I

I. What is soilless culture:
Soilless culture is the technique of growing plant using mineral nutrient solutions either in a soilless medium (artificial medium such as sand, gravel, vermiculite, rockwool, perlite, peatmoss, coir, or sawdust) to provide mechanical support or directly in an aquatic based environment. (1)

II. Advantages and disadvantages of soilless culture:

a) Advantages:

• Absence of soil-borne diseases or pests reduce pesticides and other chemicals use and lead to a sterile environment for plant production (2)

• Works under controlled environment that helps plant to grow faster resulting in a higher yield (2)

• Less space is needed, plants can be grown closer to each other due to their small roots

• Water conservation, water can be reused multiple times (closed system)

• Less labor

• Easy harvesting

• Crops can be grown all year round (2)

• No soil tillage and preparation

• Safe alternative to soil disinfection

• Limited media volume per plant allows better controlled plant nutrition

b) Disadvantages:

• High cost of initial set up

• Requires a technical knowledge

III. Feeding plants in soilless culture
Fertigation is the combination of fertilization and irrigation, through which the fertilizer is dissolved in the water used in the irrigation system. Fertigation requires a dosing system and tanks for the stock solution where nutrients are incorporated. Stock solution is a solution diluted to a lower concentration and injected in the irrigation water, it is used to conserve materials and to reduce both preparation time and storage space (4).
3 tanks are used to feed plants in soilless culture. 2 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 containing 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 (5).

IV. Soilless culture system (3)

Classification of soilless culture according to:

a) the method of managing the drainage solution
Open system:
In open system nutrients and water are introduced for each irrigation cycle, 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. An adequate run-off must be maintained in order to keep nutrient balance in the root zone. Also the concentration of the nutrient solution does not need to be regularly checked like they do when the nutrients and water are recycled. Open system is a passive system that does not require electricity and it helps growers in saving energy cost (6).

Closed system:
In 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. The concentration of the nutrient solution is regularly monitored and adjusted. A closed system is a cost-effective system because pumps are utilized to pump the nutrients and water back to the plants on a daily basis after recycling. (6).

b) the rooting medium

• Hydroponics

o Water culture

Deep water culture:
Deep water culture is a method where plant roots are suspended directly into a 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 root level. The solution to this problem is either through the use of air pumps to oxygenate the nutrient solution or through the application of recirculating deep water culture systems (RDWC) that use a reservoir to provide nutrient solution to multiple buckets (7).

Floating Hydroponics: Plants are grown on a ?oating raft
of expanded plastic
Floating Hydroponics: Plants are grown on a ?oating raft
of expanded plastic
Floating Hydroponics: Plants are grown on a ?oating raft
of expanded plastic
Float hydroponics:
Plants are grown on a raft of expanded plastic floating in tanks filled with nutrient solution. This system is characterized by its low set up and management cost and it is easy to build, it does not require lots of automation to monitor and adjust the nutrient solution.
The tank is constructed from a low cost material (concrete, bricks) and it is covered with a polyethylene film that helps in holding the nutrient solution. A Styrofoam sheet is added to create a floating platform. A single-tank or multiple-tank system may be used. 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 (8).

Nutrient film technique:
NFT is a hydroponic technique where bare rooted plants lie in waterproof channels in which flows a very thin layer of nutrient solution. Channels are installed on a slope that helps in eliminating ponding, the slope should not be severe and it may be provided by the floor itself, or by using adjustable benches so the required elevation can be obtained. 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 because they are present on the bottom of the channel. 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 catchment pipe located at the lower end of the channels, to return the solution to the cistern and to recirculate it (9).

Deepflow technique:
In deepflow technique plants are suspended on Styrofoam boards floating on the surface of the nutrient solution, which was circulated around the root zones of the plants (10).

Aeroponics:
Aeroponics is a technique to grow plants where their root system is suspended in air or mist environment without the use of soil. The nutrient solution is constantly sprayed or fogged on the fully exposed roots of the plants (11).

o Cultivation on inert aggregates or chemically inactive substrate:
Plants are grown in in pots, containers, channels or bags filled with chemically inactive substrate (sand, rockwool, perlite, pumice, expanded clay, etc).

• Soilless cultivation on chemically active substrates (inorganic or organic chemically active substrate): Plants are grown in pots, containers, channels or bags filled with chemically active substrate. There is 2 types of chemically inactive substrate: inorganic substrate (zeolite, vermiculite, tuff) or organic substrate (peat , coir , sawdust).

V. Substrate or growing media:

a) Substrate:
In hydroponic system the soil is substituted by a substrate which is a porous material that provides functions essential for plant growth:
• Physical support to the plants
• Water to the roots
• Oxygen to the roots
• Nutrients to the roots
The substrate should have the following desired characteristics:
• good water holding capacity
• good air capacity
• does not decompose rapidly, it has a stable structure
• uniformity
• pathogens and weed seeds free
• easy handling
• low cost
• environmentally friendly
• recyclable

b) Physical properties of substrates:
Porosity:
Plants need both macropores and micropores. The good growing medium should contain a mixture of different particle sizes (small and large particles) and characteristics. Large particles have more aeration and less water-holding capacity whereas small particles have less aeration and more water-holding capacity. The growing medium is characterized by the total porosity which is a combination of different particle sizes and which gives good water holding capacity and good air capacity that are necessary for plant growth (12).

Bulk density (BD):
Bulk density is defined as the dry mass of the substrate per unit of substrate volume in a dry state. ¬¬¬¬¬¬ Excessive bulk density indicates compaction. Bulk density and porosity are indirectly related, porosity decreases when bulk density increases.

Aeration:
Oxygen is necessary for good healthy roots. It is supplied through the larger macropores, which allow the dissipation of carbon dioxide during respiration. A good growing medium is characterized by a high percentage of macropores.

Water holding capacity:
The maximum water content that can be retained by a particular containerized substrate which is allowed to drain after saturation.

c) Chemical properties of substrates:
• Chemical composition
• Cation exchange capacity
• pH