**Abstract**

Synthetic polymer hydrogels and modified natural polymer hydrogels are widely and increasingly used in agriculture, health care textiles, effluent treatment, drug delivery, tissue engineering, civil concrete structure, etc. Among them, the use of hydrogels in agricultural and horticultural sectors as matrices for the controlled release of water, various primary and secondary nutrients has drawn significant attraction from researchers, scientists, and industry persons due to their smartness with reference to controlled release characteristics based on plant requirement. Since the use of these hydrogels for controlled release application ensures the minimum utilization of water and plant nutrients in fields. Besides, this will bring down the overloading of fertilizer, soil contamination, and water pollution such as eutrophication, nitrate pollution, and micronutrient imbalance. This chapter is focused on the class of hydrogels that are used for the controlled release application in the agricultural and horticultural sectors as matrices, the possible methods of fine-tuning their structures for improving their fertilizer uptake and release behavior, safety aspects, and environmental issues.

**Keywords:** swellability, controlled release, reusability, environmental protection, water conservation

### **1. Introduction**

The smart polymers hydrogels are the class of functional polymers, which finds extensive use [1, 2] in diverse areas like agricultural, medical, pharmaceutical, effluent treatment, textile, etc. They have physicochemically crosslinked three-dimensional network, which are derived from water-soluble acrylic monomers, crosslinkers and natural pre-polymers. These smart hydrogels are capable of imbibing and retaining water or aqueous fluids such as urine, blood, electrolyte solution, etc. to the extent of 200 g to 1–2 kg of fluids without dissolving [3–5]. This hydrophilic nature of hydrogel leads to managing drought conditions in arid and semi-arid regions as a matrix for the controlled release of water and fertilizers [5]. To serve this, polymers with different chemical architecture are essential for diverse soil characteristics [5].

Agrochemicals such as primary and secondary fertilizers are used to hike crop yield with substantial quality foodstuff [6]. However, the traditional method of growing foodstuffs using synthetic fertilizers will not ensure a high-quality

environment [6]. Depending on the method of application and climatic conditions, about 90% of conventionally applied fertilizers never reach their objectives to realize the desirable biological response at the precise time and in the quantities required [6]. Such a mode of application provides a higher initial concentration than required for quick results. The conventional method of fertilizer amendment provides an initial concentration far above that required for immediate results to ensure the availability of sufficient nutrients. But such overdosing will result in waste of fertilizers [6] and produce undesirable side effects in the environment. Hence, there is a need for more controlled application of fertilizer, affording lower amounts of active ingredients without diminishing the efficacy. Controlled-release formulations were used to maintain an effective local concentration of active ingredients in the soil and to reduce runoff [6]. Besides, the application frequency required in the growing season could be minimized through controlled release technology. The controlled release was defined [6] as a technique or a method by which water or active chemicals were made available to a specified target at a definite rate and duration designed to accomplish an intended effect [7–15]. The method of choice to achieve controlled release in a particular application depends on the cost, release rate, potency and properties of the active compounds [14–16]. This chapter addresses the synthesis, characterization and controlled release applications of synthetic and natural polymer modified hydrogels in agriculture as matrices [16], different types of hydrogel used for controlled release, advantages, limitations and challenges.
