Hydrogels are hydrophilic cross-linked polymer chains.
They are formed through physical, ionic or covalent interactions.
Due to hydrophilicity, hydrogels have the ability to absorb water and swell growing in weight and size.
Depending on the hydrogel type they can absorb and hold up to even 1500 times their weight. Similarly, they can release this water.
The speed of water absorption depends on the type of the hydrogel. Some of the synthetic polymers can take up to 6 hours to swell to complete hydration while natural ones may require only 2 h.
The volume transitions – absorbing and releasing water may depend on the external stimuli which can be physical (temperature, electric or magnetic field, light, pressure, and sound) or chemical (pH, solvent composition, ionic strength, and molecular species).
Hydrogels have various applications: in agriculture, drug delivery, coal dewatering, food additives, tissue engineering…
Hydrogels are extremely useful in agriculture since they can save water and prevent land erosion.
They prevent the soil from being washed away and hydrate it, helping it retain moisture and increase the water-holding capacity. This makes plants survive for longer periods in drought or require less frequent irrigation.
Frequent and intense irrigation can damage the soil making it erode, form crust or harden. The hydrogel gives soil stability, increases permeability and enables better plant growth.
Adding hydrogels to the surface of the soil increases the soil water-holding capacity and the rainfall percolates the soil quite easily.
Not only this, but they reduce the need of fertilization, since the nutrient loss is prevented by reducing runoff.
Alternatively, some hydrogels are made as fertilizers and even have controlled water release so that the dose of the fertilizer is adjustable in time. The nutrient is available for the plant over a longer period of time rather than a rapid availability that ammonium nitrate, ammonium phosphate or potassium chloride provide.
The controlled or slow release of fertilizers from hydrogels do not have official differences. They are achieved by coating or encapsulating the conventional soluble fertilizers. The coatings can be water insoluble, semi permeable or impermeable with pores.
The same principles apply to pesticides and herbicides .
The use of hydrogels is extremely beneficial in cases where post plantation irrigation is limited.
Water release is controlled so that the fertilizer is released according to plants’ needs.
Hydrogels can be made of:
combination of natural and synthetic polymers.
The synthetic polymers are mechanically stronger and durable and can absorb more water. They can, therefore be more efficient in agriculture as they hold more water and lost longer.
On the other hand, hydrogels made from natural polymers are biodegradable and often do not leave any bi-products so they are more environmentally friendly.
Drying out before the next hydration makes the hydrogels less efficient so good irrigation is a necessity for the maximum effect.
It is noticed that in saline soil the hydrogels have lower efficiency.
Hydrogels can be applied by being mixed with the soil or by spraying.