Nutritionally Speaking

Whichever type of hydroponic system you select or create, you must supply the plants with nutrients. In soil, these elements come from rock and mineral leaching and organic matter decomposition. They are "held" by the soil particles and dissolved in the surrounding water. In hydroponics, the liquid solution is taken in directly by the roots and provides the leaves with nutrients through the transportation system in the stem. These nutrients or minerals are not actual food, but elements vital to helping the plant utilize the sugars (the real food) that it produces during photosynthesis.

Important Nutrients

Plants need about 16 different essential elements for optimum growth. Macronutrients, which are ordinarily found in soil, are needed by plants in rather large amounts. (Hydrogen, oxygen, and carbon are also necessary in large amounts, but are available to plants from the air and water.) The following are essential macronutrients:

Trace elements, or micronutrients, including manganese, iron, copper, and others, are important to the total well-being of the plant, but in much smaller amounts.

Classroom Tips: Supplying Nutrients

Hydroponic gardeners provide plant roots with a nutrient solution containing an appropriate balance of necessary nutrients -- a "super nutrient soup," suggests one fifth grade teacher. The easiest way to supply them is to purchase prepared hydroponic nutrients in dried or liquid form. Most are concentrated and must be mixed with water. Some classrooms have used commercial houseplant fertilizers for hydroponics, with mixed results. Students may want to compare the effects of different types of fertilizers on plant growth. Upper-grade students might want to experiment with varying proportions of individual nutrients to make their own "super secret soup."

Mixing Solutions - When mixing nutrient solutions, always dilute them to the concentration recommended by the manufacturer, typically 1 or 2 teaspoons per gallon of water. Water between 65 and 75 degrees F makes nutrients most available to plants. Tap water may contain significant concentrations of chlorine, which can adversely affect plant growth. If your water has a lot of chlorine, you can use distilled water or simply let water stand uncovered for a couple of days before using it. Your students might want to explore this themselves by comparing plants grown with distilled- versus tap water-based nutrient solutions.

Nutrient Disposal Caution Take care where you dispose of nutrient solutions. Houseplants, indoor plants, and container gardens are fine places to recycle the liquid. However, aquatic ecosystems are quite sensitive and the balance of minerals is very delicate. If there is a stream, lake, or other water source nearby, do not dispose of liquid nutrients on the ground.

How Much to Use - The amount of nutrient solution you use depends on the type of system, temperature, light, and other factors. If you're growing plants like lettuce, herbs, or flowers in a simple system such as a floating raft, a good rule of thumb is to provide 2 quarts of nutrient solution per plant. If you're trying to raise larger, fruiting crops in a more sophisticated system, you'll need to supply closer to 2 gallons of nutrient solution per plant.

Maintaining Nutrients - You'll have to replace the nutrient solution at different intervals depending on the type of system you set up, because nutrient concentration will vary as nutrients are taken up by the plant and as water evaporates and transpires from plant leaves. (Commercial growers use special equipment to measure the concentration of nutrients in a solution.) A good general rule for most classroom systems is to replace the mixture with a fresh batch every 10 to 21 days. Invite students to consider ways in which these solutions can be recycled, such as by watering other classroom or outdoor plants. As the water in your system evaporates and transpires, you may also want to top off the solution with more water to avoid building up concentrations of mineral salts.

 

Super Soup: Mixing the Nutrient Solution

 

 

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