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ThesisItem Open Access EVALUATION OF CRITICAL LIMIT OF ZINC AND RESPONSE OF RICE TO ZINC APPLICATION IN SOILS OF ASSAM(AAU, Jorhat, 2014-07) Goswami Kandali, Gayatri; Barua, N. G.Georeferenced and representative soil samples were collected from 14 districts of Assam for assessment of zinc status of soils. Altogether, 4110 numbers of surface (0-0.15m) soil samples were collected from rice growing fields of fourteen districts randomly at an interval of 2 to 2.5 km. using Global Positioning System. Thirty numbers (30) of surface (0-0.15m) soil samples were collected from rice growing fields at block level from each district randomly at an interval of 2 to 2.5 km. Based on DTPA-Zn content , soils were categorized as low, medium and high. Nutrient index and fertility ratings were determined following the procedure of Bajaj & Ramamoorty (1969). The highest Zn deficiency (32.21 %) was observed in Lakhimpur district and the lowest ( 2.66 %) in Dhemaji district. The lowest nutrient index (NI) of 1.38 in Nalbari district and the highest (1.63) in Dibrugarh district were recorded. Deficiency of zinc as a whole in the state of Assam represented by the fourteen districts was 26.22%. Twenty five georeferenced bulk surface soil samples (0-30 cm) with variable zinc status were collected from rice growing areas of Assam representing the major soil orders viz. Inceptisol, Alfisol and Entisol to find out the critical limit of Zn in rice soils. A pot culture experiment was conducted taking these soils with four levels of zinc viz., 0,2,4 and 6 kg Zn/ha to find out the response of rice (cv. Ranjit) to applied zinc and the critical limit of zinc in soils and crop. Dry matter yield of rice in control pots ranged between 7.5 to 21.5 g/pot with a mean value of 13.71g/pot. The mean dry matter yield increased significantly over control with rates of zinc application. The highest was recorded in 4 kg/ha of applied zinc. Bray’s percent yield ranged from 57.18 to 95.7 and the highest Bray’s percent yield of 95.7 was observed in the soil having DTPA-Zn of 0.4mg/kg and the lowest value of 57.18 was observed in the soil with DTPA-Zn 2.49 mg/kg. Higher zinc concentration corresponds to higher dry matter yield and higher zinc uptake. The bulk soil samples were also extracted by different extractants viz., DTPA, 0.5N HCl, 0.1N HCl, Ammonium Acetate, EDTA- Ammonium Carbonate, and Ammonium Bicarbonate-DTPA( AB-DTPA), 0.04M EDTA and 2M MgCl2 solution to find out a suitable extractant / method for available zinc in soil . DTPA showed the highest correlation with all the plant parameters followed by AB-DTPA, 0.05 N HCl and EDTA- Ammonium Carbonate. The critical limits( Cate & Nelson,1965) of DTPA, 0.5N HCl , 0.1N HCl, Ammonium Acetate, EDTA- Ammonium Carbonate, AB-DTPA, 0.04M EDTA and 2M Mgcl2 in soil were found to be 1.24, 1.25, 4.8, 0.9, 2.8, 1.74, 2.8 and 0.37mg/kg respectively. The critical limit of Zn for rice was 35 ppm based on DTPA method. In order to study the distribution of different forms of zinc in rice soils and their relative importance to zinc nutrition of rice , Zn fractions of bulk soil samples were extracted by adopting the sequential extraction procedure of Murthy(1982).Water soluble and exchangeable Zn(WSEX-Zn) fraction in soils ranged from 0.32 to 1.92 ,Complexed Zn (COMP-Zn) from 1.46 to 4.5 , Amorphous Sesquioxide bound Zn from 1.30 to 6.21,Crystalline Sesquioxide bound Zn ( CBD-Zn) from 0.45 to2.25 and Residual Zn(Res-Zn) from 101.79 to285.28 mg kg-1 with mean values of 0.87, 2.71, 2.92, 1.12 and 186.98 mg kg-1, respectively. WSEX-Zn, COMP-Zn, Amorphous Sesquioxide bound Zn, CBD-Zn and Res-Zn constituted 0.45, 1.57, 1.62, 0.63 and 95.70 % of total Zn of soils. WSEX-Zn , COMP-Zn and Amorphous Sesquioxide bound Zn was positively and significantly correlated with clay, OC and CEC of soils. However, CBD-Zn was significantly negatively correlated with pH (-0.522**) and Res-Zn was positively correlated with organic carbon (0.425*) and clay (0.397*) c ontent of the soils. Multiple regression of physico-chemical properties of soil with different Zn-fractions showed the highest R2 value (0.616) with COMP-Zn. Multiple regression of different Zn fractions with plant parameters showed the highest contribution of WSEX-Zn to Zn concentration (76.78%), Zn-uptake ( 63.03 %) and dry matter yield ( 36.15 %) of rice. Path analysis of different Zn-fractions with Zn-concentration in rice showed the highest direct effect (0.5610) of WSEX-Zn on Zn concentration and indirect effect of COMP-Zn (0.3731) and Amorphous Sesquioxide bound Zn (0.2508) via WSEX-Zn. A field experiment was conducted for two seasons (2011 and 2012) in a Zn deficient field of the Instructional cum Research Farm of Assam Agricultural University, Jorhat, to study the response of sali rice (var- Ranjit) to zinc application with six levels of ZnSO4 (0, 5, 10, 15, 20 ,25 and 30 Kg/ha) which corresponds to 0, 1.05, 2.1, 3.15, 4.2, 5.25, and 6.3 Kg Zn ha-1 in a randomised block design with three replications. Addition of 25 kg ZnSO4 ha-1 showed the highest increase in plant height by 3.46 %, total tillers by 42.05%, effective tillers by 46.65%, filled grains by 26.86%, and thousand grain weight by 4.96% over control. The percentage of chaffy grain was also lower by 24.43% over control. The grain yield varied from 26.4 q/ha to 45.9 q/ha and straw yield ranged from 55.1 q/ha to 66.4 q/ha due to zinc application. The highest grain yield (45.9 q/ha) and straw yield (66.4 q/ha) was recorded in 25 kg ZnSO4 ha-1 which was significantly higher than all other treatments. Grain yield varied from 34.47 to 73.86%, while straw yield varied from 3.62 to 20.51 % over control. Maximum mean zinc uptake of 97.57 and 311.75 g ha-1 in grain and straw was recorded in 25 kg ZnSO4 ha-1, respectively. The residual zinc was found to be lower in the second year compared to the first year in all the treatments. The treatment with 25kg ZnSO4/ha registered maximum net returns (Rs.23,198.7) and the highest benefit:cost ratio (2.43)