Recovery of sorbed fertilizer phosphorus by three water extraction methods
György. Füleky and László Tolner
Szent István University Institute of Environmental Sciences, Gödölló, Hungary
The replenishment of P in the soil solution is of considerable importance from the standpoint of plant nutrition, for the reason that the quantity of inorganic P in the solution phase at any time is usually insufficient to meet crop requirements over the whole of the growing season.
Through this study we are concerned in the phosphorus recovery from the soil solid phase that have received different levels of P-additions and of acidification applying three different water extraction methods.
A calcareous loamy soil of, pHH2O = 7.9 was incubated with 0, 80 and 320 mgP.kg-1 soil for one month. Four levels of acidification (pH 7.9, 7.3, 6.0 and 4.5) were adjusted by adding calculated volumes of HCl and incubating the soil samples far further one month, then equilibrated with different rates of phosphorus as 0, 80, 160 and 240 μg P.10 cm-3. The desorbed quantity of phosphorus was determined applying EUF, HWP and multistep desorption methods.
The multistep phosphorus desorption study was applied to the soils with different previous P-additions under two levels of acidity (pH 7.9 and 4.5). For this purpose, phosphorus adsorption was determined with different P-levels: 0, 80, 160 and 240 μgP. 10 cm-3 (recently additions) as 1 g soil and 10 cm3 of aqueous phosphate solution was shaken for 24 hours, and after that phosphorus desorption was measured by extraction with distilled water through 12 steps.
The EUF procedure was applied to the incubated soil samples under two levels of acidification pH (7,9 and 4.5) after adsorption experiments with phosphorus solutions (0, 80, 160 and 240 μgP.10 cm3 (recently additions) as described in multistep water extraction technique. The residual solid phase was transferred quantitatively into the container of the EUF apparatus. The desorption has been conducted under a constant field strength of 400 V 5 cm-3 and temperature of 20 oC. Ten- minute fractions were collected for 400 minutes (Németh, 1976).
Hot Water Percolation (HWP) was carried out on the sols that have only received previous P-additions (0, 80 and 320 mgP.kg-1 soil) with a wide range of pH (7.9, 7.3, 6 and 4.5) in the HWP instrument.
Some part of the added phosphorus can be recovered by all methods, depending on the dose and time of P addition (previous or recent), on acidity of the soil and on the method used. The desorbable phosphorus amount percentages are approximately 19% of the sum of P-loading at 0 mgPkg-1 level, and rise to about 31 and 64% at 80 and 320 mgPkg-1 levels, respectively. With general conclusion, in acidified cases, the phosphorus recovery are higher than those of non-acidified soils, while with the increasing of P-loading the apposite is true.
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Tolner L., Anas A. Wahdan, Füleky Gy. (1996): Model of the Multistep desorption of the phosphate content adsorbed by the soil (in Hun.). Agrokémia és Talajtan, 45. (3-4). 295-306.