Maataloustieteellinen Aikakauskirja
Vol. 62: 213—219, 1990

Comparison of a bioassay and three chemical methods for determination
of plant-available P in cultivated soils of Finland

MARKKU YLI-HALLA
Kemira Oy, Espoo Research Centre, Luoteisrinne 2,
SF-02271 Espoo, Finland

Abstract. Phosphorus was extracted from 32 field soil samples with water (P w), with 0.5
M NH4 -acetate-0.5 M acetic acid at pH 4.65 (Paaac) and by a method in which freshly precipi-
tated iron hydroxide is used as the sink for P desorbing from the soil (P ( method). Pi is sup-
posed to be the quantity of reversibly adsorbed P in the soil. The results of the three chemical
methods were compared with the ones obtained in a bioassay in which four yields of ryegrass
were grown in 0.2 dm 1 of soil. The grass took up P effectively and the P reserves of the soils
were probably the growth limiting factor. The quantities of P taken up corresponded to 5—21 %

(median 10 %) of soil inorganic P. The uptake ofP by the grass was approximately twice the
quantity of P, (median 25.9 mg/dm J) and several times higher than the quantities of P„
(median 6.3 mg/dm') and PA aac (median 6.4 mg/dm3 ). The results of all three chemical
methods predicted P uptake by ryegrass accurately, the correlation coefficients (r) ranging from
o.BB*** to o.93***. However, in soils low in P (PAAAc <median), P uptake correlated more
closely with P, (r =o.B7***) than with P„ (r =0.57*) or Paaac (r =0.64**). The phosphorus tak-
en up by ryegrass and extracted by the chemical methods probably originated in the same type
of P reserves of the soil. The extractions with water or AAAc seem to be sufficient for ranking
Finnish soils according to their reserves of plant-available P. If a more quantitativemeasure
of the size of desorbable reserves of P is needed, the determination of P| may be of use.

Index words: pot experiment, water extraction, ammonium acetate extraction, reversibly adsorbed P

Introduction

Assessment of reserves of plant-available P
by soil analysis is important for crop produc-
tion as well as from the environmental point
of view. Even though soil testing methods
used as the basis for fertilizer recommenda-
tions need eventually be calibrated against

results from field experiments, pot experi-
ments are practical for preliminary screening
of various methods. Extraction methods like
water and resin extraction, which do not de-
stroy the microstructure of the soil, have in
pot experiments been more in agreement with

213

JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND

https://www.c-info.fi/en/info/?token=Q_yQZUkXGmQGB-TC.xtk7_LBwqf4h_DxCt5aMKw.S6Od7Kpv9di5e4-BFFyMRRHteDvBP4uHABPcq1jTslB-Y9zZ5EniCqcNgYOdXtlFraca55SBQuxd9H6xX-6uGSK166XCcoz0DTDfPg8GmhrA3DjaO9pRXqOiAm3Uc09BrS_YV41g_RZp-Y95Zq9tiW2kcPiO60o6nD-j9H0J


P uptake by plants than have methods in
which strongly dissolving extractants are used
(Sibbesen 1983). Also in Finnish soils, water
and resin extractions have proved successful
in predicting the uptake of soil P by plants in
pot experiments (Aura 1978, Sippola& Jaak-
kola 1980).

A new method (Pj), which is claimed to
give the quantity of reversibly adsorbed P, has
recently been introduced (Zee et al. 1987,
Menon et al. 1989). In this method, the P
desorbing from the soil is trapped by strips of
filter paper which are impregnated with fresh-
ly precipitated iron hydroxide. Owing to the
high affinity of phosphate for iron hydroxide,
the concentration of P in the liquid phase dur-
ing the extraction is extremely low, which pro-
motes desorption of P from the soil. When
this method was tested with soil samples from
Finland (Yli-Halla 1989), the mean (31.2
mg/dm 5 ) of Pj was three times higher than
the mean of water-extractable P. The objec-
tive of the present investigation was to study
the ability of the new Pj method to predict P
uptake by ryegrass in a pot experiment as
compared to extraction with water or acid
NH 4-acetate which is the standard method
for soil testing in Finland. In addition, some
remarks on the relationship between P uptake
and the fractions of inorganic P in the soil
were made.

Materials and methods

The soil material consisted of 32 samples
taken from plough layers (A p horizon) of
cultivated fields in different parts of Finland.
There were 11 clay soils, 18 coarse mineral
soils and three organogenic soils. All mineral
soil samples except one were used in the previ-
ous study (Yli-Halla 1989) on reversibly ad-
sorbed P. Phosphorus extractable with water
(P w) was determined according to the meth-
od of Hartikainen (1982). Phosphorus was
also extracted with 0.5 M NH4-acetate 0.5
M acetic acid at pH 4.65 (P AAAc ) (Vuorinen
and Mäkitie 1955) which is the method used
in soil testing in Finland. Reversibly adsorbed
P (Pj) was determined according to the meth-
od of van der Zee et al. (1987) in which
freshly precipitated Fe hydroxide is used as the
sink for P desorbing from the soil while us-
ing 0.01 M CaCl2 as the supporting electro-
lyte. The method has been discussed in more
detail previously (Zee et al. 1987, Menon et
al. 1989, Yli-Halla 1989). Other soil ana-
lyses performed, including the Chang and
Jackson fractionation of inorganic P, have
also been reported earlier (Yli-Halla 1989).
Chemical and physical characteristics of the
current soil material are presented in Table 1.

For the pot experiment, two 200 ml portions
of each soil were measured, fertilized and

Table 1. Physical and chemical properties of the experimental soils.

Soil Mean Median Range Standard
characteristics deviation

Clay, % 29 25 2—71 19
Organic C, % 5.9 3.2 1.9—30 7.7
pH (CaCl2) 5.1 5.0 4.0—6.9 0.6
Fe*, mmol/dm J 64.2 61.0 30.0—136 23.3
Al*, mmol/dm 1 70.2 52.0 26.0—240 46.2
NH4F-P (“Al-P”), mg/dm 1 123 76 13—401 105
NaOH-P (“Fe-P”), mg/dmJ 171 151 33—419 86
H2 S04 -P (“Ca-P”), mg/dm J 178 161 41—531 106
Paaac, mg/dm’ 11.2 6.4 1.7—51.5 12.0
Pw ,

mg/dm J 10.4 6.3 2.3—39.0 9.8
Pi, mg/dm J 33.0 26.0 7.6—64.2 (149)** 16.5

* extracted with 0.05 M NH 4-oxalate at 3.3
** xhe value in parentheses refers to a finesand soil deviating from the rest of the material

214



poured into 0.5 dm3 plastic boxes. Forty seeds
of Italian ryegrass {Lolium multiforum, Lam.)
were sown, and the pots were covered with 250
g of acid-washed quartz sand. Deionized wa-
ter was used for watering. The following
quantities of nutrients (mg/dm 3 of soil) were
initially mixed with the soil: N 142 as
NH4NQ 3 , K 100 as KCI, Mg 52 as
MgS04 • 7H 20, Ca 126 as CaS04

• 2H 2 G, S
175 mainly as CaS04 • 2FLO and MgS04 ■

7H 20, Mo las Na 2Mo04 ■ 2H20, B 0.5 as
H,803 , Zn 3 as ZnS04 • 7H 20, Cu 3 as
CuS04

• 5H 20, Mn 4as MnSQ4 • 4H 20 and
Fe 2as FeS04

• 7H 20. For the three succes-
sive crops, N (200 mg/dm3) as NH 4N0 3 and
K (100 mg/dm3 ) as KCI were applied onto the
surface of the pots. The growing periods for
the four crops were 34 d, 30 d, 22 d and 27
d, respectively. The plant material was dried
at 60°C. In order to reduce the number of
plant analyses, the first two and the last two
yields were combined. The plant material was
analyzed for P by a vanadomolybdate meth-
od (Saari and Paaso 1980). At the end of the
experiment, the pH(CaCl 2 ) of the soils taken
from the pots was determined.

Results

Due to the small volume of the soil in the
pots and the high rate of N fertilization, the
growth of the grass was very intensive; the
quantity of dry matter produced (Table 2) cor-
responded to 50—95 tons/ha, provided the
plough layer contains 2 million dm3 of soil.
In general, the P content of the plant mate-
rial was low (Table 3). The plant material
produced in 17 soils had a P concentration

Table 2. The size of the four consecutive dry matter yields
(I, 11, 111, IV) produced by ryegrass in the pot experiment.

Yield Mean Median Range
g/dm J g/dm ! g/dm J

I 7.1 7.6 2.8—10.1
II 11.8 12.2 6.9—14.3
111 8.9 9.3 3.9—14.2
IV 9.3 10.1 3.5—12.5
Total yield 38.1 39.5 17.1—47.5

equal to or less than 1.0 g/kg in the last two
yields, suggesting P deficiency. On the other
hand, in one finesand soil the concentration
of P in the plant material was maintained at
the normal level of 2.9—3.0 g/kg throughout
the experiment; the quantity of P, was as
high as 149 mg/dm3

, i.e. more than double
the second highest result, revealing the large
reserves of plant-available P in that specific
soil. Quantities of P taken up by the four
yields were several times larger than those of
P AAAc or P* and nearly twice as large as the
quantity of P,. Phosphorus uptake cor-
responded to 5—21 % (median 10 °/o) of the
sum of inorganic P fractions (“Al-P” 4- “Fe-
P” + “Ca-P”) in the soil.

Linear correlation coefficients (r) between
dry matter yields, P uptake and soil charac-
teristics were calculated. In these calculations,
the finesand soil extremely rich in P was ex-
cluded due to its drastic deviation from the
bulk of the material. The correlation coeffi-
cients calculated separately for different yields
or for the sums of the yields were of equal
magnitude. Therefore, only the correlation
coefficients for the total yield and total P up-
take are presented (Table 4). P uptake by the
grass correlated more closely with P w , P AAAc

Table 3. P concentration of the plant material and the uptake of P by the crop in the pot experiment. The results of a
finesand soil very rich in P are given in parentheses.

Yield P concentration, g/kg P uptake, mg/dm J

Mean Median Range Mean Median Range

I+ 11 1.4 1.2 0.8—2.9 (3.0) 27.9 22.5 6.0—72.0 (75.5)
UI +IV 1.2 1.0 0.7—2.0 (2.9) 22.4 17.8 7.0—50.5 (77.5)
All yields 49.4 40.3 15.0—123 (151)

215



Table 4. Linear correlation coefficients (r) between dry
matter yields and the different indicators of soil P status.

Indicator of Dry matter yield P uptake
soil P status

P w 0.54** o.92***
P AAAc 0.46** o.Bß***
p. o.6l*** o.93***
“Al-P” 0.15" * 0.38*
“Fe-P” 0.40* 0.33n!

“Ca-P” o.64*** o.66***
“Al-P’VAI 0.39* o.79***
“Fe-P”/Fe 0.40* 0.47**

and Pj than did the dry matter yields. The
quantities of P taken up correlated statistically
very significantly with the results of all three
chemical methods (Fig. 1). In pot experiments
very poor in P (usually P w<4 mg/kg), the P,
method has been superior to water extraction
in predicting P uptake by soybean and maize
(van der Zee, personal communication). There-
fore, also in the current material the soils of
P AAAc< mg/dm (i.e. below the median)
were studied separately. Among these soils,
the results of the P| method predicted the up-
take of P by ryegrass more accurately than did
the P w and P AAAc methods. The last two
methods tended to overestimate the P status
of coarse mineral soils and to underestimate
that of clay soils. The linear correlation coeffi-
cients (r) between P uptake and P is Pw and
Paaac f°r these soils (n= 16) were as follows:

r
o.B7***
0.57*

Pi
P.w

0.64**P AAAc

Among the fractions of inorganic P, “Ca-
P” correlated most closely with dry matter
yields and P uptake by plants (n = 31) (Table
4). A reason for this may lie in the acidifica-
tion of soil in the pots. The decrease of pH
during the experiment ranged from 0.1 to 0.9
pH units, mean 0.5 pH units. Calcium phos-
phates are acid-soluble and this fraction may
partly have dissolved and contributed to P up-
take by the grass in the pot experiment. The
absolute quantities of “Al-P” and “Fe-P” did

Fig. I. Relationships between the quantities of P extracted
with different methods (P w , P Aaac P.) and P uptake by
ryegrass in a pot experiment.

216



not correlate statistically significantly or cor-
related only weakly with P uptake by the
grass. But the molar ratio “Al-P’VAI, reflect-
ing the P coverage of poorly crystalline A 1
compounds in the soil, correlated positively
with P uptake.

Discussion

The current P concentrations of the grass
were low, with quite a few extreme values, as
compared to the results by Kähäri and Nis-
sinen (1978) on field-grown timothy. In that
study, the bulk of P concentrations in 2 015
timothy samples was in the range of 2.0—3.0
g/kg, which is approximately twice the con-
centration commonly found in the present
study. Low P concentrations in the grass as
well as the positive correlation between the
yield and the indices of soil P status suggest
that P supply to the plants has been the
growth-limiting factor in the pot experiment.
When plants take up P from soil, the most
easily soluble reserves are depleted first, and
gradually the less soluble fractions are utilized.
These sparingly soluble P reserves maintain a
lower P concentration in the soil solution.
Consequently, after the P concentrationof the
soil solution decreases below the critical level,
P nutrition of the plants is reduced and growth
is depressed. Owing to the apparent P defi-
ciency experienced by the grass, it can be as-
sumed that the reserves of plant-available P
were practically used up in the course of the
pot experiment. The quantities of P taken up
in the experiment cannot, however, be re-
garded as the estimates of the reserves of P
available to plants in practical farming. In a
pot experiment, the dense root system with its
exudates and the acidification of the soils in
the pots have probably caused dissolution of
phosphates which would be virtually insolu-
ble in the field. Furthermore, utilization of soil
P to the extent measured in the pot experiment
would probably result in a seriously limited
P supply to the plants and in a loss of the
potential yield. Yet, the results may give an

estimate for the maximum quantity of P
which, in aerobic conditions, can be dissolved
from the soil by plants at extreme P starva-
tion. The study showed that the plants were
able to utilize only a small fraction of soil P.
This suggests that the bulk of soil P is proba-
bly of no practical significance in P nutrition
of plants.

The bioassay and the chemical methods
(Pw , Paaac a°d Pj) probably extracted P from
the same reserves, since theresults of all three
chemical methods correlated highly with P up-
take in the pot experiment, as far as the whole
soil material was concerned. According to
earlier results, the P coverage of poorly crys-
talline A 1 compounds in the soil (i.e. molar
ratio “Al-P”/A1) seems to control the level
of P w (Hartikainen 1982) as well as that of
Paaac and Pi (Yli-Halla 1989 ). The results
also agree with the view obtained in pot ex-
periments by Novais and Kamprath (1978)
that “Fe-P” is of lesser importance than is
“Al-P”, as far as the P supply of the soil to
plants is concerned. The quantities of P w ,

Paaac and Pj also correlated closely (r =

o.Bl***—o.B4***) with each other (Yli-
Halla 1989) in a soil material which con-
sisted principally of the same samples as were
used in the present study. Therefore, the high
correlation between the results of the bioas-
say and all three chemical methods is under-
standable.

The Pj method was introduced for tropical
soils, partly due to the fact that the soils stud-
ied by van Zee (personal communication) were
in most cases too low in soluble P to allow a
precise determination of P w . In Finnish soils
the level of Pw in most cultivated soils is high
enough to be measured without any analyti-
cal difficulty. Nevertheless, P uptake by the
grass from the poorer half of the present soil
material was more accurately predicted by the
Pi method as compared to the other two
chemical methods. Owing to the small num-
ber of soils (n= 16) on which this conclusion
was drawn, it can be regarded as a tentative
result only. However, there are certain differ-

217



ences in the nature of the extraction methods
used in this study which may partly explain
this observation. Primarily and also Pw

are measures of soil P intensity (i.e. P con-
centration of soil solution) and do not express
the size of the capacity factor (i.e. the abso-
lute quantity of plant-available P). Instead,
the Pj is supposed to be the actual amount of
reversibly adsorbed P (Zee et al. 1987) and it
is probably more closely related to the capac-
ity factor. In the present experiment in which
the reserves of plant-available P were used
very effectively, especially in the soils poor in
P, the high positive correlation between P up-
takeby plants and an index closely connected
with the capacity factor can be comprehended.
Yet, in field trials performed in Finland (Sip-

pola & Saarela 1986) in which the supply
of P to plants is not as critical for growth as
was the case in the current pot experiment,
P AAAc and Pw rather successfully predicted
yield responses to P fertilization. Therefore,
determination of Pw or P AAAc seems to be
satisfactory in ranking soils to form a basis
for P fertilization recommendations. If a more
quantitative measure for the size of these
reserves is needed than are P„ and P AAAc , the
determination of P ( should be considered.
Even though somewhat laborious, the P|
method may prove useful, for example, when
studying the capacity of erosion material to
load water courses with P and for the deter-
mination of residual effect or fixation rate of
P fertilization.

References

Aura, E. 1980. Determination of available soil phospho-
rus by chemical methods. J. Scient. Agric. Soc. Finl.
50: 305—316.

Hartikainen, H. 1982. Water soluble phosphorus in
Finnish mineral soils and its dependence on soil
properties. J. Scient. Agric. Soc. Finl. 54: 89—98.

Kähäri, J. & Nissinen, H. 1978. The mineral element
contents of timothy (Phleum pratense, L.) in Finland.
I. Elements Ca, Mg, P, K, Cr, Co, Cu, Fe, Mn, Na,
Zn. Acta Agric. Scand. Suppl. 20: 272—284.

Menon, R.G., Flammond, L.L. & Sissinoh, H.A. 1989.
Determination of plant-available phosphorus by the
iron hydroxide impregnated filter paper (P|) soil test.
Soil Sei. Soc. Amer. J. 53: 110—115.

Novais, R. & Kamprath, E.J. 1978. Phosphorus supply-
ing capacities of previously heavily fertilized soils. Soil
Sei. Soc. Amer. J. 42: 931—935.

Saari, E. & Paaso, A. 1980. Mineral element composi-
tion of Finnish foods. 11. Analytical methods. Acta
Agric. Scand. Suppl. 20; 80—89.

Sippola, J. & Jaakkola, A. 1980. Maasta eri menetelmil-
lä määritetyt typpi, fosfori ja kalium lannoitustarpeen

osoittajina astia- jakenttäkokeissa. Maatalouden tut-
kimuskeskus. Maanviljelyskemian ja -fysiikan laitos.
Tiedote 13: 24—41.
& Jansson, H. 1979. Soil phosphorus test values ob-
tained by acid ammonium acetate, water and resin ex-
traction as predictors of phosphorus content in timo-
thy (Phleum prantense, L.) Ann. Agric. Fenn. 18:
225—230.
& Saarela, I. 1986. Some extraction methods as in-
dicators of need for phosphorus fertilization. Ann.
Agric. Fenn. 25; 265 —271.

Vuorinen, J. & Mäkitie, O. 1955. The method of soil
testing in use in Finland. Agrogeol. Pubi. 63. 44 p.

Yli-Halla, M. 1989. Reversibly adsorbed P in mineral
soils of Finland. Commun. Soil Sci. Plant Anal. 20:
695—709.

Zee, S.E.A.T.M. van der, Fokkink, L.G.T. & Riemsdijk,
W.H. van 1987. A new technique for assessment of
reversibly adsorbed phosphate. Soil Sei. Soc. Amer.
J. 51: 599—604.

Ms received December 18, 1989

218



SELOSTUS

Kolme uuttomenetelmää raiheinän
fosforinoton kuvastajina

Markku Yli-Halla,
Kemira Oy, Espoon tutkimuskeskus.
Luoteisrinne 2, 02271 Espoo

Viljelysmaiden muokkauskerroksesta eri puolilta Suo-
mea kerätyistä 32 maanäytteestä uutettiin fosforia deio-
nisoidulla vedellä (P w) ja viljavuusanalyysissä käytettä-
vällä [tappamalla ammoniumasetaattiliuoksella (P AAAc)-

Lisäksi fosforia uutettiin menetelmällä, jossa liuokseen
tuleva fosfori kerättiin rautahydroksidilla käsiteltyihin
suodatinpaperisuikaleisiin. Tässä menetelmässä (Pj)
liuoksen fosforipitoisuus pysyi koko ajan alhaisena, mi-
kä edistää fosforin liukenemista. Samoissa maanäytteis-
sä kasvatettiin neljä satoa Italian raiheinää, jonka fos-
foripitoisuus määritettiin. Raiheinä otti 5 —21 % maa-
näytteiden epäorgaanisesta fosforista. Raiheinän ottamat
fosforimäärät olivat n. kaksinkertaiset P-menctelmän
tuloksiin verrattuna (mediaani 25.9 mg/dm3 ) ja monta

kertaa suuremmat kuin Pw (mediaani 6.3 mg/dra 3 ) tai
Paaac (mediaani 6.4 mg/dm 3). Kaikkien kolmen mene-
telmän antamat tulokset olivat kiinteässä vuorosuhtees-
sa (korrelaatiokerroin r o.BB***:sta o.93***:een) kasvien
ottamien fosforiraäärien kanssa. Kuitenkin niukasti fos-
foria sisältävissä maissa P|-menetelmän tulosten perus-
teella voitiin kasvien P-ottoa ennustaa hieman tarkem-
min kuin kahden muun menetelmän tulosten avulla. Pw
ja PAAAc ovat hyviä menetelmiä luokiteltaessa maita
esim. P-lannoitussuosituksia varten. Näiden menetelmien
tulokset kuvastavat fosforin intensiteettiä maassa. Työ-
läämpi Pr menetelmä voi sen sijaan antaa arvokasta tie-
toa kapasiteettitekijän suuruudesta eli siitä, kuinka pal-
jon liukoista fosforia maassa kaikenkaikkiaan on.

219