrients:

  • Primary: Nitrogen, Phosphorus, Potassium
  • Secondary: Calcium, Magnesium, Sulfur

Micronutrients:

  • Iron, Manganese, Zinc, Copper, Boron, Molybdenum, Chloride

Essential, but not applied (not absorbed by plant roots):

  • Carbon, Hydrogen, Oxygen

Nutrient Solution pH

  • Optimum nutrient solution pH: 5.5-6.5

pH is a unitless measurement of hydrogen ion concentration of a solution that affects plant nutrient availability (i.e., H2, PO4- vs. HPO4 2- ), fertilizer solubility, and individual ion uptake. pH is one of the most critical components of the nutrient solution and plays and a very large role in plant nutrient uptake. Maintaining pH within optimum range is necessary to maximize plant quality and yields. Plant nutrient uptake will also cause changes to pH. As plants absorb cation (positively charged) nutrients (i.e. NH4 + ), plant roots will extrude an H+ ion to balance the charge, causing pH to decrease. Conversely, as plants absorb anion (negatively charged) nutrients, roots will extrude an OH- ion, increasing the pH of nutrient solution. Most plant nutrients are cations. However, boron, sulfur, phosphorus, molybdenum, and nitrate-nitrogen are nutrients absorbed by roots as an anion. Whether pH drifts up or down will largely depend on the nitrogen source found in fertilizer. Nitrate uptake, a negatively charged form of nitrogen will cause pH to rise whereas ammonium uptake, a positively charged form of nitrogen will cause pH to decrease. Most commercially available fertilizers will have a certain ratio of each form of nitrogen in order to avoid fluctuations. However, the extent of these fluctuations will depend on water source, quality, alkalinity, buffering capacity, and volume of nutrient solution to plant ratio. Water with little no alkalinity or buffering capacity will fluctuate greatly even when nitrate to ammonium ratio is optimized.

Nutrient Solution Electrical Conductivity (EC):

  • Typical nutrient solution EC: 1.0-2.5 dS m-1 
  • 670 ppm = 1 dS/m

Electrical conductivity (EC) is a measure of the ability of a substance to conduct electricity and provides information on the general fertility level of nutrient solution and whether salinity problems are likely to exist. Electrical conductivity is measured in dS m-1 (=mS cm-1=1 mmho cm-1=1,000 uS cm-1 ). Parts per million (ppm) is an average of all salts (fertilizer) in solution and another common reading used to measure fertility levels of nutrient solution. However, ppm can cause some confusion due to manufacturers of ppm meters using different conversion factors to generate ppm values. For this reason, electrical conductivity should be used to avoid confusion. Electrical conductivity and ppm are a measure of all salts in solution, not individual salts in solution. So, concentration of specific nutrient (i.e. concentration of nitrogen in solution) cannot be determined by EC or ppm measurements.

Nutrient Solution Dissolved Oxygen (DO):

  • Optimum nutrient solution DO: +6 ppm

Dissolved oxygen (DO) is a measure of the presence of free 02 molecules in water and is measured in mg L-1 (ppm). Water temperature will impact DO, as warm water holds less DO than cooler water.

Nutrient Solution Temperature:

  • Optimum nutrient solution temperature: 20-22 °C (68-72 °F)

Nutrient solution temperature is typically very difficult to control. However, nutrient solution temperature can impact plant growth, development of pathogenic organisms, and dissolved oxygen levels. For most home growers, monitoring and measuring nutrient solution temperature is not necessary, but If possible, can result in increased yields and decreased disease.

MEASURING/MONITORING NUTRIENT SOLUTION COMPONENTS

Measurement instrumentation is not required for hydroponic crop production but can be very useful in providing information on pH, electrical conductivity (EC), and dissolved oxygen (DO) levels of nutrient solution. If money is to be spent on any type of meter, a pH probe will likely be the most useful and justifiable.

Ensuring accurate measurements

When performing any type of nutrient solution measurement, it is best to gently stir the probes/meter around in the nutrient solution. This will ensure an accurate measurement of the bulk nutrient solution, rather than the single location where the meter is submerged in the solution. Additionally, meters will need to be calibrated periodically to ensure accurate readings. Method and frequency of calibration will depend on make and model.

Sanitizing meters

When performing any type of nutrient solution measurement, it is important to ensure that probes/meters are sanitized to avoid inoculating nutrient solution with pest or plant diseases. Most meters can be sanitized using a 50% hydrogen peroxide solution, without causing any damage. However, it is best to check with meter manufacturer to be certain. Alternatively, a clean sample cup can be used to take a sample of nutrient solution for measurement which can be discarded after measuring. In this case, meter sanitation is less important since nutrient solution encountering meters will be discarded and will not come into contact with plants. Just be sure that anything sample cup or anything that comes into contact with nutrient solution that will be provided to plants is sanitized.

Meter Storage

After taking a measurement all meters should be rinsed with clean water (ideally reverse osmosis or distilled water) before being stored. This will prolong meter life and wash away excess fertilizer salts and microorganisms. Most dissolved oxygen and EC meters can be stored dry. Alternatively, the probe of pH meters must be stored wet to avoid dehydrating the probe. Once a probe is dehydrated, it may be rendered useless, but can typically be rehydrated with pH buffer solution. However, rehydration will typically take at least 24 hours. pH probes should be stored in pH storage solution, but pH buffer solution and water will also prevent probes from dehydrating. Most pH meters come with a probe cap that can be filled with storage solution.

Liquid vs. Substrate Measurements

In liquid culture, measurements can be taken directly in the reservoir or sample cup. In substrate culture, it is important to be aware of levels present in irrigation water, as well as the levels present in excess nutrient solution that drains from the substrate (drain solution). Drainage solution Combo pH/EC Meter $75- $1 EC meter pH Storage Solution Dissolved Oxygen Meter measurements will give an indication of pH and EC levels in the actual root zone – what plant is actually experiencing. Even if pH or EC of nutrient solution being provided to plants is optimized, this will change as it passes through substrate. The level of this change will depend on plant water/nutrient uptake, organic matter in substrate, makeup of substrate, and how often/much nutrient solution is being provided to the substrate. Drainage solution can be tested by collecting solution that drains from substrate with a drip pan contained under substrate/pot.

pH Meters/Measurement:

pH can be measured using a calibrated pH probe/meter, pH test indicator solutions, or pH test strips. pH probes and meters are much more accurate than pH indicator solution or test strips, but also considerably more expensive. pH probes/meters should be calibrated every 1-2 weeks to ensure accuracy. Calibration is performed using pH buffer solutions that are typically provided with pH probes. Ideally, pH would be monitored daily, but two-three readings per week should be enough for most home growers. But, again, pH fluctuations will depend on water source, quality, buffering capacity of substrate, and alkalinity.

Electrical Conductivity Meters/Measurement:

Electrical conductivity can be measured using an electrical conductivity meter. Electrical conductivity meters are much more durable than pH meters. If nutrient solution is consistently discarded and replenished with fresh nutrient solution, EC meters are typically not necessary for home growers. However, if growers are using the same nutrient solution for extended periods of time or long-term crops are being grown in the same solution/substrate/or pot, EC meters may be worth the investment. In addition, checking EC after mixing nutrient solution can confirm that the correct rate of fertilizer was applied. EC meters do not require nearly as much calibration or maintenance as pH meters. However, it is good to calibrate/check the accuracy of EC meters at least once per year.

Dissolved Oxygen & Temperature Meters/Measurements:

Dissolved oxygen meters are typically very expensive and unnecessary for hobby growers. Dissolved oxygen levels are dependent upon aeration/movement of solution and temperature of solution. Thus, temperature can also typically be measured with dissolved oxygen meters. Temperature can also be measured with less expensive options but is not necessary for most situations. 

NUTRIENT SOLUTION MANAGEMENT

pH Management

Typically, water/liquid culture systems will experience more fluctuations than substrate culture, as water usually contains less buffering capacity than substrate. Thus, changes to nutrient solution can occur much more rapidly in liquid culture systems. Therefore, even if pH meters are not available, pH of liquid culture systems should still be monitored with test strips or indicator solution.

Adjusting pH

If one finds that pH is outside desired range (5.5-6.5), pH control kits can quickly bring pH back into range. Many pH control kits are available and are essentially made of up acid or base solutions. If pH becomes too high, a pH down solution (acid) can be added to solution whereas pH up (base) can be used to raise pH. Make sure to be very careful when handling acids and bases and be very careful not to add excess acid or base. The amount of pH down/up required to achieve desired pH will be different for almost all solutions. Again, this will depend on water source, quality, buffering capacity, and volume of liquid. Most products will provide recommendations on how much solution to add, which is a good starting point, but the best way to achieve desired pH is to perform a titration with a known volume of nutrient solution. This can be done by taking a sample of nutrient solution (known volume), measuring the initial pH, adding very small amounts of pH up/down, stirring solution, remeasuring pH, and repeating until desired pH range is achieved. Once you know how much pH up/down you added to the sample of nutrient solution, you can calculate the amount of pH up/down need to achieve desired pH in reservoir by multiplying the amount needed to reach desired pH in sample solution by the volume of nutrient solution reservoir. For example, if it takes 3 mL of pH down/up to reach desired pH in 1-liter sample of nutrient solution and nutrient solution reservoir is 20 liters, I will need 60 mL of down/up to achieve desired pH in my 20-liter reservoir (3 mL of pH up/down • 20 L reservoir = 60 mL of pH up/down need to reach desired pH in reservoir). Alternatively, one can add very small amounts to reservoir and remeasure pH until desired range is achieved but adding too much solution can result in overshooting pH. Therefore, very small amounts of pH down/up should be added at a time to avoid overshooting.

Nutrient solution to plant ratio

When growing in liquid culture systems the volume of nutrient solution to plant ratio can greatly impact the amount nutrient solution pH will fluctuate. If small volumes of nutrient solution are being used, one can expect pH to drift out of desired range more frequently, thus pH should be monitored more closely with small volumes of solution. Alternatively, large volumes of solution will increase buffering capacity which will reduce pH swings. Ensuring a minimum of 4 liters of nutrient solution per plant can aid in buffering pH fluctuations.

Substrate pH

As stated previously, substrates contain more buffering capacity than water, thus changes to pH are less frequent and less rapid in most substrates when compared to liquid culture. However, continuous use of a substrate will inevitably lead to changes in pH and warrant management at some point.

When managing substrate culture pH, it is important to know the pH of the drain solution in addition to the pH of irrigation nutrient solution. Drain solution provides information on what is occurring in the root zone. Even if irrigation solution is maintained in desired pH range, drain solution may begin to drift outside of desired range due to nutrient uptake, rates of irrigation, and frequency of irrigation. If drain pH drifts outside of desired range, action should be taken. In order to correct drain pH, pH of irrigation solution will need to be adjusted accordingly. If pH of drain solution becomes too high, pH of irrigation solution should be adjusted to low end of desired range or high end of desired range if pH becomes too low. Once irrigation solution pH is properly adjusted, it should be overapplied to substrate at a high volume. This will result in excess drain solution and eventually, pH of substrate/drain solution should acclimate to pH of the irrigation solution. It is a good idea to do this over a couple of days to avoid overshooting desired pH. If pH is still out of range, irrigation pH should be further adjusted and repeated.

If pH fluctuates outside of 5.5-6.5 for extended periods of time, nutrient disorders and growth inhibitions in plants will begin to manifest themselves, thus pH is one of the most critical components to monitor and management to ensure high quality and high yielding plants.

Electrical Conductivity Management

Electrical conductivity will typically stay relatively stable throughout a single cropping cycle but will need to be maintained if solution is used for multiple crops or long periods of time

Increasing nutrient solution EC: Adding fertilizer to nutrient solution reservoir at recommended rates.

Decreasing nutrient solution EC: Fresh tap water can be added to dilute fertilizer. This will cause pH to change and should be accounted for.

Nutrient accumulation and depletion

If the same nutrient solution is used for extended periods, specific nutrients will eventually reach toxic or deficient concentrations in nutrient solution. Plants absorb different nutrients at different rates. For example, nitrogen may be taken up at very high rates depleting nitrogen levels in nutrient solution, but manganese uptake may be very low, leaving manganese concentration in solution quite high. Thus, if fertilizer is added to replenish nutrient solution, manganese is being added to solution where manganese concentration is already quite high. Eventually, if this same nutrient solution is continuously used and replenished with full strength fertilizer, manganese will accumulate to toxic concentrations and induce toxicity responses in plants. For this reason, many growers opt to discard nutrient solution and remake fresh solution. This method is likely the simplest way to manage EC and can eliminate the need for EC meters and ensure that target nutrient levels are being maintained.

Substrate electrical conductivity

In substrate culture, it is important for substrates to have adequate drainage to avoid buildup of fertilizer salts to toxic levels. If drainage EC becomes too high (buildup of salts), plants will begin to experience drought stress in a relatively short period of time. Even if substrate is wet, excessively high EC will inhibit plant uptake of water and nutrients. Excess salt accumulation almost always occurs as a result of underwatering or poor drainage. When substrate is underwatered, there is little to no solution Healthy basil being grown in a deep-water liquid culture system. being passed through substrate or being drained from substrate, thus fertilizer salts will begin to accumulate in substrate.

Level of salts in substrates is monitored by measuring drain EC. If drain EC begins to rise above 2.5 dS/m, action should be taken. To decrease substrate drain EC, substrate should be irrigated heavily with tap water to flush and drain salts out of substrate. Additionally, alternating irrigation between water and nutrient solution can greatly decrease the risk of salt accumulation (i.e. one day of nutrient solution followed by two days of tap water).

Dissolved Oxygen Management

The necessity of aeration may depend on the type of system (i.e. Recirculating nutrient film technique systems may not need the addition of aeration due to the constant movement of water), however, adding aeration can never hurt (unless aeration is violent and damaging plant roots). Aeration must not be too close to plants roots, as violent bubbles or aeration can cause mechanical damage to plant roots.

Aeration is not necessary for substrate culture, as roots are exposed to oxygen contained within substrate pores. However, overwatering can lead to anaerobic (low oxygen) conditions that can negatively affect plant growth. For this reason, it is important not to overwater your crops. However, as stated previously, underwatering crops can lead to salt accumulation and drought stress. It is best for substrate to be continuously moist (not soaking wet). That is, plants prefer frequent small volumes of nutrient solution throughout the day, as opposed to one large volume watering per day. However, unless one has the capability to automate their irrigation system, quality crops can still be achieved by one-time watering. Just be sure not to over/under water. Substrate should be moist, but not soaking wet or dry. Typically, moist substrate can be crumbled in hand whereas moist substrate will stick together, and dry substrate will turn to dust.

SUMMARY

In short, nutrient solution pH, EC, and DO will greatly impact plant quality and yields and in order to ensure high-quality crops and yield, nutrient solution components should be monitored and managed as frequently as possible. Different monitoring and management strategies are required for substrate and liquid culture systems. However, if these components are managed frequently and correctly, high-quality crops and yields should be achieved.

This article was produced by Herbal House Ltd Hydroponic Consultant © Daniel Gillespie, 2019. Unauthorized use and/or duplication of this material without express and written permission from this author is strictly prohibited.