The No-Soil Solution
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There are a number of different hydroponic systems. One is NFT, or nutrient film technique, in which plants are supported in narrow channels filled with vermiculite or another lightweight medium. These can be store-bought or constructed by slicing PVC pipes lengthwise. The fertilizing liquid is pumped from a reservoir to the top of the channel. A small amount of the liquid, only two millimeters deep, travels down the channel, pulled by gravity. The tips of the plants’ roots barely sit in the liquid, while the remainder is aerated. Capillary action pulls water upward across the surfaces of the medium, delivering more water to the plants.
Other growing mediums include sand, sawdust, rock wool, expanded clay, crushed granite, volcanic cinders or a mixture. An ideal medium holds onto moisture while providing good aeration. The more porous the medium, the greater amount of surface area it has and the more moisture it can retain. Materials used for growing mediums should be nontoxic and durable. They periodically need to be sterilized, either with steam or chemicals, as diseased organisms may build up inside them. Rock wool, a mineral-based product resembling household insulation, cannot be sterilized and reused; it must be disposed of after each growing cycle because it is susceptible to mold.
Flood-and-drain systems function as they sound: A bed filled with plants and a growing medium undergoes cycles of flooding and draining of the liquid nutrients. The cycles are automated by pumps with float-valve switches or bell siphons. This method allows the vegetable roots to alternate from being immersed in liquid solution to being in an oxygen-rich, well-drained environment.
Another growing method, used mainly for lettuces, is the deep-water raft method. In this method, Styrofoam sheets with holes cut into them are floated in a 12-inch-deep trough. Plants are placed in net pots that sit in the holes in the Styrofoam board. As liquid solution from the nutrient tank is circulated through the trough, plant roots grow deep and utilize the nutrients in the full depth of the water column.
aeroponicIn aeroponic systems, the nutrient solution is sprayed directly on the vegetable roots, and no growing medium is used at all.
Without soil to act as a reserve tank of nutrients for plant growth, care must be taken to ensure that all plant needs are being met. Critical parameters to monitor are pH, temperature and electroconductivity
- The pH should be kept between 5.8 and 6.5. It can be brought down using phosphoric or nitric acid or raised with potassium hydroxide.
- The ideal temperature should be 20 to 22 degrees Celsius. It’s important that the reservoir tank of nutrient solution doesn’t become too hot. It can quickly overheat if exposed to the sun, making it a good idea to locate the reservoir tank in a shaded location
- Electroconductivity is essentially a measure of the ratio of water to nutrients in the liquid fertilizer. A special electroconductivity meter can be used to monitor these levels. As the nutrients are used up, the meter will register a drop in total dissolved salts, or TDS.
Plants are categorized as being low, medium or heavy feeders. The heavy feeders, like tomatoes, require the most nutrients. Low feeders, like lettuces, need the least. Because the liquid fertilizer of macro- and micronutrients is mixed especially for the particular category of feeder, plants grown together in a system must have the same nutrient requirements.
The fertilizer mix is made of specific ratios of fertilizer to water each time it’s made. The nutrient levels in the fertilizer reservoir drop as they are consumed by the plants. However, plants use the nutrients they need, which may not exactly match the proportions in the mix.
Because there is no way to know exactly what nutrients are in greater or lesser supply in the reservoir, the nutrient solution needs to be replaced every two to three weeks. In the summer, when plant growth is more vigorous and evaporation rates higher, this may need to be done weekly.
The NFT Set-up
NFT systems are common and simple. To create one, you’ll need the following items:
- Reservoir tank: You’ll need a bucket, fish aquarium or plastic stock tank ranging from 5 to 100 gallons to store the liquid nutrient solution. The larger the tank, the more you can grow and the less frequently it needs to be refilled. (If this was an aquaponics system, the fish would live in here; see “The Dirt on … Aquaponics” on page 26 in the 2011 November/December issue of Urban Farm.)
- NFT channels: The plants are placed in the channels. They can be either store-bought channels, old rain gutters, large PVC tubing cut in half or any other 2- to 4-inch-deep channel. The biggest advantage to using manufactured channels is that they have covers and predrilled holes in which to place plants, which provide support for the plants. Manufactured channels are also designed to provide the optimal mix of air, nutrients and water.
- Water pump: A small water pump moves the nutrient solution from the bottom of the tank to the top of the NFT channels.
- Tubing: Tubing carries the liquid from the outlet of the pump to the top of the channels.
1. Build a table, rack or wall-mounted system to support the NFT channels. The channels should be placed at a slight angle (dropping at least 1/4 inch per foot), with the far end higher than the tank.
2. If a source of natural lighting exists, place the channels to best utilize this light. South-facing windows are preferable, providing the most light throughout the course of the day. In the absence of natural light, artificial lighting, such as fluorescent bulbs or grow lights, must be used over the channels. (Read “Portable Sunshine” by Bill Bradley in the March/April 2011 issue of Urban Farm to learn how to build your own portable light stand. – Ed.)
3. Place the reservoir tank under the lower lip of the channel, so the liquid can return to its source via gravity. Place the water pump into the tank.
4. Run tubing from the pump to the high end of the channel. Attach the end of the tubing to the channel so that water will run through it without spilling out. A “T” connector can be used to divide the water through multiple channels.
5. Plug in the pump, and adjust the water flow to a thin trickle.
6. Fill the channels with growing medium, and add plants or seeds. Seeds can also be germinated in separate seed starting trays and transplanted to the channels when they are a few inches tall, depending on the plant. Follow planting depth and spacing instructions on the seed package. Seeds should be kept moist, but not immersed in water. Seedlings need a special nutrient solution (available from any hydroponics supplier) formulated for their specific growth needs.
Hydroponic and soil-less gardening practices can be fun and educational. They are highly scalable and can be easily located in an apartment, classroom or commercial greenhouse. As more people become interested in intensive urban-food production, the technology will no doubt evolve in many fascinating directions.
Scott Kellogg and Stacy Pettigrew run the Radix Ecological Sustainability Center (www.radixcenter.org) in Albany, N.Y., and co-authored Toolbox for Sustainable City Living: A Do-it-Ourselves Guide (South End Press, 2008).
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