AVAILABILITY OF A SOLUBLE PHOSPHORUS SOURCE APPLIED TO SOIL SAMPLES WITH DIFFERENT ACIDICITY LEVELS ( 1 )

Considerations on the interactions of P in the soil-plant system have a long history, but are still topical and not yet satisfactorily understood. One concern is the effect of liming before or after application of soluble sources on the crop yield and efficiency of available P under these conditions. The aim of this study was to evaluate the effect of soil acidity on availability of P from a soluble source, based on plant growth and chemical extractants. Nine soil samples were incubated with a dose of 200 mg kg-1 P in soil with different levels of previously adjusted acidity (pHH2O 4.5; 5.0; 5.5; 6.0 and 6.5) and compared to soils without P application. After 40 days of soil incubation with a P source, each treatment was limed again so that all pH values were adjusted to 6.5 and then sorghum was planted. After the first and second liming the P levels were determined by the extractants Mehlich-1, Bray-1 and Resin, and the fractionated inorganic P forms. In general, the different acidity levels did not influence the P availability measured by plant growth and P uptake at the studied P dose. For some soils however these values increased or decreased according to the initial soil pH (from 4.5 to 6.5). Plant growth, P uptake and P extractable by Mehlich-1 and Bray-1 were significantly correlated, unlike resin-extractable P, at pH values raised to 6.5. These latter correlations were however significant before the second liming. The P contents extracted by Mehlich1 and Bray-1 were significantly correlated with each other in the entire test range of soil acidity, even after adjusting pH to 6.5, besides depending on the soil buffering capacity for P. Resin was also sensitive to the properties that express the soil


INTRODUCTION
In view of the growing global fertilizer demand for agricultural production, the understanding and enhancement of the efficiency of fertilizer use in different soils and crops has increasingly been sought.Among the nutrients, P is one of the most limiting to growth and development of plants (Taiz & Zeiger, 2004), especially in tropical environments, given the great power of these more electropositive soils to retain P in forms not available to plants (Novais et al., 2007, Fernandez R. et al., 2008a, b).
Long-standing questions about the interaction of P in soil-plant system are still pertinent and mostly inadequately investigated.One issue is the time to apply soluble P sources to soils with high drainage/ adsorption potential (Novais & Smyth, 1999), whether before or after liming would be most ideal (Kaminski & Mello, 1984), to minimize precipitation of P by Al 3+ under acidic conditions, fixing the nutrient (Neto Rolim et al. 2004).On the other hand, hydrolysis of Al 3+ with liming, with recent formation of hydroxides would cause, similarly, P fixation by adsorption with ligand exchange (OH -↔ H 2 PO 4 -or HPO 4 2-) with practical consequences similar to the previousprecipitation due to Al 3+ (Haynes, 1984).
Studies conducted to evaluate P uptake or plant growth in response to the application of soluble P sources to the soil, before or after liming, suggest that soil acidity reduces plant growth and P uptake (Bataglia et al. 1984;Kaminski & Mello, 1984).The interpretation of this result, which would indicate decreased P concentration in a solution of acid soils and a likely increase after liming, seems to be misleading.Indeed, the toxicity caused by Al under acidic conditions (Masion & Bertsch, 1997) creates unfavorable condition for the efficiency of plant P uptake, even if a high concentration of the nutrient in the solution is ensured, due to the toxicity caused by Al (Foy, 1984;Silva et al., 2002).
Therefore, the efficiency of soluble P sources in acidic soils is governed particularly by the greater or lesser plant tolerance to Al and not necessarily by the characteristics of soil adsorption for P, modified by varying degrees of acidity (Foy, 1984).In fact, studies that measured P availability under these acidic conditions, in chemical determinations such as maximum P adsorption (Vasconcellos et al., 1974, Gonçalves et al., 1985), P adsorption energy (Ernani et al. 1996), and extractants (Novais et al., 2007), etc., are consistent when addressing the minimal or little influence of soil acidity on the chemical changes in soil P, caused by reactions of precipitation/adsorption.The high value of the point of zero charge (PZC) of oxyhydroxides in tropical soils, highly affected by weathering (Mello & Novais, 2007), is the main reason for this fact.
Another question concerns inorganic P forms in soil and P availability for plants.It could be more convenient for plants to have more P-Ca in soils with high P adsorption capacity than P-Al and P-Fe, which are theoretically more stable forms than the first (P-Ca), generated from the dissolution of applied P -Ca (rock phosphates).It can be concluded that low levels of P-Al and P-Fe are more limiting to plant growth than high levels of P-Ca (Novais & Smyth, 1999;R. Fernández et al, 2008a, b).
The extractant Mehlich-1, used nation-wide in laboratory routine analyses, is composed of strong diluted acids and may overestimate the available P levels of soils treated with rock phosphate or in which the levels of inorganic P-Ca are higher due to low weathering (Novelino et al., 1985).On the other hand, it can underestimate the values of available P in soils with high clay content, as a consequence of the exhaustion of the extractant under such conditions (Novais & Kamprath, 1979a;Muniz et al., 1987).However, the Resin method is also questioned with regard to the probable underestimation of labile P, especially in soils with high P adsorption capacity (Campello et al., 1994).A recent study confirms this fact (Schlindwein & Gianello, 2008).
The purpose of this paper was to evaluate the effect of soil acidity on the availability of P from a soluble source, measured by plant growth and by chemical extractants.

MATERIAL AND METHODS
Samples were collected from the 0-20 cm layer of nine soil types, in the state of Minas Gerais, for chemical and physical analyses (Table 1).The maximum P adsorption capacity (MPAC) and the constant related to P adsorption energy (PAE) were estimated using the Langmuir isotherm (Olsen & Watanabe, 1957), adjusted with the points of region II (Novais & Smyth, 1999).
The samples were air-dried, sieved through 2 mm mesh, homogenized and treated with increasing doses of a mixture of CaCO 3 and MgCO 3 (molar ratio 4:1), defined by an incubation curve, required to raise the pH (H 2 O) of these subsamples to 4.5; 5.0; 5.5, 6.0 and 6.5 and then packed in plastic bags.After 30 days of incubation at moisture adjusted to field capacity the samples were air-dried, sieved through 2 mm mesh and homogenized.Each sample was treated with 200 mg kg -1 P as triple superphosphate, sieved through 0.25 mm mesh, and homogenized with the whole soil mass.Soil samples under natural conditions, without P addition, were used as control.The treated samples were incubated for 40 days (moisture at field capacity).After this period, the samples were air-dried and sieved again.Then the pH H 2 O of each sample was determined by adding corrective lime to initially raise all pH values to 6.5.The samples were incubated again for 30 days.After this period they were dried, sieved and separated into portions of 1.7 kg of soil and filled in plastic pots, with four replications.At this stage, standard fertilization was applied according to Alvarez V. (1974), minus P, and sorghum (Sorghum bicolor L. Moench) hybrid "Br 300" was sown, thinned to 10 plants per pot after emergence.Five days after thinning K was topdressed in KCl solution to increase the nutrient content of each soil sample to 150 mg kg -1 K.
During crop growth, daily irrigations maintained soil moisture near field capacity, and weekly N topdressings of 75 mg/pot were applied in solution, in the form of NH 4 NO 3 .
Thirty-five days after emergence, the plant shoots were cut at the ground and dried to constant weight in a forced-air oven at 70 °C.After determining the shoot dry weight, the plant material was ground and homogenized.These samples were then subjected to nitroperchloric digestion (Sarruge & Haag, 1974), analyzed, and the P levels in this extract, as well as in the others obtained in this study, were determined using a colorimetric method of ascorbic acid, proposed by Braga & Defelipo (1974).
For analysis of the P content and inorganic P forms, soil samples were taken from each treatment before and after the second liming.Phosphorus levels were determined by the extractants Mehlich-1, Bray-1 and Resin.For Mehlich-1 and Bray-1, the soil: solution ratio was 1:10, with a shaking time of 5 min.For resin, the method proposed by Raij et al. (1987) was used, with a soil: solution ratio of 1:1, and 16 h of shaking.Then the inorganic P forms were subdivided, as proposed by Chang & Jackson (1957) and adapted by Braga (1980).
The treatments were arranged in a randomized block design with four replications.The experiment had a 9 x 6 factorial design, corresponding, respectively, to the soil and pH levels, including the control.
Correlations between variables were calculated and regressions fitted to the data of dry matter production as the dependent variable of different acidity levels at which the different soil samples were incubated with soluble P source and, on this basis, the pH H2O values related to the lowest production estimated.Regression equations were selected by testing the coefficients of the components of each model to a level of 5 %.

Plant growth and phosphorus uptake
Incubation of soil samples with triple superphosphate, not considering the control (no P application), at different pH values, appears to influence plant growth and P absorption in most soils significantly (Table 2).For some soils, these two variables increased or decreased with rising pH during the initial incubation period of the P source in the soils at increasing pH levels.Thus, the different regression models adjusted for dry matter production as dependent variable of the pH in soil incubated with superphosphate, suggest a differentiated behavior of P applied in relation to the chemical particularities of some soils (Table 3).Estimation of the soil pH, corresponding to the lower dry matter production (Table 3) indicated accentuated variation, ranging from 4.5 to 6.5.This fact, coupled with the occurrence of non-significant differences in dry matter production in most soils, indicates that increased P availability for plants is not always verified by applying a soluble source of this nutrient to limed soils, since the transformation of labile into a nonlabile P happens in very short time intervals and is apparently little related to the initial soil pH (Novais et al., 2007).
Table 6.Regression equations adjusted to plant P uptake as related to P extracted by Mehlich-1, Bray-1 or resin and properties that reflect the buffering capacity of P in soil ns , a , °, * and **: non-significant, significant at 12;10;5 and 1 %, respectively.
It is observed that in certain situations and more clearly in Tb soil (clayey LVA), both dry matter production and P uptake by sorghum decreased with increasing pH (Table 2).This fact is possibly associated to changes caused by liming on soil constituents associated to P adsorption.The new P adsorption sites formed (Al-OH and Fe-OH) as a result of liming, become less active over time (Sims & Ellis, 1983).Considering some methodological aspects used in this study and, in particular, those associated to the time factor, the hypothesis that this effect was more pronounced in an acidity level corresponding to pH 6.5, compared to pH 4.5, seems reasonable.

Chemical extractants
The correlations between the mean dry matter production and P uptake by sorghum with the P levels recovered by extractants for Mehlich-1 and Bray-1, unlike for resin, were significant (Table 5).This is possibly due to the higher sensitivity of Mehlich-1 and Bray-1 to the buffering capacity of soil P, similarly to the plant Plant buffering capacity or plant P uptake (Novais & Smyth, 1999).However, when regressions of plant P uptake are adjusted as related to P extracted by Mehlich-1, Bray-1 or resin, and are included in the regression properties that reflect the P buffering capacity in soil (clay content, remnant P or MPAC), particularly those related to Resin-extracted P, the entry of these properties in the model is significant (Table 6).This fact is in agreement with data found by Campello et al. (1994) and Schlindwein & Gianello (2008), indicating that resin is sensitive to the P adsorption capacity factor, however less expressive than Mehlich-1 and Bray-1.
According to Holford &Mattingly (1979) andHoldorf (1980), an extractant is considered sensitive to the P buffering capacity in soil if P is extracted in direct relation to the P-labile content, but inverse to the P buffering capacity in soil.Thus, a comparison of the estimated percentage of P-triple superphosphate, extracted by the plant, with P contents evaluated by Mehlich-1 and Bray-1, indicated highest P contents in soils with lowest clay levels and presumably the lowest buffering capacity of P (Table 1, 2 and 4).Thus, in soil Di1 (loamy LVA), with the lowest MPAC value, dry matter production and P accumulated and extracted by sorghum were highest (Tables 1 and 2) showing, particularly in this soil, the greater efficiency of the plant drain compared to the soil drain.

Fractionation of inorganic P forms
Applying triple superphosphate to the soil similarly increased the values for P-Al, P-Fe and P-Ca (Table 7).
For the same P dose applied, the levels of this nutrient extracted by Mehlich-1, Bray-1 and resin were variable among soils (Table 8), due to the difference in chemical composition between extractants, resulting in an unequal extraction of P from inorganic forms in soils.The acidity of Bray-1 and especially Mehlich-1 causes a specific unequal action for P-Ca solubilization.However, the presence of fluoride in Bray-1 leads to Al complexation and a more efficient solubilizing action for the P-Al form, although with lower efficiency for the P-Fe form (Thomas & Peasle, 1973;Novais & Kamprath, 1979b).Thus, increased P levels extracted by Bray-1 reflect a higher incidence of P-Al and P-Fe forms in soils (Table 9).Furthermore, the significant (p < 0.01) correlation of P-Al with the P content extracted by Melhich-1 indicates, as theoretically expected, the extraction of P in this form by the sulfate of Mehlich-1.
The adjustment of correlations between dry matter yield and P accumulated in the plant shoots with the levels of P-Al, P-Fe and P-Ca (Table 10) shows that these correlations tend to be significant (p < 0.1) and positive for P-Al, thus highlighting the importance of this P form for plant growth and development (Novais et al., 2007).In the case of P forms P-Fe and P-Ca, these correlations tend to be negative and not significant.
Table 7. Fractionation (1) of the inorganic P forms in soil as related to the pH (4.5; 5.5 and 6.5) and the application of 200 mg kg -1 of triple superphosphate in the samples studied before and after the second liming, to raise the pH of samples to 6.5 (1) Only in soils with pH incubation values 4.5; 5.5 and 6.5. (2)Soils with pH value adjusted to 6.5 in the first liming were incubated for 30 days, then fertilized with P and incubated again for 40 days, without a second liming, since their pH values had been adjusted previously to 6.5 in the first liming.Table 8.Phosphorus content in soil as determined by the extractants Mehlich-1, Bray-1 and Resin, before and after the adjustment of different pH values to 6.5 (1) Soils with pH value adjusted to 6.5 in the first liming were incubated for 30 days, then fertilized with P and incubated again for 40 days, with out a second liming, since their pH values had been adjusted previously to 6.5 in the first liming.SL: Sete Lagoas Table 9. Simple linear correlation coefficients between the forms P-Al, P-Fe and P-Ca, and the P content determined by Mehlich-1, Bray-1 and Resin extractants before and after the adjustments of different pH values to 6.5 ns , * and **: non-significant and significant at 5 and 1 %, respectively.
The correlations between the P levels obtained by Mehlich-1 and Bray-1 (Table 11) suggest similar selectivity to the P-Al form, which may be related to preferential extraction of certain forms of inorganic P (Bahia Filho and Braga, 1975a;Novais & Kamprath, 1978).It was also observed that correlation coefficients obtained among the P levels extracted by Mehlich-1 and Bray-1 increased with increasing pH, and decreased when considering the correlations between these extractants and resin (Table 11).This behavior is possibly due to the consumption of acidity and of exchange anions SO 4 2-(Mehlich-1) and F - (Bray-1) in the extractants by soil constituents, a phenomenon responsible for the exhaustion or "wear" of the extractant (Bahia Filho , 1982), as stated before.
When the different incubation pH values were raised to 6.5, it was observed that the correlations obtained between the resin and Mehlich-1 and Bray-1 extractants were not significant (Table 11).This behavior suggests a lower sensitivity of Resin, when compared to Mehlich-1 and Bray-1, to physical and chemical characteristics, which reflect the buffering capacity of P in soil (Table 12), as stated by Novais et al. (2007).When the different pH values were raised to 6.5, P levels by Mehlich-1 and Bray-1 decreased, more expressively in the latter (Table 8).Besides the consumption of acidity in the extractants, this decrease is possibly due to the reductions observed in the P-Al levels (Table 7), since this behavior was different from P-Fe, where, in some cases, levels were higher, and from P-Ca, with similar contents to those observed at different pH values.
The trend of decreases of Resin-extracted P to a minimum value at pH 5.5 was detected in the soils with the highest initial exchangeable Al contents (Tables 1 and 8).This suggests that reactions involving Al, with increasing pH, must have caused substantial changes in the adsorption surface for P in the soil, due to the creation of new active adsorption sites.According to Haynes & Ludecke (1981), there is a very close relationship between the lime effect on Al precipitation, as amorphous Al hydroxide, and the increase of P adsorption by soil.Robarge & Corey (1979) studied P adsorption by hydroxide species of Al in Al-saturated resin and observed a similar behavior as found here.Sims & Ellis (1983) studied P adsorption and availability, after liming of an acid soil with high Al 3+ levels, and observed an increase in P adsorption in a period ranging from to 240 days.For these authors, a plausible explanation for the increase observed is possibly a continuous polymerization of Al-OH precipitate with time, would produce new surfaces of adsorbing more P.
The results of this study do not indicate the main factor behind the increasing trend of P-labile at pH 6.0 and higher, for most soils, detected by Resin (Table 8).However, this behavior may be partly due to the dissolution of crystalline phosphates of Fe and/or Al, original soil constituents, or due to competition of OH - for new P adsorption sites, as suggested by Fernandez et al. (2008a).

CONCLUSIONS
1.For most soil types, initial values of varying pH do not alter the availability from an applied soluble P source, measured by plant growth and P uptake.For other soil types, these results increased and decreased with increasing soil pH.
2. The correlations between plant growth, absorbed P and P extracted by Mehlich-1 and by Bray-1 were significant, but non-significant for resin-extracted P.
3. P retrieved by acid extractants depended strongly on the buffering capacity of P in soil, while for resin this dependency was smaller.
4. The P contents obtained by the extractants Mehlich-1 and Bray-1 were significantly correlated with each other in the entire acidic range of the soil tested.However, when these contents are compared to those extracted by resin, the correlations decrease with increasing soil pH.

Table 10 . Simple linear correlation coefficients between the forms P-Al, P-Fe and P-Ca, and P accumulated (P-ac) and dry material (DM) at different pH values
ns, ° ,* and **: non-significant and significant at 10 and 5 %, respectively.