Abstract

Eutrophication of the Baltic Sea is a serious problem. A major contributing factor is diffuse losses of phosphorus (P) from agricultural land in surrounding countries. In order to estimate P losses, environmental monitoring of small agriculture-dominated catchments is being carried out
in most of these countries. Evaluation of the risk of P leaching to waters is usually based on chemical tests originally developed to quantify the amount of soil P available for plant production. The tests are performed in different ways in the different countries and a number of different
extraction agents are in use. The ammonium lactate method (P-AL) is used in Sweden and Lithuania, the double lactate method (P-DL) in Latvia and Poland, the Mehlich 3 method (P-M3) in Estonia and the Olsen method (P-Olsen) in Denmark.

A total of 99 soil samples from five agricultural catchments and two field trials in the Baltic States and Sweden were extracted according to the four methods listed above. The amount of P was then quantified either colorimetrically or by inductively coupled plasma (ICP) spectrometry
in accordance with the practices of the respective country. The amount of P determined by ICP spectrometry was nearly always (in 98% of cases) higher than that determined colorimetrically, with an average difference of 19% in plant-available P. The amount of P extracted by the four
methods increased in the order Olsen-P < P-DL . P-M3 < P-AL, with Olsen-P values being on average only 24% of P-AL values. The different active agents used in the four methods differ in their efficiency in desorbing and releasing P from minerals and organic compounds.

In the Baltic Sea area, neither a P adsorption index (PSI) or the amounts of P in relation to aluminium (Al-AL) and iron (Fe-AL) in the acidic AL extract is suggested to be a general good predictor of soil capacity to adsorb P or release dissolved reactive P (DRP) to water, based on the
results from the limited number of sites in the present study.

Table of contents

Introduction ................................................................................................................................................................. 1
Materials and Methods ............................................................................................................................................... 2
　Soil samples .............................................................................................................................................................. 2
　P extraction methods ................................................................................................................................................ 5
　P analysis methods ................................................................................................................................................... 6
　Calculations ............................................................................................................................................................. 6
　Statistics ................................................................................................................................................................... 7
Results .......................................................................................................................................................................... 7
　P analysis by colorimetry and ICP spectrometry ...................................................................................................... 7
　Comparison of different extraction methods ............................................................................................................. 8
　Soil P characterisation as environmental index ...................................................................................................... 11
Discussion ................................................................................................................................................................... 12
　Extraction methods ................................................................................................................................................. 12
　Environmental P index ........................................................................................................................................... 14
Conclusions ................................................................................................................................................................ 15
Acknowledgements .................................................................................................................................................... 15
References .................................................................................................................................................................. 16
　Literature ................................................................................................................................................................ 16
　Personal communications ...................................................................................................................................... 19
　Internet references .................................................................................................................................................. 19

Appendix 1 ................................................................................................................................................................. 20
Appendix 2 ................................................................................................................................................................. 21
Appendix 3 ................................................................................................................................................................. 25
Appendix 4 ................................................................................................................................................................. 29
Appendix 5 ................................................................................................................................................................. 31
Appendix 6 ................................................................................................................................................................. 32
Appendix 7 ................................................................................................................................................................. 33
Appendix 8 ................................................................................................................................................................. 35
Appendix 9 ....................................................................................................................... Error! Bookmark not defined.
Appendix 10 ..................................................................................................................... Error! Bookmark not defined.

P extraction methods

The ammonium lactate method (P-AL) according to Egner et al. (1960) and Swedish standard SS 02 8310 (Svensk Standard, 1993, 1995) is used in Sweden and Lithuania. In the present study, 5 g air-dried soil were weighed and placed in a 200 mL shaker bottle together with 100 mL extract
solution (0.01 M ammonium lactate (NH4CH3CH(OH)COO) and 0.40 M acetic acid (CH3COOH)). The samples were shaken by end-over-end rotation and the extract was filtered (OOH-filter) and analysed colorimetrically for P. The same extraction method was used in a commercial laboratory and the extract was analysed for P by ICP spectrometry.

The double lactate (P-DL) method was performed according to Riehm (1943) and Thun & Herrmann (1953). Two grams of air-dried fine soil were placed in a 200 mL shaker bottle with 100 mL extract solution (0.02 M calcium lactate (Ca[CH3CH(OH)COO]2) and 0.02 M HCl). The samples were shaken by end-over-end rotation and the extract was filtered (OOH-filter) and analysed colorimetrically for P.

The Mehlich 3 method is commonly used in some parts of the U.S. and in 2004 it was adopted in Estonia. The procedure according to Mehlich (1984) and Sims (2000) was used in the present study. Two grams of air-dried soil were placed in Erlenmeyer flasks with 20 mL extract solution
(0.2 M acetic acid (CH3COOH), 0.25 M ammonium nitrate (NH4NO3), 0.015 M ammonium fluoride ([NH4]F), 0.013 M HNO3 and 0.001 M EDTA). Flasks were shaken on a shaking table and the extracts were then filtered (OOH-filter) and analysed colorimetrically for P.

The Olsen-P method was performed according to Olsen et al. (1954). One gram of air-dried soil and 20 mL extraction solution (0.5 M NaHCO3) (pH 8.5) were mixed by shaking at 200 or more vibrations per min for 30 min. The suspension was filtered (OOH-filter) and analysed for P by
ICP. Extraction and analysis were carried out at a commercial laboratory.

The main cation, its concentration and the pH in the different extracts varied as shown in Table 5.

P analysis methods

In the colorimetric method for analysis of the amount of P in the extract solution, 5 mL filtrate were transferred into a 50 mL calibrated flask together with 8 mL mixed reagent according to Murphy & Riley (1962) containing 1.25 M sulphuric acid (H2SO4), 0.03 M ammonium
molybdate ((NH4)2MoO4), 0.01 M ascorbic acid (C6H8O6) and 0.0002 M potassium antimony tartrate (C4H4O7SbK). The volume of solution in the flask was made up to 50 mL with deionised water. The reagent formed a molybdate blue complex together with orthophosphates, also called
molybdate reactive P (MRP), giving an intense pure blue colour, which was analysed on a spectrophotometer (904 nm) after 1 h. The series with standard solutions was prepared with extraction solutions.

With ICP spectrometry, the solution is converted at a very high temperature into an aerosol, or plasma, where the atoms exist in an ionised state. Ions in this state emit light, which is converted into an electronic signal that can be measured quantitatively by a photomultimeter, which
measures the intensity of light of a specific wavelength (Bradford & Cook, 1997).