Hydroponics is the soilless growth of plants by immersing the roots in a nutrient solution. This nutrient solution can be administered in an open system (a one-time flushing of the roots) or a closed system (where the solution is re-circulated repeatedly). For hydroponic growers, the relationship between roots and shoots is paramount.
Roots need carbohydrates produced by the leaves for growth. Likewise, leaves depend on the roots for water (maintaining turgor pressure) and nutrients for assisting photosynthesis and various other biological processes. Since these plants are growing mostly in an aqueous environment, monitoring the water quality is integral for obtaining positive results.
Just as water quality is important to aquarium enthusiasts, so too should hydroponic growers have knowledge of certain parameters. Those parameters include temperature, dissolved oxygen (DO), pH, total dissolved solids (TDS), electrical conductivity (EC) and hardness and alkalinity. While most of these parameters do not need to be monitored on a weekly basis, all are important in regard to initial set-up. First, let’s discuss the role of temperature on plant growth.
Temperature impacts plant growth directly through kinetics. Lower temperatures will reduce absorption of water/ions by the roots while increased temperatures will have the opposite effect. In addition, temperature has a direct impact on cellular respiration and DO in water. For example, if the temperature increased from 68oF to 86oF, the saturation of DO decreases by 17 per cent while the rate of oxygen (O2) consumption via cellular respiration doubles.
Therefore as temperature increases, the level of DO decreases and the demand for oxygen increases. As a general guideline, optimal temperature between 68oF and 86oF will facilitate ideal plant growth in hydroponic systems. Now let’s review the connection of temperature and DO in water.
Arguably, one of the most important albeit sometimes overlooked water quality parameters in hydroponic systems is dissolved oxygen. Without adequate levels of oxygen, cellular respiration of root cells is reduced or ceases entirely. This reduction in oxygen leads to decreased water/ion absorption, which causes nutrient deficiencies and decreased growth. In anaerobic conditions, root necrosis (death) can occur, leading to total loss of the plant.
Dissolved oxygen is less problematic in open systems than closed systems because of the constant inundation of water to the roots. As temperature increases, there is a decrease in the saturation of dissolved oxygen in water coupled with an increase in metabolic oxygen demand.
The dissolved oxygen saturation point for temperatures of 68oF to 86oF is 8.84 and 7.53 ppm, respectively. When looking at water sources, inherent dissolved oxygen can range from 20 to 40 per cent saturation; however, levels below 60 per cent saturation can lead to decreased vigor. Thus mechanical mixing of water or adding O2 via air pumps might be a necessary step to maximize plant growth.
As for the impact of pH on nutrient availability, the pH of water directly affects the availability of essential elements. Water and nutrient solutions are best kept slightly acidic, with a value of 6 to 6.5—though it is acceptable to maintain pH between 5 to 7. Extremes of pH should be avoided. If the solution is too acidic (below 5) then toxicity might occur due to excessive absorption. If the solution is too basic (above 8) then nutrient deficiencies may occur due to precipitation of certain micronutrients.
Another method of monitoring nutrient content is with TDS. Total dissolved solids is the measure of both organic (proteins, carbohydrates, etc) and inorganic (most of the nutrients) content of water. Like many of the parameters discussed, extreme values of TDS should be avoided. TDS should not exceed 1,400 ppm with an acceptable range between 200 and 500 ppm. Levels below 100 ppm could indicate reduced nutrient availability and additional “fertilizer” is required.
Electrical conductivity (EC) is the main way of estimating nutrient content. Electrical conductivity provides information pertaining to the concentration of ions and/or salt content in solution. For general purposes your water source should have an EC value below 1 dS/m. Water that is hard (contains Mg2+ and Ca2+) will have a larger EC value than soft water. Once you add nutrients (fertilizer) to the water, the EC value should be maintained above 1 dS/m and below 3 dS/m depending on the salts used. Ideal EC values of the nutrient solution should be between 1.5 and 2 dS/m. Carefully monitor water levels as quick evaporation can lead to salt toxicity due to a concentrating effect.
Finally, let’s look at the role of hardness and alkalinity on water sources used in hydroponics. Hardness and alkalinity are important factors when evaluating ideal water sources. Hardness measures the amount of calcium and magnesium ions dissolved in water. Water that has a high amount of Ca2+ and Mg2+ (>61 ppm) is said to be hard while water that contains low or no amounts (< 60 ppm) is soft. It is the author’s opinion that there is not an ideal hardness, but you have to take into account the levels of magnesium and calcium in your source water when determining your nutrient solution. Indeed, the water source may contain the necessary amount of both ions so that supplementation is not necessary.
Alkalinity measures the acid neutralizing ability of the water. A high alkalinity (above 100 ppm, if calculations are based on mg/L of CaCO3) will be very resistant to changes in the pH while a low alkalinity or low buffering capacity of water can lead to dramatic swings in pH. The ideal level of alkalinity also depends on the grower. For some, the ability to adjust the pH quickly and easily is more ideal than the alternative.
This is especially the case when reverse osmosis filtration is used to create soft water in hard water areas. Avoid using ion exchange resins (water softeners) as these units replace Ca2+ and Mg2+ with sodium (Na+) often at concentrations that are detrimental to plants.
Hydroponics is a dynamic method for growing plants and hopefully this article has provided helpful guidelines and useful information, whether you are novice or an expert in hydroponics. Many of these recommendations are based on general principles and, as such, are not applicable to every plant. Always research the plant of interest, as well as your water source, prior to starting up a hydroponic system.