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Fluid Journal : Spring 2014
9 The Fluid Journal Spring 2014 The management of P nutrition is still a challenge in acidic high P-fixing soils. In these soils, precipitation and adsorption reactions are the processes that remove P from solution, reducing its availability to plants. The precipitation reactions occur at high P concentrations. In acidic soils, Al and Fe are normally the cations involved in the reaction; whereas at low P concentrations adsorption reactions into Al/Fe oxyhydroxide minerals predominate. The dissolution of granular fertilizers renders an acidic highly P concentrated solution that, when in contact with Al and Fe from the soil, may precipitate in the form of Al and Fe phosphates. If this is the case, it has been hypothesized that application of fluid P fertilizers may be beneficial as P is spread in a larger volume of soil, therefore diminishing the formation of highly concentrated P environments. On the other hand, if P sorption is the predominant mechanism reducing P availability, then banding of P will be beneficial as it limits the contact of P with a large number of strongly P-sorbing sites. Objective This study aimed at investigating the chemical reactions that limit fertilizer P availability. The experiment was conducted to evaluate the diffusion, lability, and solubility of a range of granular and fluid fertilizers applied in acidic high P-fixing soils. Methodology Soils. Air-dried and < 2 mm sieved surface samples (1-10 cm) of soils from the orders Andisols (North and Chile), Oxisols (Greenwood and Redvale), The Fluid Journal • Official Journal of the Fluid Fertilizer Foundation • Spring 2014 • Vol. 22, No. 2, Issue #84 Ms. Daniela Montalvo, Dr. Fien Degryse, and Dr. Mike J. McLaughlin Tests are showing that P nutrition is still a challenge in acidic high P-fixing soils. Summary: Recommended P management strategy for soils where adsorption reactions predominate is banding the fertilizer to limit its contact with the soil. There is still much in the range of work needed to improve the efficiency of P fertilizer in acidic high P-fixing soils. calcic Inceptisol (Pt Kenny) and Alfisol (Monarto) were used in the study (Table 1). The calcic Inceptisol is a soil rich in calcium carbonate, which promotes precipitation of P with Ca and the Alfisol is a weak P-sorbing soil. These soils were included for comparison. Site evaluation. A 35 d Petri dish incubation experiment was performed. Briefly, Petri dishes (5.5 cm diameter and 1.1 cm height) were filled with soil to obtain a bulk density of 0.5 g cm-3 for Chile soil, 0.7 g cm-3 for the North soil, 1.1 g cm-3 for Greenwood, Redvale, and Kenny soils, and 1.3 g cm-3 for the Monarto soil. Soils were wetted to field capacity and after an equilibration time of 24 h the fertilizer treatments were placed at the center of the Petri dish and 3 mm below the surface. Fertilizer treatments included four granulars and two fluids. The granular sources were: • Single super phosphate(SSP; 0-8 -8- 0 [NPK]) • Triple super phosphate (TSP; 0-20- 0) • Mono-ammonium phosphate (MAP; 10-22-0) • Diammonium phosphate (DAP; 18- 20-0). The fluid sources were: • A technical grade MAP (TGMAP; 12-26-0) applied in a volume of 200 or 100μL. In addition, a control treatment without fertilizer was included. The treatments were applied at the rate of 9.2 mg P per petri dish. All treatments were replicated three times. At the end of the incubation period, two concentric soil sections inner (0-7.5 mm radius) and outer (7.5-27.5 mm radius) surrounding the fertilizer application point were removed using stainless steel cylinders to perform the chemical analysis. Sub-samples of each soil section were assessed for total P and labile P determined by the isotopic dilution method. For this analysis, soil was weighed into a centrifuge tube and equilibrated with deionized water in a 1:10 soil:water ratio. What About P Fluids In Strongly P-sorbing Soils? ▼ DOWNLOAD