PHOSPHOROUS FIXATION AS INFLUENCED BY
SOIL CHARACTERISTICS OF SOME MAURITIAN SOILS

B Lalljee

University of Mauritius

ABSTRACT

Phosphorus fixing capacity of some soils from Mauritius was determined. Simple regression studies showed that amorphous Mn, amorphous Fe, organic matter, pH and clay content were dominant factors affecting P fixing capacity. Cation exchange capacity, exchangeable Na, K, Ca, B, Fe, Mn did not show any significant correlation. Liming, use of rock phosphate and calcium ammonium nitrate are recommended for acid soils of Mauritius to control P fixation.

INTRODUCTION

Without phosphorus in the environment, no living organism could exist. Phosphorus is present in all plant and animal tissues. It is now well understood that P-nutrition of crop plants is more of a soil problem and a higher dose of P is necessary for soils having high P -fixing capacities. In view of the economic desirability of having single fertiliser applications, the P-fixation capacity of our soils are of considerable importance in determining the requirement for applied P and may be indicative of possible adverse environmental effects.

The P fixation in soils depends upon many factors, namely the pH of the soil, organic matter content, type of clay and sesquioxides. Owusu - Bennoah and Acquaye (1989) studied the phosphate sorption characteristics of some Ghanaian soils and found that the phosphate sorption maxima were highly correlated with the soil properties in the order : Al2O3, clay content, free Fe2O3 and organic carbon. Morel et al. (1989) evaluated the phosphate fixing capacity of soils by the isotopic exchange techniques in north-east France and reported that there was a significant correlation between amount of phosphorus fixed, pH, exchangeable cations, clay content and soluble phosphate. Soon (1991) studied the solubility and retention of phosphates in soils of north western Canada prairies and found that correlations showed between P sorption capacity and clay content, Al-organic matter complexes and amorphous iron oxides were significant. Soils around Beemanique near Rose-Belle, were found to have a maximum P-fixation of 95.7% (MAFNR 1987).

MATERIALS AND METHODS

Twenty four representative top soil samples (0-20cm) under different types of cultivation (Table I) were collected around the island. The samples were air dried and analysed as follows: pH was determined on a 1 : 2.5 soil : water suspension using a pH meter. The international pipette method was used for particle size determination. Organic matter was determined by the Walkley and Black method as outlined by Jackson (1973). Exchangeable cations were extracted and estimated by flame emission and atomic absorption spectrophotometry as described by Cottenie (1980). Hot water soluble boron was estimated by the method of Sillanpa (1990). Amorphous Fe (Oxalate extractable ) and Mn were extracted by 0.2 M ammonium oxalate (pH 3) at a soil:extracting solution ratio of 1:50 (0.2 g soil extracted by 10 ml of extractant).

Table 1 Soil type and location

Soil location Region SoilTypes
Mauritius USDA

Sugarcane fields

Balaclava
Balaclava
BonAccueil
Cluny

Medine
Tamarin
TerreRouge

LHL
Regosols
LBF
HFL
LRP
DMC
LHL

Tropeptic Haplustox
Tipic Ustipsamment
Gibbsioxic Humitropept
Dystropetic Gibbsiaquox
Ustic Eurropept
Tropetic Torret
Tropeptic Haplustox

Tea fields

Bois Cheri

LaPipe

Midlands
Wootun

HFL
HFL
HFL
LBF

Dystropetic Gibbsiaquox
Dystropetic Gibbsiaquox
Dystropetic Gibbsiaquox
Gibbsioxic Humitropept

Experimental Station
Abercrombie
Barkly
Curepipe
Plaisance
Richelieu

LHL
LRP
LBF
LRP
LHL

Tropeptic Halustox

Ustic Eutropept
Gibbsioxic Humitropept
Ustic Eutropept
Tropeptic Haplustox

Grass cultivation GrandGorges
BaieduCap

Lithosols
Regosols

Lithic Ustropept
Tipic Ustipsamment
Vegetable production University Farm

LHL

Tropeptic Haplustox
Bareland RiviereChampagne

HL

Oxic Humitropept

DMC : Dark Magnesium Clay LBF : Latosolic Brown Forest

HFL : Humic Ferruginous Latosol LHL : Low Humic Latosol

HL : Humic Latosol LRP : Latosolic Reddish Prarie

Available P was determined by the modified Truog method, using 0.1 M H2SO4; the soil : extracting solution ratio was 1:50; and the shaking time 1 hr; estimation was done using the molybdenum blue colour method.

The phosphorus fixing capacity was evaluated by adding 25 ml aliquots of sodium dihydrogen phosphate (NaH2PO4) containing 100 micrograms P to 1 g of soil. The mixture was incubated for 1 week at room temperature and then centrifuged. The supernatant solution was carefully decanted and residues washed with distilled water and recentrifuged. The process was continued until the clear supernatant liquid showed no trace of phosphorus. The soil was then extracted as above with 0.1 M H2SO4. Amount of P fixed was obtained by subtracting the original P concentration from the value of P obtained after fixation (incubation).

RESULTS AND DISCUSSION

The physicochemical properties and phosphorus fixing capacity of the soils are given in Table 2.

The P fixing capacity varied from 143 ppm to 549 ppm, with a mean of 339 ppm.

The pH values were in the range of 3.8 to 8.1 with a mean of 6.3.

Values of organic matter were in the range 2.5% to 6.5% with a mean of 4.3%

Clay content varied from 15% to 38%, with a mean of 23.5%.

Amorphous Fe was found to be in the range of 0.50% to 1.50%, with a mean of 0.72%, while amorphous Mn was in the range of 100 ppm to 1121 ppm, with a mean of 602 ppm.

Table 2 Physicochemical properties and Phosphorus fixing capacity of the soils

Location

P Fixed

ppm

pH

Clay
%

Amorphous Fe %

Amorphous Mn

ppm

Abercrombie

433

5.0

38

1.30

1050

Alma

383

6.2

20

0.75

625

Baie du Cap

433

5.3

32

0.92

950

Balaclava

283

8.1

20

0.64

370

Barkly

366

6.7

21

0.85

650

Belle Mare

350

6.5

23

0.78

670

Bois Cheri

258

6.7

25

0.50

350

Bon Accueil

143

7.5

18

0.51

100

Chamarel

416

5.6

34

0.87

870

Cluny

325

6.0

25

0.69

642

Curepipe

500

4.1

25

1.50

1121

G. Gorges

241

7.4

18

0.59

328

La Pipe

275

6.2

15

0.62

250

M.Aux Vacoas

257

6.3

20

0.64

350

M. Longue

350

6.1

15

0.83

510

Medine

175

7.1

18

0.54

350

Midlands

466

6.4

35

0.79

925

Plaisance

400

5.7

30

0.84

940

R.Champagne

258

7.3

15

0.39

390

Richelieu

241

7.0

20

0.45

420

Tamarin

274

7.2

18

0.52

410

Terre Rouge

391

7.0

24

0.75

544

U. Farm

366

6.5

25

0.77

525

Wooton

549

3.8

30

1.20

1115

Mean

339

6.3

23.5

0.76

603

SD

101

1.0

6.6

0.27

294

Figure. 1 shows a significant relationship (r = 0.78) between pH and P-fixation. The lower the pH, the more available are the metallic ions, especially Mn, Fe and Al. These elements then combine with soluble P and form insoluble compounds. Similar results have been reported by Kanwar and Grewal (1990). Naidu et al. (1990) explained the increase in P- fixation with decrease in pH through interactions between added P, negative charge and the electrostatic potential in the plain of sorption.

The correlation coefficient between organic matter and P-fixation was also quite high (r = 0.83) (Figure 5).This may be due to the formation of phosphohumic compounds (Dolui and Gangopadhyay 1984). However other workers ( Awad and Al-Obaidy 1989) have found that addition of organic matter to soils increases the availability of the element (Dhargawe et al. 1991). The results show that P-fixation is quite a complex phenomenon depending upon many interacting factors

The relationship between P-fixation and clay content is shown in Figure 3. Our soils are mostly old soils (Oxisols and Ultisols). Most of them are rich in iron oxides and gibbsites. In oxide or oxide-coated layer silicate systems, P-fixation increases with increase in clay content. Woodruff and Kamprath (1965) found that sandy Ultisols retain much less phosphorus than clayey Ultisols of similar mineralogy

.

There was a significant correlation between amorphous Mn and P-fixation (r = 0.88), and between amorphous Fe and P-fixation (r = 0.86) (Figures 4 and 5). From Table 2 it can be seen that the levels of these two elements are quite high in our soils. The high correlation between them and phosphate fixation are due to the formation of insoluble Mn and Fe phosphates, especially in acidic soils. In similar studies, Thomazi et al. (1990) found that iron oxides and clay were the main factors contributing to P-fixation in some Brazilian soils.

The other properties of the soils studied, namely exchangeable Na, K, Mg, Zn, Cu, B, Fe and Mn did not show any significant correlation with P-fixation.

CONCLUSION

The results suggest that P-fixation depends upon many factors and is a quite complex phenomenon. In this study P-fixation was found to be significantly correlated with amorphous Mn, amorphous Fe, organic matter, pH and clay content. Considering the economic and environmental impact of phosphorus fertilisation, it would be desirable to control the process of P-fixation. Using lime or coral sand on acidic soils raises the pH and reduces Fe, Mn and Al responsible for P-fixation. Since ammonium sulphate has a soil-acidifying effect, CAN should be applied wherever possible. Rock phosphate is preferable to TSP in acid soils in order to minimise P-fixation.

REFERENCES

AWAD KM and AL-OBAIDY KS. 1989. Effect of organic residues on phosphate adsorption by some calcareous soils. Mesopotamia Journal of Agriculture 21 (4) : 53 - 67.

COTTENIE A. 1980. Soil and plant testing as a basis of fertilizer recommendation. FAO Soils Bulletin No. 38. Rome, Italy : FAO.

DHARGAWE GN, MATTUR DB, BABULKA PS, KENE DR and BORKAR DK. 1991. Availability of soil phosphorus as affected by organic matter. Journal of Soils and Crops 1 (2) : 142 - 146.

DOLUI AK and GANGOPADHYAY SK. 1984. Fixation of phosphate in relation to properties of some red and lateritic soils of West Bengal. Indian Journal of Agricultural Chemistry XVII (2) : 177 - 182.

JACKSON ML. 1973. Soil chemical analysis. India : Prentice - Hall.

KANWAR JS and GREWAL J. 1990. Phosphorus fixation in Indian soils. 2nd edition. New Delhi, India : Indian Council of Agricultural Research.

MAFNR see under Ministry of Agriculture, Fisheries and Natural Resources

Ministry of Agriculture, Fisheries and Natural Resources:1987. Annual Report of the Agricultural Services for the year 1984.

MOREL JL, FARDEAU JC, BERUFF MA and GUCKERT A. 1989. Phosphate fixing capacity of soils : a survey using the isotopic exchange technique of soils from north-eastern France. Fertiliser Research 19 (2) : 103 - 111.

NAIDU R, SYERS JK, TILLMAN RW, and KIRKMAN JH. 1990. Effect of liming on phosphate soption by acid soils. Journal of soil science 41 (1) : 163 - 175.

OWUSU-BENNOAH E and ACQUAYE DK. 1989. Phosphate sorption characteristics of selected major Ghanaian soils. Soil Science 148 (2) : 114 - 123.

SILLANPA M. 1990. Micronutrient assessment at the country level : an international study. FAO Soils Bulletin No. 63. Rome, Italy : FAO.

SOON YK. 1990. Solubility and retention of phosphate in soils of north western Canada prairie. Canadian Journal of Soil Science 70 ( 2 ) : 227 - 237.

THOMAZI MD, MELLO FAF, ARZOLLA S and MELLO FA. 1990. Phosphate fixation in soils of the Piracicaba Municipality. Revista de Agricultura 65 : (1) : 45 - 53.

WOODRUFF JR and KAMPRATH EJ. 1965. Phosphorus adsorption maximum as measured by Langmuir Isotherm and its relationship to phosphorus availability. Soil Science Society of America Proceedings 29,.p.148 - 280.

COMMENTS

Q. Why is Calcium Ammonium Nitrate (CAN) used in acid soils as compared to ammonium sulphate?

A. The lowering of pH with the application of ammonium sulphate is due to ammonium ions. The presence of calcium ion in CAN helps to prevent acidification.