Choosing the right substrate is crucial to the success of hydroponic growing, as it influences plant health, irrigation and drainage efficiency, as well as nutrient availability. Below are the key parameters to consider when assessing the suitability of a substrate for hydroponic growing.
- Water Retention, Porosity and Drainage
An optimal substrate must be able to retain enough water to provide the plants with a constant supply of moisture, but it must also allow good drainage to avoid waterlogging and root suffocation. The relationship between water retention and drainage must be balanced to avoid problems such as salt accumulation and lack of oxygen to the roots. In simple terms, it is generally considered that a substrate should have around 70% of its volume made up of macro- and micropores, these in turn being equally distributed. To evaluate this, the water release curve of the substrate is used, which determines the existing volume of water, air and solid fraction according to the water matrix tension of the substrate. An ideal substrate would have a pore space of 75-80%, a bulk density of less than 400 g/L, an aeration capacity of 20-30% of the volume and a percentage of easily usable water (between 10 cm a.c. and 50 cm a.c. of matrix tension) of around 20-30% of the volume.
- Ion Exchange Capacity
The substrate must be able to retain and release nutrients efficiently so that plants can access them when they are needed. The nutritional capacity of the substrate is determined by the parameter Cation Exchange Capacity (CEC), which represents the capacity of the cantions of the different nutrient elements to be retained on the negatively charged surface of a solid particle of substrate and which can therefore be stored in the substrate and released into the substrate solution and be available for absorption by the roots. A good CEC is considered to be above 20 meq/100g substrate.
- Stability and Durability
A substrate must be sufficiently stable and durable to support plants throughout their growth cycle. Biodegradation reactions of organic substrates affect the availability of nutrients, especially nitrogen, creating a competition between the micro-organisms in the substrate and the plant if the material from which the substrate is derived is easily decomposable. It also affects the physical properties, because it would lead to a situation where the macropores of the substrate would tend to shrink and the aeration capacity of the roots would be reduced.
- pH and Neutrality
The pH of the substrate influences the availability of nutrients for plants. A substrate that has the ability to maintain an adequate pH for plant growth can prevent nutrient deficiencies and toxicity problems. An ideal pH is considered to be between 5.2 and 6.3, as this is the pH range in which nutrient uptake is most available, depending, of course, on the species to be grown.
- Availability and Cost
The choice of substrate should also take into account local availability and cost. Some substrates may be expensive or difficult to obtain in certain regions, which may affect the economic viability of the crop.
- Crop Compatibility
Different crops may have different requirements in terms of substrate. Some crops may prefer lighter substrates, while others may thrive in denser substrates. It is important to choose a substrate that is compatible with the type of plant to be grown, which is why versatility in terms of the particle size that a substrate can have is so important, as particle size leads to more or less aeration, which contributes to the plant adapting correctly to substrate growing. The most versatile substrates in terms of particle size are perlite, coconut fibre and almond shells.
- Environmental Sustainability
Consideration of the environmental sustainability of the substrate is essential. Some substrates can have a greater environmental impact due to their extraction, manufacture and disposal, such as substrates of mineral origin. Opting for renewable and environmentally friendly substrates is an important consideration.
In summary, the selection of an optimal substrate for hydroponic cultivation is a comprehensive process that involves the evaluation of several interrelated parameters. A careful balance between water retention, drainage, aeration, nutrient availability and other factors is essential to ensure healthy plant growth and long-term success in hydroponics.
