REVIEW ARTICLE CURRENT SCIENCE, VOL. 102, NO. 9, 10 MAY 2012 1272 *For correspondence. (e-mail: samardattaiari@gmail.com) Use of modified clays for retention and supply of water and nutrients B. B. Basak 1,2 , Sharmistha Pal 1,3 and S. C. Datta 1, * 1 Division of Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New Delhi 110 012, India 2 Present address: Directorate of Medicinal and Aromatic Plant Research, Boriavi, Anand 387 310, India 3 Present address: Central Soil and Water Conservation Research and Training Institute, Research Centre, Madhya Marg, Chandigarh 160 019, India Clay minerals are natural materials well known for their role in retention and persistence of organic and inorganic compounds in soil. Clay minerals are being modified through different processes to alter their charge and surface properties tailored for specific purposes. Modified clays in general include pillared layered clays, organoclays, nanocomposites, acid and salt-induced and thermally and mechanically induced modified clays. Pillared organoclays and nanocompo- sites constitute a novel class of materials, mainly one kind of modification of expanding 2 : 1 clay minerals. Acid and salt treatments lead to modification of the catalytic properties of clays. Clay mineral–polymer nanocomposites are the hybrids which exhibit a change in composition and structure on a scale of nanometres. Modified clay imparts slow-release pro- perty to fertilizers and enhances the water-holding capacity of soil. Therefore, understanding the methods of clay modification and properties of modified clays may facilitate development of agricultural management systems that ensure long-term sustainability of soil resources. Keywords: Clay minerals, modified clay, nanocompo- sites, slow-release fertilizers. THE modification of cationic characteristics of soil clays for soil reclamation purposes is an ancient practice that is still in use today 1 . However, there has been steady pro- gress in clay materials technology through modification of the properties of natural clays, for example, changing from hydrophilic to hydrophobic, or forming clays with an interlayer characterized by selective pore sizes 2 . Recent work has shown that the preparation of clay mineral– polymer nanocomposites and superabsorbents, which are hybrids, leads to changes in composition and structure of the mineral over a nanometre scale 3 . There are works which show that the preparation of clay–polymer com- posites by the hybridization of inorganic clay minerals with organic superabsorbent polymer, leads to changes in composition and structure of the product which impart higher water-holding capacity, swelling rate and slow- release property. Superabsorbents are loosely crosslinked hydrophilic polymers (CPs) that can absorb and retain aqueous fluids up to thousand times their own weight. In the hybridized clay–polymer composite, CPs get fixed between the clay structure which imparts more stability to clay composite and improves the slow-release property of the product. Like natural clay, modified clay also imparts slow-release property to a fertilizer and enhances water- holding capacity of the soil. Preparation and evaluation of superabsorbent polymers with slow-release phosphate fertilizer (SAPSRPF) by Zhan et al. 4 have found to influ- ence water absorbancy and phosphorus release property. Incorporation of a small amount of pure montmorillonite into the copolymeric gel was reported to increase water absorbency 5 . Slow-release fertilizer with superabsorbent and moisture preservation (SRFSMP) property showed good retention and release of water and nutrient 6 . Studies with pillared clay-modified phosphate fertilizers reported decreased fixation and increased bioavailability of soil phosphorus 7 . In this review a general picture will be pro- vided on the modified clay materials and their mecha- nisms of interaction with water and fertilizer nutrient which will play a significant role in improving water re- tention and nutrient use efficiency in soil. Methods of clay modification Clay minerals are natural materials well known for their effect on retention and persistence of organic and inor- ganic compounds in soil. Modification of clay minerals like organoclay and organo-modified clay is the new ave- nue of clay mineral research. Actually these organic and inorganic hybrids are produced by exposing the purified clay to Na + and H 2 O, leading to separation of individual platelets; and on treatment with the polymer, it gets fixed between two platelets. The most important property re- garding the improved water and nutrient retention capac- ity of modified clays is the barrier property. In general, barrier property refers to the hindrance of natural release of any material from clay structure. The organically modified clay creates a unique structure with the compos- ite (termed as intercalated morphology) resulting into a complex microstructural diffusion pathway 8 . The interca- lated morphology and tortuous pathway of this composite limit natural movement of water molecules as well