Aggregate associated zinc and zinc fractionation after six years of rice wheat cropping system with different doses and frequency of zinc application
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Date
2019
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Dr. Rajendra Prasad Central Agricultural University, Pusa, Samastipur
Abstract
A six-year long-term field experiment was initiated during Kharif 2012-13 with rice-wheat cropping system under AICRP on micronutrients at nursery jhilli area of Dr. Rajendra Prasad Central Agricultural University, Pusa, Bihar. The experiment was planned in RBD with 13 treatments and 3 replications consisting of four doses of zinc i.e., 2.5, 5.0, 7.5, and 10.0 kg ha-1 applied as zinc sulphate (ZnSO4) in Kharif with three different combinations of frequencies i.e., in the first year, alternate year and every year of zinc application along with control. Soil samples were collected and analyzed after 6 years i.e. after the harvest of wheat crop in 2018.
Increasing doses and frequency of zinc application increased DTPA extractable zinc in all aggregate fractions and initial adsorption was more in small aggregate fractions than larger sized fractions. The order of DTPA extractable zinc was: 8 - 5 mm (1.01mg/kg) < 5 – 2 mm (2.37 mg kg-1) < 2 – 1 mm (3.81 mg kg-1) < 1 - 0.5mm (5.34 mg kg-1) < 0.5 - 0.25 mm (5.42 mg kg-1) < 0.25 - 0.1mm (6.13 mg kg-1).
Most of the total zinc remained in residual form (80.03%) followed by crystalline form (9.19%) and the quantity of plant available form like water soluble + exchangeable (0.6%), complexed (3.16%), organically bound (1.19%), amorphous
oxide bound zinc (6.6%) were very low. The order of dominance of different zinc fractions in soil was: Total-Zn (31.43 mg kg-1) > RES-Zn (25.13 mg kg-1) > CRY-Zn (2.89 mg kg-1) > ORG-Zn (2.09 mg kg-1) > COM-Zn (1.00 mg kg-1) > AMO-Zn (0.38 mg kg-1) > WS + EX-Zn (0.19 mg kg-1). All the fractions of zinc except crystalline and residual zinc had positive correlation with wheat grain yield and zinc uptake. Among all fractions Water soluble + Exchangeable and complexed fractions were the predominant forms that contributed most of the available zinc for wheat grain uptake and yield.
Improvement in soil physical properties viz. bulk density and aggregate stability coupled with increase in organic carbon content and nutrient availability viz. nitrogen, phosphorous, Sulphur with application of 7.5 kg Zn ha-1 in alternate year resulted in optimum wheat grain and straw yield (45.6, 76.5 q ha-1) in the sixth year of experiment and also for rice-wheat cropping system (982.9 q ha-1).
DTPA extractable zinc decreased with increasing depth of soil. The order of DTPA extractable zinc was 0-15 cm (0.99 mg kg-1) > 15-30 cm (0.81 mg kg-1) > 30-45 cm (0.31 mg kg-1) > 45-60 cm (0.21 mg kg-1)> 60-90 cm (0.19 mg kg-1). Most of the applied zinc was distributed in the upper layers of the soil (0-30 cm) than the lower layers (>30 cm).