Transformation of Zinc and Boron and their Availability to Rice, Wheat and Jute Crops in Ganges River Floodplain Soils

Abstract

Deficiencies of micronutrients particularly, zinc (Zn) and boron (B), in the intensively cropped soils of Bangladesh has long been established. An in-depth knowledge of the forms of Zn and B, their transformation in soils and interaction with soil components is desirable to determine the effects of Zn and B fertilizers on crops and their fate in the deficient soils. Therefore, field and laboratory experiments were conducted to study the fractionation of Zn and B and their transformation under variable field and laboratory conditions, in soils of Low Ganges River Floodplain (Agro ecological zone, AEZ 12) that contain low available Zn and B. The cropping patterns of the study were Wheat-Jute-Transplanted Aman (T. Aman) rice (W-J-R) and Boro-Fallow-T. Aman rice (R-F-R). Responses of wheat, jute and rice crops to various rates of Zn and B and the relationship between crop uptake of Zn and B and their fractions in soil were also studied. In addition, the status of micronutrients, namely, iron (Fe), manganese (Mn), copper (Cu), Zn and B in the High and Low Ganges River Floodplain (AEZs 11 and 12) soils were studied. A large number of soil samples from AEZs 11 and 12 contained low concentrations of plant available Zn and B. On the other hand, the average concentrations of plant available Fe, Cu and Mn in the Ganges River Floodplain soils were too high for plant growth. In the W-J-R pattern, significant effect of Zn and B was observed in grain yield of wheat (BARI Gom 26). The Zn effect was dominant over B effect. However, straw yield of wheat was unaffected by Zn and B application. The stick and fiber yield of jute (JRO 524) and grain yield of T. Aman rice increased but not consistently according to the rate of Zn and B application. Similar trend of inconsistent yield response was also observed in Boro and T. Aman rice crops of R-F-R pattern. The residual effect of Zn and B applied in wheat was realized in the yields and nutrient uptake of jute and T. Aman rice crops, suggesting that all of the applied Zn and B are not utilized by the current crop. However, a fresh addition of the nutrients yielded more jute or rice grain than that of the residual nutrients. The positive response of T. Aman rice to B applied in jute indicates that B application should not be ignored in Low Ganges River Floodplain soils. A significant response of crops to Zn and B application in R-F-R soil with sufficient available Zn and B implied a decrease in their availability in the soil during the crop season caused by interactions with various soil components. Similar to grain yield, the effect of Zn and B on the uptake of Zn and B in wheat, jute and rice crops were insignificant, although some remarkable variation was observed. A marked re-distribution in the amorphous and crystalline Fe-oxide bound Zn and residual Zn was observed in the incubation study with added Zn indicating a transformation of the added Zn. In the incubation of the soil with added B, the readily soluble and specifically adsorbed B increased substantially and residual B, decreased greatly implying a shift of added B to plant available forms. Fractionation of the field soil under W-J-R soil after two years of cropping revealed significant variations in the exchangeable, organic and Mn oxide-bound Zn forms (which are comparatively labile) due to application of Zn and B implying changes in the chemistry of native soil components. The increase in organically bound Zn in the native soil could be a contribution of the crop residue or the impact of applying N, P, K and S fertilizers. The fractionation of B in the same soil revealed that the concentration of B in the various soil fractions decreased from that of the initial soil and the added B could not be traced out in any of the B fractions practically. Correlation between plant tissue Zn and the soil Zn fractions varied with plant type, season and soil properties. The Zn fractions that are positively correlated with wheat tissue Zn was negatively correlated with rice tissue Zn. Concentration and uptake of grain B in wheat and rice were significantly correlated with exchangeable and specifically adsorbed B implying that crops absorbed B from these B pool in the soil. The concentration of exchangeable-Zn of the soils from different AEZs decreased with the increase in soil pH. The reactivity of MnOx was reflected proportionately with the increase in available Zn of the soils, impliying that MnOx-bound Zn acts as an important source of available Zn in Bangladesh soils. Higher residual Zn was observed in Ganges River Floodplain soils indicating fixation by clay minerals. Likewise greater quantity of specifically adsorbed B in Ganges River Floodplain indicates adsorption of B by smectite type clay minerals. Further investigations of the factors that interact with the fractions of Zn and B can reveal the details about the fate of the fertilizers applied under flooded and upland soil conditions. Development of appropriate soil management practices can make use of the potential sources in soil (e.g., labile pool) of these micronutrients for increased crop yield and increased levels of Zn and B in the food crops.