Introduction to Hydroponic Crop Cultivation

Hydroponics is the process of plant cultivation without the use of natural living soils. It is used because soils are inconsistent from place to place and time to time, and because they can contain microorganisms and insects that are harmful to plants. In order to achieve maximal growth rates a grower or farmer must be able to control moisture, air, temperature, and nutrients in the root zone, and hydroponic systems function as tools for high precision root zone management. They may employ a simple fertilizer solution wherein the roots sit or flow, or they may include aggregates and substrate media (soilless cultivation) that anchor plant roots and provide oxygen from pores. Regardless of the system, research has shown that crops grown hydroponically may have a higher phytochemical and nutritive content than conventionally grown crops.

Nutrient Film Technique (NFT)

The nutrient film technique is one of the oldest methods for hydroponic growing and was heavily incorporated into the ancient Hanging Gardens of Babylon. It functions on the premise that a shallow(2-3 mm), constantly flowing film of water can be effectively oxygenated by passive diffusion (Under 25C) due to the shallow depth. Today, it is the most commonly used hydroponic system and takes numerous conformations, including a single horizontal canopy, A-frame, towers, spirals, and others. NFT systems are fairly inexpensive to purchase/build and can be easily assembled. Moreover, they do not require very powerful pumps relative to size, though the pumps will be running continuously (24/7). They perform quite well with a wide variety of vegetable crops when environmental conditions are right, but may experience issues with clogging in the irrigation manifold and must be checked. The shallow film of water also normalizes with the ambient temperature readily and is virtually impossible to heat or cool efficiently under extreme environmental conditions.

In nature, NFT would be the equivalent of plants growing on a shallow river/stream or its banks. The roots are constantly wet, but their shallow position and the turbulence of the water keep them aerated and happy. 

Deep Flow Technique (DFT)

The deep flow technique (sometimes referred to as DWC by hobby gardeners) is also an ancient growing method and has been used in rice paddy swamps as well as the gardens in Babylon. This system employs large raceways or tubs filled with fertilizer solution to a depth of 5 cm to 50 cm, on which floating rafts hold the plants upright as their roots flow through the solution. DFT is the second most common hydroponic method for growing plants nowadays, and is valued for its exceptional thermal stability. DFT systems are also fairly inexpensive to purchase/build and can be easily assembled. The weight of a deep solution means that DFT can not easily be used for vertical stacking or vertical systems, however, and maintaining high dissolved oxygen that is well distributed can be challenging. Luckily, technological advances have been made in chemical and electronic products that enhance bubble suspension and oxygen diffusion in the solution, though a compressor must be pumping through well distributed air stones continuously (24/7).

In nature, DFT would be comparable to plants growing in a swamp or on a pond, wherein algae and bacteria produce gases including oxygen that aerate the water, which is otherwise generally stagnant.

Aeroponics

Aeroponics is a newer method of hydroponic crop cultivation that employs air chambers and high pressure nozzles to atomize the irrigation solution and micro-pulse for maximal root absorption. It is generally only used for high value/exotic fruit and vegetable production as well as in medicinal plant production. While they exhibit exceptional performance under ideal conditions, Aeroponic systems are more energy and maintenance intensive and are considered the most expensive and complex hydroponic systems. A very powerful pump is needed to deliver highly pressurized solution to the nozzles and the use of a good particle filtration system is advised to help avoid clogs. These nozzles are generally removed and cleaned in between each crop cycle, and some companies even employ individual sensors to detect clogs and avoid crop damage as quickly as possible.

In nature, Aeroponics can be seen in caves under waterfalls, where plants growing on the mineral walls receive a fine mist from the crashing river water as their only source of irrigation.

Aggregate Beds (ABs)

Aggregate beds are commonly used in amateur aquaponics systems because they are fairly fool proof when employed correctly. In common usage, a bed about 50 cm deep would be constantly flooded to about 10 to 20 cm deep, creating a moisture gradient fairly similar to real soil conditions. Commercially, these systems are flooded to the top of the aggregate and used to grow semiaquatic plants like watercress.

In nature, ABs can be seen where river and lake plants grow in completely flooded gravel/sand beds. These plants must be tolerant of low dissolved oxygen conditions at their root zones.

Substrate Systems

Substrate systems include Dutch (or Bato) buckets as well as slab, bag, and trough culture. These systems are often ideal for growing fruiting vine or berry plants. There are numerous options for horticultural substrates including coco croutons, pine bark, expanded clay aggregate, calcined clay, and vermiculite, though the most popular are sphagnum peat moss, coco coir, and perlite. In substrate systems, the sterile and inert substrate functions to anchor plants and retain moisture, air, and fertilizer in the root zone. 

Substrate systems are the most similar to soil growing, as the substrate particle size is usually small and comparable to clay, sand, and loam soils. The main difference is that soilless substrates will have been sterilized to avoid contaminating growers' crops with pathogenic organisms.

Shallow Aggregate/Aero Ebb-and-Flow (SAEF)

Recently developed and still in testing, the SAEF system borrows components and characteristics from NFT (shallow irrigation), DFT (rafts), ABs (Tray/aggregate), and Aeroponics (Air chamber) while using ebb-and-flow irrigation benching, which can be used with rolling benches or Dutch trays for maximal space usage efficiency. The SAEF system is relatively low cost to set-up and operate, very energy efficient, and can be made to be very space efficient. It can take multiple conformations and can be easily stacked for vertical applications. This system must be perfectly leveled in order to work properly.

In nature, the SAEF mechanism can be seen in fields as the soil moisture level rises during rain and drops when the rain stops, with the added advantage that plant foliage does not become wet during irrigation in a SAEF system. It can also be seen in plants growing in rocky cracks in the mountains and being irrigated by mineral rich melt water pushed up through the cracks periodically.

Hybrid Systems

Some hybrid hydroponic systems that combine NFT and DFT have also been developed. These function by pulse irrigating through a manifold with micro-tubes into a table containing stagnant water to 4 cm deep. Rafts float on the table, securing plants and contributing to root oxygenation with air cavities on the undersides of the rafts.

Dry Hydroponics (DH)

Dry hydroponics is a method similar to DFT but with one slight variation. Each seedling is planted in the floating raft so that the upper portion of its roots remain dry, and only the lower potion of the roots are immersed in water. Allowing the plant to develop air roots and a drier stem in this way allows for a reduction in the power consumption attributed to aerating the stagnant solution.

Aquaponics

Aquaponics is simply growing plants in hydroponic systems while deriving the majority of plant nutrients from aquaculture effluent. Since this is the case, all hydroponic systems are equally compatible with proper aquaponic growing!