INTRODUCTION:

Hydrogels which are also known as superabsorbents, this are crosslinked polymers that can absorb 400 to 1500 times its original dry weight in water^[1] this makes approx 95 per cent of stored water available for crop absorption^[2]. When this hydrogel gets mix with the soil, it forms an amorphous gelatinous mass on hydration and is capable of absorption and desorption for prolong period of time, therefore helps in slow release of water in soil.^[3]

Hydrogels are a cross linked system and they may be strong or weak based on flow behaiviour in steady state^[4]. These are three dimensional structure obtained from synthetic or natural polymer which retain sufficient amount of water^[5].

The structure of this polymer is formed by hydrophilic groups upon hydration in aqueous environment. Because of the presence of three dimensional structure this are insoluble in water. Hydrophilicity in hydrogels is induced by the presence of hydrophilic groups, such as hydroxyl, carboxyl, amide, and sulfonic groups, along the polymer chain; whereas the cross-links are formed by either covalent bonds, electrostatic or dipole–dipole interactions.^[6]

Hydrogels have wide spread applications ranging from contact lenses, drug delivery, tissue engineering, wound dressings, soil water retention to disposable diapers. In many food industries edible gels are widely used mainly refer to gelling polysaccharides (i.e. hydrocolloids)^[7].

Water is one of important factor for supporting life. Agriculture use 70% of total fresh water and because of this we need increased irrigation efficiency for more productivity.^[3]Many nations have experienced drought stress, mass starvation, cessation of economic activity particularly in developing countries which is based on agriculture. Therefore study of polymers has helped to deal with this types of situations and lead to generate better facilities for same.

Johnson (1984) reported using hydrogels leads to contain more moisture in root part and therefore longer interval of time can be taken for irrigation. Under rainfed condition, crops can better withstand drought condition without moisture stress. The main factors are seed germination, crop production and growth. This will help to ensure uniform and healthy crop stand as well as achieve high crop yield. When water leaches out from hydrogel it losses some volume but it create pore for the space for air and water infiltration, storage and root growth. Soil porosity, soil permeability and water infiltration are the effect of hydrogels that reduce surface runoff and soil erosion, especially when soil forms semi hydrophobic crusts under compacted soil condition^[3]. Hydrogel reduced 15% irrigation necessity for crop improvement with better yield. The improvement in properties like water absorbency and water retention by hydrogel proved it’s essential in agricultural field.

The objective of this study was to investigate the growth of plants under SAPs condition, and compare with plants growth without the use of SAPs.

The largest consumer of water is our agricultural sector and the demand for water is constantly rising from sector like domestic, industry etc due to the rise in population and such uses can often be at the cost of agriculture. In large part of country irrigation is carried out for agriculture practices and in those crops just utilizes half of the water released and other hail part gets lost conveyance, application, runoff and evaporation.^[9]

APPLICATIONS OF HYDROGELS IN AGRICULTURE:

Since 1950s, agricultural sector concerned about improving physical properties of soil for increasing crop productivity. This led to the development of hydrogels^[10]. In early 1960s American Company, Union Carbide introduce super absorbents in market. This are the material which can absorbs 20 times more weight than its original weight and this are considered as superabsorbent^[12]. Later due to the development of more cross link polymer with high water retention capacity (400 times & even up to 2000 times of their weight) and comparatively low cost increases the interest for the use of polymer in agriculture.^[3]

The polymers are developed in such a way that it should have following properties which help in increasing soil physical properties i.e. Increasing both water-holding capacity and water use efficiency, also enhancing soil permeability and infiltration rates, reducing both irrigation frequency, and compaction tendency, reducing soil erosion and water run-of, increasing plant performance.^[10]

CHARACTERISTICS OF SUPERABSORBENT POLYMER- HYDROGEL:

There are two types of polymers used for marketing purposes:

(1) Water-soluble Polymers:

(2) Gel forming Polymers:

Water soluble Polymers are firstly developed polymer which aggregate together and stabilizes soils, reduce soil erosion and improves percolation. Examples of this type are homopolymers and copolymers such as poly (ethylene glycol), poly (vinyl alcohol), polyacrylates, polyacrylamide, poly (vinyl acetate-alt-maleicanhydride).^[10]

Except poly ethyleneglycol all other polymers are synthesized by polymerization process and the later one is prepared by petroleum industry and is of low cost than others. polyacrylamide is one of the most widely used polymer. Polyacrylamides are divided into two types of hydrogels: Soluble (linear) and Insoluble (cross-linked).

Soluble PAM gets easily dissolved in water and it reduces irrigation-induce erosion in agricultural field. Insoluble PAM do not get dissolve easily, and it forms gel when water is mixed to it and this type has application in garden, landscape, and nursery situations for retaining moisture. Insoluble PAM is used as superabsorbents or hydrating crystals, as they absorb water much more times than their original times^[14].

Calcareous non fertile land in France near Dijon is an example of PAM application. Tropical soil because of heavy rainfall suffers from problem like low aggregate stability and bulk density. And because of this water intake and storage decreases and surface drainage increases. The most effective solution against soil erosion is proved by PAM. Permeable layer of soil made by hydrogels makes soil stabilizes and thus reduces the runoff^[10].

Gel-forming polymers were first introduced in agriculture in early 1980s. Three types of water absorbing hydrogels are developed still now for agriculture purposes:

(1) Starch-graft copolymers

(2) Cross-linked polyacrylates

(3) Cross-linked polyacrylamides ^[10]

Water holding capacity of hydrogels depends upon five factors such as:

· Texture of soil

· Type of hydrogel

· Particle size

· Salinity of soil solution

· Presence of ions.

Cross-linked polyacrylamides can hold water 400 times of its original weight in water and release 95% of water retained in granules for the growth of plant^[10].

DIFFERENT FORM OF HYDROGEL:

Many scientific literatures are there since 1970 about chemical and physical structure, properties and application of the hydrogels. Recently many scientists are working on preparing hydrogels from biopolymers like guargum and chitosan^[6]. Barbucci et al.shown the preparation of porous hydrogel from guargum by cross linking with polyethylene glycol diglycidyl ether; and this hydrogel remain unaffected by PH of the solution in which it is present. Ozcelik et al. (1996) studied and prepared ultrathinchitosan –polyethyleneglycol based hydrogel films. This have application in corneal tissue engineering application. Xiao et al. (2007) investigated the properties of hydrogel films that was prepared from chitosan and hydroxypropyl guar gum; and noticed many effect like tensile strength improvement, elongation at break, when concentration of hydroxypropyl guargum in chitosan increased. Lim et al. shows application of hydrogels nanoparticals in transdermal drug delivery system by utilizing hyaluronic acid and polyethylene glycol as effective carriers in transdermal delivery systems.

Soppimath et al. prepared a series of anionic hydrogels from polyacrylamide grafted with guar gum and it has application in pH sensitive drug delivery systems. Huang and Xiao prepared hydrogel membrane from cationic guar gum with acrylic acid.

Fujioka et al. investigated the preparation of superabsorbent by using guar gum and succinic anhydride and also using 4-dimethylaminopyridine as the esterification and this lead to maximum absorbance of water, 200 g water/g dry gel. Wang et al. prepared superabsorbent hydrogel composites by graft polymerization process using guar gum, acrylic acid and attapulgite clay. guar gum-g-poly (acrylic acid)/attapulgite complex have showed the presence of many improved characteristics like water absorbing and water retention capability, swelling rate, pH-resistant property and reusability. Wang et al. prepared micro spherical hydrogels synthesized by free radical grafting mechanism using chitosan, acrylic acid, gelatin and N, N- methylene-bisacrylamide. Liu and Lin studied the properties of carboxymethyl and hexanoyl substituted chitosan based hydrogels and found having pH sensitivity.

Performance of gel on plant growth only depends upon the method of application used. Scientists’ attention is towards inserting NPK fertilizer directly in cross linked structure of hydrogel. Cross-linked polyacrylamide has application which acts as carrier molecule for fungicides, herbicides and insecticides.^[10]

EFFECT OF HYDROGEL IN ARID AND SEMI ARID REGION:

Only 42% of total dry land area is used for food grain production. Failure of crop yielding is mainly caused due to soil moisture. Hayat and Ali in (2004), reported that in arid and semi arid region yield is very less due to low and negligible moisture content.

In arid and semi arid region, uncertainty and increased temperature is prominent and therefore encourages using the water conservative irrigation technology of SAPs. This can absorb water, much more times than their dry weight which leads to improvement in moisture capacity of soil and plant will not face any water stress condition during drought and irrigation intervals. Woodhouse and Johnson, (1991), investigated that SAPs hold take 400-1500g of water which is much more than their original weight. Johnson and Veltkamo, (1985) found that the 95% of water retained in the granules are available to the plants which helps them in better growth. Irrigation necessity is also reduced from 15-50% when soil conditioning with SAPs with different proportion when used. SAPs are safe, bio degradable, nontoxic and inert with increased self life^[9].

On steep hill slopes planting of citrus plants is very difficult due to lack of supplementary irrigation and nutrients and thus the productivity decreased. Experiment was conducted by mandarin orchards of the village Mirem. Evaluation is done for growth of Khasi mandarin under the effect of hydrogels. The result was positive growth of plant using hydrogel application^[26].

VARIOUS EFFECTS OF HYDROGEL:

Johnson and Veltkamp (1985) investigated that using SAPs leads to the growth of plant. Mikkelsen et al. (1993) investigated that adding fertilizer solution with the polymer showed 45%reduction N leaching losses from soil. Fescue growth was seen for these studies and found that the growth was increased by 40% when fertilizer is mixed with polymer as compared to fertilizer alone. Magalhaes et al. (1987) studied that leaching of NH4, Ca, Mg, and K elements has been reduced due to the presence of polymer.

Sandolo et al. studied the effect of various range of temperature (7, 15, 25 and 37 ºC) on the rheological property of hydrogel prepared by using guar gum cross-linked with glutaraldehyde. With increase in moisture content hydrogels reduced the water stress of plants and thus resulting in plant growth with better performance^[8].

Batyuk et al. 1973 investigated that the yield increases and irrigation necessity decreases for sugar beet production using hydrogels. This is due to the following reasons better soil aeration, which enhances activity of microbes; it delayed the dissolution of fertilizers, it increases the uptake of nutrients by plants. Sendur et al. (2001) found increased in the number of fruits, fruits weight and yield per plant in tomato when hydrogel is used with soil, similar findings was seen by Dhumal (1993) in chilli and tomato and Cookson et al. (2001) in okra. Similarly, Yazdani et al. (2007) have seen the increased level in yield of soyabean productivity by using hydrophilic polymers (225 kg ha^-1) as compared to control, and in water stress condition yield level may be enchenced by using this hydrogels. Abdulaziz and Al-Harbi (1996) investigated that addition of hydrogels in cucumber plant under low soil moisture condition results in higher productivity. Similar finding was seen in in mahogany plant by Swenson (1993). In tomato plant due to increase in concentration of hydrophilic polymer root parameters like root length, root volume, root fresh, and dry weight gets increased^[3]. The rate of germination also give positive effect in tomato, lettuce, maize by the use of hydrogels.

Pesticide release devices are also prepared based on hydrogel application and number of patent based on this has been developed^[30]. Most of the acrylate based hydrogels are not biodegradable and causes pollution to the environment and therefore more attention is now giving on producing pollution free hydrogels^[31].

HYDROGEL EFFECT IN SOIL TEXTURE:

Positive effect of hydrogels is also depend upon soil texture. Fine texture soils can retain more water as coarse textured soil because in corsed textured large pores are present and therefore retain less amount of water.

With polyacrylamide addition the bulk density of loamy and sandy soil gets reduced as compared to control but there was small increase in claley soil. With increase in poly-acrylamide doses porosity also increases for clay loam and sandy soil^[3]. Nazrali et al. (2011) reported positive aspect of SAPs as high water retention capacity and protection during drought condition.

HYDROGEL EFFECT ON ECONOMICS:

Use of 1.75 g superabsorbent polymer per plant could be recommended for increasing per unit productivity and also net profit ^[3].Cost of hydrogel ranges from 150-160rs for 100gm,but if we go for irrigation only meter rolls cost starting from 250rs for 10m length, then amount of water we give and loss by plant also become one of the problem.

CONCLUSIONS:

The beneficial effects of hydrogels on agricultural field are a well-established fact. As in arid and semi arid region rainfall is uncertain and temperature is always high so irrigation facility cannot be maintained properly and thus yield is low and for this, the region encourages the use of water conservative irrigation technology of SAPs. SAPs having great water absorption capacity thus improve the moisture capacity of soil and plant will get appropriate amount of water, during the drought conditions, and between intervals of irrigation. There are numerous publications that shows positive effect on growth of various plant and also the productivity with the use of hydrogel. Hydrogel showed very suitable for agriculture use particular for short cycle growing crop.

Most of the farmers still do not know about this technology for various reasons and therefore need special attention. In some places SAPs are not available so farmers cannot be able to buy for agricultural purposes. So the challenges are many but if we adopt this technology it will be the part of long term water management program. As many applications of hydrogels are present in today’s world and modification in these will lead to success in various field like drug delivery, tissue engineering, pH resistant, greater swelling capacity, etc. Fertilizer solution are mixed with hydrogel helps in reducing N leaching loss from soil and further scientists are looking for inserting fertilizer directly in the cross linked structure of hydrogels. Other experimental investigation like use of SAPs to optimize the yield of cash crop to improve the economic and also to help the small farmer’s social conditions are ought to carried out.

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Received on 13.12.2018 Modified on 16.01.2019

Research J. Pharm. and Tech. 2019; 12(5):2566-2570.

DOI: 10.5958/0974-360X.2019.00431.1