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An Algorithm for Estimating Amounts of Rock Phosphate Needed to MeetCrop Phosphorus Requirements in West African SoilsAminata S. Diarraa; Russell S. Yostb; Mamadou D. Doumbiaa; Tasnee Attanandanac; Adama Bagayokoa;Aliou B. Kouyatea; Richard Kablanb; Xinmin Wangb
a LaboSEP, Institut d'Economie Rurale BP 262, Bamako, Mali b Department of Tropical Plants and SoilScience, University of Hawaii at Manoa, Honolulu, Hawaii, USA c Department of Soil Science, Facultyof Agriculture, Kasetsart University, Jatujak, Bangkok, Thailand
Online publication date: 18 March 2004
To cite this Article Diarra, Aminata S. , Yost, Russell S. , Doumbia, Mamadou D. , Attanandana, Tasnee , Bagayoko,Adama , Kouyate, Aliou B. , Kablan, Richard and Wang, Xinmin(2004) 'An Algorithm for Estimating Amounts of RockPhosphate Needed to Meet Crop Phosphorus Requirements in West African Soils', Communications in Soil Science andPlant Analysis, 35: 3, 385 — 397To link to this Article: DOI: 10.1081/CSS-120029720URL: http://dx.doi.org/10.1081/CSS-120029720
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COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS
Vol. 35, Nos. 3 & 4, pp. 385–397, 2004
An Algorithm for Estimating Amounts of Rock
Phosphate Needed to Meet Crop Phosphorus
Requirements in West African Soils
Aminata S. Diarra,1 Russell S. Yost,2,*Mamadou D. Doumbia,1
Tasnee Attanandana,3 Adama Bagayoko,1 Aliou B. Kouyate,1
Richard Kablan,2 and Xinmin Wang2
1LaboSEP, Institut d’Economie Rurale BP 262, Bamako, Mali2Department of Tropical Plants and Soil Science, University of
Hawaii at Manoa, Honolulu, Hawaii, USA3Department of Soil Science, Faculty of Agriculture, Kasetsart
University, Jatujak, Bangkok, Thailand
ABSTRACT
An incubation study was conducted (i) to investigate the influence of
soil properties on the dissolution of Tilemsi rock phosphate (PNT)
and extractability of phosphorus (P) after dissolution and (ii) to
develop an algorithm for estimating amounts of rock P needed tomeet
crop P requirements in West African soils. Subsequently, the amount
*Correspondence: Russell S. Yost, Department of Tropical Plants and Soil
Science, University of Hawaii at Monoa, 3190 Maile Way, Honolulu, HI 96822;
Fax: 808-956-3894; E-mail: rsyost@Hawaii.edu.
385
DOI: 10.1081/CSS-120029720 0010-3624 (Print); 1532-2416 (Online)
Copyright & 2004 by Marcel Dekker, Inc. www.dekker.com
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of phosphate rock (PR) that had dissolved and changes in Bray-P and
Olsen-P were respectively determined by measuring the increase in
extractable P (�NaOH-P) and changes in plant-available P (�Olsen-P
and� Bray-P). The extent of dissolution of PNT ranged from 3.3 and
38.2% of the total P added to the soils. The greatest PNT dissolution
was obtained in Niessumana and Longorola soils, which are acid
lowland soils. The highest amount of plant-available P was also
measured in these soils. Exchangeable acidity, calcium (Ca) satura-
tion, and effective cation exchange capacity appeared to be the factors
controlling the PNT dissolution. The amount of rock phosphate
dissolved in contrasting Malian soils could be predicted by a
modified MacKay’s equation of the form: Y¼Aþ (B � e�c � time),
where Y¼ amount of PNT dissolved, measured by NaOH extraction;
A¼ 78.1627þ 30.62149 acidity� 0.63335CaSat; B¼�16.58786�
21.74709 acidityþ 4.07675ECEC and c¼ curvature coefficient. The
amount of P to reach the optimal yield for a given crop, which is the
difference between P critical level andmeasured P in a given soil, could
be estimated using the plant available-P/NaOH-P ratio and the above
modified MacKay’s equation.
Key Words: Phosphate rocks; Tilemsi rock phosphate (PNT);
PR dissolution; NaOH-P and soils.
INTRODUCTION
Many research programs have provided evidence that P is the mostdeficient and plant growth-limiting nutrient in West African soils.[1–5]
Phosphate rock (PR) materials from around the world have beenevaluated for their suitability as low-cost, direct application P fertilizers.However, many PRs have not proven to be sufficiently soluble for directapplication in intensive agricultural production.[6]
The PR dissolution depends on chemical and physical properties ofthe PR and the soil properties.[7] Several scientists[6,8,9] pointed out thatPR dissolution is highly related to PR reactivity. The reactivity dependson the isomorphous substitution of CO3/PO4 in the PR.[10] Mokwunye[9]
founded that West African PRs are not very reactive.Regarding soil properties that might be expected to affect the extent
of dissolution of rock phosphate in soils, Khasawneh and Doll[6] listedthe following:
1. Concentration of Hþ in the soil and the Hþ buffer capacity insoil: the dissolution of PR in soils is driven by the supply of Hþ,
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which is controlled by the initial soil pH and the pH buffercapacity of the soil. The presence of Hþ ions in the soil results inthe rapid conversion of the PO3�
4 to H2PO�4 and HPO2�
4 . Severalauthors[11–16] also found that the rate of dissolution of PRincreased with increasing pH buffer capacity of the soil. In anyevent, the ability of a soil to supply Hþ ions appears to be animportant factor controlling dissolution of PR.
2. Concentration of P in soil solution and P buffering capacity ofthe soil: PR dissolution increases as the P retention capacity ofsoil increases.[6]
3. Concentration of Ca2þ in soil solution and Ca2þ bufferingcapacity of the soil: PR dissolution increases with lower exchange-able Ca in soil. Robinson and Syers[17] found the Ca sink hada large influence on PR dissolution.
4. Organic matter content: organic matter is related to the bondingof Ca and provides an effective sink for Ca.[6]
5. Soil texture: coarse-textured soils and sands are, in general, notgood sinks for P and Ca.[6]
Wright et al.[14] reported a low dissolution of PR in soil with high pH,high Ca saturation and a low P retention capacity. Using 30 contrastingsoils MacKay et al.[18] found that percent Ca-saturation, P-sorptioncapacity, and Ca-exchange capacity of the soil were the three most impor-tant parameters influencing Sechura PR (from Peru) dissolution in soils.They found that the dissolution of Sechura PR increased as exchangeableCa decreased and as P-sorption capacity increased. Chu et al.,[19] Chienet al.,[20] and Smyth and Sanchez[21] also found that the dissolution of PRincreased as the P-sorption capacity of the soil increased. Bolland andBarrow[22] demonstrated that the capacity of the soil to adsorb P mightpotentially play amajor role in the dissolution of apatite rock phosphate insoil. In the computer simulationmodel of PR dissolution[18] the capacity ofthe soil to adsorb P was more important than the pH buffering capacity ofthe soil. Hammond et al.[7] and Wright et al.[14] suggested that dissolutionof PR depended on pH, Ca status in soil (exchangeable Ca and soil Casaturation), and P status of soil (soil solution P and P-sorption capacity).
Some of the early studies compared the effects of clay percentageon increasing rock phosphate dissolution and increasing P retention.[23]
The simulation results suggested that while increased clay percentageincreased rock phosphate dissolution, it increased P retention even more,resulting in reduced P availability (Wang 1997, personal communication).
The purpose of this study was to develop an algorithm (method)that would produce quantitative estimates of the amount of rock
Estimating Amounts of Rock Phosphate 387
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phosphate necessary to provide nutrient P for crops in West Africa.The indicator of adequate amounts of nutrient P can be extractable P suchas Bray 1 or Olsen methods. Other extractants could be used after alaboratory incubation and calibration with NaOH extractable P. Theexample developed in this article uses Tilemsi rock phosphate from Mali,West Africa.
MATERIALS AND METHODS
In developing the algorithm, it was recognized that factors thatincrease dissolution sometimes also increase P sorption or retention.Therefore, the two processes of dissolution and retention were separated.Dissolution processes include soil factors (soil pH, soil calcium status,etc.), type of rock phosphate, and many of the management factors suchas type of crop and weather conditions. The P retention process, however,seemed to be relatively unaffected by variable management factors, at leastas proposed by Cox.[24]
Incubation studies were carried out with five soils representative of thefood and fiber production agriculture in Mali. Selected properties of thesoils used in the investigation are shown in Table 1. The soils were air-driedand passed through a 2-mm sieve. The following chemical parameterswere determined: organic carbon by combustion,[25] pH in water at a soil:solution ratio 1:1, exchangeable cations by extraction with 1M NH4OAc(pH 7),[26–28] exchangeable acidity,[26] ECEC (effective cation exchange
Table 1. Soil properties of samples selected for rock phosphate study.
Soil
pH(H2O)
1:1
Clay
(%)
ECECa
(cmolc kg�1)
Exchangeable
(cmolc kg�1)
CaSat
(%)
5b 10b
Ca Acidity kg PNT/ha
Cinzana plateau 5.0 5 1.14 0.54 0.33 47 50 100
Cinzana valley 6.47 36 11.53 9.11 0 79 3568 7136
Cinzana slope 5.6 4 1.31 1.02 0 78 134 267
Niessumana 4.3 36.5 6.35 1.37 4.2 22 39 78
Longorola 4.9 36.5 7.34 2.08 3.83 28 72 143
aECEC¼ effective cation exchange capacity.bEstimates of amounts of rock phosphate (Tilemsi) to increase Bray 1 by 5 and
10mgP/kg, respectively.
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capacity) was estimated by summing the exchangeable cations and
exchangeable acidity, clay content by the pipette method,[29] and plant
available P by extraction with sodium bicarbonate[30] and Bray
1-extractable P (0.025M HClþ 0.03M NH4F).[31]
The rate of dissolution of Tilemsi PR over 60 days was investigated.
Tilemsi PR (PNT) was ground to pass a 250-mm sieve. The characteristics
of the PR material are given in Table 2.The PNT was added to the soils at a rate of 500mgkg�1 soil,
thoroughly mixed, moistened to field capacity with deionized water, and
incubated at 22–24�C. A control was included for each soil and was
similarly treated. Each treatment was in 3 replications. Soils were aerated
and the moisture content adjusted to field capacity every two days during
the incubation. Treated and untreated (control) soils were sampled after
0, 3, 7, 15, 30, and 60 days of incubation to estimate the rate of dissolution
of PNT. Extent of PR dissolution and available P (Olsen- and Bray
1-extractable P) were determined at each sampling time. The extent of
dissolution in the various soil samples was estimated by the 0.5M NaOH
method.[18] Soil samples were extracted for 16 h with 0.5MNaOH at a soil
to solution ratio of 1:100 following a prewash with 1M NaCl for 1 h.
Phosphorus in the extracts was determined colorimetrically by the method
of Murphy and Riley[33] and absorbance was measured at 880 nm. The
difference in P level (�P-NaOH) between treated and untreated samples
was used as an estimate of PR dissolution. The available P was determined
in treated and untreated soils at each sampling time. Extractable P was
estimated by the Olsen and Bray 1 methods. The difference in extractable
P (�P-Olsen and �P-Bray) between treated and untreated samples was
used as an estimate of P available for plants after PR dissolution and
subsequent reaction of the released P with soil. Data were statistically
analyzed using the General Linear Model procedure of SAS.[34]
Table 2. Selected chemical characteristics of Tilemsi PR.
Location Solubility
Total P
(%)
ApatiteCitrate
soluble P
(% of rock) CO3/PO4
Length of
a-axis A�
Empirical
formula
Tilemsi,
Mali
Medium 12.22 9.331 Ca9.63 Na0.27Mg0.10 (PO4)4.96(CO3)1.04 F2.41
1.65 0.210
Source: Truong et al.[32]
Estimating Amounts of Rock Phosphate 389
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RESULTS AND DISCUSSION
The influence of soil properties on the rate and extent of dissolution ofa PR can be seen clearly in Fig. 1. The increase in NaOH-extractable P in asoil, to which PNT has been added, relative to the control soil, can be takento represent the amount of P dissolved from the PR. The PNT dissolved toa differing extent in different soils. The rate of dissolution of PNT rangedfrom 3.3 to 38.2% of the total P added to the soils. The shapes of thedissolution curves were similar. The rate of dissolution rapidly slowedafter about 7–14 days incubation. In each case, the reaction reachedequilibrium after 40–50 days. Other authors[17,21,23,35,36] obtained similarshaped dissolution curves. The maximum PNT dissolution, in this study,followed the sequence: Niessumana>Longorola>Cinzana plateau>Cinzana slope>Cinzana valley, which is consistent with the decreasingCa-saturation of the soils. Dissolution of PNT has often been shown toincrease with decreasing Ca-saturation of soil.[17,18]
The dissolution curves obtained for 5 soils could be described by amodified MacKay’s equation of the form:
Y ¼ Aþ ðB � e�c�timeÞ ð1Þ
in which Y¼ amount of PNT dissolved, measured by NaOH extractionat time; A: asymptote; B: difference between the asymptote and theintercept; and c: curvature coefficient.
0
50
100
150
200
250
0 20 40 60 80
Incubation time, Days
NaO
H-P
, m
g/k
g
Cinzana plateau - pH 5.0
Cinzana Valley - pH 6.47
Cinzana slope - pH 5.6
Niessumana - pH 4.3
Longorola - pH 4.9
Figure 1. Predicted dissolution of Tilemsi rock phosphate in soils of varying
acidity, Ca saturation, and effective cation exchange capacity. Symbols-
experimental data, lines-model predictions. (View this art in color at www.
dekker.com.)
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When the Cinzana valley soil with pH 6.5, was excluded from theanalysis, the stepwise multiple linear regression gave the followingequations to predict A and B.
A ¼ 78:1627þ 30:62149 acidity� 0:63335CaSat (Residual
Mean Square ¼ 0:16132Þ ð2Þ
B ¼ �16:58786� 21:74709 acidityþ 4:07675 ECEC (Residual
Mean Square ¼ 0:01463Þ ð3Þ
where CaSat¼Ca-saturation, %. CaSat can be calculated from the datain Table 1. Acidity and ECEC are in cmolc kg
�1 as in Table 1.By using Eqs. (1)–(3) for acid soils (pH 5.6 or below), the rate and
extent of dissolution of PNT can be determined. However, the modelmust be evaluated under field conditions.
These equations relate the PR dissolution to the percent Ca satura-tion, effective cation exchangeable capacity and soil acidity. Khasawnehand Doll,[6] MacKay et al.,[18] and Robinson and Syers[17] demonstratedthat Ca saturation had a large influence on PR dissolution.
An Algorithm to Predict Amounts of Rock Phosphate
Necessary to Meet Plant Requirements
The proposed method begins with an analysis by the PDSS(Phosphorus Decision Support System), in which the user selects thecrop and indicates the soil test and soil test values of the field or land tobe managed (Fig. 2). If the soil test value is below the critical level then
PredictionNo P deficiency - EXIT
Yes P deficient
Rock P Rock P module
Recommendation
DiagnosisChoose P materiel
Soluble P (Treble Super)
Economic Analysis
Figure 2. Phosphorus decision support system and linkages of PR module.
Estimating Amounts of Rock Phosphate 391
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P is recommended. The amount of P must now be determined. As in thestandard PDSS format, the difference between the critical level and themeasured P is the amount of P that must be added in the form of eithersoluble or rock phosphate fertilizer. This value will be called the DeltaP(Pc-P0). Now the amount of rock phosphate that will provide thisamount of soluble P needs to be determined. This approach now con-tinues by converting the estimate of the extractable P needed to reach theP critical level into the amount of 0.5M NaOH-P that needs to be increased.This value is now a multiple variable function that can be predicted fromsoil and site properties using equations such as that proposed by MacKayet al.[18] and the revision to MacKay’s equation as proposed above(Eqs. (2) and (3)) (Table 1). As the factors in the equation indicated, thisequation will require the input of Ca saturation, effective cation exchangecapacity (ECEC), and soil acidity. Calcium saturation can be approxi-mated by measuring exchangeable Ca and ECEC.
Thus, it is hypothesized that the amount of rock phosphate can bepredicted by relating soil test P levels to changes in NaOH extractable P.The first predictions of this algorithm are given in Table 1 on the far rightcolumns of 5 and 10, indicating the predicted amounts of Tilemsi rockphosphate required to increase Bray 1 by 5 and 10mgP/kg, respectively,after 30 days of incubation. The modified MacKay’s equation can predictthe amount of dissolved PR. This equation needs testing in greenhouseand field conditions.
Predicting Amounts of Dissolved P That Is Plant Available
As well documented in the literature, estimates of the amounts ofNaOH-extractable P are not usually sufficient to estimate plantavailability. For PR to be viable alternative fertilizer, it should dissolvein soil. The second requirement is that the P released remains available toplants. Syers and Mackay[35] pointed out that the soil properties thatincrease the extent of PR dissolution in soil may not necessarily result inan increase in the amount of plant available P.
In this study, Olsen- and Bray1-P were used as the indicators of plantavailable P. Levels of Olsen-P and Bray1-P varied with soils and time ofincubation (Fig. 3). The change in available P (� Olsen- and � Bray-P)ranges from 1.09 to 59.3mg kg�1 and 0.5 to 65.6mg kg�1, respectively.The � Olsen- and � Bray-P in soil where PNT was added generallyincreased as � NaOH-P increased (Figs. 1 and 3).
It is proposed to measure one of the extractable P methods such asBray 1 or Olsen P to establish the relationship between extractable P pools
392 Diarra et al.
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of these methods with the pools of NaOH. This proposed relationshipmay not be as highly affected by management factors, as is the rockphosphate dissolution. It may also, after an initial period, assume somenearly constant value, which occurred in the incubation study after about30 days (Fig. 4). It is noted that for these soils, the ratios of Bray 1 andOlsen P to NaOH were quite different among soils, suggesting that thebuffering of these extractable P measured may differ substantially. Thisalso needs testing in field conditions.
Delta Olsen-P.
0
10
20
30
40
50
60
70(a)
(b)
0 10 20 30 40 50 60 70
Time, days
De
lta
Ols
en
-P,
mg
kg
-1
Cinzana platea
Cinzana slope
Cinzana valley
Niessumana
Longorola
Delta Bray-P.
0
10
20
30
40
50
60
70
0 10 20 30 40 50 60 70
Time, days
De
lta
Bra
y-P
, m
g/k
g
Cinzana plateau
Cinzana slope
Cinzana valley
Niessumana
Longorola
Figure 3. Changes in extractable P in soil over 60 days following PNT addition
at 500mgPkg�1 of soil.
Estimating Amounts of Rock Phosphate 393
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Perhaps this relationship will be sufficiently constant that it needs tobe estimated only once for a particular soil or perhaps it can be predictedfrom other soil properties such as those used to predict buffer coefficientsuch as percent clay. The relationship between Olsen P/NaOH-P ratioand clay content in soils could be described by the following equation:Olsen P/NaOH-P¼ 0.58� 0.0095 �Clay (R2
¼ 0.69).
CONCLUSIONS
In summary, results from this study demonstrated that thedissolution rate of PR varied from soil to soil but was fairly rapid overfirst 7–14 days of incubation until a steady state was reached afterapproximately 50 days. The incomplete dissolution of PNT is probablydue to the saturation of the Ca and P sinks in the conditions of closed,nonleached system.
The extent of PNT dissolution in 5 Malian soils after 60 daysincubation ranged from 3.3 to 38% of the total P added. Exchangeableacidity, calcium saturation and the effective cation exchange capacitywere the most important factors influencing the PNT dissolution. Nosingle soil property was responsible for promoting the dissolution of PRover all soils investigated. Limited solubility of Tilemsi phosphate rock
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60 70
Time, days
Delt
a B
ray-P
/Delt
aN
aO
H
b) Delta Olsen/Delta NaOH
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 20 40 60 80
Time, days
Delt
a O
lsen
/delt
aN
aO
H
Cinzana plateau
Cinzana valley
Cinzana slope
Niessumana
Longorola
a) Delta Bray-P/Delta NaOH
Figure 4. Variation of delta available-P/deltaNaOHduring 60 days of incubation
period.
394 Diarra et al.
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occurred in soil with combinations of low exchangeable acidity, high soilpH, and high Ca saturation.
Bray1 and Olsen extractable-P were used as estimates of P availablefor plant after dissolution of PR. The general pattern of available P overtime was similar to that of PR dissolution. Soils with the greatest extentof PR dissolution had the highest levels of extractable P.
The algorithm can be used to predict the amount of PR that shouldbe applied in order to supply nutrient P required to meet croprequirements except in soil with pH 6.4.
More research will be required, especially under field conditions tomodify and validate the algorithm of the PR dissolution in the presenceof the plant and Ca and P removal.
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