Effects of Cu and Zn Coated Urea on Eh, pH and Solubility of Cu and Zn in Rice Soils

The concentration of Cu (Copper) and Zn (Zinc) decreases upon flooded conditions of rice soil. To assess the effects of flooding and application of Cu and Zn coated urea on changes in Eh, pH and solubility of Cu and Zn, a glasshouse experiment was conducted at Universiti Putra Malaysia. Rice plants (30 days old seedlings of type MR-219) on two soils (riverine and alluvium and marine alluvium) were transplanted. Nine treatments with variable rates and combinations of Cu and Zn coated urea were applied. The sources of fertilizers were copper sulfate and zinc sulfate. Eh values decreased with flooding time in both soils. The changes of Eh values were more negative in control treatments and stabilized after 3 weeks of submergence. The Eh variation was not observed affectively in the treated soils however, soil pH increased with flooding time. During the 3 week of submergence, pH was neutral (pH 7.0). In both soils, Cu and Zn treated soil showed lower Eh and higher pH values as compared to untreated soil. Concentration of Cu and Zn in soil solution decreased with flooding. The higher Cu and Zn contents in soil were recorded in treated soils. Reduced solubility of Cu and Zn in control soils was related to larger changes in Eh and pH values. Mean comparison with Tukey’s HSD (Honest Significant Difference) test showed that Cu and Zn solubility decreased with decreased Eh and increased pH in the soil solution (p < 0.05%).


INTRODUCTION
NO 3 is reduced, iron reduction is initiated and is completed at Eh -100mV [2].
The differences between aerobic and anaerobic soils are the alteration of oxygen supply, which is mainly due to the changing redox potential and pH [3]. When oxidation and submergence combined together results in anaerobic and reduced soils which tends to converge soil pH to neutrality Authors E-Mail: (saima_babar2006@yahoo.com, naheedtalpur@gmail.com, khooharo@gmail.com) S 2takes place at a low redox potential and eventually end-up immobilizing them. The metal sulfides came into existence are sturdily insoluble even in strong acidic conditions [7]. Generally, micronutrients availability increased with the increase of soil acidity. Soils in Malaysia are acidic in nature and found deficient in micronutrient contents. This is due to the continuous subsequent planting pattern; paddy to paddy [8][9]. The cropping trend and fertilization practices are influenced on micronutrients availability [10]. The chemistry of rice soils is different as compared to normal acidic soils because of their submerged conditions. Due to pH changes the Fe 3+ reduced to Fe 2+ and Mn 4+ to Mn 2+ therefore, the uptake of Fe and Mn increased in flooded soils and Fe concentration can reach toxic level [11]. Zn and Cu availability generally decreased in lowland rice soils because of pH increased and redox potential decreased. Boron remains unchanged even after flooding and Mo concentration was found to increase in rice soils [12].
In this regard, the relationship between soils Eh, pH and Cu and Zn availability has been investigated upon the application of Cu and Zn coated urea in two main rice zones of Malaysia (Kedah and Kelantan). The main objective of this research was to evaluate the effect of Cu and Zn coated urea on the availability of Cu and Zn in paddy soils.

MATERIALS AND METHOD
The experiment was conducted in a glasshouse at Universiti Putra Malaysia. Two soil types used for this study were derived from riverine alluvium situated in the north-east and marine alluvium situated in the north-west  Table 2.
Soil redox potential (Eh) and pH were determined directly from the situ in each pot with portable Eh and pH meter (Hanna instrument Hl 8424 portable pH/ORP meter). The extractable Cu and Zn in soils were determined by using Mehlich-I (soil to solution ratio 1:5, shaking time 15 minutes) and their concentrations were analyzed by using Atomic Absorption Spectrophotometer (Perkin Elmer AAnalyst 400) [13][14]. Soil texture was determined by the pipette method [15]. Total N in the soil was determined by the Kjeldhal method on Auto Analyzer (Lachat Instrument Quik Chem FIA+ 8000) [16]. Soil extractable P was obtained by the method of Bray 2 [17].
The data were analyzed using Statistix 8.1 software. All data were subjected to the ANOVA (Analysis of Variance) for the RCBD, followed by the mean comparison by using Tukey's standardized range test (HSD) at 5% level of significance. where common urea is being applied [19].

RESULTS AND DISCUSSION
Soil pH and Eh were measured according to the scheduled time frame (weekly basis till fourth week of sowing).
Results revealed that the soil pH increased and the Eh  [20], who have reported that pH at its peak, occurred at day 5 after application of un-coated urea, which associated with the accelerated urea hydrolysis.
After three months of sowing, pH becomes acidic as it was before. The chemical changes regarding redox potential and pH in this study were almost similar to the values reported by Rostaminia [21].

FIG. 1. SOILPH AT FIRST FOUR WEEKS UNDER TREATED SOILS
The Eh and pH are the key factors in terms of soil fertility [21]. Eh and pH remained fluctuated in paddy fields and turns as compels towards the availability of applied and native elements [22]. Zn at the rate of 10 kg ha -1 ). The redox potential was between +10 mV to -80 mV from the first day of sowing till the third week, for surface applied soils. Whereas, Cu and Zn coated urea applied soils, Eh ranged between +15 mV to -40 mV (Fig. 2). The slow release mechanism of such fertilizer reduces the rate of hydrolysis and provides the nutrients accordingly [23]. Therefore, the application of essential nutrients in coating fertilizer is indispensable technique.