The definition of hydroponic gardening is that no soil is used. Therefore all nutrients need to be provided directly through the nutrient solution. This has the advantage of being able to completely control what nutrients are accessible to the plant, and in what quantities. Luckily for those of us who don’t have the time or knowledge to make our own nutrient solutions, they are readily available from hydroponics suppliers and online. Nutrient solutions typically come in combinations of two or three concentrate bottles. This is because certain minerals can not be combined in high concentration because the precipitate.
Hydroponic Nutrient Components
The main components of hydroponics nutrients are calcium, magnesium, potassium, nitrate, sulfate, and phosphate. Many micronutrients are also present such as iron, manganese, copper, zinc, boron, chlorine, nickel, and molybdenum. Nutrient solutions can also come in single bottle concentrates, which contain chelating agents to keep the iron soluble.
Hydroponic Nutrient Concentration
The concentration of nutrients needs to be maintained at a certain level. This is easily monitored with a handheld meter that measures the concentration of minerals in water (measured in parts per million; ppm). With the nutrient solution that I use, a concentration of about 1,000 ppm is ideal. With time, plants use up both nutrients and water. If both nutrients and water are used up at the same time, the volume of water will drop but the concentration of nutrients in the water will remain the same as they are being used up at the same time. If not as much nutrients are being used, for example when plants are still young, the water will be used up quicker than the nutrients and the concentration will increase. The opposite occurs when plants are growing strongly and require a large amount of nutrients. Regular monitoring and correcting concentration by either adding more nutrients or water keeps this under control. After a reasonable period of time, nutrient solutions need to be completely replaced. This is because individual nutrients e.g. nitrogen would be used up from the solution, even though the overall concentration may be stable. It is recommended that nutrient solutions be replaced every 1-2 weeks. I replace mine every 2 weeks.
Hydroponic Nutrient pH
The other important aspect of nutrient solutions is pH – the balance between acidity and alkalinity. The importance of pH is in the ability of plants to absorb nutrients. Each mineral is absorbed by plants within a certain pH range. The optimal pH range for absorption of most nutrients is between pH 6 and 7 (see Figure 1). As can be seen from the figure, movement away from this pH in either direction results in greatly reduced availability of many nutrients, which results in sub-optimal plant growth.
Plants also have pH ranges within which they grow best. The ideal growth pH ranges for a selection of plants is shown in Figure 2. As you can see, this varies between pH 5 and pH 8. Scanning through the pH ranges, it can be seen that for most plants, a pH of 6 is within the ideal range. This is the pH that is recommended by most hydroponics gardeners, and is what I use in my hydroponics systems.
Hydroponic Nutrient Calculators
For those gardeners that want to get into the nitty gritty of making their own nutrients or tinkering with compositions, there are a couple of nutrient calculators available for free online. These are HydroBuddy and Hydrocal. While it is possible to make one’s own nutrient solution, the large range of commercially available solutions means that there really isn’t a need for gardeners to go into this level of detail. I myself have never tinkered with the composition of nutrient solutions, other than to change pH.
Images are from (1) Roberto K. 2003, “How to hydroponics”, 4th edition, and (2) “Hydroponics for the home gardener”.