Dynamics and interaction of zinc and boron with phosphorus in ultisol

Abstract

The recent soil fertility assessment of the entire state has revealed that more than 60% of the soils in the state are having high P status due to continuous application of P fertilizers like factomphos and bone meal. It was also established that 15% of the soils are deficient in zinc and about 60% of soils are deficient in boron. Antagonistic interaction of P with zinc has already been well established. However studies on interaction of P with boron are limited. Thepresent study was undertaken in the above background in the Dept. of Soil Science and Agricultural Chemistry during the period from 2013-2015 to understand the chemistry, dynamics and bioavailability of zinc and boron with respect to the P status of the soil, which in turn will help in modifying the fertilizer prescription in terms of quantity and method of application of these nutrients. In order to achieve the objective of elucidating the dynamics of Zn and B as influenced by P status in lateritic soils and to optimize the level of P for balanced nutrition of cowpea with respect to Zn and B, 18 lateritic soil samples (Ultisol), six each coming under low, medium and high P status were identified from an initial 100 soil samples and characterized. A potculture experiment with cowpea as a test crop was conducted in three soils, one each with low, medium and high average P status. Soil and plant samples were collected at flowering and at harvesting stages and analyzedfor nutrient content. The distribution of fractions of inorganic P in the three soils showed that Fe bound P was the dominant fraction contributing to more than 50% of the total inorganic P. The soluble P fraction was about 6% in all the three soils. Fe and Al-P were the main fractions contributing to the available pool initially.Among the fractionsof boron, readily soluble boron recorded the lowest, where as the contribution of residual boron was the highest. Available P status in soils with low and medium P increased due to the application of P while it decreased in soil with high P. The soil with high P soil showed that the application of phosphorus lead to the fixation of phosphorus in to insoluble forms whereas, if P was not applied there was solubilisation of Fe-P and Al- P resulting in increased its availability. Plant adsorbed P from soluble P led to its depletion at the end of vegetative phase. Applied P got transformed into Fe-P and Al- P initially, which along with native occluded P got transformed to calcium bound P which is contributing to the available pool at later stages. Application of Zn was found to reduce Al-P and Fe-P due to the formation of insoluble zinc phosphate. Application of Zn and B reduced the Ca-P, probably due to the formation of zinc phosphate and Calcium borate. In case of zinc fractions, water soluble + exchangeable fraction and organic matter occluded zinc was directly contributing to the available pool. The other Zn fractions except amorphous iron oxide occluded zinc were contributing to the available pool indirectly through water soluble + exchangeable fraction. Application of P reduced the water soluble + exchangeable zinc fraction where as the application of boron enhanced the transformation of zinc into this fraction especially when boron was applied without P. Application of P resulted in adsorption of zinc into specifically adsorbed zinc. With respect to boron fractions, readily soluble boron and oxide bound boron were directly contributing to the available pool where as binding of boron with organic matter as well as its transformation to residual boron reduced boron availability. All the fractions of boron were contributing to available pool indirectly through readily soluble fraction of boron. Application of phosphorus was found to reduce the readily soluble boron due to anion competition. The applied boron either remained in the soluble form or getting transformed to specifically adsorbed, oxide bound, organic matter bund boron. Boron application along with P reduced the readily soluble boron. Application of Zn increased the Zn content in plant. However, the application of P with and without B reduced the Zn content in plants. Application of P and Zn reduced the boron content in plants and application of boron with and without phosphorus recorded the highest boron content. The highest grain yield was recorded in soil with medium P, while the high P status in soil either due to native P or due to applied P reduced the yield resulting from induced lower uptake of zinc and boron. Thus, at high levels of P, enough quantities of soluble zinc should be assured, over and above the quantities of this element precipitated as zinc phosphate, both by optimizing the pH and applying enough quantities of Zn. Similarly H3BO3 and H2BO3 - ions should be enough to overcome competition from H2PO4 - ion at root surface.