Farm Operations Management

Fundamentals of nutrient solution management: How to think about and measure EC, pH, and dissolved oxygen

Articles for Farm Operations Managers

In hydroponics, the water itself that the roots are in contact with is the growing environment. When any of EC, pH, or dissolved oxygen falls out of balance, the plant cannot use the fertilizer you think you are supplying.

What beginners need to grasp first is not memorizing numerical values. It is understanding what each indicator shows: EC as a proxy for fertilizer concentration, pH as the condition for uptake, and dissolved oxygen as the foundation that supports root respiration.

This article organizes the items you should watch in nutrient solution management, the basics of EC, pH, and dissolved oxygen, how to think about source water and nutrient solution renewal, and the procedure for checking when trouble occurs.

What should you actually do in nutrient solution management

Unlike soil cultivation, plants take up the nutrients they need only from a “nutrient solution” in which those nutrients are dissolved in water. Because there is no buffer like soil, the condition of the nutrient solution is directly reflected in crop growth. Put simply, the goal of nutrient solution management is to maintain water uptake and root respiration while preserving the optimal nutrient balance.

The main elements to manage are five: EC (a proxy for fertilizer concentration), pH (the balance between acidic and alkaline), dissolved oxygen (the oxygen required for root respiration), nutrient solution temperature, and sanitation. These influence one another. For example, when nutrient solution temperature rises, dissolved oxygen decreases, microbial proliferation becomes active, and pH fluctuates more readily. If any one of them is neglected, crops will not grow properly no matter how accurate the others are.

Source water quality matters too

Source water quality is not something you adjust on a daily basis, but it affects the success of nutrient solution management. The main items to check are three: hardness, pH, and chlorine.

Hard water contains large amounts of calcium and magnesium ions, so you need to take this into account when designing the nutrient solution. Tap water pH varies by region, and if you are using strongly alkaline water, the amount of adjuster you add will increase. Chlorine contained in tap water can adversely affect roots; it can be removed by leaving the water to stand for about a day or by using an activated carbon filter. Understanding the characteristics of your source water before starting hydroponics reduces the daily management burden.

EC (electrical conductivity) management

EC stands for “Electrical Conductivity” and is an indicator showing the total amount of dissolved ions (mainly from fertilizer salts) in the nutrient solution. If components other than fertilizer are present in the water, those components are also reflected in the EC value.

Pure water hardly conducts electricity at all, but once fertilizer-derived ions dissolve into it, it becomes able to conduct electricity. The higher the fertilizer concentration, the higher the EC value, so by measuring the degree to which electricity flows through the water, you can indirectly grasp the fertilizer concentration in the nutrient solution. The unit of EC is generally expressed as “mS/cm” or “dS/m”, but these two are the same unit.

However, the EC value only shows the total amount of fertilizer; it cannot tell you the balance or the types of individual nutrients. Even when some components are in excess and others are deficient, nutrient solutions with different component balances can end up with the same total EC value. This is the limitation of EC, the flip side of how easily it can be measured.

How to adjust the EC value

To raise the EC value, add fertilizer to the nutrient solution; to lower it, dilute with fresh water or replace part of the nutrient solution. Managers measure the EC value daily and make adjustments when anything abnormal is seen.

The larger the cultivation scale, the harder it is for people to keep monitoring constantly, so automation using sensors and control equipment becomes essential.

Common troubles in EC management

The following table lists common problems related to EC values and how to deal with them.

pH (hydrogen ion concentration) management

pH (hydrogen ion concentration) is an indicator showing how acidic or alkaline an aqueous solution is. It is expressed on a scale from 0 to 14, where 7 is neutral, below 7 is acidic, and above 7 is alkaline. In hydroponics, pH directly affects the solubility of nutrients and their uptake by plants.

Each fertilizer component has its own pH range in which it is easily absorbed. Micronutrients such as iron and manganese are absorbed more easily in acidic environments, calcium and magnesium are absorbed more easily in alkaline environments, and phosphorus is absorbed most easily at a pH close to neutral. Many crops absorb nutrients most efficiently in a pH range of 5.5 to 6.5; outside this range, specific nutrients become insoluble and unusable by the plant.

Why pH changes during cultivation

In hydroponics, pH fluctuates over time. The main factors are five: selective ion uptake by plants, microbial activity, the nature of the fertilizer, water quality, and temperature change.

The nature of the fertilizer in particular is often overlooked. When ammonium nitrogen is taken up by the plant, the pH of the nutrient solution falls; when nitrate nitrogen is taken up, it rises. Also, when temperature rises, microbial activity becomes more active and accelerates pH fluctuation. Small swings in pH are unlikely to cause problems, but once it falls far outside the appropriate range, the damage to crops becomes large.

When pH is too low (strongly acidic), you see heavy-metal toxicity from aluminum and iron, inhibition of calcium, magnesium, and phosphorus uptake, and root damage. When pH is too high (strongly alkaline), micronutrients such as iron, manganese, copper, and zinc become insoluble, leading to leaf yellowing (chlorosis) and poor growth.

How to manage pH

For pH measurement, a digital pH meter is the basic tool, the most accurate and reliable. For emergency quick checks, test paper can also be used, but its precision is low and it is not suited to daily management.

pH is adjusted using dedicated adjusters. To raise pH, you can use potassium carbonate solution (gentle rise, with the added benefit of supplying potassium), potassium hydroxide (strong), or sodium bicarbonate (with a buffering effect). To lower pH, you can use phosphoric acid (with the added benefit of supplying phosphorus), nitric acid (with the added benefit of supplying nitrogen), or citric acid (mild adjustment).

When using an adjuster, add it in small amounts, measure each time, and avoid rapid changes. Acidic adjusters must always be diluted before use.

Ideally, you do not use pH adjusters

When you use a pH adjuster, only the specific component it contains increases in the nutrient solution, and the balance can be thrown off. That is why the ideal approach is to control pH to some extent by adjusting the fertilizer balance of the nutrient solution itself. This kind of on-the-ground know-how is covered in our other content as well.

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Dissolved oxygen management

Dissolved oxygen (DO: Dissolved Oxygen) refers to the oxygen molecules (O₂) dissolved in the water, and the unit is generally “mg/L” or “ppm”.

In hydroponics, because the roots are in the water, the plant absorbs oxygen dissolved in the water to carry out energy production through root cell respiration, active uptake of nutrients (especially calcium and phosphorus), root growth and metabolism, and maintenance of resistance to pathogens. When dissolved oxygen is insufficient, root function declines, and that affects the growth of the whole plant.

When dissolved oxygen is insufficient, healthy white or cream-colored roots turn brown or black, and you begin to see thick, short roots and dying root tips. It can progress to overall growth delay, wilting or yellowing of leaves, calcium deficiency (such as blossom-end rot), and root rot.

The ideal dissolved oxygen concentration in hydroponics is 5 mg/L or higher, and preferably around 8 mg/L. The main factors that reduce dissolved oxygen are temperature (the higher the temperature, the lower the solubility), total dissolved salts (the higher the EC value, the lower the dissolved oxygen), microbes (which consume oxygen in the water), and changes in oxygen demand according to the plant’s growth stage. Summer in particular calls for caution.

How to increase dissolved oxygen

In large-scale hydroponics facilities, “cascade aeration”, in which water is dropped from a high position to entrain air, is the most common method. It is easy to keep additional power consumption down, and because oxygen dissolves in proportion to the water flow rate, it is well suited to long-term operation. When increasing the surface area or circulation volume is still not enough, aeration that sends air into the water is also effective. It is also easy to introduce in small-scale cultivation.

Nutrient solution renewal when you are in trouble

When growth problems or disease occur in hydroponics, one of the most effective responses is “nutrient solution renewal”. In recirculating hydroponics, replacing the nutrient solution with a fresh one can resolve many problems at once.

When you keep using a nutrient solution for a long time, plants take up each nutrient selectively, so the component balance becomes skewed, and you end up in a state where some are deficient while others are in excess. Micronutrients in particular are difficult to manage and tend to go out of balance. In addition, allelopathic substances and other organic acids and waste products secreted from the plant roots accumulate and inhibit root function. Furthermore, pH becomes unstable, and as the use of adjusters increases, the nutrient solution balance breaks down even more, falling into a vicious cycle.

Through nutrient solution renewal, you can reset these problems all at once. You can simultaneously replenish deficient nutrients, remove excess components, and eliminate allelopathic substances, and you also get the effect of cleaning the piping and cultivation tanks.

The cycle and timing of nutrient solution renewal

To prevent trouble before it occurs, regular nutrient solution renewal is effective. In recirculating hydroponics, once every 2 to 3 months is generally the guideline, but it is adjusted according to the crop, planting density, and season. Fast-growing leafy greens consume nutrients quickly, so the renewal frequency is raised; in summer, microbial activity becomes more active, so deterioration speeds up.

Apart from regular renewal, consider renewing the nutrient solution early when you see any of the following signs.

Summary

The essence of nutrient solution management is to keep maintaining an “environment in which the roots can function properly” through the three indicators of EC, pH, and dissolved oxygen. They look like independent indicators, but in fact they influence one another via temperature, microbial activity, and fertilizer balance.

In practice, the starting point is measuring EC and pH at the same time every day to grasp the trend of fluctuation. A rapid change in the numbers is a sign that something is wrong with the plant, and judging it together with not only the measured values but also the condition of the roots and the appearance of the leaves leads to early detection of problems.

From the perspective of long-term risk management, before you increase your dependence on pH adjusters, it is effective to build regular nutrient solution renewal every 2 to 3 months into your plan. Nutrient solution renewal is both a response to trouble and a preventive measure that resets accumulated problems in one go.

Quick check sheet for EC/pH management

Recording template for nutrient solution management

We also share various templates on this site, so please check those out too.