Management of the nutrient solution applied to hydroponic plants falls into two basic categories: those that are closed and recirculate the solution, either continually or intermittently, and those that don’t recirculate the solution, termed open systems.
Closed systems are more widely used amongst indoor growers, while in the past, drip-irrigated open systems were commonly seen in commercial greenhouse production of a wide range of hydroponic crops.
A degree of solution management skill, monitoring, and regular adjustment are the basis of both open and closed systems, however, each has different advantages when it comes to plant production.
Closed systems that recirculate the nutrient solution are often based around solution culture and include NFT, DFT, float/raft/pond systems, aeroponics, aquaponics, and ebb and flow. In these systems the nutrient solution is made up to working strength, EC and pH are adjusted, and the same solution is applied to the plants either continually or intermittently. Recirculating systems are regularly topped up with water, the solution is managed on a regular basis and is only partially or fully replaced when required.
Drip-irrigated nutrient delivery systems may be either open or closed depending on whether the solution is collected and recirculated or drained to waste.
Open systems are those that do not recirculate or reuse the nutrient solution that drains from the base of the growing substrate. The waste nutrient solution, once it has flowed past the roots and out the base of the substrate, is channeled away to be disposed of. While this may seem wasteful, drain-to-waste solutions often find a second use as liquid fertilizer sources applied to other crops outdoors such as lawns, pasture, tree crops, vegetable gardens, potted plants, and ornamentals.
In some countries, nutrient discharge is prohibited or carefully controlled so traditional open systems can’t be used, and any waste nutrient drainage must be collected and reused or disposed of correctly to avoid environmental issues.
Closed systems may seem preferable when it comes to hydroponics as they make efficient use of water and fertilizers and, in theory at least, the same solution can be recirculated for an extensive length of time. In reality, however, closed systems can be complicated when it comes to solution management as the ratio of individual elements can change over time without the grower being aware of problems developing.
For example, if the nutrient product or formulation used is not precisely matched to plant uptake of each element, accumulation and depletion can occur over time in the nutrient solution. This most commonly occurs with macro elements such as nitrogen or potassium and it is possible for a rapidly growing crop to completely strip these elements from a recirculating nutrient solution if monitoring is not regularly carried out.
Because it is difficult to predict the exact ratio of nutrients taken up by a crop (and as this varies between growing environments, systems, and even growth stage), changes in the composition of a recirculating nutrient solution occur over time, even when fresh stock solution concentrates are added to maintain the EC level.
Commercial growers with large closed systems that recirculate the nutrient solution manage this issue by having regular solution analysis carried out by an agricultural lab, and some may even carry out testing onsite. This gives valuable information about which elements are accumulating and which may be depleting over time, so that correct adjustments can be made as required.
For smaller growers where regular solution analysis is not cost effective, a simpler method is to partially or completely replace the recirculating solution in closed systems on a regular basis before any imbalances begin affecting plant growth.
A second issue with closed recirculating systems is when the water source or a growing substrate contains unwanted minerals that may accumulate in the nutrient solution over time. A common example is where sodium and chloride are present in a water supply — these unwanted elements can accumulate rapidly in recirculating systems, particularly where the volume of solution in the system is low and where water use is high, thus introducing more and more of the unwanted elements with each water top up. Other examples include water sources which may be hard and high in calcium and magnesium or have levels of trace elements that can accumulate in closed systems over time creating imbalances and even toxicities.
Ideally, closed systems are the most efficient when a high-quality, low-mineral water source is used, and this, combined with a carefully balanced nutrient product or formulation, gives the longest usable life of a nutrient solution. Reverse osmosis water has no unwanted elements present, thus the nutrient solution can be fine tuned to the crop being grown and the requirement for solution dumping and replacement can be minimized. If well-managed, closed systems with high-quality water and a well-managed nutrient formulation can run the same nutrient for a considerable length of time, many growers only replace this once per crop.
Open systems are often thought of as being wasteful of both water and fertilizers while creating an environmental risk through the disposal of waste nutrient solution. Despite this, open systems are still in widespread use in commercial horticulture due to the ease of operation and nutritional management.
Open systems typically use drip irrigation. Once the nutrient is applied at regular intervals, it flows through the root system, replenishing moisture and nutrients in the substrate and any excess then flows out the base of the growing container/bed/slab/bucket. This amount of waste solution should only be a small percentage of the volume initially irrigated onto the plant with most growers working on a 10-30 per cent drain basis. The drainage solution is disposed of and not reused on the hydroponic crop but may be collected for use on outdoor soil grown plants as a useful fertilizer.
The main advantage of drain-to-waste open systems is that at each irrigation, the root zone receives fresh, completely balanced nutrient solution so ratios and nutrient levels are usually close to optimal. This is particularly useful where regular nutrient solution analysis is not possible as nutrient imbalances are far less likely to occur. Commercial growers, however, will collect samples of the nutrient drainage and regularly have this analysed to determine if the solution they are applying at each irrigation is matching the needs of plant growth and make regular adjustments to their nutrient formulation to optimize this.
Management of such closed systems is usually carried out to minimize waste, and this includes running the lowest volume of drainage possible while at the same time ensuring enough run off to keep nutrients in balance in the root zone. Some growers manage to run drainage volumes as low as five per cent in carefully controlled systems with precise nutrient formulation and a high-quality water source.
Open systems are particularly useful where the water supply contains certain unwanted elements such as sodium, as the drainage of the excess nutrient at each irrigation not only flushes through these minerals but prevents accumulation in the root zone and nutrient solution. This allows growers to make considerable cost savings on having to treat and demineralize certain water supplies if using an open system.
Systems Differences and EC
The main EC differences between an open substrate-based system and a closed system such as NFT are that generally drain-to-waste systems are run at a lower EC than if the same solution was running in a recirculating solution culture system. This is because the solution applied to a substrate-based open system is different to the actual EC in the root zone (and hence drainage solution). With open systems, the EC and pH should be measured in a sample of the drainage solution flowing from the substrate after each irrigation as this is what these will be directly surrounding the root system. The EC and pH of the drainage solution can then be used to make regular adjustment to the irrigation solution to maintain the correct levels directly around the root zone. Apart from differences in EC, nutrient formulations don’t differ much between most open and closed systems unless the substrate in an open system has an effect on the balance of elements in the root zone.
Open and closed hydroponic systems both have their own pros and cons and suitability to different growing situations and inputs. Understanding how each system influences the composition of the nutrient solution over time and the correct monitoring and management technique for each is essential in optimizing plant nutrition from whichever system is being used.