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Soil Science Society of America Journal 63:1455-1462 (1999)
© 1999 Soil Science Society of America

DIVISION S-8-NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Soil Characteristics and Management Effects on Phosphorus Sorption by Highland Plateau Soils of Ethiopia

Miressa Dufferaa and Wayne P. Robargea

a Dep. of Soil Science, North Carolina State Univ., Box 7619, Raleigh, NC 27695-7619 USA

wayne_robarge{at}ncsu.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusion
 REFERENCES
 
Differences in crop and fertilizer management are known to influence P retention by soils. An experiment was conducted to study the effect of soil characteristics and management practices on P sorptoin behavior of the highland plateau soils of Ethiopia. Surface samples from two Vertisols, an Andisol, and an Alfisol were collected from farmers' fields, research station farms, and from non-cultivated-non-fertilized areas. Phosphorus sorption data were obtained by equilibrating 3-g soil samples in 30 mL of 0.01 M CaCl2 containing various amounts of KH2PO4. Inorganic P fractions were determined by the Hedley P fractionation scheme. There was little variation in P sorption among Vertisol samples of alluvial origin as a result of cultivation-fertilization practices. For soils of volcanic origin (Vertisol2 and Andisols), and the Alfisol, samples collected from farmers' fields sorbed more P than the non-cultivated and research station samples. Least amounts of applied P sorbed by the non-cultivated Andisol samples reflect the relatively large amounts of resin extractable P initially present in these soils and demonstrate that labile P initially present in the soil can influence subsequent P sorption. Stepwise regression analysis of the P sorption data showed that resin P accounts for 81% of the variation in P sorption at 0.2 mg P L-1 in solution. The highest amount of P was sorbed by samples collected from farmers' fields and was mainly due to the practice of continuous cropping with minimal P fertilization, which depletes labile P, and therefore requires higher levels of P fertilization for optimum crop yield.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusion
 REFERENCES
 
FOR THE HIGHLAND PLATEAU SOILS OF ETHIOPIA, P is a potentially limiting element in crop production (Desta, 1982; Tekalign and Haque, 1987). Several authors have reported that between 70 and 75% of the agricultural soils of the highland plateau region of Ethiopia are P deficient (Desta, 1982; Tekalign et al., 1988; Asnakew et al., 1991). However, limited detailed work has been done on the P status of the soils of Ethiopia, with most studies focusing on the rates and timing of P applications for optimum yields. Because P sorption processes are believed to govern the behavior of labile inorganic P in soils, an understanding of P sorption and release characteristics of the soils of Ethiopia is important in determining the fate of applied P fertilizer.

The highland plateau soils of Ethiopia present different conditions of soil formation compared with soils of the humid tropical lowlands. The climate of the highland plateau of Ethiopia is characterized by summer-long monsoon seasons followed by a prolonged drought, a phenomenon conducive to the formation of Vertisols. It is estimated that Vertisols comprise about 24% of all cropped highland soils of Ethiopia (Jutzi and Haque, 1985). Volcanic activity in the region has resulted in the formation of Andisols, although Vertisols are also found associated with volcanic ash as a parent material. Of the remaining soils, Alfisols formed from granitic material occupy the majority of the highland plateau region. However, similar to other agricultural soils of the tropics, the highland plateau soils of Ethiopia are generally deficient in P to support optimum crop yields, except for the Andisols derived from volcanic ash (Desta, 1982; Tekalign et al., 1988; Asnakew et al., 1991). Under the prevailing conditions of high fertilizer costs and low agriculture-based incomes, correcting P deficiencies by fertilizer application must take into account the P fixation characteristics of these soils.

Reserves of plant-available P in soils are the result of indigenous P and past P fertilization. Duffera and Robarge (1996) reported that cultivation without adequate fertilizer application has decreased the labile P pool in the highland plateau soils of Ethiopia. Soil chemical properties can be affected by fertilization and management, and some of these fertilizer induced changes may also affect P sorption (Mullins, 1991). Heavy P fertilization of field plots 1 yr prior to measuring P adsorption caused adsorption isotherms to be shifted so that there were higher amounts of sorbed P at a given P concentration in solution (Fox et al., 1974).

Phosphorus sorption studies have been reported for many tropical soils (Juo and Fox, 1977; Loganathan et al., 1987), but few studies have been conducted for the highland plateau soils of Ethiopia. So far, the only reported work on P sorption in Ethiopian soils is that of Sertsu and Ali (1983) and Tekalign and Haque (1987). They compared the relative added P retention capacity of different soils, but did not consider the influence of P originally present in the soil on P sorption characteristics. In this study, an attempt was made to describe and compare the P sorption characteristics of some of the highland plateau soils of Ethiopia as a function of the general soil characteristics and management practices (cultivation-fertilizer application) being carried out in the region.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusion
 REFERENCES
 
Soils
Twelve representative surface soil samples (0–15 cm) were collected from four locations in the highland plateau region of Ethiopia. For each location, single composite samples were collected from a non-cultivated-non-fertilized field, a farmer's field, and a site on a research station farm. The soil order at each location was Vertisol, Andisol, or Alfisol. Of the Vertisols, one is from the Akaki area (9.02°N, 38.45°E) and developed on alluvial deposits, the other is from the Debre Zeit area (8.48°N, 39.38°E) and developed on parent material of volcanic origin. According to Murphy (1959) soils occurring between Akaki and Mojo areas of Ethiopia are of volcanic origin. An Alfisol sample formed from granite was collected from the Eastern highland region (Hamaressa area) (9.26°N, 42.23°E). These three soil orders are believed to be the major soil types occurring in the highland plateau of Ethiopia (Murphy, 1959).

The soil samples were collected by the Debre Zeit Agricultural Research Center, Soil Science Research Program staff, as part of a larger P characterization study, and shipped to North Carolina State University in May of 1993. The Debre Zeit Research Center was established in early 1960s and the use of mineral fertilizers began in the late 1960s. The other research stations were established in the 1980s. On the research station farms, deep plowing with tractor drawn implements is practiced, and P fertilizer is applied every year at a recommended rate that varies depending on soil type and crop grown. Most farmers in Ethiopia practice shallow cultivation with oxen-drawn implements and fertilizers are applied sparingly at much less than the recommended rate. The sampling area described as non-cultivated-non-fertilized are currently used as unmanaged pastures. Farmers may have cultivated these areas in the past, but it is highly unlikely that they have received significant amounts of P fertilizer additions.

The soil samples were air dried and ground to pass a 2-mm sieve. The pH was measured in distilled water (1:1 soil-solution ratio). Particle size distribution was determined by the pipette method after pretreatment to remove soluble salt, organic matter, and carbonates (Gee and Bauder, 1986). Organic carbon contents were determined by the dichromate oxidation method of Walkley and Black (Nelson and Sommers, 1982). The cation exchange capacity was determined with 1 M NH4OAc buffered at pH 7 (Thomas, 1982). Dithionite-extractable Fe and Al were determined by the method of Coffin (1963). The surface area of the soil samples was measured by the N2 adsorption method (Carter et al., 1986) using a Quantachrome Monosorb surface area analyzer (Quantachrome Corporation, Syosset, NY).

The method of Hedley et al. (1982), as modified by Beck and Sanchez (1994), was used to sequentially fractionate soil P. The procedure is designed to progressively remove less plant available soil P fractions with each subsequent extraction. Resin and bicarbonate extractable P are assumed to be the labile forms of P obtained with this sequential procedure, and are thought to consist of P adsorbed on surfaces of more crystalline P compounds, hydrous Fe and Al oxides or carbonates, including P present in soil solution (Bowman et al., 1978; Bowman and Cole, 1978). Sodium hydroxide extractable P is assumed to be P associated with amorphous and crystalline Al and Fe phosphates. Results for the resin-P, and the inorganic bicarbonate-P, and sodium hydroxide-P fractions are presented in Table 1 . The remaining results and subsequent interpretation from the sequential fractionation of P in these soil samples are presented elsewhere (Duffera and Robarge, 1996).


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  		Table 1 Sampling locations, physical and chemical properties, and extractable P fractions in the surface soil samples (0–15 cm) collected from the highland plateau areas of Ethiopia

 
Phosphorus Sorption Isotherms
Phosphorus sorption data were obtained by equilibrating 3-g duplicate soil samples with 30 mL of 0.01 M CaCl2 in 50-mL plastic centrifuge tubes. Various amounts of P (0–20 mg L-1) were added as KH2PO4 and the samples were equilibrated for either 24 h or 6 d. Differences in the amount of P sorbed between a 24-h or 6-d equilibration time were found to be less than 5% for the range of P concentrations considered in this study. Therefore, the P sorption data reported here were obtained using a 24-h equilibration period. A few drops of toluene were added to suppress microbiological activity and the samples were shaken on a reciprocal shaker at a speed of 100 cycles per min. At the end of the equilibration period, the aqueous solution was separated by a combination of centrifugation (4800 x g rpm for 10 min) and filtration (Whatman No. 40). The concentration of P in solution was determined by the molybdate-ascorbic acid method (Murphy and Riley, 1962). The amount of P sorbed was calculated as the difference between the amount of P added and that remaining in solution (Fox and Kamprath, 1970).

Adsorption Model
The P sorption data, including the solution P concentration (C) and the amount of sorbed P (qi, mg kg-1 or µg m-2), were fitted to the Langmuir equation. The Langmuir adsorption equation, originally developed by Langmuir (1918) to describe adsorption of a gas on a clean solid, often has been adopted to interpret reactions between the adsorption of ions by a solid and the concentration of ions in solution (Olsen and Watanabe, 1957). The common derived form of the equation as applied to liquids or ions in solution is:

(1)
where k1 and k2 are constants: k1 is designated as the Langmuir sorption maxima, and k2 is related to binding energy. The parameters were derived by transformation of the equation:

(2)

A linear regression was used to fit the data to the Langmuir isotherm.

Statistical Analyses
Relationships between the amount of added P sorbed to obtain solution P concentration of 0.2 mg L-1 and soil physical and chemical properties were analyzed with simple correlation and tested for significance at P = 0.05. Multiple regression analysis, conducted with SAS STEPWISE and REG procedures (SAS Institute Inc., 1990), was used to test the interrelationships between the amount of P sorbed at 0.2 mg L-1 in solution, resin P as determined by the sequential fractionation procedure, and other soil properties.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusion
 REFERENCES
 
A summary of the physical and chemical properties of the soil samples used in this study is presented in Table 1. The pH values varied from 6.2 to 7.9. The Vertisol samples were highest in clay content and CEC. The Andisol samples had the lowest amounts of clay, but CEC values were greater than CEC values for the Alfisols. The Alfisols had the highest percentage of dithionite extractable Fe and Al, with more variation in the extractable Al than the extractable Fe among the remaining soil samples. Clay contents were similar among soil samples selected from different management practices for Vertisol1 and Alfisols. However, there were some variations in clay content among soils from Debre Zeit, with a lower clay content associated with the Andisol and a higher clay content for the Vertisol from the research station compared with the other sites. This is likely due to the topographic location of the two soil types collected from Debre Zeit Research station. The Andisols are located on the upper slope areas followed by Vertisols occupying the lower flat area of the landscape. This may have resulted in eroding the smaller size fractions from the upper areas and depositing them on the lower flat land.

Phosphorus Sorption Isotherms
The rates of P addition were adjusted to yield solution P concentrations generally less than 2.0 mg L-1. Within this range, the P concentration in solution increased with increasing rates of P addition. The solution P concentration at the same level of P applied was higher in the non-cultivated soil samples compared with the samples collected from research station or farmers' fields for each soil type, except for the Alfisols (Fig. 1) . Soil samples with volcanic influence (Andisol and Vertisol2) had the highest solution P concentration at the same level of P applied compared with the other two soil types. The solution P concentration in the non-cultivated Andisol samples without P application was more than 15 times greater than that found in the samples collected from farmer's field and more than two times than that found in the samples collected from the research station (Fig. 1). This reflects the relatively large amounts of labile P initially present in these soils. The non-cultivated Andisol samples had the highest amounts of resin, bicarbonate, and sodium hydroxide extractable P (Table 1), even though the total P content was greater (1271 mg kg-1 vs. 1569 mg kg-1 in non-cultivated and research station samples, respectively) in the samples collected from the research station (Duffera and Robarge, 1996). Differences in the amount of solution P concentration in the Andisol samples reflect the relatively large amounts of labile P already present in these soils and demonstrates that labile forms of P initially present in the soil can influence subsequent P sorption studies.



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  		Fig. 1 Solution P concentration as a function of P applied for surface soil samples (0–15 cm) from the highland plateau areas of Ethiopia. (Standard errors are calculated on the basis of duplicate samples analyzed)

 
Phosphorus sorption isotherm plots enable comparison of the soil P sorption capacity corresponding to observed solution P concentrations that lead to optimum crop growth (Fox and Kamprath, 1970; Klages et al., 1988). Klages et al. (1988) have reported that the fertilizer P requirement for dry-land wheat was predicted more accurately by P sorption isotherms than by the Olsen soil P test. The amount of P retained to obtain 0.2 mg P L-1 in solution was calculated for all soils by the Langmuir equation (Eq. [2]). Solution P concentration of 0.2 mg L-1 was selected following the suggestion of Beckwith (1965) that this concentration, if maintained continuously in soil solution, will provide adequate P to many plants. Even though the P concentration required by plants varies, the P sorption at a solution P concentration of 0.2 mg L-1 (standard P requirement) can be used as a standard for comparing the P requirement of different soils (Fox and Kamprath, 1970; Juo and Fox, 1977; Loganathan et al., 1987). The amount of added P sorbed by the soil samples at 0.2 mg P L-1 in solution, range from 0 to 201 mg P kg-1 soil (Table 2) . Except for the Andisols, cultivated soils from research stations and farmer's field required more sorbed P to maintain the same solution P concentration than samples from non-cultivated fields. Generally, soil samples collected from the farmer's field retained the highest amount of P or were similar to the research station soils. For Vertisol1, there was little difference between the three samples, but less P was sorbed by the non-cultivated sample (Fig. 2) . Relatively less P was also sorbed by the non-cultivated Vertisol sample of volcanic origin. Large differences in the amount of P sorbed were observed for the Andisol samples in which the sample collected from the farmer's field sorbed greater amounts of applied P apparently because this sample had much less labile P initially present than the other Andisol samples.


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  		Table 2 Phosphorus sorbed at 0.2 mg L-1 in solution, Langmuir constant (k2), and regression coefficient (r2) for P sorption isotherm studies conducted on surface soils (0–15 cm) from the highland plateau areas of Ethiopia with and without inclusion of resin-P as an estimate of initially sorbed P

 


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  		Fig. 2 Phosphorus sorption isotherms for applied P sorbed expressed per unit mass of soil for surface soil samples (0–15 cm) from the highland plateau areas of Ethiopia. (Standard errors are calculated on the basis of duplicate samples analyzed)

 
When expressed on a unit surface area basis (Fig. 3) , the P sorption isotherm trends were essentially similar to isotherms expressed on a unit mass basis (Fig. 2). The difference in the density of surface coverage (expressed on surface area basis assuming monolayer adsorption) observed between samples of a given location is likely due to differences in the amount of P initially present in the soil samples. This supports the conclusion that the difference in amount of P sorbed among soils within a given location is due primarily to continuous cropping, with the samples from the farmer's field requiring more sorbed P to maintain the same solution P concentration.



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  		Fig. 3 Phosphorus sorption isotherms for applied P sorbed expressed per unit surface area for surface soil samples (0–15 cm) from the highland plateau areas of Ethiopia. (Standard errors are calculated on the basis of duplicate samples analyzed)

 
The soil samples from the research stations and the non-cultivated fields in general contained more resin P than the soil samples from the farmer's field (Table 1). The resin P from the modified Hedley et al. (1982) P fractionation scheme was added as part of the sorbed P in the P sorption isotherms to observe the effect of P initially present in the soil samples on P sorption characteristics of the soil. When resin P is included as initially sorbed P in the P sorption isotherms, the difference among the P sorption isotherms as a function of cultivation-fertilization practice becomes less evident, especially for the Andisol and Vertisol2 soil samples (Fig. 4) . When the resin-P estimate is included in the calculation, the amount of P sorbed by the soil samples at 0.2 mg P L-1 in solution range from 41 to 206 mg P kg-1 soil, but with little difference among the different cultivation-fertilization practices (Table 2). Results obtained by Mullins (1991) showed that sorption of newly added P is affected by long term P fertilization primarily by having a larger percentage of P sorption sites already occupied in fertilized soils. Barrow (1978) discussed the difficulty of taking adequate account of the P already present in the soil and concluded that the simple Freundlich equation described sorption well because many Australian soils are deficient in P and hence the value of initially sorbed P was small.



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  		Fig. 4 Phosphorus sorption isotherms considering the resin-P as initially sorbed P for surface soil samples (0–15 cm) from the highland plateau areas of Ethiopia. (Standard errors are calculated on the basis of duplicate samples analyzed)

 
The quantitative description of soil P retention can be used to determine the critical solution or labile P content of soils for optimum plant growth (Holford and Mattingly, 1976). Soils usually contain a certain quantity of phosphate ions already present on their surfaces. The presence of this initially bound P must be taken into consideration when fitting P sorption data (Bache and Williams, 1971; Tolner and Fulkey, 1995). The amount of bound P initially present has been determined with the aid of ion exchange resins (Fitter and Sutton, 1975), or estimated by isotopic exchange techniques (Bache and Williams, 1971). Tolner and Fulkey (1995) found a linear correlation between P treatments and the originally adsorbed P calculated using an adsorption model. In studying the effect of lime and silicate amendments on P sorption, Smyth and Sanchez (1980) concluded that previous P applications were more effective in decreasing P sorption than the amendments.

The results shown in Fig. 4 suggest that resin P as determined in the Hedley et al. (1982) procedure, provides an excellent estimate of P sorbed to sites that would react with P addition. Inclusion of the bicarbonate and sodium hydroxide extractable P as an estimate for initially sorbed P generally reversed the order of the P sorption isotherms for the different cultivation-fertilization practices, especially for Andisol and Vertisol2 samples in which samples collected from farmers' field sorbed the least amounts of P (Fig. 5) . It appears that the bicarbonate and sodium hydroxide extractable P, although shown to be highly correlated with resin P (Duffera and Robarge, 1996), extracts P from sorption sites or other solid phases that will not react with P addition, assuming that the samples have equal adsorbent characteristics at three different cultivation-fertilization practices within each soil.



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  		Fig. 5 Phosphorus sorption isotherms considering the sum of resin, NaHCO3 Pi, and NaOH Pi as initially sorbed P for surface soil samples (0–15 cm) from the highland plateau areas of Ethiopia. (Standard errors are calculated on the basis of duplicate samples analyzed)

 
Relationship with Selected Soil Properties
The correlation of soil properties with applied phosphate sorbed to obtain 0.2 mg P L-1 in solution was calculated for all soils (Table 3) . The amount of added P sorbed to obtain 0.2 mg P L-1 in solution was negatively correlated to resin P (r = -0.90) showing the influence of this P fraction on P sorption behavior of the soil samples. Stepwise regression analysis of the amount of P sorbed at 0.2 mg P L-1 in solution as a function of soil physical and chemical properties as well as different P pools showed that resin P was responsible for 81% of the variability in P sorption capacity of the soil samples. This suggests that resin P provides an excellent estimate of initially sorbed P when constructing P sorption isotherms to estimate the P status and fertilizer requirements of soil. When all of the soil samples were considered, the amount of P sorbed (per unit mass) at 0.2 mg P L-1 in solution was directly correlated to clay (r = 0.73), dithionite extractable Al (r = 0.89) and surface area of the soil (r = 0.64). Several workers (Loganathan et al., 1987; Solis and Torrent, 1989; Bennoah and Acquaye, 1989) have reported correlation of P sorption with clay content, and this may be a reflection of the effect of specific surface area on P sorption (Loganathan et al., 1987). Indeed, the positive significant correlation obtained between surface area, measured by the BET-N2 gas adsorption method, and clay content (r = 0.94), is consistent with this relationship. Extractable forms of Fe and Al have been reported as key factors controlling P sorption in Ethiopian soils, although the correlation involving Al is higher than those involving Fe (Tekalign and Haque, 1987). Differences in the sorption capacity observed between the different soil types could be attributed to the differences in soil physical and chemical properties. The Alfisols, containing more than two times dithionite extractable Fe (Table 1) compared with the other three soil types, showed higher Langmuir sorption constant (k2) (Table 2). Correlation of soil pH with P sorbed to obtain 0.2 mg L-1 in solution was not significant which is likely due to the samples having a very narrow pH range within which pH is not expected to have much effect on P sorption.


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  		Table 3 Correlation coefficients between P sorbed at 0.2 mg L-1 in solution and selected soil properties (n = 12)

 

   Conclusion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusion
REFERENCES
 
The difference in P sorption capacity of the soil samples collected from the highland plateau region of Ethiopia was mainly due to differences in labile P originally present in the soil samples, as measured by the resin-P and inorganic bicarbonate-P and sodium hydroxide-P fractions of the modified Hedley fractionation scheme. There was little variation in the P sorption among the Akaki Vertisols (Vertisol1) as a result of soil cultivation-fertilization practices. These soil samples also showed little differences in labile P fractions as well as soil physical and chemical properties. However, there were large variations among the soils of volcanic origin (Vertisol2 and Andisols) in P sorption. Greater differences were also observed in labile P fractions among these soil samples. The Alfisol samples collected from research station and non-cultivated area showed similar trends in P retention, while samples collected from farmer's field sorbed relatively higher P, even though the labile P fraction in these samples seem to be the same. Dithionite-extractable Fe and Al, clay, and surface area are a little higher in the sample from farmer's field, which tend to increase P sorption. Generally, the differences in P retention observed among the different cultivation-fertilization practices within a given soil type are likely due to the variations in the labile P present in the samples rather than the differences in soil physical and chemical properties.

On the basis of the quantity of added P sorbed at 0.2 mg P L-1 in solution, the order of P sorption of the highland plateau soils of Ethiopia is Alfisols > Vertisols > Andisols. The low added P sorption of the Andisols is due to the presence of large amounts of labile P in the soil and probably low Al and clay content. The Alfisol samples had relatively higher amounts of dithionite-extractable Fe and Al as compared with the other soil types. Within a given soil, the highest amount of P was sorbed by samples collected from farmers' field. This is mainly due to the practice of continuous cropping with minimal P fertilization, which depletes labile P. From this study, it can be concluded that soils from the highland plateau region of Ethiopia under different management practices have different capacities to retain P and therefore require different levels of P fertilization for optimum crop yield. Therefore, P calibration studies conducted on research station farms, destined for farmers' utilization, may not reflect the local farmers condition and may require due caution when fertilizer recommendations are given to farmers from studies conducted on research station farms. However, trends in P sorption reported here are based on limited observations and further studies with a detailed characterization of P sorption isotherms leading to better management and efficiency of P fertilizer use are needed.SAS Institute 1990


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusion
 REFERENCES
 




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B. L. Allen and A. P. Mallarino
Relationships between Extractable Soil Phosphorus and Phosphorus Saturation after Long-Term Fertilizer or Manure Application
Soil Sci. Soc. Am. J., February 2, 2006; 70(2): 454 - 463.
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