ABSTRACT
The choice of not only the hydroponic system but also the substrate is determined by the crop grown� When making this decision, consider the expected growth of the plant during its cropping period and the form of the crop (low-profile vs� vine-crop) stature� Small plants have less extensive root systems and can hence grow in smaller containers� Many plants simply do not like to grow in a nutrient solution alone but must have a medium into which to spread their roots� These include many herbs such as sage, rosemary, thyme, and oregano that are accustomed to a dry medium around their bases (crowns)� Long-term crops that grow for many months cannot tolerate a water solution as they soon develop an oxygen deficit that causes their roots to die�
Plants that can tolerate a water culture system include lettuce, basil, watercress, mint, chives, arugula, and bok choy� Lettuce, arugula, basil, and bok choy have a relatively short cropping cycle of less than 2 months� For this reason, they will grow well in water culture systems such as the floating raft system or nutrient film technique (NFT)� Most herbs, if grown on a short cycle, especially for “live” herbs, are happy in water culture systems� “Live” herbs are those that are harvested with their growing cube and roots intact and marketed in small plastic bags or sleeves as “living” plants� This method of harvesting prolongs their shelf life� It is very popular for basil, arugula, and lettuce� Even for your own use, keep the roots on these plants if you are not going to use the entire plant at one preparation� You will be aware of the lettuce in the supermarkets that are packaged in semi-rigid (clam-shell) containers with a lid and a small reservoir in the bottom to contain the roots in their growing cube� The most popular method of growing “live” herbs is in NFT channels, as they can easily be removed at harvesting without damaging a lot of their roots�
Long-term crops growing more than several months must have a substrate and be containerized or grown in beds� These include all vine crops and melons� Rockwool and coco coir slabs are the most common systems of hydroponics with these types of crops� They all use drip irrigation� The advantage of the drip irrigation system is to apply the solution near the base of the plants bringing oxygen with it as it percolates through the substrate� With the moisture homogeneously distributed within the substrate, the roots can spread around to seek water, nutrients, and oxygen� With any open or re-circulating system, it is critical to get good drainage that conducts the leachate away from the plant substrate and container so that oxygenation is not restricted to the plant roots� With ebb and flow systems, this complete drainage between irrigation cycles is one factor that will determine the success or failure of the crop� While smallscale systems of ebb and flow crops will do well as drainage is fairly rapid, with much
larger systems of ebb and flow it is more difficult to get adequate and complete drainage in order to avoid restricting plant growth and production� For that reason, with long-term crops I do not recommend using ebb and flow systems� If crops are stressed by environmental limitations, they become more susceptible to pests and diseases� This is another reason to select the correct growing system for the specific crop�
Certain plants have preferences for specific substrates� Some plants like a higher water-holding capacity, while others need very rapid drainage and less moisture contained within the medium� Most vine crops thrive in most media, providing the medium has good drainage with available oxygen to the plants’ roots� A few herbs such as rosemary and sage like very good drainage so do best in a coarse welldrained substrate like perlite, vermiculite, or mixtures of these with peat or coco coir at a 1:1 ratio� Knowledge of the type of soil these plants prefer will indicate the nature of medium to use with hydroponics� For example, if the plants do best in a sandy soil, they prefer a substrate with very good drainage, so we would use perlite, vermiculite, or the mixtures of these with some peat or coco coir�
Table 15�1 summarizes the most suitable hydroponic systems and substrates that some commonly grown hydroponic crops prefer� The raft culture system is not suitable to many crops due to the logistics of harvesting� It is used only for one-time harvest plants, so even though some low-profile plants may thrive in the water culture it is not feasible from a labor stand point to grow them as the boards would have to be taken out to harvest and then replaced again with the mature plants intact� This is difficult to do without damaging the boards or the plants� The roots of the plants could become broken or infected with disease organisms during the removing and replacing of the boards into the pond or raceways� Also, with a single harvest crop the boards are then sterilized before placing them back into the raceways for transplanting�
Aeroponics can grow most plants well, but due to the cost of such a system and the inconvenience of supporting vine crops, it is not used for vine crops� It is feasible from a growing point of view for low-profile plants, including strawberries, lettuce, arugula, basil, and many herbs, but again it may not be the correct choice due to capital costs� Aeroponics is the culture of preference for any crop whose harvestable portion is high value roots or tubers� For example, to produce medicinal products from some herb plant roots, aeroponics is both practical and economically feasible� Another very specific application is the growing of seed potatoes as they must be kept free of any disease� This system of aeroponics is used in the International Potato Research Center at Universidad de La Molina in Lima, Peru (Figures 15�1 and 15�2)� That center ships certified potato seed tubers to all parts of the world in establishing mother plants for local planting� These seed potatoes are a very high-value crop with strict control of introducing seed stock certified free of diseases to various countries�
Table 15�1 shows that vine crops of tomatoes, peppers, European cucumbers, and eggplants grow in most hydroponic systems with the exception of water culture and aeroponics� They also do well in most substrates that offer good drainage� While melons are included in the list of crops, they normally are not grown unless they can be trained vertically by support strings� Their production is usually limited to five to six fruits per vine over the cropping period� For this reason, they are not economically feasible to grow commercially except in countries such as Japan, where they demand a very high price in the marketplace (up to $30-40 per fruit)�
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