Published in Agron. J. 96:761-768 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
PHOSPHORUS MANAGEMENT
Greenhouse Evaluation of Phosphorus Sources Produced from a Low-Reactive Brazilian Phosphate Rock
L. I. Prochnowa,
S. H. Chien*,b,
G. Carmonab and
J. Henaob
a Dep. of Soil and Plant Nutrition, Univ. of São Paulo/ESALQ, C.P. 9, 13418-900, Piracicaba, Brazil, and IFDC, P.O. Box 2040, Muscle Shoals, AL 35662 during 1999–2001
b IFDC, P.O. Box 2040, Muscle Shoals, AL 35662
* Corresponding author (nchien{at}ifdc.org).
Received for publication June 30, 2003.
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ABSTRACT
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The Patos de Minas phosphate rock (PR) in Brazil is not suitable for direct application due to its low reactivity or for commercial production of P fertilizers because of its high Fe-Al content. The objective of this study was to investigate the potential agronomic effectiveness of P sources with low water solubility produced from the PR. The P treatments were (i) compacted granular Patos PR with a single superphosphate (SSP) containing high water-soluble P content at a P ratio of 50:50, (ii) powdered low-grade single superphosphate (LG SSP) produced from Patos PR with low water solubility, (iii) powdered Patos PR, (iv) powdered mixture of PR and SSP at 50:50 P ratio, and (v) compacted granular SSP. The P sources were incorporated into an acid soil with pH 5.3 at 0, 10, 25, 50, and 100 mg P kg–1 to grow wheat (Triticum aestivum L.) and ryegrass (Lolium perenne L.) as test crops in a greenhouse study. We confirmed that Patos PR was low in agronomic effectiveness, only 1 and 30% as effective as SSP in producing dry matter yield of wheat and ryegrass, respectively. There were no significant differences between LG SSP or compacted PR+SSP [PR+SSP(C)] and SSP in dry matter yields of both crops, whereas mixed PR+SSP was less effective than PR+SSP(C) compared with granular SSP. Compacted PR+SSP and LG SSP can be potential P sources to be produced from the low-grade PR that could not be used either for direct application or acidulated P fertilizers.
Abbreviations: LG SSP, low-grade single superphosphate • PR, phosphate rock • PR+SSP(C), compacted phosphate rock with single superphosphate • PR+SSP(M), mixture of powdered phosphate rock with powdered single superphosphate • RAE, relative agronomic effectiveness • RE, relative effectiveness • SSP, single superphosphate
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INTRODUCTION
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DIRECT APPLICATION of PR often does not give satisfactory results when the PR's reactivity is too low, soil pH is too high, or the period of crop growth is too short (Chien and Menon, 1995a). One practice that has been reported to increase the utilization of low-reactive PR for crop production is the supply of water-soluble P to meet early P requirement of plant. By doing so, the plants would have better early root development and, in turn, would be able to utilize PR more effectively than could a plant treated with PR alone (Chien et al., 1996). Additionally, the acidity produced from the hydrolysis of monocalcium phosphate in superphosphates to H3PO4 in soil may enhance PR dissolution and thereby increase P availability from PR (Mokwunye and Chien, 1980).
Dry compaction of PR with water-soluble P fertilizers represents an alternative means for producing agronomically and economically more effective P fertilizers from indigenous PR resources that may otherwise be unsuited or of high cost for use as a fertilizer (Chien and Menon, 1995b). Positive agronomic results from compacting low-reactive PRs such as Pesca PR (Colombia), Hahotoe PR (Togo), Dorowa PR (Zimbabwe), and Sukulu Hills PR (Uganda) with water-soluble P sources have been reported by several researchers (Chien et al., 1987; Kpomblekou et al., 1991; Govere et al., 1995; Butegwa et al., 1996).
The sedimentary Patos de Minas PR in Brazil is recognized as low in reactivity, low grade in total P content, and high in Fe-Al oxide content. Consequently, this PR is not suitable for direct application because of its low reactivity or for chemical acidulation because the high Fe-Al content that can result in a P fertilizer that does not meet the minimum water solubility requirement of the Brazilian legislation. A minimum of 78 g kg–1 of total available P, i.e., (water + citrate)-soluble P (AOAC, 1999), and 70 g kg–1 of water-soluble P in fully acidulated SSP fertilizers is generally required (Brasil Ministério da Agricultura Secretaria Nacional de Defesa Agropecuária, 1982). This is equivalent to about 90% of total available P as water-soluble P. However, Engelstad and Hellums (1992) reviewed the literature on water solubility requirements of P fertilizers and showed that 60% water solubility is adequate to produce 90% of maximum yield for most crops. To meet the high water solubility requirement of P fertilizers produced from the PR, the Fe-Al oxide impurities of the PR must be removed through an expensive beneficiation process. This adds additional cost to the production of acidulated P fertilizers from this PR. Therefore, there is a need to evaluate the agronomic effectiveness of SSP with low water solubility produced from the PR without beneficiation. There is also a need to determine whether compaction of PR with water-soluble P can enhance the agronomic effectiveness of the PR.
The objective of this greenhouse study was to assess agronomic effectiveness of low-water-soluble P sources produced from Patos PR. The information is also relevant to the utilization of low-reactive PR deposits in other countries with characteristics similar to those of Patos PR and with legislation for P fertilizers similar to that of Brazil. This information will especially be relevant to developing countries where resource-poor farmers cannot afford to use costly imported water-soluble P fertilizers for crop production.
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MATERIALS AND METHODS
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Phosphorus Sources
The following P sources were used in the study: (i) pow dered Patos PR, (ii) compacted SSP manufactured from beneficiated Araxa PR (hereafter called SSP) with 74 g kg–1 of water-soluble P, (iii) powdered LG SSP manufactured from Patos de Minas PR with 28 g kg–1 of water-soluble P, and (iv) a mixture of PR and SSP at 50:50 P ratio. Mixture iv was used as a source of P in two ways: (i) compacted into pellets [PR+SSP(C)] and (ii) mixed in the powder form [PR+SSP(M)]. In total, there were five P treatments of which PR+SSP(C) and PR+SSP(M) were the same except in different physical forms. The sources of SSP and PR+SSP(C) were compacted at 140 MPa using a laboratory-scale Carver press (Carver, Wabash, IN), crushed, and sieved to produce particle sizes between 3.35 mm (6 mesh) and 2.00 mm (10 mesh). The SSP with high water solubility was used as a reference for comparison with P sources produced from Patos PR. Some chemical properties of the P sources used are shown in Table 1. All analyses of P sources (total P and water- and citrate-soluble P) were determined after Johnson (1973). For PR+SSP(C) and PR+SSP(M), data were based on calculations since no changes in chemical properties would be expected by physical mixing of the P sources (Prochnow et al., 2003).
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Table 1. Chemical properties of phosphate rock, single superphosphate, low-grade single superphosphate, compacted phosphate rock with single superphosphate, and mixture of powdered phosphate rock with powdered single superphosphate used.
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Greenhouse Evaluation
A greenhouse study using wheat and ryegrass as the test crops was conducted with a Hiwassee clay-loam soil (thermic Rhodic Kanhapludults). The soil contained 39 g kg–1 of sand, 22 g kg–1 of silt, and 39 g kg–1 of clay. Resin-extractable P (Van Raij and Quaggio, 1983) was 3 mg kg–1, and P by Pi test (Menon et al., 1989) was 1.7 mg kg–1. Effective cation exchange capacity was 3.5 cmolc kg–1 by 1 M KCl (Sumner and Miller, 1996), and pH in water (1:1 ratio) was 5.3. The P sources were thoroughly mixed with 4 kg of soil at rates of 0, 10, 25, 50, and 100 mg P kg–1. Nitrogen and K were added at 200 mg N kg–1 as urea and 200 mg K kg–1 as KCl. A nutrient solution containing secondary nutrients and micronutrients was added to all the treatments at the rate of 200 mg MgSO4·7H2O pot–1, 50 mg CuSO4·5H2O pot–1, 20 mg ZnSO4·7H2O pot–1, and 20 mg Na2B4O7·10H2O pot–1. The pots were arranged in a randomized block design with three replications.
Ten seeds of wheat (Pioneer 2684) were planted per pot at a depth of 
7 mm and thinned to six plants 10 days after germination. Approximately 0.2 g of ryegrass seeds (100 seeds) was planted on the soil surface. All pots were watered daily using deionized water to maintain 80% field moisture capacity during the entire experiment. The aboveground wheat plants were harvested 137 d after planting. The ryegrass was first cut 1.5 cm from the soil surface 101 d after seeding. The second and third cuts were performed 137 and 184 d after seeding.
After harvest, the plants were dried at 60°C for 2 wk, ground, and weighed. Concentration of P in ground tissue samples was determined after digestion with H2SO4–H2O2 (Prochnow et al., 2003). Dry matter yield and P uptake by ryegrass were determined by combining three cuts. Soil samples were taken from each pot after harvest, followed by extraction with anion exchange resin as described by Van Raij and Quaggio (1983). Analysis of total P concentration for the plant and soil samples was determined by the Murphy and Riley (1962) method.
Data Analysis
The relationship between dry matter yield or P uptake and rate of P applied was evaluated using regression procedures (SAS Inst., 1996). A combined regression analysis using a dummy variable, as described by Prochnow et al. (2003), was performed for all P sources. The dummy variable takes the value of 1 for the P source being considered and 0 for other P sources. This resulted in a common intercept and a single value of root mean square of error (RMSE) and R2 for the five regression equations (one for each P source). Three response functions (linear, semi-log, and square root) were tested. The one representing the lowest RMSE and the highest R2 value was chosen. The models tested were as follows:
 | [1] |
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 | [3] |
where Yi is dry matter yield or P uptake obtained with source i, X is the rate of P applied, ßi is the slope of the response function for source i, ß0 is the common intercept, and 
i is the error term of the fitted model.
The relative agronomic effectiveness (RAE) was calculated for each P source i (RAEi). The RAE was defined as the ratio of the two slopes:
 | [4] |
where ßi is the slope of the response function of the P source i tested and ßSSP is the slope of the response function of SSP. This expression ranks the P sources with respect to SSP, i.e., RAE of SSP = 100%, according to their agronomic potential to produce a yield response (Chien et al., 1990).
The relative effectiveness (RE) of PR in the presence of SSP, i.e., PR+SSP(C) and PR+SSP(M), with respect to SSP using PR+SSP(C) as an example was calculated according to the following formula:
 | [5] |
where REPR is the relative effectiveness of PR in the presence of SSP, [PR+SSP(C)]X and SSPX are dry matter yield or P uptake at application rate X, and SSPX/2 is dry matter yield or P uptake at application rate X/2. The conceptual theory for this approach is demonstrated in Fig. 1
, which shows the effectiveness of 25 mg P kg–1 applied as PR with 25 mg P kg–1 applied as SSP (
Y3) with respect to the effectiveness of additional 25 mg P kg–1 applied as SSP after the first 25 mg P kg–1 applied as SSP (Y1). It should be noted that this definition differs from the conventional definition of RAE of PR, which would be the ratio of yield increase due to PR alone (Y4) and yield increase due to SSP (Y2), both at 25 mg P kg–1. The REPR defines the relative effectiveness of PR in the presence of SSP with respect to SSP, whereas the RAEPR defines the relative effectiveness of PR alone with respect to SSP.
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Fig. 1. Relative effectiveness of phosphate rock (REPR) in compacted phosphate rock with single superphosphate [PR+SSP(C)] and relative agronomic effectiveness of phosphate rock (RAEPR) alone with respect to that of water-soluble P [single superphosphate (SSP)] in a conceptual figure.
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By assuming the models:
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 | [7] |
with n = 1 for linear model and n = 1/2 for square root model, Eq. [5] can be redefined as:
 | [8] |
where REPR is the relative effectiveness of PR and ßPR+SSP(C) and ßSSP are the slopes of the response functions for PR+SSP(C) and SSP, respectively.
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RESULTS AND DISCUSSION
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The square root model, i.e., Eq. [3], was found to best describe the relationship between dry matter yield and P rate, whereas the linear model, i.e., Eq. [1], described the best relationship between P uptake and P rate for wheat (Table 2 and Fig. 2)
. The RAE of Patos PR was very low (1%) due to its low reactivity. A significant P response was observed with all other P sources. There were no significant differences among SSP, PR+SSP(C), and LG SSP in terms of dry matter yield whereas PR+SSP(M) was less effective than the former P sources. The RAE values of LG SSP (91%) and PR+SSP(C) (99%) were very high for dry matter yield (Table 2). For P uptake, the RAE values of LG SSP (87%) and PR+SSP(C) (88%) were lower than that of SSP. The lower RAE values of PR+SSP(M) for dry matter yield (69%) and P uptake (57%) can be attributed to a reduced effectiveness of powdered SSP in PR+SSP(M) because of a greater soil P fixation compared with compacted SSP in PR+SSP(C). This was confirmed in another study (Prochnow, unpublished data, 2000) that showed powdered SSP was only 54% as effective as compacted SSP in terms of increasing dry matter yield of wheat.
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Table 2. Regression estimates for the models describing the relation between dry matter yield (DMY) or P uptake and rate of P applied, and relative agronomic effectiveness (RAE) of various sources of P for wheat.
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The RE values of Patos PR in PR+SSP (C) with respect to compacted SSP in terms of dry matter yield of wheat and P uptake were calculated with Eq. [8] and are shown in Table 3. These values represent the percentage effectiveness of compacted PR in relation to compacted SSP at the same P rate, as defined in a conceptual Fig. 1. For example, the RE value of Patos PR at 25 mg P kg–1 in PR+SSP(C) applied at a total 50 mg P kg–1 was 90% (Y3/Y1) as effective as the second increment of 25 mg P kg–1 of SSP applied after the first 25 mg P kg–1 in increasing dry matter yield of wheat. In the absence of SSP, Patos PR was useless, being only 1% (Y4/Y2) as effective as SSP (Table 2). Apparently, SSP had a significant enhancement effect on P availability from Patos PR when the two P sources were mixed together. The same trend was also observed with P uptake by wheat, except RE of Patos was 76% as effective as SSP (Table 3). For PR+SSP(M), it was not possible to calculate RE of PR because SSP in PR+SSP(M) was in powder form whereas SSP alone was in compacted form.
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Table 3. Relative effectiveness of Patos de Minas phosphate rock (REPR) in the compacted phosphate rock with single superphosphate with respect to single superphosphate (SSP) (RE = 100%) for wheat and ryegrass.
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Although LG SSP had only 49% of total available P as water soluble compared with SSP, which had 87% (Table 1), LG SSP was as effective as SSP in increasing dry matter yield of wheat (Fig. 2A). This agrees with other reports (Bartos et al., 1992; Mullins and Sikora, 1995; Mullins et al., 1995) that it is not necessary to always have high water solubility in fully acidulated P fertilizers to reach maximum crop yield. Apparently, the water-insoluble but citrate-soluble P compounds in LG SSP were also able to provide effective plant-available P to wheat and increase dry matter yield. Although PR+SSP(C) and LG SSP were as effective as SSP in dry matter yield (Fig. 2A), they were less effective at increasing P uptake (Fig. 2B).This is probably the result of differences in P uptake during early plant growth when soil solution P concentration was higher from SSP than that from PR+SSP(C) and LG SSP, and these differences had a significant effect on P uptake (Chien et al., 1987).
Figure 3
shows that P uptake by wheat from PR+SSP (C) was higher than that from SSP and LG SSP with the same amount of resin P extracted from the soil. A similar result was also obtained with dry matter yield (not shown). The results suggest that resin tended to underestimate available P in the soil treated with PR+SSP(C) with respect to SSP and LG SSP. Since P uptake from PR greatly depends on the contact between PR particles and plant roots, the enhancement effect of water-soluble P on PR effectiveness is possible through an increased physical contact of PR particles by a better early development of plant roots that is stimulated by water-soluble P (Hammond et al., 1986). In this case, any soil test would not be able to measure this physical effect since it does not involve chemical reactions in the soil. Consequently, PR+SSP(C) and SSP would not follow the same relationship between P uptake or dry matter yield and soil available P. Furthermore, the relationship curve of PR+SSP(C) would be above the curve of SSP. Data shown in Fig. 3 thus provide additional evidence that SSP indeed enhanced PR effectiveness in PR+SSP(C) for wheat.
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Fig. 3. Relationship between P uptake by wheat and resin P of soil samples treated with three P sources. SSP, single superphosphate; LG SSP, low-grade single superphosphate; PR+SSP(C), compacted phosphate rock with single superphosphate.
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The RAE values of Patos PR alone were 30% for dry matter yield and 15% for P uptake by ryegrass (Fig. 4
and Table 4), whereas the PR was only 1% for wheat (Table 2). Chien and Menon (1995a) also showed that ryegrass was more effective than wheat in utilizing PR in an acid Ultisol. They attributed the efficient use of PR by ryegrass to its high root density. However, due to its low reactivity, Patos PR was still a poor P source for ryegrass compared with SSP. Similar to the results of wheat, there were no significant differences among SSP, LG SSP, and PR+SSP(C) in terms of dry matter yield (Fig. 4). The RAE values of LG SSP and PR+SSP(C) were 99 and 95% in dry matter yield, respectively (Table 4). For P uptake, LG SSP was as effective as SSP, but PR+SSP(C) was less effective than SSP (Fig. 4). Ryegrass was also able to utilize PR+SSP(M) more effectively than wheat as evidenced by the higher RAE values. The RAE of PR+SSP(M) increased from 69% with wheat to 86% with ryegrass in dry matter yield and from 57% with wheat to 72% with ryegrass in P uptake (Tables 2 and 4). In terms of dry matter yield and P uptake, there were also no significant difference between PR+SSP(C) and PR+SSP(M) for ryegrass (Fig. 4), whereas PR+SSP(C) was more effective than PR+SSP(M) for wheat (Fig. 2).
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Table 4. Regression estimates for the models describing the relation between dry matter yield (DMY) or P uptake and rate of P applied, and relative agronomic effectiveness (RAE) of various sources of P for ryegrass.
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The RE values of Patos PR in PR+SSP(C), with respect to compacted SSP calculated according to Eq. [8], were 82 and 54% for dry matter yield of ryegrass and P uptake, respectively (Table 3), higher than the RAE values of Patos PR in the absence of SSP (30 and 15% for dry matter yield and P uptake, respectively) (Table 4). This comparison suggests that SSP likely enhanced the effectiveness of PR in PR+SSP(C) for ryegrass. It can be seen that P uptake from Patos PR in PR+SSP(C) was much higher than the PR applied alone at the same PR-P rate applied for both crops (Fig. 5)
. For example, P uptake by ryegrass at 25 mg P kg–1 of PR-P applied increased from only about 1 mg P pot–1 with PR alone to about 9 mg P kg–1 with PR in PR+SSP(C).
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Fig. 5. Phosphorus uptake from Patos de Minas phosphate rock (PR) by (A) wheat and (B) ryegrass in the soils treated with PR alone or compacted PR with single superphosphate [(PR+SSP(C)] at 0, 25, and 50 mg P kg–1.
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As the P fertilizer industry is expected to use more and more of low-grade P ores containing Fe and Al impurities (Cathcart, 1980; Lehr, 1984), there is a concern shown in literature that the agronomic effectiveness of these acidulated P fertilizers produced from the low-grade P ores will decrease because of decreased P water solubility. The results obtained in this study suggest that P sources with lower water solubility such as LG SSP and PR+SSP(C) can be as good as those with highly water-soluble acidulated SSP for wheat and ryegrass and that the water-insoluble P compounds present in LG SSP and PR+SSP(C) should not be deemed as totally undesirable. However, it should be pointed out that the present results were obtained in greenhouse pots that may differ from the actual field trials. Nevertheless, the results suggest these P sources are potentially attractive to the P industry.
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CONCLUSIONS
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In this study, two P sources (49 and 78% of available P as water-soluble P, respectively) were produced from a low-grade (low reactivity and high Fe-Al impurities) Brazilian Patos PR by compacting the PR with water-soluble P or acidulating with H2SO4. These low-water-soluble P sources were found to be as effective as SSP with 87% of available P as water-soluble P in increasing dry matter yield of wheat and ryegrass. The results suggest that there are ways to produce agronomically and economically effective P fertilizers from low-grade P ores that otherwise could not be used for either direct application or acidulation to form high-water-soluble P sources. If feasible, these techniques should have a significant impact to the P fertilizer industry, especially in developing countries where many farmers cannot afford to use the acidulated P fertilizers with high water solubility. However, more research is needed to evaluate (i) their agronomic effectiveness (especially in field trials for long-term effect), (ii) compacted fertilizer properties (especially physical compatibility of ingredients and hardness of compacted granules), and (iii) the fertilizer production costs for these types of low-water-soluble P sources before they can be commercially used in crop production.
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TOP
ABSTRACT
INTRODUCTION
0.05). PR, phosphate rock; SSP, single superphosphate; LG SSP, low-grade single superphosphate; PR+SSP(M), mixture of powdered phosphate rock with powdered single superphosphate; PR+SSP(C), compacted phosphate rock with single superphosphate.
