Many indoor horticulturists store a large volume of nutrient solution in a reservoir until it is needed by their plants. For hydroponic growers, these reservoirs are the heart of their systems, delivering nutrition and dissolved oxygen to the plant's roots. Many soil growers take advantage of nutrient reservoirs as well. Soil gardens with many containers require a lot of water. Using a nutrient reservoir makes it easier and more practical to deliver a consistent nutrient solution to all the plants in the garden.
Whether a grower is using the reservoir for soil or hydroponics, there are several parameters that should be followed to ensure the nutrient solution contains adequate essential elements and dissolved oxygen, and maintains a consistent pH for maximum nutrient absorption.
Nutrient Solution Temperature
The temperature of a nutrient solution in a reservoir is one of the most important factors affecting the dissolved oxygen content of the solution. In fact, water's ability to hold oxygen is directly related to its temperature. As the temperature of the water (or the nutrient solution) gets warmer, the dissolved oxygen potential decreases. As the temperature of the water gets colder, the dissolved oxygen potential increases. Since oxygen is good for the plant's roots, a grower should try to have the coldest nutrient solution possible, right? Well, not exactly. If the temperature gets too cold, a whole new set of problems are created. Temperatures that are too cold will cause the plant's growth to slow and eventually stop altogether.
Most indoor horticulturists experience nutrient reservoir temperatures that are too warm. This causes a decreased dissolved oxygen content, which allows anaerobic pathogens to make their home. Pythium, or root rot, is the most common pathogen that plagues the indoor grower with warm nutrient reservoir temperatures. The optimal temperature for a nutrient reservoir is 63 to 72°F. Within this temperature range, the dissolved oxygen content is high and the temperature is warm enough to facilitate nutrient uptake.
It may be necessary for a grower to employ one or more devices to make sure the nutrient reservoir stays in the desired temperature range. Submersible titanium heaters are a great way to keep temperatures from dropping below the desired range. Many of these heaters come equipped with a thermostat, which makes this a set-it-and-forget-it device. For growers experiencing reservoir temperatures that are rising above the desired range, a water chiller is the best automated device to remedy this problem. Water chillers are like air conditioners but are made for cooling water instead of air. The water from the reservoir is recirculated through the water chiller, which cools the nutrient solution's temperature. A dedicated pump is usually required for a water chiller to operate most efficiently. However, growers experiencing excessive heat should be aware that the chiller unit, much like an air conditioner, will create heat as waste. Some growers remedy this by placing the chiller in a separate room. This way the heat generated by the chiller won't be absorbed by the nutrient solution in the reservoir, creating a counterproductive process.
Growers who intermittently experience heat issues in their reservoirs can implement the ice-bottle technique. By filling a two-liter plastic bottle with water and freezing it, a grower can fix a hot reservoir in a pinch. Another plastic bottle can be put in the freezer so the two can be rotated as needed. In some cases, removing the reservoir from the actual garden area is all a grower needs to do to control the heat. HID lighting creates a lot of radiant heat which, if the reservoir is in the garden room, can be absorbed by the nutrient solution. Many hydroponic growers will create "control rooms" where the reservoir is stored. This allows for heightened control over all the reservoir parameters. Submersible or inline pumps deliver the nutrient solution to the garden room(s) as needed.
Dissolved Oxygen Content
The temperature of the nutrient solution determines how much oxygen the water can hold, but how do growers replace the oxygen that is used by the plants? The best way a grower can replace the oxygen in the solution is to somehow aerate the reservoir. As long as the reservoir's surface tension is continually agitated, the solution will be absorbing oxygen. This can be done a few different ways. One of the most common ways is to use an air pump with an air diffuser (air stone). Air pumps should operate 24 hours a day to provide a constant supply of air (oxygen) to the reservoir. As long as the nutrient solution's temperature is kept in the desired range, the air pump and diffuser will be more than sufficient to replace the dissolved oxygen used by the plants.
Read also: Dissolved Oxygen: The Hidden Necessity
Potential Hydrogen (pH)
Another huge determining factor of plant performance in any garden is the nutrient solution's pH. The pH of the solution basically determines the solubility of each nutrient. Growers experiencing pH fluctuations out of the desired range run the risk of hindered growth or, worse, nutrient lockout. Unlike temperature, the optimal pH for a nutrient solution depends on the particular medium that the grower is using. For growers using soil, the optimal pH range is 6.5 to 6.8. For growers using hydroponic systems, the optimal pH range is a little more acidic and comes in at 5.5 to 5.8. These pH ranges are where all the essential elements are available for absorption.
The best way a grower can be sure the nutrient solution's pH stays in the desired range is to continuously monitor it. This is especially true right after nutrient changes or the addition of nutrient additives to the reservoir. Usually the pH will stabilize in a few hours after a change has been made to the solution. At that time growers can adjust the solution's pH with the help of pH buffers (pH Up and pH Down).
Unfortunately, the optimal concentration of a nutrient solution is an arbitrary number—many factors will determine the optimal nutrient concentration for any given system. The plant variety, stage of growth, amount of light energy and type of growing system will all dramatically affect the optimal nutrient concentration. Nutrient monitors are a must-have tool for any serious indoor horticulturist. With a nutrient monitor and a little experimentation, any grower can find the optimal nutrient concentration. For beginner growers, it is wise to start fertilizing at a lower dosage than recommended by the manufacturer. From there, incremental increases can be made until the optimal concentration is found. Close attention should be paid to the plants as the concentration of the nutrients is increased. Any sign of burning on the outer edge of the leaves is a good indicator that the concentration is on the verge of being too potent.
By keeping the nutrient reservoir parameters in check, a grower guarantees that the nutrient solution delivered to the plants is optimal. With the proper reservoir set-up, including monitoring and automation devices, growers can get the most out of their nutrient solutions (and their gardens). A grower who understands the way variables are affected in a nutrient reservoir is better able to make proactive decisions regarding the reservoir. After all, the nutrient solution is literally the food and water source for the entire garden. A grower who masters the parameters of the nutrient reservoir is one step closer to mastering indoor horticulture as a whole.