Transformation of zinc in soil and zinc nutrition in lowland rice under different levels of Phosphorus

Abstract

Phosphorus and zinc are two important essential elements required by the plants for various metabolic activities, functions and associated with membrane structure. Continuous use of phosphatic fertilisers with less zinc, aggravates zinc shortage also phosphorus applied will be accumulated in the soil leading to formation of zinc phosphate complex that affect zinc translocation to various plant parts. There are contradictions regarding phosphorus zinc interaction that is controlled by native P and Zn status. In this context an investigation entitled “Transformation of zinc in soil and zinc nutrition in lowland rice under different levels of phosphorus” was conducted. Two separate experiments were carried out at Regional Agricultural Research Station, Pattambi. In the first experiment, fractions of Zn under different P levels was studied in Long Term Fertilizer Experiment (LTFE) with six treatments on rice in RBD with four replications. The soil samples were taken after the paddy harvest of kharif 2020 and sequential extraction of Zn (water soluble + extractable, organically bound Zn, amorphous sesquioxide Zn and crystalline sesquioxide Zn) and P (sol-P, Al-P, Fe-P, sesquioxide occluded P and Ca-P) were carried out. In the second experiment phosphorus and zinc interactions was studied using pot culture experiment on rice in factorial CRD with 3 replications. Samples from different locations of Palakkad district having native P <12, 12- 24, >24 kg ha-1 and native Zn <1, 1-3, >3 mg kg-1 were collected. Sixteen treatment combinations were applied with 4 levels of P (0, 25, 50,100 mg kg-1 ) as KH2PO4 and 4 levels of Zn (0, 5, 10, 15 mg kg-1 ) as ZnSO4.7H20 through solution for studying the nutrient uptake and yield. The results of fractionation study showed that continuous use of inorganic fertilisers with organic manures in T5 (100% NPK + FYM) increased various Zn fractions except amorphous sesquioxide Zn fraction which was highest in the control. Even though the increase in P application increased different P fractions significantly higher content was observed in 100% NPK+ FYM and these were proved to be beneficial towards increasing different P fractions. But Ca-P fraction was more in the lime treated plot. Correlation study concluded that most of the phosphorus fractions have negative impact on different zinc fractions. The results of the pot experiment elucidated that P and Zn application increased number of productive tillers and thousand grain weight but higher zinc application decreased plant height. Among different soils, soil type S2 (P > 24 kg ha-1 ) registered higher grain and straw yield, P in grain and straw, total P uptake of paddy, but Zn uptake and Zn in grain and straw were more in soil type S3 (Zn > 3 mg kg-1 ). Combined application of 100 mg kg-1 P and 10 mg kg-1 Zn recorded higher grain (57.27 g pot-1 ) and straw yield (66.82 g pot-1 ). Phosphorus application significantly increased the P content in grain and straw but zinc addition had antagonistic effect on them and treatment combination of P100Zn0 registered higher P content in grain (0.26%) and straw (0.214%). The Zn in grain and straw significantly increased by the application of zinc, but phosphorus was having an adverse effect and treatment combination of P0Zn15 registered higher Zn content in grain (61.28 mg kg-1 ) and straw (44.19 mg kg-1 ). Total P and Zn uptake of paddy increased by the application of both phosphorus and zinc with higher content obtained at P100Zn15 (28.47 mg pot-1 ) and P50Zn10 (0.458 mg pot-1 ) treatment combination. Correlation study concluded that phosphorus and zinc application affected P and Zn uptake in high zinc containing soil (S3) and high P soil (S3), respectively. In future this work can be used to study the changes in various P and Zn fractions under continuous crop removal, various management practices to mobilise fixed P in high phosphorus containing soil and also to study Q/I relationship of both phosphorus and zinc.