Agriculture Reference
In-Depth Information
polymer network) is made up of two polymers, where one is cross-linked to the
polymeric chain of the other, thereby getting their chains intertwined at the molec-
ular level. Hydrogel may also be semi-IPN; in this case, the polymer is also formed
by the combination of two different polymers, but one in cross-linked form and
another linear (Yoshunari et al. 2005 ; Ma et al. 2007 ).
Over the last decades, natural hydrogels have been gradually replaced by
semisynthetic hydrogels, which exhibit greater durability, high capacity to water
absorption, higher mechanical strength, and biodegradability.
Within the various application areas, an aspect that has been highlighted is the
application of hydrogels in agriculture. Some studies reported in literature show
that hydrogels began to be studied conditioners from the 1980s (Willingham and
Coffey 1981 ; Wallace 1987 ; Sayed et al. 1991 ). Thereafter, it was shown that the
application of hydrogels optimizes water availability in soil, reduces nutrient losses
by leaching and percolation, improves aeration and soil drainage (Henderson and
Hensley 1986 ; Lamont and O
Connell 1987 ), increases the seedling budding index,
and accelerates the root development and plant aerial part, leading to a significant
increase in the final production per hectare (Nissen 1994 ).
Recently, several works started reporting hydrogels as carriers of nutrients. Guo
et al. ( 2005 ), in a study about encapsulation of urea fertilizer in starch hydrogels,
found that 40-70 % of nitrogen present in the hydrogel and urea capsule is released
into the soil, so it can be absorbed by plants. Mikkelsen et al. ( 1993 ) studied the
efficiency of hydrogels regarding the loss of nitrogen by leaching and concluded
that the presence of the hydrogel reduced by 45 % the nutrient losses by leaching
and increased the growth of a grass (test plant) by 40 % in comparison to grass
planted in standard conditions. Bajpai and Giri ( 2003 ) studied the potential of
controlled nutrients release in graft-polyacrylamide hydrogels without
carboxymethyl cellulose chains. The authors observed that the release was highly
dependent on the chemical structure of the hydrogel, pH, and temperature of
swelling.
Despite the good performance of hydrogels in agriculture, their application is
still limited to the final price and low biodegradability of the product, often
preventing this material from using in scaled-up applications. In order to improve
the potential of hydrogels in agriculture, various studies have been performed to
obtain hydrogels combined to polysaccharides (Nie et al. 2004 ; Leone et al. 2008 ).
Polysaccharides present a high number of hydroxyl and carboxylic groups that
could improve the hydrophilic character of hydrogels; they also increase the
hydrogel biodegradability because their structures have glycosidic groups, which
facilitate biodegradation by bacterial or fungal attacks (Leone et al. 2008 ; Wallace
1987 ).
Another strategy for the modification of hydrogel properties is to design it as a
clay-based composite or nanocomposite. As abovementioned, clay is widely pre-
sent in soil and has high hydrophilicity, high cation-exchange ability, and, there-
fore, high affinity to the hydrophilic hydrogel chains. Thus, clay minerals generally
can be properly incorporated into the polymeric network, during the hydrogel
synthesis, allowing improvements on the mechanical properties, but also on
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