Introduction

Super Absorbent Polymers, also known as SAP, hydrogel, absorbent polymers, absorbent gels, super soakers, super slurpers, water gel, is a new type of macro molecular synthetic water absorbing polymer material. It has a water uptake potential as high as 100,000% of its own weight in a short period of time by osmosis and form granules in soil to enhance soil properties. SAPs are generally white sugar-like hygroscopic materials that swell in water to form a clear gel made of separate individual particles and can retain moisture even under pressure without risk of conflagration or rupturing/blasting.

Super Absorbent Polymers used in agriculture are mostly prepared from acrylic acids and a cross-linking agent like potassium by solution or suspension polymerization. The polymer so formed is called a polyacrylate whose swelling capacity and gel modulus depends greatly on the quantity and type of cross-linker used. Polyacrylates are non-toxic, non-irritating and non-corrosive in nature and tested to be biodegradable with a degradation rate of 10%-15% per year. They demonstrate high water absorbance potential and can freely release 95% of the same under suction pressure by plant roots.

Application and Benefits

The three most common soil conditions that hinder plant growth and crop yield are low water retention capability, high evapo-transpiration rate and soil moisture leaching. Apart from these, factors like unforeseen drought conditions, degradation and salination, overuse of synthetic fertilizers and pesticides and improper irrigation practices severely affect soil and plants, often rendering permanent damage to soil biota as well.

Desirable characteristics for applications in agriculture

High absorption capacity in saline and hard water conditions
Optimized absorbency under load (AUL)
Lowest soluble content and residual monomer
Low price
High durability and stability in the swelling environment and during storage
Gradual biodegradability without formation of toxic species
pH‐neutrality after swelling in water
Photostability
Re‐wetting capability

Hydrogel agriculture technology, as it is popularly called, has the following advantages.

Improves soil quality, preserves water and resists drought stress
Increases seed sprouting and seedling development leading to better farm success
From the environmental aspects, it is non-polluting and biodegradable, helps in reducing irrigation frequency and water consumption and creates a simple cyclic process to provide water directly to roots and prevent soil compaction.
In agriculture and agroforestry, SAPs act as micro water reservoirs at plant roots. They absorb natural and supplied water 400-500 times their own weight and release it slowly on account of root capillary suction mechanism thus preventing water loss in soil by leaching and evaporation.
SAPs form a consistent cyclic process of absorption and release of water; the water so released can provide optimum moisture for quick germination and seedling maturation. Thus it reduces seedling mortality by several folds in nurseries.
In cold regions, death during germination and maturation is common due to moisture freezing in & around plant root tissue. Absorbed moisture in hydrogels does not freeze and makes easy accessibility to plants. It also regulates seedling growth temperature preventing death by freezing.
SAPs can help save water and labor by reducing irrigation frequency, help overcome drought conditions and act as soil conditioners, prevent leaching in sandy soils, runoffs in mountainous and sloping fields, improve virescence efficiency and restore soil biota.
SAPs can reduce overuse of fertilizers and pesticides in fields. The chemicals so absorbed with water are slowly released thus extending the operational life and uptake efficacy by root systems.
SAPs act as soil matter flocculants. They closely bind loose soil thus forming loams that can help better root latching. Simultaneously, the repeated absorb-release mechanism prevents over compaction of soil minerals and provides space for aeration and development of soil edaphon.
It has a wide area of application ranging from agriculture, forestry, industrial planting, municipal gardening, drought management, water conservation, It helps reduce soil erosion by surface run-offs, fertilizer and pesticide leaching to ground water, reducing cost of water and irrigation and success rate at growth and high yields of crops.

Examples of SAPs

Pusa Hydrogel

The Indian Agricultural Research Institute, New Delhi has developed an absorbent polymer called "Pusa Hydrogel" to meet the requirements of water productivity in Agriculture.

Salient features of Pusa Hydrogel

Natural polymer backbone based water absorber based on cross-linked Potassium polyacrylate polymer
Exhibits maximum absorbency at temperatures (40- 50 ⁰ C) characteristic of semi-arid and arid soils.
Absorbs water 400 times its dry weight and gradually releases the same.
Stable in soil for a minimum period of one year
Less affected by salts
Low rates of soil application – 1-2 kg / ha for nursery horticultural crops; 2.5-5 kg/ ha for field crops
Reduces leaching of herbicides and fertilizers
Improves physical properties of soils and the soil less media

Improves seed germination and the rate of seedling emergence
Improves root growth and density
Helps plants withstand prolonged moisture stress
Reduces nursery establishment period
Reduces irrigation and fertilization requirements of crops with improved input use efficiency
No undesirable effect on crops and soil as reported by experimenters and farmers.

How to use?

During Sowing

Recommended Dose : 1 - 2 kg per Acre.
Method of application :

Apply the Pusa Hydrogel at a depth of 6 to 8 inches of soil. For clay soil, apply at a depth of 4 inches from the soil surface, preferably at the root zone.
For Field Crops:

Prepare the field for sowing.
Add 1 kg Pusa Hydrogel to 10kg of fine dry soil and mix properly.
The mixture should be applied along with seeds /fertilizer or in the opened furrows before sowing.
For best results, the Pusa Hydrogel should be applied close to seeds.

In Nursery Bed for Transplants:

Apply 2 gm mix of Pusa Hydrogel in nursery bed uniformly for every sq. Meter. It should be placed at a depth of 2 inches of the nursery bed from the top.
In pot culture , mix 3 – 5 g / kg of soil before planting

While Transplanting

Thoroughly mix 2 gms of Pusa Hydrogel in one litre of water to ensure a free flowing solution and allow it to settle for half an hour.
Dip the roots of the transplanting plant in the solution and then transplant in the field.

A semi‐synthetic High capacity super absorbent polymer based on starch from CTCRI

Key characteristics

Uses cassava starch backbone (other starches can also be used, but absorbency may vary)
Contains no detectable level of the monomer, acrylamide.
Absorbency ranges from 400-425 g/g of the dry sample
Application rate: 0.5% based on soil weight in pots.
Maintains about 40-50% of absorbency even after 4-5 cycles of swelling- deswelling-reswelling.
Improves physical properties of soil such as porosity, water holding capacity, and electrical conductivity.
Improved soil nutrient and organic carbon status.
Improvement in soil microflora.
Reduces watering interval of plants, especially in pots.
Suitable for cultivation of medicinal plants, garden plants etc and application in nurseries.

Salient features of cassava starch based superabsorbent polymer

Colour	White
Form	Granules
Odor	Odorless
pH	6.9 ± 0.5
Solubility in water	Insoluble (swells in water)
Absorbency (g kg -1 )	42500
Nature	Partially hygroscopic

Source : CTCRI

Related resources

NPETL lecture

Applications of Super Absorbent Polymers in Agriculture