Soilless Vegetables Cultivation

In vegetables crop production, soilless cultivation refers to a range of systems that support plant growth without the presence of natural soil. Instead, water and essential nutrients are supplied through nutrient solutions, either with or without the use of a growing medium like stone wool, peat, perlite, pumice, or coconut fiber. These soilless culture systems, often known as hydroponic systems, can be categorized as either open systems, where surplus nutrient solutions are not recycled, or closed systems, where excess nutrients from the plant roots are collected and reused within the system.

 

The adoption of soilless culture systems has emerged as a viable solution to combat soil-borne diseases that have historically plagued greenhouse cultivation practices. Presently, soilless growing systems have become commonplace in horticultural practices across many European countries, though not universally on a large scale.

The advantages of soilless systems over traditional soil-based cultivation methods include:

 

  • Elimination of pathogens at the start, achieved through the use of non-soil substrates or more accessible management of soil-borne pathogens.
  • Independence of growth and yield from the quality or type of soil in the cultivation area.
  • Enhanced growth control facilitated by precise nutrient solution delivery.
  • Potential for reusing nutrient solutions, optimizing resource utilization.
  • Improved produce quality due to better control over environmental factors such as temperature, relative humidity, and pests.

 

Commonly, in the realm of soilless cultivation, open loop or run-to-waste systems are more prevalent than closed loop or recirculation systems, although the latter are increasingly mandated in numerous European countries. In open systems, the surplus or spent nutrient solution is either discharged into the environment, affecting ground and surface water, or employed in open field cultivation. However, for economic and environmental reasons, it is advantageous for soilless systems to operate in a closed manner, where nutrient solutions are recirculated, substrates are reused, and sustainable materials are favored.

The benefits of closed systems are numerous, including a reduction in waste material, decreased pollution of water sources, more efficient water and fertilizer usage, increased production due to better management options, and cost savings from reduced material usage and higher yields. However, there are also notable disadvantages, such as the necessity for high-quality water, substantial initial investments, the risk of spreading soil-borne pathogens through recirculation, and the accumulation of potentially harmful substances and organic matter in the recirculating nutrient solution.

 

Commercial systems address the issue of pathogen dispersal through various disinfection techniques, including physical, chemical, and biological filtration. However, one significant challenge hindering the wider adoption of recirculating nutrient solution culture, particularly in aquaponic settings, is the accumulation of salts in the irrigation water. The electrical conductivity (EC) of the water tends to increase steadily due to the buildup of ions that are not fully absorbed by the crops

Soilless systems

 

Extensive research conducted in the realm of hydroponic cultivation has yielded a wide array of cultivation systems (Hussain et al., 2014). Due to the extensive range of available systems, it becomes essential to categorize the various soilless systems (Table 4.1) to facilitate better understanding and organization.

 

 

Table 1 – Classification of hydroponics systems according to different aspects.
 

Solid substrate systems

 

During the inception of soilless cultivation in the 1970s, numerous substrates underwent testing (Wallach 2008; Blok et al. 2008; Verwer 1978). However, many of them proved unsuitable due to issues like excessive moisture, dryness, lack of sustainability, high costs, and the release of toxic substances. Despite these challenges, several solid substrates managed to endure, including stone wool, perlite, coir (coconut fiber), peat, polyurethane foam, and bark. Solid substrate systems can be categorized as follows:

 

Fibrous Substrates: These substrates may either be organic, such as peat, straw, and coconut fiber, or inorganic, exemplified by stone wool (Fig. 2). Characterized by the presence of fibers of varying sizes, fibrous substrates exhibit high water-retention capacity (60-80%) and a modest air capacity (free porosity) (Wallach 2008). A substantial percentage of retained water is readily available to plants, leading to a reduced volume of substrate per plant required to ensure sufficient water supply. The absence of significant water and salinity gradients along the substrate profile allows roots to grow vigorously, uniformly, and abundantly, utilizing the entire available volume.


Granular Substrates: Generally consisting of inorganic materials like sand, pumice, perlite, and expanded clay, granular substrates vary in particle sizes, resulting in different textures. They exhibit high porosity and excellent drainage capabilities. However, their water-holding capacity is relatively poor (10-40%), and a significant portion of the retained water remains inaccessible to plants (Maher et al., 2008). As a consequence, a larger volume of substrate per plant is required compared to fibrous substrates. In granular substrates, a distinct moisture gradient develops along the profile, causing roots to predominantly grow at the bottom of containers. Smaller particle sizes enhance water retention capacity, moisture uniformity, electrical conductivity (EC), and reduce the required substrate volume for optimal plant growth.

These substrates are often enveloped in plastic coverings, known as grow bags or slabs, or placed in variously sized containers made of synthetic materials. Before planting, it is essential to saturate the substrate to achieve uniform distribution of water and nutrients throughout the slab, consistent EC and pH levels, and the removal of air pockets for homogenous wetting of the material.


Additionally, allowing the substrate to dry after planting is crucial to stimulate the plants’ root exploration throughout the substrate, promoting a well-developed and evenly distributed root system at different levels and exposure to air.
Reusing substrates can be challenging, especially for granular substrates with drain holes in the plastic coverings, preventing saturation.

 

Organic substrates like coir can be more easily recovered and reused by adopting short and frequent irrigation cycles, as they regain water-retention capability. Inert substrates like stone wool and perlite pose more difficulty in rehydration for second-time use (Perelli et al., 2009).

Types of Hydroponic Systems Based on Water/Nutrient Distribution

 

Deep Flow Technique (DFT)
The deep flow technique (DFT), also referred to as deep water technique, involves cultivating plants on floating or hanging supports, such as rafts, panels, or boards, placed in containers filled with a nutrient solution of approximately 10-20 cm deep (Van Os et al., 2008) (Fig. 4.3). In aquaponics, this nutrient solution depth may be increased to about 30 cm. There are various application forms of DFT, primarily distinguished by solution depth and volume, as well as recirculation and oxygenation methods.

 

A straightforward DFT system includes tanks with a depth of 20-30 cm, which can be constructed using different materials and waterproofed with polyethylene films. These tanks are fitted with floating rafts, available in multiple types from suppliers, that support the plants above the water level while enabling the plants’ roots to extend into the nutrient solution. This system is particularly appealing due to its cost-effectiveness and ease of management. Automation of control and correction processes is relatively limited in such systems, reducing complexity and maintenance requirements.


In the context of short-duration crops like lettuce, the Deep Flow Technique (DFT) offers an advantage due to its relatively high nutrient solution volume. This facilitates the replenishment of the nutrient solution only at the end of each growth cycle, with periodic monitoring required mainly for oxygen levels. Ensuring oxygen levels above 4-5 mg/L is crucial, as low oxygen content may lead to suboptimal root system performance and nutrient deficiencies. Solution circulation usually aids in adding oxygen, and Venturi systems can be incorporated to significantly increase air supply into the system. This becomes especially critical when water temperatures exceed 23°C, as elevated temperatures can trigger the premature flowering of lettuce (bolting).

Nutrient Film Technique (NFT)
The Nutrient Film Technique

 

(NFT) represents a widely used and classic hydroponic cultivation system. In this method, a nutrient solution flows along troughs with a shallow water layer of about 1-2 cm (Cooper 1979; Jensen and Collins 1985; Van Os et al., 2008) (Fig. 4.4). The recirculation of the nutrient solution and the absence of a growing substrate are major advantages of the NFT system. Its potential for automation allows for cost-saving benefits in terms of labor (planting and harvesting) and enables precise management of optimal plant density during the crop cycle. However, the lack of substrate and low water levels render the NFT system susceptible to pump failures, such as clogging or power supply issues.

 

Temperature fluctuations in the nutrient solution may lead to plant stress and increase the risk of diseases. A significant limitation of the Deep Flow Technique (DFT) lies in the root system’s development, where part of the roots remains exposed to air above the nutrient flow.

 

This exposure can lead to premature aging and reduced functionality of the roots, making it challenging to cultivate long-cycle crops lasting over 4-5 months. Moreover, the system is not well-suited for environments with high levels of irradiation and temperature, such as regions in the southern areas of the Mediterranean basin. To address these issues, a multilayer NFT trough has been designed (known as NGS), allowing for extended production cycles without facing clogging problems.

 

The NGS system comprises interconnected layers positioned in a cascading manner, ensuring that even in strongly rooting plant species like tomatoes, the nutrient solution can reach the roots by bypassing any root-clogged layers through lower positioned layers.

Aeroponic Systems

 

The aeroponic technique primarily caters to smaller horticultural species and, as of now, has not gained widespread adoption due to its high investment and management costs. Plants are supported by plastic panels or polystyrene arranged horizontally or inclined on top of growing boxes. These panels are upheld by a structure made from inert materials like plastic, steel coated with plastic film, or polystyrene boards, creating enclosed boxes for the suspended root system’s development.

 

In the aeroponic technique, the root system of plants is suspended in the air within these modules. Nutrient solution is delivered directly to the roots through a process of spraying, which typically lasts for 30 to 60 seconds. The frequency of spraying is variable and depends on factors such as the stage of cultivation, growth phase of the plants, species being cultivated, and even the time of day. Some aeroponic systems employ vibrating plates that create micro droplets of water, generating a fine mist that condenses on the roots, effectively delivering nutrients to the plants.

 

As the nutrient solution is sprayed onto the roots, any excess solution or leachate is collected at the bottom of the box modules. This collected leachate is then channeled to a storage tank, where it can be recycled and reused. The process of nutrient recycling contributes to the efficiency and sustainability of aeroponic cultivation systems.

 

References

– Blok C, de Kreij C, Baas R, Wever G (2008) Analytical methods used in soilless cultivation. In: Raviv, Lieth (eds) Soilless culture, theory and practice. Elsevier, Amsterdam, pp 245–290
– Cooper A (1979) The ABC of NFT. Grower Books, London
– Hussain A, Iqbal K, Aziem S, Mahato P, Negi AK (2014) A review on the science of growing crops without soil (soilless culture)-a novel alternative for growing crops. Int J Agric Crop Sci 7:833–842
– Jensen MH, Collins WL (1985) Hydroponic vegetable production. Hortic Rev 7:483–558
– Maher MJ, Prasad M, Raviv M (2008) Organic soilless media components. In: Raviv, Lieth (eds) Soilless culture, theory and practice. Elsevier, Amsterdam, pp 459–504
– Maucieri C, Nicoletto C, Junge R, Schmautz Z, Sambo P, Borin M (2018) Hydroponic systems and water management in aquaponics: a review. Ital J Agron 13:1–11
– Perelli M, Graziano PL, Calzavara R (2009) Nutrire le piante. Arvan Ed., Venice.
– Van Os EA, Gieling TH, Lieth JH (2008) Technical equipment in soilless production systems. In: Raviv, Lieth (eds) Soilless culture, theory and practice. Elsevier, Amsterdam, pp 157–207
– Wallach J (2008) Physical characteristics of soilless media. In: Raviv, Lieth (eds) Soilless culture, theory and practice. Elsevier, Amsterdam, pp 41–116