Suitability of methods for soil P testing

Many factors influence plant growth and its P uptake. Plant roots take up orthophosphate ions from the soil solution. However, mass flow (the absorption of water from soil solution by the plant) can only explain 5–10% of the total P uptake. This means that a large fraction of phosphate ions taken up by the roots is desorbed from the solid phase of the soil during plant growth. Thus, available P can be defined as the phosphate ions that may leave the soil particles and enter into soil solution for uptake by the root hairs. The quantity of available P is therefore, the fraction of total soil P that could be made available for plant uptake and not the actual quantity of P extracted or removed by a crop [20].

Phosphorus fertilizer recommendations are based on the crop yield responses to increasing applications of P fertilizer in defined soils and environments. To calculate the optimum P fertilizer rate to be applied to a crop, it is necessary to estimate the soil available P. There are many routine methods for soil P testing to determine available soil P. Most routine methods are based on the extraction capacity of several chemical compounds simulating the P uptake by roots. A chemical extractant

40

when added to a soil will react with the predominant P forms of that soil; thus when changing either soils or extractants, the P extracted will differ widely. No single chemical method is suitable for all soils. In this study two common extractant methods were utilized which included Olsen and Bray II.

Other methods for soil P testing are based on the exchange reactions of phosphate ions in the soils.

One of these utilized in this study measures available P by anion exchange resins. Another exchange method employed for the quantification of available soil P is the ³²P exchange kinetics method, based on the Brownian motion principle of phosphate ions in a soil suspension (1:10 soil:water ratio ).

The soil P status is characterised through the determination of the intensity, quantity and capacity factors. The quantity factor (E₁ value), that is the amount of phosphate ions exchangeable after one minute of exchange is considered the available soil P.

Another method utilises iron oxide-coated filter paper (Pi) strips as sinks or collectors for the P in the soil suspension. From preliminary studies, this Pi method appears attractive for routine P testing in a wide range of soils.

The soils in study were classified into categories of low, medium and high content of available soil P based on arbitrary critical limits. The distribution of the soils is shown for the Olsen, Bray II, Pi, P resin, and ³²P exchange kinetics methods in Table X.

Data on amounts of available P obtained by Olsen and Bray II methods are different due to contrasting extracting power of the solutions utilised in these methods. However, the distribution pattern of the soils according to the available amounts is quite similar. The majority (some 60%) of soils have low P availability, whereas soils with medium and high availability are more or less equally represented (20% each). Considering their natural soil P fertility (total P content), it may be inferred that some of them, in particular those with high available P have received P fertilizers in the past.

In general, the results of iron strip P (Pi) showed a similar trend as indicated above for the chemical extraction methods but with a slightly higher number of soils (Ultisol and Inceptisol) on the medium and high availability category.

The distribution of soils according to P availability by the resin was similar to the one described above for the extraction and Pi methods; with about 60% for the low available P category and the rest more or less similarly distributed between medium and high available P categories. However, the distribution within the soil groups was most close between the Pi and P resin methods.

The amounts of available soil P determined by the exchange methods of resin and isotopic exchange kinetic varied widely, as indicated by the range and critical limits. A close examination of the E values revealed that some data, in particular those of the Inceptisols and one Oxisol with very high P fixation capacity were overestimated. The parameters determined by the isotopic method are largely influenced by the P fixing capacity of the soils [21]. Reclassifying these outliers as low available, the distribution of soils was as follows: low (23 or 74%), medium (6 or 19%) and high (2 or 6%). A particular limitation in most tropical acid soils is their very low P concentration in soil solution complicated by their high P fixing capacity and interference of colloidal particles, making it difficult to obtain accurate and reliable data on intensity and quantity factors (JC Fardeau, personal communication). Further studies are needed to improve these determinations such as use of ionic chromatography [22]

In this study, a preliminary evaluation of methods for available P was made but limited to a relative comparison of the laboratory tests. No greenhouse and field data from the agronomic testing were included. The participants of the project have conducted this study as part of their own work.

In summary, no conclusion can be made in this study on the suitability of the methods for soil P testing due to the lack of data on the agronomic evaluation. Further investigations on PR application to tropical acid soils, under well-defined management conditions, i.e: cropping system, cultivation, fallow, and P fertilization history, are needed to: a) establish relationships between crop yields and soil P supply in field experiments, and ii) group soil P supply into categories based on the probability of crop response to P fertilization.

41 TABLE X. DISTRIBUTION OF SOILS ACCORDING TO

OLSEN P, BRAY II P, IRON STRIP P (Pi), RESIN P, AND E₁ VALUES OLSEN P

<7 7–14 >14 mg P/kg Range

ULTISOLS 14 2 4 1.6–26.9

INCEPTISOLS 2 4 3 3.3–42.3

OXISOLS 5 1 - 3.2–7.9

TOTAL 21 (60%) 7 (20%) 7 (20%)

BRAY II P

<20 20–40 >40 mg P/kg Range

ULTISOLS 14 2 4 2.4–184.6

INCEPTISOLS 4 2 3 4.7–118.3

OXISOLS 5 1 - 13.1–21.2

TOTAL 23 (66%) 5 (14%) 7 (20%)

IRON STRIP P (Pi)

<2 2–4 >4 mg P/kg Range

ULTISOLS 10 5 5 0–21.6

INCEPTISOLS 3 2 4 0.5–15.5

OXISOLS 6 - - 0.1–1.5

TOTAL 19 (54%) 7 (20%) 9 (26%)

RESIN P

<10 10–20 >20 mg P/dm³ Range

ULTISOLS 12 4 4 1.8–81.9

INCEPTISOLS 3 1 5 2.5–89.4

OXISOLS 6 - - 3.7–9.0

TOTAL 21 (60%) 5 (14%) 9 (26%)

E₁ VALUES

<2 2–4 >4 mg P/kg Range

ULTISOLS 9+2 4 2 0.29–8.70

INCEPTISOLS 2 6 2.0–9.0

OXISOLS 5 1 - 0.81–3.9

TOTAL 16 (52%) 7 (23%) 8 (26%)

ACKNOWLEDGEMENTS

This work was carried out in the frame of the FAO/IAEA Co-ordinated Research Project “The use of nuclear and related techniques for evaluating the agronomic effectiveness of P Fertilizers, in particular rock phosphates”. The financial support of the IAEA and the “Institut Mondial du Phosphate”

(IMPHOS) is greatly acknowledged.

42

REFERENCES

[1] KHASAWNEH, F.E., DOLL, E.C., Use of phosphate rock for direct application. Adv. Agron.

30 (1978) 159-206.

[2] ZAPATA, F., Evaluating agronomic effectiveness of phosphate rocks using nuclear and related techniques: Results from an FAO/IAEA Coordinated Research Project. In: IAEA TECDOC 1159, IAEA, Vienna (2000) 91-100.

[3] AFNOR, Recueil de Normes “Qualité des sols”, volume 1 (566 pages) and 2 (408 pages). ISBN 2-12-213141-1, 1999 (http://www.afnor.fr).

[4] ORSINI, L., REMY, J.C., Utilisation du chlorure de cobaltihexamine pour la détermination simultanée de la capacité d’échange et des bases échangeables des sols. Bulletin de l’AFES, Science du Sol (1976) 269-275.

[5] OLSEN, S.R., COLE, C.V., WATANABE, F.S., DEAN, L.A., Estimation of available phosphorus in soils by extraction with sodium bicarbonate. USDA Circ. 939. US Gov. Print Office, Washington DC. (1954).

[6] BRAY, R.H., KURTZ, L.T., Determination of total, organic, and available forms of phosphorus in soil. Soil Sci. 59 (1945) 39-45.

[7] MENON, R.G., CHIEN, S.H., HAMMNOND, L.L., The Pi soil phosphorus test. A new approach to testing for soil phosphorus. IFDC Ref. Manual R-7 International Fertilizer Development Center, Muscle Shoals, Alabama, USA (1989) 10 p.

[8] HABIB, L., CHIEN, S.H., MENON, R.G., CARMONA, G., Modified Iron Oxide-Impregnated Paper Strip Test for soils treated with Phosphate Fertilizers. Soil Sci Soc Am. J. 62 (1998) 972-976.

[9] RAIJ, B. VAN, QUAGGIO, J.A., Metods de analise de solo para fins de fertilidade. Boletim Tecnico No.81. Instituto Agronomico de Campinas, Brasil (1983) 30 p.

[10] FARDEAU, J.C.; GIRAUD, G., MAROL, C., Bioavailable soil P as a key to sustainable agriculture. Functional model determined by isotopic tracers. In: Proc. Int. Symp. on Nuclear and related techniques in soil-plant studies on sustainable agriculture and environmental protection. IAEA, STI/PUB/947, Vienna (1995) 131-144.

[11] SHARPLEY, A.N., TIESSEN, H., COLE, C.V., Soil phosphorus forms extracted by soil tests as a function of pedogenesis. Soil Sci. Soc. Am. J. 51 (1987) 362-365.

[12] CHIEN, S.H., Reactions of phosphate rocks with acid soils of the Humid Tropics. In: Proc.

Workshop “Phosphate sources for Acid Soils in the Humid Tropics”, Kuala Lumpur, November 1990. Malaysian Soc. of Soil Science, Kuala Lumpur (1992) 18-29.

[13] GILKES, R.J., BOLLAND, M.D.A., The Australian Experience with Rock Phosphates:

Limitations and Explanations. In: Proc. Workshop “Phosphate sources for Acid Soils in the Humid Tropics”, Kuala Lumpur, November 1990. Malaysian Soc. of Soil Science, Kuala Lumpur (1992) 177-205.

[14] KANABO, I.A.K., GILKES, R.J., The role of soil pH in the dissolution of phosphate rock fertilizers. Fert.Res.12 (1987) 165-174.

[15] YAMUREMYE, F., DICK, R.P., Organic amendments and phosphorus sorption by soils. Adv.

Agron. 56 (1996) 139-185.

[16] HUGHES, J.C., GILKES, R.J., The effect of soil properties and level of fertilizer application on the dissolution of Sechura Rock Phosphate in some soils from Brazil, Colombia, Australia and Nigeria. Aust.J.Soil Res.24 (1986) 219-227.

[17] FROSSARD, E., FELLER, C., TIESSEN, H., STEWART, J.W.B., FARDEAU, J.C., MOREL, J.L., Can an isotopic method allow for the determination of the phosphate-fixing capacity of soils? Commun. Soil Sci. Plant Anal. 24 (1993) 367-377.

[18] OWUSU BENNOAH, E., SZILAS, C., HANSEN, H.C.B., BORGAARD, O.K., Phosphate sorption in relation to Aluminium and Iron Oxides of Oxisols from Ghana. Commun. Soil Sci.

Plant Anal. 28 (1997) 685-697.

[19] BOLLAND, M.D.A., BARROW, N.J., Effect of level of application on the relative effectiveness of rock phosphate. Fert. Res.15 (1988) 181-192.

[20] FARDEAU, J.C., Dynamics of phosphates in soils. An isotopic outlook. Fert. Res. 45 (1996) 91-100.

43 [21] XIONG LIMING, FARDEAU, J.C., ZHOU ZHIGAO, Suitability of isotopic kinetic approach to

assess phosphorus status and bioavailability of major acidic soils soil in subtropical China.

Pedosphere 7 (1997) 11-118.

[22] SINAJ, S., MACHLER, F., FROSSARD, E., FAISSE, C., OBERSON, A., MOREL, C., Interferences of colloidal particles in the determination of orthophosphate concentration in soil water extracts. Commun. Soil Sci. Plant Anal. 29 (1998) 1091-1105.

Part II

STUDIES FROM WESTERN EUROPE AND NORTH AMERICA

47 THE USE OF ³²P DILUTION TECHNIQUES TO EVALUATE