AVAILABILITY OF P FROM PHOSPHATE ROCK, THERMOPHOSPHATE AND TRIPLE SUPERPHOSPHATE AFTER DIFFERENT INCUBATION PERIODS
W. B. SCIVITTARO EMBRAPA,
Pelotas
T. MURAOKA, A.E. BOARETTO CENA,
Piracicaba E.C. BRASIL EMBRAPA, Belem Brazil
Abstract. Inorganic forms of phosphorus were determined in two Red-Yellow Latosols, which were incubated with P fertilizers for a month, after equilibration with 32P for 60 hours. The methods used were soil P fractionation combined with the 32P isotopic dilution technique. The fertilizers applied were: H3PO4, 10-30-0 suspension, MAP and triple superphosphate, at the rate of 92 mg P kg-1 of soil. In both soils, the isotopically exchangeable inorganic phosphorus fractions decreased in the following order: water soluble P >
Al bound P > Fe bound P > calcium bound P > occluded-P. The water- soluble and Al bound P were the main source of available P for the newly fertilized soil. The Fe bound phosphate was also an important source of available P in both soils when fertilizer was not applied. The soil P fixing capacity affected the availability of native and added phosphorus.
1. INTRODUCTION
In most Brazilian soils, as in other tropical soils, P deficiency is the main limitation to optimum crop yields due to low natural P fertility and high P retention capacity. The application of phosphate fertilizers in these soils is essential to supply P to crops. Rational utilization of P fertilizer, however, requires a better understanding of the availability of this element to plant roots in the soil. Usually, less than 10% of applied fertilizer P is utilized by an annual crop [1], the rest being fixed in several forms of inorganic P [2]. Information on the fate of fertilizer P remaining in the soil is scarce in Brazil. Dunbar and Backer [3] presented interesting results, evaluating the inorganic phosphate transformations in the soil using 32P. They observed that the increase in the active P of solid phase is associated with greater availability of phosphorus in determined soil types. The objective of this research was to determine, by isotopic dilution, the availability of native and added phosphates in two Latosols with different P fixation capacity.
2. MATERIAL AND METHODS
Two surface (0-20 cm) soils, classified as very clayey Alic Red-yellow Latosol (soil A) and medium texture Red-Yellow Latosol (soil B), with different physical and chemical properties were utilised (Table I). Prior to P fertilizer application, the soil was limed and incubated for 20 days. Then, the soil was transfered to 200 g capacity plastic pots. Four P sources, namely H3PO4, 10-30-0 coloidal P suspension (susp.), Mono-ammonium phosphate (MAP), and triple superphosphate (TSP) were applied at a rate of 92 mg P/kg soil. A control without P application was also included. After mixing with fertilizers, the soil was alternatively moistened and air-dried weekly for one month.
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TABLE I. PHYSICAL AND CHEMICAL CHARACTERISTICS OF THE SOILS
Soil Sand Silt Clay pH P* Pfc** SiO2 Al2O3 Fe2O3 TiO2
---%--- ---mg P cm-3--- ---%---
A 36 8 56 4.1 3 1118 19.0 19.2 19.7 6.5
B 86 0 14 4.2 2 525 2.6 4.1 2.1 0.7
* Resin extractable P [10]
Soil H+Al Al3+ Ca2+ Mg2+ K+ OM
---cmol dm-3---
A 15 1.4 0.39 0.13 0.11 6.3
B 6.1 0.7 0.25 0.10 0.10 2.9
** Pfc: fixing capacity [11]
TABLE II. SOIL PHOSPHORUS CONTENT (TOTAL AND INORGANIC FRACTIONS)
---P fraction---
P source Total P
PH2O PAl PFe PCa POccluded
---mg P/g soil--- (Soil A)
Control 1 47 b* 50 a 18 b 239 386 b
H3PO4 1 88 a 74 a 21 b 214 473 a
Susp. 1 84 a 78 bc 22 b 238 458 a
MAP 1 93 a 88 a 21 b 210 481 a
TSP 1 88 a 88 ab 29 ab 218 475 a
CV (%) 17 8 5 9 8 9
(Soil B)
Control 1 b 42 b 23 c 7 b 52 b 180 b
H3PO4 2 a 91 a 29 b 6 b 66 a 267 a
Susp. 1 b 99 a 34 a 10 b 52 b 257 a
MAP 2 a 91 a 28 b 7 b 59 ab 273 c
TSP 2 a 93 a 31 ab 18 a 70 a 236 a
CV (%) 17 6 7 29 10 4
133 TABLE III. PERCENTAGE OF ISOTOPICALLY EXCHANGEABLE P OF
SOIL INORGANIC FRACTIONS AFTER INCUBATION WITH FERTILIZERS
---P fraction---
P source Total P
PH2O PAl PFe PCa POccluded
---%--- (Soil A)
Control 100 21 b 20 6 b 2 a 7 b
H3PO4 100 35 a 18 9 a 1 b 10 a
Susp. 100 32 a 17 9 a 1 b 10 a
MAP 100 32 a 16 9 a 1 b 10 a
TSP 100 34 a 16 7 ab 1 b 10 a
CV (%) - 13 21 16 20 8
(Soil B)
Control 100 31 30 a 7 0.4 12 b
H3PO4 100 41 17 b 7 0.3 16 a
Susp. 100 39 15 b 4 0.4 18 a
MAP 100 35 18 b 6 0.4 15 a
TSP 100 37 16 b 2 0.3 15 a
CV (%) - 16 10 24 10 7
Soil samples (1.0g) were shaken with 5 mL carrier free 32P solution (206 KBq) for 60 hours for attaining isotopic equilibrium [3]. The suspensions were then centrifugated and the soils were analysed for P fractions by the Chang & Jackson technique [4]. Total P was also determined in each soil sample, by perchloric-nitric-hydrofluoric acid digestion [5].
The 32P activity was determined by counting in a liquid scintillation counter using the Cerenkov effect and total P was analysed by colorimetry. The amount of solid phase P of each P fraction, which equilibrated with 32P was considered as available P.
3. RESULTS
The total soil P contents, which include the native and added P, were higher in soil A than soil B, as a consequence of the higher clay content of soil A (Table II). In both soils, the lowest amount of P was observed in the water-soluble fraction (PH2O). In the soil A, the native phosphates are constituted mainly by the occluded P fraction (Pocluded), followed by iron bound P (PFe) and aluminum bound P (PAl), both in equivalent amounts, and calcium bound P (PCa). For soil B, the sequence obtained for native P was Poccluded > PAl > PFe > PCa > PH2O. The predominance of Pocluded , PFe and PAl reflect the advanced weathering of these soils, as with the evolution of pedogenesis, the calcium phosphates are converted to aluminum and iron phosphates and finally, as ocluded iron and aluminum phosphates [6, 11].
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TABLE IV. AVAILABLE P IN THE SOIL P FRACTIONS, AS DETERMINED WITH 32P
---P fraction---
P source Total P
PH2O PAl PFe PCa POccluded
---mg P/g soil--- (Soil A)
Control 1 10 b* 50 a 18 b 239 386 b
H3PO4 1 31 a 74 a 21 b 214 473 a
Susp. 1 27 a 78 bc 22 b 238 458 a
MAP 1 30 a 14 a 21 b 210 481 a
TSP 30 a 13 a 29 ab 218 475 a
CV (%) 17 8 5 9 8 9
(Soil B)
Control 1 b 42 b 23 c 7 b 52 b 180 b
H3PO4 2 a 91 a 29 b 6 b 66 a 267 a
Susp. 1 b 99 a 34 a 10 b 52 b 257 a
MAP 2 a 91 a 28 b 7 b 59 ab 273 c
TSP 2 a 93 a 31 ab 18 a 70 a 236 a
CV (%) 17 6 7 29 10 4
The low calcium bound phosphate content observed even after fertilizer additions could be a consequence of the acid conditions predominant in these Latosols. It is interesting, however, to note that soil B fertilized with TSP presented a significantly higher PCa fraction than the other sources.
Probably part of this fertilizer had not reacted with the soil components, remaining as calcium phosphate.
A considerable fraction of applied fertilizer P was converted to PAl and PFe fractions, altering the original distribution of P fractions. This was especially the case in soil B, where the PAl fraction became predominant. Yuan et al. [7]; Chang and Chu [8]; and Gonçalves [9] have reported similar results.
The isotopically exchangeable P determined in the P fractions in the soils (A and B) is presented in Table III. The PAl fraction is the most available form. However, in the control, which represents the availability of native P, the contribution of PFe to available P is comparable to PAl fraction. For both soils, the portion of P fractions which equilibrated with 32P could be ordered as follows: PH2O >
PAl > PFe > PCa > Pocluded. Dunbar and Backer [3] observed the some sequence for soils with contrasting chemical and mineralogical characteristics.
Although the amount of total available P found in both soils were similar, comparing this value with total P content (Table IV), the availability of P in soil A is proportionally lower than soil B. This
135 result corroborates the work from Chang and Chu [8], where soils with high P fixing capacity have P adsorbed in less available form.
4. CONCLUSIONS
The main conclusions from the study were:
The inorganic P fractions which equilibrated with solution 32P followed the sequence: PH2O > PAl > PFe
> PCa > Poccluded, in both soils, indicating that the availability of inorganic P in each fraction decreases from PH2O to the Poccluded fraction.
The PH2O and PAl fractions constituted the main sources of available P in freshly fertilized soil. In non-fertilized soils, PFe is another main source, beside PH2O and PAl.
The availability of native and added phosphates was higher in the soil B, with smaller P retention capacity.
ACKNOWLEDGEMENTS
Financial and technical support of the International Atomic Energy Agency (IAEA) under research contract BRA-7500 is greatly acknowledged.
REFERENCES
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