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The intense research carried out in the field of hydroponic cultivation has led to the development of a large variety of cultivation systems (Hussain et al. 2014). In practical terms all of these can also be implemented in combination with aquaculture; however, for this purpose, some are more suitable than others (Maucieri et al. 2018). The great variety of systems that may be used necessitates a categorization of the different soilless systems (Table 4.1).
Table 4.1 Classification of hydroponic systems according to different aspects
table thead tr class="header" thCharacteristic/th thCategories/th thExamples/th /tr /thead tbody tr class="odd" td rowspan="6"Soilless system/td td rowspan="3"No substrate/td tdNFT (nutrient film technique)/td /tr tr class="even" tdAeroponics/td /tr tr class="odd" tdDFT (deep flow technique)/td /tr tr class="even" td rowspan="3"With substrate/td tdOrganic substrates (peat, coconut fibre, bark, wood fibre, etc.)/td /tr tr class="odd" td Inorganic substrates (stone wool, pumice, sand, perlite, vermiculite, expanded clay) /td /tr tr class="even" tdSynthetic substrates (polyurethane, polystyrene)/td /tr tr class="odd" td rowspan="2"Open/closed systems/td tdOpen or run-towaste systems/td td The plants are continuously fed with “fresh” solution without recovering the solution drained from the cultivation modules (Fig. 4.1a) /td /tr tr class="even" tdClosed or recirculation systems/td td The drained nutrient solution is recycled and topped up with lacking nutrients to the right EC level (Fig. 4.1b) /td /tr tr class="odd" td rowspan="2"Water supply/td tdContinuous/td tdNFT (nutrient film technique) DFT (deep flow technique)/td /tr tr class="even" tdPeriodical/td tdDrip irrigation, ebb and flow, aeroponics/td /tr /tbody /table
At the start of soilless cultivation in the 1970s, many substrates were tested (Wallach 2008; Blok et al. 2008; Verwer 1978). Many failed for reasons such as being too wet, too dry, not sustainable, too expensive and releasing of toxic substances. Several solid substrates survived: stone wool, perlite, coir (coconut fibre), peat, polyurethane foam and bark. Solid substrate systems can be divided as follows:
Fibrous Substrates These may be organic (e.g. peat, straw and coconut fibre) or inorganic (e.g. stone wool). They are characterized by the presence of fibres of different sizes, which give the substrate a high water-retention capacity (60—80%) and a modest air capacity (free porosity) (Wallach 2008). A high percentage of the retained water is easily available for the plant, which is directly reflected in the minimum volume of substrate per plant required to guarantee a sufficient water supply. In these substrates there are no obvious water and salinity gradients along the profile, and, consequently, the roots tend to grow faster, evenly and abundantly, using the entire available volume.
Granular Substrates They are generally inorganic (e.g. sand, pumice, perlite, expanded clay) and are characterized by different particle sizes and thus textures; they have high porosity and are free draining. Water-holding capacity is rather poor (10—40%), and much of the water retained is not easily available to the plant (Maher et al. 2008). Therefore, the required volume of substrate per plant is higher compared to the fibrous ones. In granular substrates, a marked gradient of moisture is observed along the profile and this causes the roots to develop mainly on the bottom of containers. Smaller particle sizes, increase in the capacity for water retention, moisture homogeneity and greater EC and a lower volume of the substrate are required for the plant.
Substrates are usually enveloped in plastic coverings (so-called grow bags or slabs) or inserted in other types of containers of various sizes and of synthetic materials.
Before planting the substrate should be saturated in order to:
Provide adequate water and nutrients supply in the entire substrate slab.
Achieve uniform EC and pH levels.
Expel the presence of air and make a homogeneous wetting of the material.
It is equally important for a substrate dry phase after planting to stimulate the plants to evolve homogeneous substrate exploration by roots to obtain an abundant and well-distributed root system at the various levels and to expose the roots to air. Using a substrate for the second time by rewetting may be a problem because saturation is not possible due to drain holes in the plastic envelope. In an organic substrate (such as coir), adopting short and frequent irrigation turns, it is possible to recover the water-retention capability to use it for the second time, more easily than inert substrates (stone wool, perlite) (Perelli et al. 2009).
A substrate is necessary for the anchorage of the roots, a support for the plant and also as a water-nutritional mechanism due to its microporosity and cation exchange capacity.
Plants grown in soilless systems are characterized by an unbalanced shoot/root ratio, demands for water, air and nutrients that are much greater than in open field conditions. In the latter case, the growth rates are slower, and the quantities of substrate are theoretically unlimited. To satisfy these requirements, it is necessary to resort to substrates which, alone or in mixture, ensure optimal and stable chemical—physical and nutritional conditions. An array of materials with different characteristics and costs can be used as substrates as illustrated in Fig. 4.2. However, as yet, there is no one substrate that can be used universally in all cultivation situations.
Fig. 4.2 Materials usable as substrates in soilless systems
Bulk Density (BD) BD is expressed by the dry weight of the substrate per unit of volume. It enables the anchorage of the roots and offers plant support. The optimum BD for crops in a container varies between 150 and 500 kg msup-3/sup (Wallach 2008). Some substrates, because of their low BD and their looseness, as is the case of perlite (ca. 100 kg msup-3/sup), polystyrene in granules (ca. 35 kg msup-3/sup) and non-compressed Sphagnum peat (ca. 60 kg msup-3/sup), are not suitable for use alone, especially with plants that grow vertically.
Table 4.2 Main chemical—physical characteristics of peats and coconut fibre. (dm = dry matter)
table thead tr class="header" th rowspan="2"Characteristics/th th colspan="2"Raised bogs/th thFen bogs/th th rowspan="2"Coconut fibre (coir)/th /tr tr tdBlond/td tdBrown/td tdBlack/td /tr /thead tbody tr class="even" tdOrganic matter (% dm)/td td94–99/td td94–99/td td55–75/td td94–98/td /tr tr class="odd" tdAsh (% dm)/td td1–6/td td1–6/td td23–30/td td3–6/td /tr tr class="even" tdTotal porosity (% vol)/td td84–97/td td88–93/td td55–83/td td94–96/td /tr tr class="odd" tdWater-retention capacity (% vol)/td td52–82/td td74–88/td td65–75/td td80–85/td /tr tr class="even" tdFree porosity (% vol)/td td15–42/td td6–14/td td6–8/td td10–12/td /tr tr class="odd" tdBulk density (kg msup3/sup)/td td60–120/td td140–200/td td320–400/td td65–110/td /tr tr class="even" tdCEC (meq%)/td td100–150/td td120–170/td td80–150/td td60–130/td /tr tr class="odd" tdTotal nitrogen (% dm)/td td0.5–2.5/td td0.5–2.5/td td1.5–3.5/td td0.5–0.6/td /tr tr class="even" tdC/N/td td30–80/td td20–75/td td10–35/td td70–80/td /tr tr class="odd" tdCalcium (% dm)/td td<0.4/td td<0.4/td td>2/td td–/td /tr tr class="even" tdpH (Hsub2/subO)/td td3.0–4.0/td td3.0–5.0/td td5.5–7.3/td td5.0–6.8/td /tr /tbody /table
Source: Enzo et al. (2001)
Porosity The ideal substrate for potted crops should have a porosity of at least 75% with variable percentages of macropores (15—35%) and micropores (40—60%) depending on the cultivated species and the environmental and crop conditions (Wallach 2008; Blok et al. 2008; Maher et al. 2008). In small-sized containers, total porosity should reach 85% of the volume (Bunt 2012). The structure should be stable over time and should resist compaction and the reduction of volume during dehydration phases.
Water-Holding Capacity Water-holding capacity ensures adequate levels of substrate moisture for crops, without having to resort to frequent irrigations. However, the water-holding capacity must not be too high in order to avoid root asphyxia and too much cooling. The water available for the plant is calculated by the difference between the quantity of water at the retention capacity and that retained at the wilting point. This should be around 30—40% of the apparent volume (Kipp et al. 2001). Finally, it must be considered that with the constant increase of the root system biomass during growth, the free porosity in the substrate is gradually reduced and the hydrological characteristics of the substrate are modified.
Cation Exchange Capacity (CEC) CEC is a measure of how many cations can be retained on substrate particle surfaces. In general, organic materials have a higher CEC and a higher buffer capability than mineral ones (Wallach 2008; Blok et al. 2008) (Table 4.2).
pH A suitable pH is required to suit the needs of the cultivated species. Substrates with a low pH are more suitable for crops in containers, as they are more easily modified towards the desired levels by adding calcium carbonate and also because they meet the needs of a wider number of species. Moreover, during cultivation the pH value tends to rise due to irrigation with water rich in carbonates. The pH may also vary in relation to the type of fertilizer used. It is more difficult to correct an alkaline substrate. This can however be achieved by adding sulphur or physiologically acid fertilizers (ammonium sulphate, potassium sulphate) or constitutionally acid fertilizers (mineral phosphate).
Electrical Conductivity (EC) Substrates should have a known nutrient content and low EC values, (see also Table 4.4). It is often preferable to use a chemically inert substrate and to add the nutrients in relation to the specific crop requirements. Particular attention has to be paid to the EC levels. High EC levels indicate the presence of ions (e.g. Nasup+/sup) that, although not being important as nutrients, can play a decisive role in the suitability of the substrate.
Health and Safety Health in the systems and safety for operatives are provided by the absence of pathogens (nematodes, fungi, insects), potentially phytotoxic substances (pesticides) and weed seeds. Some industrially produced materials (expanded clay, perlite, stone wool, vermiculite and polystyrene) guarantee high levels of sterility due to the high temperatures applied during their processing.
Sustainability Another important characteristic of a substrate is its sustainability profile. Many commonly used substrates face ecological challenges relating to their provenance, production process and/or subsequent processing and end-of-life footprint. In this regard, substrates originating from materials with a low ecological footprint (modified in an environmentally friendly way and ultimately biodegradable) are an extra characteristic to consider. Reusability of the substrate can also be an important aspect of the sustainability of a substrate.
Cost Last but not least, the substrate must be inexpensive or at least cost-effective, readily available and standardized from the chemical—physical point of view.
The choice of substrates ranges from products of organic or mineral origin which are present in nature and which are subjected to special processing (e.g. peat, perlite, vermiculite), to those of organic origin derived from human activities (e.g. waste or by-products of agricultural, industrial and urban activities) and of industrial origin obtained by synthesis processes (e.g. polystyrene).
This category includes natural organic substrates, including residues, waste and by-products of organic nature derived from agricultural (manure, straw, etc.) or, for example, industrial, by-products of the wood industry, etc. or from urban settlements, e.g. sewage sludge, etc. These materials can be subjected to additional processing, such as extraction and maturation.
All the materials that can be used in hydroponics can also be used in AP. However, as the bacterial load in an AP solution may be higher than in conventional hydroponic solutions, it can therefore be expected that organic substrates may be prone to an increased decomposition rate, causing substrate compaction and root aeration problems. Therefore, organic materials can be considered for crops with a shorter growth cycle, whilst mineral substrates may be preferred for crops with a long growth cycle.
Peat, used alone or with other substrates, is currently the most important material of organic origin for substrate preparation. The term peat refers to a product derived from residues of bryophytes (Sphagnum), Cyperaceae (Trichophorum, Eriophorum, Carex) and others (Calluna, Phragmites, etc.) transformed in anaerobic conditions.
Raised bogs are formed in cold and very rainy environments. Rainwater, without salts, is retained on the surface by mosses and vegetable residues, creating a saturated environment. In raised bogs we can distinguish a deeper, much decomposed layer of dark colour (brown peat) and a slightly decomposed, shallower layer of a light colour (blond peat). Both of the peats are characterized by good structural stability, very low availability of nutrients and acidic pH whilst they mainly differ in their structure (Table 4.2).
Brown peats, with very small pores, have a higher water-retention capacity and less free porosity for air and have higher CEC and buffer capability. Physical characteristics vary in relation to the particle size that allows water absorption from 4 up to 15 times its own weight. Raised bogs usually satisfy the requirements needed for a good substrate. Moreover, they have constant and homogenous properties, and so they can be industrially exploited. However, the use of these peats requires pH corrections with, e.g. calcium carbonate (CaCOsub3/sub). Generally, for a Sphagnum peat with a pH 3—4, 2 kg msup-3/sup of CaCOsub3/sub should be added to increase the pH for one unit. Attention must be paid to avoid the complete drying of the substrate. It should also be taken into account that peat is subjected to microbiological decomposition processes which, over time, may increase water-retention capacity and reduce the free porosity.
Fen bogs are mainly present in temperate areas (e.g. Italy and western France), where Cyperaceae, Carex and Phragmites are dominant. These peats are formed in the presence of stagnant water. The oxygen, salts and calcium content in the water allow for a faster decomposition and humification, compared to that which occurs in the raised bogs. This results in a very dark, brown to black peat with a higher nutrient content, in particular nitrogen and calcium, a higher pH, higher bulk density and much lower free porosity (Table 4.2). They are rather fragile in the dry state, and have a remarkable plasticity in the humid state, which confers high susceptibility to compression and deformation. The carbon/nitrogen (C/N) ratio is generally between 15 and 48 (Kuhry and Vitt 1996; Abad et al. 2002). Because of its properties, black peat is of low value and is not suitable as a substrate, but can be mixed with other materials.
It should be noted that in some countries there is a drive to reduce peat use and extraction to reduce environmental effects and various peat substitutes have been identified with varied success.
Coconut fibre (coir) is obtained from removing the fibrous husks of coconuts and is a by-product of the copra (coconut oil production) and fibre extraction industry, and is composed almost exclusively of lignin. Before use, it is composted for 2—3 years, and then it is dehydrated and compressed. Prior to its use, it must be rehydrated by adding up to 2—4 times of its compressed volume with water. Coconut fibre possesses chemical—physical characteristics that are similar to blond peat (Table 4.2), but with the advantages of having a higher pH. It also has a lower environmental impact than peat (excessive exploitation of peat bogs) and stone wool where there are problems with disposal. This is one of the reasons why it is increasingly preferred in soilless systems (Olle et al. 2012; Fornes et al. 2003).
Organic substrates which are derived from wood or its by-products, such as bark, wood chips or saw dust, are also used in global commercial plant production (Maher et al. 2008). Substrates based on these materials generally possess good air content and high saturated hydraulic conductivities. The disadvantages can include low water-retention capacities, insufficient aeration caused by microbial activity, inappropriate particle-size distribution, nutrient immobilization or negative effects due to salt and toxic compound accumulations (Dorais et al. 2006).
This category includes natural materials (e.g. sand, pumice) and mineral products derived from industrial processes (e.g. vermiculite, perlite) (Table 4.3).
Table 4.3 Main chemical—physical characteristics of inorganic substrates used in soilless systems
table thead tr class="header" thSubstrates/th thBulk density (kg msup3/sup)/th thTotal porosity (%vol)/th thFree porosity (%vol)/th thWaterretention capacity (%vol)/th thCEC (meq %)/th thEC (mS cmsup1/sup)/th thpH/th /tr /thead tbody tr class="odd" tdSand/td td1400–1600/td td40–50/td td1–20/td td20–40/td td20–25/td td0.10/td td6.4–7.9/td /tr tr class="even" tdPumice/td td450–670/td td55–80/td td30–50/td td24–32/td td–/td td0.08–0.12/td td6.7–9.3/td /tr tr class="odd" tdVolcanic tuffs/td td570–630/td td80–90/td td75–85/td td2–5/td td3–5/td td–/td td7.0–8.0/td /tr tr class="even" tdVermiculite/td td80–120/td td70–80/td td25–50/td td30–55/td td80–150/td td0.05/td td6.0–7.2/td /tr tr class="odd" tdPerlite/td td90–130/td td50–75/td td30–60/td td15–35/td td1.5–3.5/td td0.02–0.04/td td6.5–7.5/td /tr tr class="even" tdExpanded clay/td td300–700/td td40–50/td td30–40/td td5–10/td td3–12/td td0.02/td td4.5–9.0/td /tr tr class="odd" tdStone wool/td td85–90/td td95–97/td td10–15/td td75–80/td td–/td td0.01/td td7.0–7.5/td /tr tr class="even" tdExpanded Polystyrene/td td6–25/td td55/td td52/td td3/td td–/td td0.01/td td6.1/td /tr /tbody /table
Source: Enzo et al. (2001)
Sands are natural inorganic material with particles between 0.05 and 2.0 mm diameter, originating from the weathering of different minerals. The chemical composition of sands may vary according to origin, but in general, it is constituted by 98.0—99.5% silica (SiOsub2/sub) (Perelli et al. 2009). pH is mainly related to the carbonate content. Sands with lower calcium carbonate content and pH 6.4—7.0 are better suited as substrate material because they do not influence the solubility of phosphorus and some microelements (e.g. iron, manganese). Like all mineral-origin substrates, sands have a low CEC and low buffering capability (Table 4.3). Fine sands (0.05—0.5 mm) are the most suitable for use in hydroponic systems in mixtures 10—30% by volume with organic materials. Coarse sands (>0.5 mm) can be used in order to increase the drainage capacity of the substrate.
Pumice comprises aluminium silicate of volcanic origin, being very light and porous, and may contain small amounts of sodium and potassium and traces of calcium, magnesium and iron depending on the place of origin. It is able to retain calcium, magnesium, potassium and phosphorus from the nutrient solutions and to gradually release these to the plant. It usually has a neutral pH, but some materials may have excessively high pH, good free porosity but low water-retention capacity (Table 4.3). The structure however tends to deteriorate fairly quickly, due to the easy breaking up of the particles. Pumice, added to peat, increases the drainage and aeration of the substrate. For horticulture use, pumice particles from 2 to 10 mm in diameter are preferred (Kipp et al. 2001).
Tuffs derive from volcanic eruptions, with particles ranging between 2 and 10 mm diameter. They may have a bulk density ranging between 850 and 1100 kg msup-3/sup and a water-retention capacity between 15% and 25% by volume (Kipp et al. 2001).
Vermiculite comprises hydrous phyllosilicates of magnesium, aluminium and iron, which in the natural state have a thin lamellar structure that retains tiny drops of water. Exfoliated vermiculite is commonly used in the horticultural industry and is characterized by a high buffer capability and CEC values similar to those of the best peats (Table 4.3), but, compared to these, it has a higher nutrient availability (5—8% potassium and 9—12% magnesium) (Perelli et al. 2009). NHsub4/subsup+/sup is especially strongly retained by vermiculite; the activity of the nitrifying bacteria, however, allows the recovery of part of the fixed nitrogen. Similarly, vermiculite binds over 75% of phosphate in an irreversible form, whereas it has low absorbent capacity for Clsup-/sup , NOsub3/subsup-/sup and SOsub4/subsup-/sup. These characteristics should be carefully assessed when vermiculite is used as a substrate. The vermiculite structure is not very stable because of a low compression resistance and tends to deteriorate over time, reducing water drainage. It can be used alone; however, it is preferable to mix it with perlite or peat.
Perlite comprises aluminium silicate of volcanic origin containing 75% SiOsub2/sub and 13% Alsub2/subOsub3/sub. The raw material is crushed, sieved, compressed and heated to 700—1000 ˚C. At these temperatures, the little water contained in the raw material turns into vapour by expanding the particles into small whitish-grey aggregates which, unlike vermiculite, have a closed cell structure. It is very light and possesses high free porosity even after the soaking. It contains no nutrients, has negligible CEC and is virtually neutral (Table 4.3) (Verdonk et al. 1983). pH, however, can vary easily, because the buffer capacity is insignificant. pH ought to be controlled via the quality of the irrigation water and should not fall below 5.0 in order to avoid the phytotoxic effects of the aluminium. The closed cell structure allows water to be held only on the surface and in the spaces between the agglomerations, so the waterretention capacity is variable in relation to the dimensions of the agglomerations. It is marketed in different sizes, but the most suitable for horticulture are 2—5 mm diameter. It can be used as a substrate in rooting beds, because it ensures good aeration. In mixtures with organic materials, it enhances the softness, permeability and aeration of the substrate. Perlite can be reused for several years as long as it is sterilized between uses.
Expanded clay is obtained by treating clay powder at about 700 C. Stable aggregates are formed, and, depending on the used clay material, they have variable values with regard to CEC, pH and bulk density (Table 4.3). Expanded clay can be used in mixtures with organic materials in the amounts of about 10—35% by volume, to which it provides more aeration and drainage (Lamanna et al. 1990). Expanded clays with pH values above 7.0 are not suitable for use in soilless systems.
Stone wool is the most used substrate in soilless cultivation. It originates from the fusion of aluminium, calcium and magnesium silicates and carbon coke at 1500—2000 ˚C. The liquefied mixture is extruded in 0.05 mm diameter strands and, after compression and addition of special resins, the material assumes a very light fibrous structure with a high porosity (Table 4.3).
Stone wool is chemically inert and, when added to a substrate, it improves its aeration and drainage and also offers an excellent anchorage for plant roots. It is used alone, as a sowing substrate and for soilless cultivation. The slabs used for the cultivation can be employed for several production cycles depending on quality, as long as the structure is able to guarantee enough porosity and oxygen availability for root systems. Usually, after several crop cycles, the greater part of substrate porosity is filled with old, dead roots, and this is due to the compaction of the substrate over time. The result is a then a reduced depth of substrate where irrigation strategies may need adaptation.
Zeolites comprise hydrated aluminium silicates characterized by the capacity to absorb gaseous elements; they are high in macro- and microelements, they have high absorbent power and they have high internal surface (structures with 0.5 mm pores). This substrate is of great interest as it absorbs and slowly releases Ksup+/sup and NHsub4/subsup+/sup ions, whilst it is not able to absorb Clsup-/sup and Nasup+/sup, which are hazardous to plants. Zeolites are marketed in formulations which differ in the N and P content and which can be used in seed sowing, for the rooting of cuttings or during the cultivation phase (Pickering et al. 2002).
Synthetic materials include both low-density plastic materials and ion-exchange synthetic resins. These materials, called "expanded", because they are obtained by a process of dilation at high temperatures, are not yet widely used, but they possess physical properties suitable to balance the characteristics of other substrates.
Expanded polystyrene is produced in granules of 4—10 mm diameter with a closed cell structure. It does not decompose, is very light and has a very high porosity but with an extremely low water-retention capacity (Table 4.3). It has no CEC and virtually zero buffer capability, so it is added to the substrate (e.g. peat) exclusively to improve its porosity and drainage. The preferred particle size is 4—5 mm (Bunt 2012).
Polyurethane foam is a low-density material (12—18 kg msup-3/sup) with a porous structure that allows absorption of water equal to 70% of its volume. It is chemically inert, has an almost neutral pH (6.5—7.0), does not contain useful nutrients available to plants and does not undergo decomposition (Kipp et al. 2001). In the market it is possible to find it in the form of granules, rooting cubes or blocks. Like a stone wool, it can also be used for soilless cultivation.
Mixed substrates can be useful to reduce overall substrate costs and/or to improve some characteristics of the original materials. For example, peat, vermiculite and coir can be added to increase water-retention capacity; perlite, polystyrene, coarse sand and expanded clay to increase free porosity and drainage; blonde peat to raise the acidity; higher quantities of organic material or suitable amounts of clay soil to increase CEC and buffer capability; and low decomposable substrates for increased durability and stability. The characteristics of the mixtures rarely represent the average of the components because with the mixing the structures are modified between the individual particles and consequently the relationship of physical and chemical characteristics. In general, mixtures with a low nutrient content are preferable, in order to be able to better manage cultivation. The right relationship among the different constituents of a mixture also varies with the environmental conditions in which it operates. At high temperatures it is rational to use components that possess a higher water-retention capacity and do not allow fast evaporation (e.g. peat) and, at the same time, are resilient to decomposition. In contrast, in humid environments, with low solar radiation, the components characterized by high porosity are preferred to ensure good drainage. In this case, it will be necessary to add coarse substrates such as sand, pumice, expanded clay and expanded polystyrene (Bunt 2012).