Reactions of Fluid and Granular Copper and Molybdenum-Enriched Compound Fertilisers in Acidic and Alkaline Soils

The reactions of copper (Cu)and molybdenum (Mo)-enriched phosphorus (P) fertilisers have rarely been studied, and we hypothesized that theses elements supplied in fluid form would behave differently in soil to those supplied in granular form, based on previous work with manganeseand zinc-enriched P fertilisers. Reactions or Cu and Mo in and around fertiliser granules or fluid bands have not been evaluated before, and we employed a combination of several laboratory based isotopic and spectroscopic methods to evaluate lability and ichemical forms of different fertilizer derived Cu and Mo and in both acidic and alkaline soils. INTRODUCTION Millions of hectares of arable land worldwide, particularly in arid and semi-arid regions, are deficient in plant available micronutrients and human populations require more trace elements in their diets (Welch and Graham 2005). Copper and Mo are essential trace elements in all agricultural systems (Kaiser et al. 2005; Welch 1995), yet these elements are often overlooked in fertilisation programs. The major reason for the widespread occurrence of deficiency of micronutrients in soils is the low availability of micronutrients to plant roots rather than their low concentration in soils. Copper and Mo deficiencies are widespread in Australasia and Asia – for example up to 30% of the soils in China are estimated to be deficient in Cu and Mo (Yang et al. 2007) and Mo was central to the agricultural development of high rainfall acidic soils in Australia for legumebased pastures (Anderson and Moye 1952). While Cu deficiency occurs predominantly in alkaline soils where availability of Cu is low, it may also occur in highly leached soils where total and available Cu concentrations are very low due to leaching over geologic time scales e.g. Western Australia (Brennan et al. 1980). By contrast, availability of Mo is lowest in acidic soils (Welch and Anderson 1962) as it is anionic and is increasingly attracted to the positive charges on clay minerals as soil pH falls (Barrow 1978). Recent field studies conducted by our group have shown an increased response to fluid Cu, Mn and Zn (concentration in grain and/or grain yield) compared to granular fertilizers in calcareous sandy loam soils (Holloway et al., 2002; 2006). Further, recent laboratory experiments using isotopic dilution techniques using Mn, and Zn revealed that granular Mn and Zn did not diffuse readily from the point of application in soil nor did they enter, or remain in, the labile (available) pool compared to fluid fertilizer micronutrients (Hettiarachchi et al., unpublished data). The aim of this study was to investigate the reaction products of Cu and Mo in fluid and granular fertilizers in highly acidic and calcareous soils using isotopic and spectroscopic procedures. MATERIALS AND METHODS The soil used in this study was a grey calcareous alkaline sandy loam from Upper Eyre Penisula in South Australia (Warrramboo, 5% clay, pH 8.5), a neutral sandy soil from Western Australia (WA) (Eneabba , 3% clay, pH 6.2) and a slightly acidic loamy sand also from WA (Kambellup, 13% clay, pH 6.1). Treatments were: Granular monoammonium phosphate with 0.6% Cu and 0.04% Mo (and 0.6% Zn), fluid technical grade MAP (TG MAP) with added Cu, Mo and Zn equivalent to that supplied in granular form, and ammonium polyphosphate (APP) with added Cu, Mo and Zn as above. Soils at 60% of maximum water holding capacity were incubated for 5 weeks at 25C in Petri dishes/experimental cells with fertilisers in the centre. After 5 weeks, concentric rings of soils were collected from Petri dishes, outwards from the fertilizer placement point (0-4 mm, 4-7.5 mm, 7.5-13.5 mm and 13.522.5mm). Soils from the 0-4 mm zone, including incubated fertilizer granules/fluid fertilizer injection point, and from the 4 mm 7.5 mm zone were dried in an oven maintained at 40oC. Soil pH and total, water-soluble and labile concentrations of Cu and Mo were determined by aqua regia digestion, and by isotope dilution, respectively. For determination of labile micronutrients (E-values) by isotope dilution, 1 MBq Cu (half-life 12.8 days) or 100 kBq Mo (half-life 67 days) were added to a pre-equilibrated (4 days end-over-end shaking) soil:water suspension (1:10 soil:water ratio) and equilibrated for 24 hours. Suspensions were then filtered (<0.2 μm) and assayed for radioisotopes by gamma spectroscopy. Concentrations of unlabelled Cu and Mo in the solutions were determined by inductively-coupled plasma mass spectroscopy. Percentages of Cu and Mo derived from the added fertilisers in each soil section were determined by subtracting the total and labile concentrations of these elements from those determined in similarly incubated unfertilized soil, and expressing this as a % of the fertiliser Cu and Mo added. Incubated soils/fertilizers as outlined above were prepared in experimental cells were used for synchrotron-based micro scale x-ray fluorescence (to determine elemental distribution), and micro scale x-ray absorption spectroscopy (to determine chemical form of reaction products). Data were collected at GSE-CARS, 13-BM, Advanced Photon Source, Argonne, IL, U.S.A. RESULTS AND DISCUSSION Movement of and lability of fertilizer copper and molybdenum A large percentage of the Cu in the fertilisers remained in the fertilised zone (0-4mm) in and around the granule/fluid injection point and this was consistent across all soils. Data for the most alkaline soil (Warramboo) and the most acidic soil (Kambellup) are shown on Figures 1 and 2. Diffusion of Cu away from the fertiliser into the surrounding soil was greatest with the fluid products, with only small differences evident between the two fluid formulations (TG MAP and APP). Figure 1. Distribution and lability of Cu in the acidic soil (Kambellup) 5 weeks after incubation with fertilisers. % o f F er til iz er C u