IX. Greenhouse soilless culture:
1) Soilless culture definition:
Soilless culture is the technique of growing plants using mineral nutrient solution, either in a substrate that provide mechanical support to the plant roots or directly in an aquatic based environment.
2) Closed and open soilless system:
a) Closed system:
In a closed system the drained nutrient solution released from the root environment after its supply to the crop is collected, replenished with nutrients and water and reused.
b) Open system:
In an open system the drained nutrient solution released from the root environment after its supply to the crop is not reused nor recirculated but it is thrown out of the system, thus an adequate run- off must be maintained in order to keep nutrient balance in the root zone.
3) Water culture:
a) Deep water culture:
Plant roots are suspended directly into the nutrient solution. The system consists of a bucket covered with a thin layer of sand placed on a net and cloth, to support the plants. The main drawback of the system is that due to the limited air-water exchange area, hypoxic conditions may occur at the root level. This problem can be solved by using air pumps to oxygenate the nutrient solution.
b) Floating system:
Plants are grown on a raft of expanded plastic floating in tank filled with nutrient solution. The tank is covered with a polyethylene film that helps to hold the nutrient solution. A Styrofoam sheet is added to create a floating platform. The nutrient solution is oxygenated by the use of pumps that play a role in driving part of the solution into a pipe connected to a Venturi tube to insufflate air. To avoid roots damage, the airflow should not be very strong.
c) Nutrient film technique:
Plant roots lie in channels that contains a very thin layer of nutrient solution. Channels are installed on a slope that is provided by the use of adjustable benches so the required elevation can be obtained. The nutrient solution is applied at the higher end and flows down through the channels to keep the roots totally wet, then it is drained to a large pipe located at the lower end of the channels to return the solution to the cistern and to recirculate it. Good aeration of the roots is provided by the thin layer of the nutrient solution, as the roots are continuously exposed to the air especially on their upper surface.
d) Deep flow technique:
Plants are grown on a polystyrene trays that float in channels containing nutrient solution with 50- 150 mm depth.
Plants are grown on trays and their roots are suspended in air. The nutrient solution is constantly sprayed or fogged on the fully exposed roots of the plants.
4) Substrate culture:
a) Chemically active substrate:
Plants are grown in pots, containers, channels or bags filled with chemically active substrate. There are 2 types of chemically active substrate: inorganic substrate (zeolite, vermiculite, and tuff) and organic substrate (peat, coir, sawdust).
b) Chemically inactive substrate:
Plants are grown in pots, containers, channels or bags filled with chemically inactive substrate (sand, rockwool, perlite, pumice, expanded clay, etc).
Fertigation is to obtain an irrigation solution that results from the combination of fertilization and irrigation. The irrigation solution also called nutrient solution is the result of the dilution of a stock solution to a lower concentration and then injected into the irrigation water. Fertigation system requires pressure regulators, filters, tanks for stock and for acid solution, fertilizer injection devices, pH and EC measuring tools and a water/solution delivering system.
Three tanks are used to feed cultivated plants. Two of them are used to separate fertilizers that can interact, the possible combination is a tank containing calcium fertilizer and another tank containing phosphate and sulphate fertilizers to avoid precipitation of calcium phosphate or calcium sulphate, and the third one contains an inorganic acid used to control pH of the nutrient solution, to wash the irrigation system and to avoid clogging of the nutrient solution emitters.
6) Nutrient solution:
A nutrient solution is a diluted water solution containing all essential nutrients, macronutrients (nitrogen, phosphorus, sulfur, potassium, calcium and magnesium) and micronutrients (manganese, zinc, copper, boron and molybdenum), in form of inorganic ions or soluble inorganic compounds, with the exception of iron, which is a micronutrient present in form of an organic chelate.
Nutrient solution characteristics:
• Electrical conductivity
• Macronutrient concentrations
• Micronutrient concentrations
7) Monitoring and adjusting the pH of nutrient solution in the root zone:
It is very important to monitor and to adjust the pH of the nutrient solution regularly because a too high or a too low pH has a negative effect on plants, it prevents them from getting minerals that they need for their growth. If the pH is too high there is phosphorus, iron, manganese, copper and zinc deficiency and if the pH is too low there is calcium, magnesium and potassium deficiency. A solution with a pH ranging between 5.5-5.8 is delivered to the crop to maintain the pH in the root zone between 5.5-6.5. The pH of the nutrient solution may change in the root zone due to selective ion uptake by the plants. If anion uptake exceeds cation uptake, HCO3- and/or OH- are excreted in the root zone thus the pH in the external solution increases. An acid is added (nitric acid, phosphoric acid, sulphuric acid, citric acid or acetic acid) to reduce the pH of the nutrient solution. If cation uptake exceeds anion uptake, H+ is excreted in the root zone thus the pH in the external solution decreases. A base KOH, KHCO3 or K2CO3 may be added to increase the pH of the nutrient solution.
8) Monitoring and adjusting the EC of nutrient solution in the root zone:
The EC indicates the total salt concentration in the nutrient solution and it is measured by means of an EC meter which is a portable instruments. It is very important to monitor and to adjust the EC of the nutrient solution regularly. For an accurate EC measuring, standard solutions should be used to calibrate the EC meter. The portable meter should be stored in a cool dry place not in the greenhouse. An increase of the EC in the root zone is a frequent problem that growers can face and it may cause an excessive top growth or may cause damage to the root tips.
A high EC can be controlled by:
• Using a good quality water containing low amount of NaCl, Ca and Mg
• Balancing the composition of the supplied nutrient solution (EC, nutrient ratios)
• Adjusting the target EC in the nutrient solution supplied to the crop by taking into consideration the EC of the drainage solution and its composition
• Increasing the irrigation frequency
• Using a correct irrigation scheduling
• Using rain water instead of irrigation water to wash out salts from substrates
9) Nutrient solution recycling in closed system:
The recycling of nutrient solution saves and conserves water but it may result in recycling of pathogens (Pythium sp., Phytophtora sp., Fusarium sp., Verticilium sp., etc) that may infect plants via the recycled nutrient solution and in the long term it may result in accumulation of Na and Cl which may cause a problem if the concentration of these ions in the solution supplied to the crop is not very low.
10) Nutrient solution disinfection:
The disinfection of the drainage solution in closed system can be done through several methods:
• Pasteurization using heat treatment
• UV radiation (ultraviolet radiation)
• Slow sand filtration
• Micro-membrane filtration
• Active hydrogen peroxide
• Ozonation (O3)