Effect of extractant pH on the release of soil phosphorus , aluminium and iron by ammonium fluoride

Release of P, Al and Fe of five mineral soils in four-hour extraction by 0.1 M NH 4F, pFI 4.2 —8.6, generally increased with decreasing pFI of the extractant. Fluoride was a rather selective extractant of Al at pH 6.1—8.6 where the extractability of iron was low. NH 4F, pH 4.2, released a great part of P solubilized in fractionation of inorganic soil P, and Al was extracted more than by Tamm’s acid ammonium oxalate. Acid fluoride solutions released OHions from soils. The initial pH of fluoride was 4.2 —5.2, and it rose in the filtrates of all soils. Index words: amorphous Al and Fe, fractions of inorganic P, ligand exchange, mineral soils


Introduction
The use of ammonium fluoride as extractant in the fractionation of inorganic soil phosphorus is based on the assumption that neutral and alkaline fluoride selectively dis- solves aluminium-bound phosphate (Chang  and Jackson 1957, Fife 1959 a).In the de- velopment of the fractionation procedure, natural and synthetic crystalline phosphates were used as model compounds.In fertilized soil, however, phosphate is sooner present as amorphous iron and aluminium phosphates, and it is sorbed on aluminium and iron oxides, these forms of phosphate being more soluble than crystalline phosphates (Yuan et al. 1960,  Laverty and McLean 1961).
The efficiency of fluoride as an extractant of phosphate is explained by the stability of the hexafluoroaluminium complex ion in neutral and alkaline solutions (Turner and  Rice 1952).The hexafluoroiron(lll) complex, formed in acid solutions, is not stable in neu- tral and alkaline solutions.
There has been disagreement about the pH at which ammonium fluoride best distin- guishes between aluminium-bound and iron- bound phosphate.Chang and Jackson (1957)  and Chang and Liaw (1962) recommended pH 7.0, while Fife (1959 a) considered that less hexafluoroiron complex is formed at pH 8.5 than at pH 7.0.A previous study showed that the quantities of aluminium and iron released in onehour extraction by alkaline ammonium fluo- ride in the phosphorus fractionation procedure were small (Niskanen 1987).The aim of this paper was to study whether the solubility of phosphorus, aluminium and iron is dependent on the pH of ammonium fluoride in prolonged extraction.

Material and methods
The material consisted of five mineral soil samples (Table 1) described in a previous paper (Niskanen 1987).Soil pH was mea- sured in soil-0.01M CaCl 2 suspension (1 : 2.5 v/v) (Ryti 1965), the particle-size distribution determined by the pipette method (Elonen 1971) and organic carbon content by a wet combustion method (Graham 1948).The amorphous aluminium and iron were extracted by acid ammonium oxalate (0.18 M ammonium oxalate, 0.10 M oxalic acid, pH 3.3, 1 : 20 w/v, 2 h) (Tamm 1922) and deter- mined by atomic absorption spectrophotometry.
To study the effect of pH of fluoride solu- tion on the release of phosphorus, aluminium and iron, the soils were extracted in duplicate by 0.1 M NH 4 F (1 :200 w/v, 4 h), the pH adjusted to different values with NaOH and HCI.The extracts were analysed for phosphorus by a molybdenum blue method modified by Kaila (1955) and for aluminium and iron by atomic absorption spectrophotometry.

Results and discussion
Phosphorus was effectively extracted by 0.1 M NH 4 F, pH 4.2 (Table 2).Four-hour ex- traction released even more P than the frac- tionation procedure (Table 3).Acid fluoride did not distinguish between aluminium-and iron-bound phosphate.Obviously, calcium- bound P was also extracted.According to Matzel and Suntheim (1977), apatite phosphorus is solubilized already by neutral fluoride.
The extractability of phosphorus generally tended to decrease with increasing pH of fluo- ride (Table 2).Especially in soils Nos 3 and 4, containing more amorphous iron than aluminium, the extractability of P decreased efficiently, the pH of fluoride being 5.2 or higher (Table 2).This was due to diminishing complexation of iron by fluoride.
Quantities of phosphorus, aluminium and iron released in fractionation of inorganic phosphorus (Chang and Jackson 1957) of experimental soils are given in a previous paper (Niskanen 1987).The total values of fractionated phosphorus and values of phosphorus extracted by 0.5 M NH 4 F (pH 8.5, 1 :50 w/v, 1 h) (Fife 1959 a) in connection with fractionation are included in Table 1.
Basic fluoride, pH 8.6, extracted more P from soil No. 3 than was released by neutral fluoride (pH 7.1) (Table 2).The same tenden- cy seemed to prevail also in the soils Nos 4 and 5, although there was no statistically signifi- iron oxide is increased by raising the pH of fluoride to 8.5 (Bromfield 1967 a, b).
cant difference in the means of P values.The lower extractability by neutral fluoride may be due to phosphate readsorption.Extraction involves the risk of dissolved aluminiumbound phosphate being partially adsorbed on iron oxides (Khin and Leeper 1960, Fife   1962, Bromfield 1967 a, b, Rajendran and   Sutton 1970).Readsorption decreases when the pH is raised from 7 to 8.5 (Khin and Leeper 1960, Fife 1963).With increasing ex- tractant to soil ratio readsorption decreases (Fife 1962, 1963).The higher extractability of P by basic fluoride may also be due to release of iron-bound phosphate (Khin and Leeper  1960).Appreciable hydrolysis of iron phosphate occurs at a pH above 7 (Chang and  Liaw 1962), and release of P adsorbed on Apart from soil No. 1, very rich in fluoride- soluble P, basic 0.1 M NH4F extracted more P during four hours than did 0.5 M NH 4 F during one hour (Tables 1 and 2).The studies of Fife (1962,1963) suggest that a one-hour extraction by NH 4 F does not remove all aluminium-bound phosphate from soils, and that a 24-hour extraction is preferable.The efficiency of P removal also increases with dilution (Fife 1962).
Aluminium was poorly extracted within one hour by basic 0.5 M NH 4 F (Niskanen 1987), while the four-hour extraction by 0.1 M NH 4 F, pH 8.6, enhanced the extractability (Table 2).This is in accordance with Fife (1959 b), who found that increasing amounts of aluminium are dissolved from aluminium oxide by fluoride as the time of extraction is prolonged.The extractability of aluminium tended to increase with decreasing pH of fluo- ride (Table 2), at pH 4.2 being even higher than by acid ammonium oxalate (Table 4).In the study of Fife (1959 b), aluminium was extracted by 0.5 M NH 4 F from aluminium oxide nearly three times more at pH 6.6 compared with pH 9.3.In the fractionation of inorganic soil phosphorus iron was poorly extracted by basic 0.5 M NH4F (Niskanen 1987), and four-hour ex- traction by basic 0.1 M NH 4 F did not en- hance the extractability (Table 2).Williams   et al. (1971) found that 0.5 M NH 4 F pH 8.2, extracted less than 2 % of oxalate-extract- able iron in non-calcareous lake sediments.The extractability of iron increased only when the pH of fluoride was 5.2 or lower (Tables 2   and 4).
In the reaction of fluoride solution with soil clays and amorphous oxides, hydroxide ions are released (Fife 1962, Huang and Jackson  1965, Brydon and Day 1970, Perrott et al.  1976).The elevation of pH in the reaction of fluoride with soil has been used as a test for allophane and aluminium hydroxide in B horizons of podzols (Brydon and Day  1970, Perrott et al. 1976).Huang and Jackson (1965) showed that neutral 1 M NH 4 F reacted primarily with aluminium and there- after with iron in soil clays and oxides, and the amounts of aluminium and iron solubilized were nearly stoichiometric to the OH ions released.In the present study, the pH values in filtrates (Table 5) showed that hydroxide ions were released in fluoride ex- traction.At an initial pH 5.2 or lower, OHĩ ons were released from all soils.Release of OH~ions, calculated on the basis of a rise in pH, was not stoichiometric to extracted Al and Fe.This non-equivalence can be ascribed partially to the buffering properties of soil.
The pH-dependence of the extraction pattern coincides with the adsorption theory presented by Hingston et al. (1972).Accord- ing to this model, adsorption of fluoride on soils and aluminium and iron oxides peaks near pH 3, corresponding to a pK a of HF.At a pH much below 3 the concentration of Fĩ s low.With rising pH the dissociation of HF and F" concentration increase and fluoride adsorption is enhanced.Any further rise of pH decreases the sorption of fluoride rather steeply after exceeding the value corresponding to the pK a of HF.This is because the concentration of H + ions needed to neutralize HO" ions liberated from oxide surface by ligand exchange with F decreases.
In a slightly acid pH region, fluoride seemed to be a selective extractant of aluminium, the extractability of iron being rather low, while acid fluoride extracted both metals.The efficiency of fluoride as extractant is due to the similar size of F and OH ions.

Table 1 .
Characteristics of soil samples.

Table 2 .
Soil P, Al and Fe (mmol/kg soil) released by 0.1 M NH 4 F in four-hour extraction*.
*Each soil and element tested separately.Values marked with the same letter do not differ at P =0.05.Table3