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Published online 28 September 2007
Published in Soil Sci Soc Am J 71:1699-1707 (2007)
DOI: 10.2136/sssaj2007.0008
© 2007 Soil Science Society of America
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SOIL CHEMISTRY

Animal Manure Reduces Aluminum Toxicity in an Acid Soil

Y. Tanga, H. Zhangb,*, J. L. Schroderb, M. E. Paytonc and D. Zhoud

a Yangzhou Univ., Jiangsu, China
b Dep. of Plant and Soil Sciences, Oklahoma State Univ., Stillwater, OK 74078
c Dep. of Statistics, Oklahoma State Univ., Stillwater, OK 74078
d Institute of Soil Science, Chinese Academy of Sciences, Nanjing, China

* Corresponding author (hailin.zhang{at}okstate.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Manures are believed to be an effective treatment to reduce Al toxicity in acidic soils. A pot experiment was conducted to study the effect of feedlot manure (FM) and poultry litter (PL) on soil Al status and wheat growth. Custer, an acid susceptible wheat cultivar (Triticum aestivum L. var. Custer), was planted in a manure amended Teller (fine-loamy, mixed, thermic Udic Argiustoll) fine sandy loam soil and relationships between soil pH, organic carbon (OC), and P added in the manure, 0.01 mol L–1 CaCl2 extractable Al (AlCaCl2), 1.0 mol L–1 KCl exchangeable Al (AlKCl), and wheat growth were investigated. An acidic surface soil was mixed with five levels of FM and PL and incubated for 30 d in an environmentally controlled growth chamber before wheat was planted. Wheat was grown for 35 d and harvested for aboveground dry matter. Both PL and FM increased soil pH, reduced AlCaCl2 and AlKCl, and increased wheat biomass. Wheat biomass was positively correlated with soil pH (r = 0.76), OC added (r = 0.63), and P2O5 added (r = 0.87) but was negatively correlated with AlCaCl2 (r = –0.83) and AlKCl (r = –0.87). Path analysis showed significant direct effects (p < 0.01) between wheat growth and OC added and P2O5 added. The direct effects of soil pH, AlCaCl2, and AlKCl were not significant (p > 0.05) but the indirect effects of OC added and P2O5 added were important contributors to the correlations between wheat biomass and soil pH, AlCaCl2, and AlKCl. These two animal manures have the potential to reduce Al toxicity in acidic soils but need to be further evaluated at a field scale.

Abbreviations: AlCaCl2, 0.01 mol L–1 CaCl2 extractable Al • AlKCl, 1.0 mol L–1 KCl exchangeable Al • Alsat, Al saturation • CEC, cation exchange capacity • ICP–AES, inductively coupled plasma–atomic emission spectrophotometer • OC, organic carbon


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil acidity is one of the major problems for agricultural production in many parts of the world (Kamprath, 1984). It controls the solubility and precipitation of chemical compounds of some essential plant nutrients. Soil acidity is harmful for plant growth due to nutritional disorders (deficiency of Ca and Mg, decreased availability of P and Mo) as well as the immediate toxicity of soluble Al, Mn, and H+ (Carver and Ownby, 1995; Haynes and Mokolobate, 2001). The relative importance of each of these factors is difficult to generalize across soils with inherently different soil solution chemistry. However, in strongly acidic soils (pH < 5), Al toxicity is considered the most detrimental problem (Foy, 1984; Kochian and Shaff, 1991) and poor crop growth has been correlated with high Al saturation (Sartain and Kamprath, 1977; Fox, 1979; Farina et al., 1980; Abruna-Rodriguez et al., 1982, Fageria et al., 1997; Costa et al., 2003). Aluminum saturation is the percentage of effective cation exchange capacity (CEC) occupied by exchangeable Al and has been used as an indicator of Al toxicity in soils and as an index for lime requirement (Fox, 1979; Farina et al., 1980). The initial and most obvious symptom of Al toxicity is the inhibition of root growth. Injured roots are characteristically stubby with reduced growth of the main axis and inhibited lateral root formation (Foy, 1988). Root growth inhibition occurs through impedance of both cell elongation and cell division (Kochian, 1995). Since root growth is restricted, plant water uptake is reduced. As a result, nutrient and/or water stresses are common in plants suffering from Al toxicity (Foy, 1984).

Wheat is the number one food grain consumed by humans with more land being committed to the production of wheat than any other crop in the world (Briggle and Curtis, 1987). The minimum recommended soil pH for wheat production is 5.5 (Johnson et al., 2000) but a recent study showed that 28% of the wheat fields in Oklahoma had a pH < 5.5 (Zhang et al., 1998). Lime is commonly used to improve conditions of acid soils for plant growth and has been used to correct acid soil conditions for wheat production (Zhang et al., 2004). Liming increases soil pH, decreases concentrations of active Al level and increases the supply of Ca and other nutrients (Adams, 1984).

An alternative approach to liming is the application of organic materials to reduce Al toxicity. As early as 1933, there were reports that the addition of organic matter could prevent Al toxicity (Hester, 1935; Mattson and Hester, 1933). Beneficial effects of organic matter application on Al detoxicification have been further confirmed by later investigations. In greenhouse experiments in a sand culture, Tan and Binger (1986) showed that in the absence of humic acid, growth of maize plants decreased linearly with increasing Al additions. However, addition of humic acid at 100 to 300 mg kg–1 greatly improved plant growth and ameliorated the negative effect of increasing Al concentrations. In solution culture experiments, Suthipradit et al. (1990) found that addition of fulvic acid reduced the amount of monomeric Al present in solution and alleviated the toxic effect of Al on growth of soybean, cowpea, and green gram. The addition of organic residues to soils can increase soil pH (Hoyt and Turner, 1975; Hue, 1992; Noble et al., 1996) and precipitate soluble Al. Additionally, PL and FM may raise pH via a liming effect because they contain large amounts of CaCO3, which originates in the animal diet (Eghball, 1999; Moore and Edwards, 2005).

Over 2.2 billion tons of animal manure is produced annually in the USA (Wright et al., 1998). Animal manures contain large amounts of organic matter, which may serve to alleviate Al toxicity and to provide nutrients for plant growth. Several researchers have shown the addition of animal manures to acid soils increased pH (Hue, 1992; Cooper and Warman, 1997, Wong et al., 1998; Whalen et al., 2000; Materechera and Mkhabela, 2002; Mokolobate and Haynes, 2002a, 2002b; Parham et al., 2002). The majority of these studies reported that the addition of animal manures also decreased some measurable form of Al. Other researchers have found that the addition of animal manures to acid soils decreased total Al or monomeric Al in soil solution and increased crop growth (Hue, 1992; Wong and Swift, 1995). However, the mechanism by which Al detoxification occurs is controversial. Haynes and Mokolobate (2001) reviewed the mechanisms involved in the amelioration of Al toxicity by the addition of organic residues. In their review, they suggested the possible mechanisms for the amelioration of Al toxicity by the addition of organic residues to soil were an increase in soil pH by decomposition of organic matter, complexation of Al in soil solution by soluble organic matter, and reduction of exchangeable Al due to complexation of solid-phase organic matter. However, manures also contain significant amounts of inorganic and organic P that may complex Al and decrease the activity of Al in soil solution (Wong and Swift, 2003).

It has been shown that soil properties are interrelated and autocorrelated (Basta et al., 1993), which makes it difficult to determine the components that contribute most to such things as heavy metal and P adsorption. It is most likely that properties of manures are also interrelated and autocorrelated making it difficult to determine which property affects crop yield the most. Therefore, simple correlation analysis inadequately explains the relationships because correlation does not imply that a direct cause-and-effect relationship exists (Wright, 1921). Rather, indirect effects may influence the correlation analysis and the resulting coefficient.

Path analysis is a statistical technique that partitions correlations into direct and indirect effects and distinguishes between correlation and causation (Wright, 1934; Afifi and Clark, 1984). Path analysis has been used extensively in agronomic studies (Gravois and Helms, 1992; Pantone et al., 1992; Cramer and Wehner, 2000; Zheng et al., 2002; Garcia del Moral et al., 2003), to investigate relationships between soil properties and adsorption of heavy metals (Basta et al., 1993; Krishnasamy and Mathan, 2001), and to investigate relationships between soil properties and adsorption of P (Zhang et al., 2005). To the best of our knowledge, no studies have utilized path analysis to examine the contributions of the different manure properties to correlations established between manure properties and crop yield. The objectives of this study were to (i) evaluate the effect of two different manures (i.e., PL and FM) on reducing Al toxicity to wheat and (ii) to use path analysis to investigate the relationships between wheat biomass and properties of manure.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil and Manure Properties
Soil used for the greenhouse experiment was a surface soil (0–15 cm) collected from the Agronomy Research Station of Oklahoma State University in Perkins, OK, where winter wheat grew poorly due to low pH and Al toxicity (Johnson et al., 2000). The soil series was Teller (fine-loamy, mixed, thermic Udic Argiustoll) (USDA-NRCS, 2001). Soil samples were air-dried, passed through 2-mm sieve, and analyzed for soil pH, OC, texture, plant available P and K, nitrate N (NO3–N), extractable Al, exchangeable Al, and Al saturation (Table 1 ). Soil pH was measured in a 1:1 soil/deionized water suspension (Thomas, 1996). Soil texture was determined using the hydrometer method (Gee and Bauder, 1986), while OC was determined by dry combustion using a LECO CN 2000 (LECO Corp., St. Joseph, MI). Both plant available P and K were extracted by Mehlich 3 solution using a soil/solution ratio of 1:10 and a shake time of 10 min (Mehlich, 1984). Phosphorus in solution was quantified by a Lachat Quickchem 8000 automated flow-injection analyzer (Zellweger Analytics, Milwaukee, WI, Lachat method # 10–115–01–1-A) while K in solution was determined by a Spectro Cirros inductively coupled plasma–atomic emission spectrophotometer (ICP–AES) (Spectro Cirros, ICAP, Fitchburg, MA). Nitrate-N in soil was extracted with 1.0 mol L–1 KCl (1:10 soil/solution ratio and a shake time of 30 min) and analyzed by automated flow injection (Lachat method # 12–107–04–1-B; Zellweger Analytics, Milwaukee, WI; Mulvaney, 1996). Extractable Al and AlKCl were extracted in 0.01 mol L–1 CaCl2 and 1.0 mol L–1 KCl (1:10 soil/solution and shaken for 30 min), respectively, (Bertsch and Bloom, 1996). Aluminum in the extracts was measured by ICP–AES (Soltanpour et al., 1996). Aluminum saturation was calculated as a measure of Al toxicity using Eq. [1] (Johnson et al., 1997).

Formula 1[1]
where AlKCl is exchangeable Al and ECEC is the effective cation exchange capacity (the sum of exchangeable Na, K, Ca, Mg, and Mn measured in 1 mol L–1 NH4OAc at pH 7.0, and expressed in cmol(+) kg–1 plus the concentration of AlKCl expressed in cmol(+) kg–1). Duplicate analyses were conducted in the measurement of physical and chemical properties in soil.


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  		Table 1. Selected chemical and physical properties and plant available nutrient status of the Teller soil used for the study.

 
Manures were characterized by measuring pH, OC, total N, P, K, Ca, Mg, nitrate N (NO3–N), and ammonium N (NH4–N) (Table 2 ). Manure pH was determined as described above for soils. Total N and OC were determined in manures by dry combustion according to procedures outlined by Watson et al. (2005) using a LECO CN 2000 (LECO Corp., St. Joseph, MI). Nitrate-N and ammonium N were determined colorimetrically according to Peters et al. (2005). Phosphorus, K, Ca, and Mg were determined using wet acid digestion according to Wolf et al. (2005). Duplicate analyses were conducted in the measurement of physical and chemical properties in manure.


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  		Table 2. Chemical properties, physical properties, and major nutrient concentrations of feedlot manure and poultry litter used in the incubation study.

 
Soil Treatments
The study soil was amended with three materials FM, PL, or urea. Poultry litter was obtained from a litter compost facility and FM was obtained from a local feedlot. Both manure materials were air-dried and ground to pass 2.0-mm sieve before addition to soil. Manure and urea rates added to soil were calculated to supply 0, 25, 50, 100, and 200 mg kg–1 available N, assuming N availability from manure is 50% of the total N (Zhang, 2005) (Table 3 .). Application rates for PL were 0.0, 1.0, 2.0, 4.0, and 8.0 g kg–1 while application rates for FM were 0.0, 1.5, 3.0, 6.0, and 12.0 g kg–1 (Table 3). Urea treatments were utilized as a positive control to evaluate if wheat was responding to a nitrogen fertilization effect. Application rates for urea were 0.0, 0.054, 0.108, 0.216, and 0.432 g kg–1 (Table 3).


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  		Table 3. Amount of amendment and available N added, mean extractable Al (AlCaCl2), mean exchangeable Al (AlKCl), mean soil pH, and mean yield (n = 5) of the amended soil. Measurements were taken at the time of wheat harvest (i.e., 65 d after amendment application).

 
Two kilograms of amended soil were placed in 15 (diam.) by 15 (height) cm plastic pots. Container capacity for the soil was determined according to Cassel and Nielsen (1986). All pots were incubated at 85% of container capacity at 25°C for 30 d. Soils were periodically weighed and brought back to initial weight to maintain moisture at 85% of container capacity during the incubation time and during the experiment.

Wheat Growth
Following incubation, 16 wheat seeds (Triticum aestivum L. var. Custer), an acid susceptible cultivar (Kariuki et al., 2007) were planted per pot. Five replicates of each treatment were planted in a completely randomized design. Pots were thinned to seven plants per pot at 7 d. Plants were grown in a controlled environmental growth chamber with 18 h of light d–1, daytime temperatures of 20°C and night temperatures of 18.5°C. Wheat shoots were harvested after 35 d by cutting at the soil surface. All plants were washed with tap water and rinsed with deionized distilled water. The clean plant samples were dried at 80°C for 24 h and dry matter weight was recorded. Soil samples were collected and analyzed for AlCaCl2 and AlKCl after the first 30-d incubation, after wheat growth (i.e., at 65 d), and again 55 d after wheat biomass harvest (i.e., at 120 d).

Statistical Analyses
Statistical analyses were performed using PC SAS Version 8.2 (SAS Institute, 2001). Two different statistical techniques (backward-stepwise regression analysis and path analysis) were utilized to evaluate the effect of soil properties and treatments on wheat biomass. Backward-stepwise regression analysis was used to generate empirical models capable of predicting wheat biomass based on soil properties. The backward-stepwise regression was used to identify crucial soil properties that explain most of the variation in wheat biomass. Soil properties that did not explain a significant part of the variation (i.e., p > 0.05) were not used as independent variables in the multiple regression equation.

Path analysis differentiates between correlation and causation by partitioning simple correlation coefficients between independent variables (manure properties) and dependent variables (wheat biomass) into direct and indirect effects (Afifi and Clark, 1984; Basta et al., 1993). Path analysis provides a numerical value for both direct and indirect effects, thus indicating the relative strength of causal relationships (Loehlin, 1987). Direct effects are referred to as path coefficients and are standardized partial regression coefficients (Basta et al., 1993).

A path analysis model was used to evaluate the relationships between wheat biomass and soil properties (Fig. 1 ). The direct effects of soil properties on yield are represented by single-headed arrows while coefficients of intercorrelations between soil properties are shown by double-headed arrows. Indirect effects of soil properties on what biomass are determined from the product of one double-headed arrow and one single-headed arrow. The independent variables of the model were OC added, P2O5 added, AlCaCl2, AlKCl, and soil pH (Fig. 1). For the model, direct and indirect effects were obtained from multiple linear regression of soil properties on wheat biomass and simple correlations between soil properties (SAS Institute, 2001). Additionally, an uncorrelated residue (U) was calculated for both models using the following equation.

Formula 2[2]
where R2 is the coefficient of determination. Path analysis results were determined from the following equations (Williams et al., 1990):

Formula 3[3]

Formula 4[4]

Formula 5[5]

Formula 6[6]

Formula 7[7]
where rij is the simple correlation coefficient between soil property and wheat biomass, Pij are path coefficients (direct effects) and rijPij are the indirect effects of soil property on wheat biomass. Subscript designations are:(i) OC added, (ii) P added, (iii) AlCaCl2 (iv) AlKCl, (v) pH, and vi) yield (Fig. 1).


Figure 1
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  		Fig. 1. Path diagram for the relationship between soil properties and wheat biomass. The direct effects (Pij) of soil properties on biomass are represented by single-headed arrows while the indirect effects (rijPij) of soil properties are shown by double-headed arrows. Subscript designations for soil properties and biomass are identified numerically as follows:(1) OC = organic carbon added (2) P = P2O5 added, (3) AlCaCl2 = 0.01 mol L–1 CaCl2 extractable Al, (4) AlKCl = 1.0 mol L–1 KCl extractable Al, (5) pH = soil pH, and (6) Biomass = biomass of wheat harvested at 35 d.

 
The path analysis results can be summarized in a concise table, which consists of a matrix with the main diagonal representing direct effects and off-diagonal elements representing indirect effects (Williams et al., 1990). The position of each element in the matrix corresponds to its position in the normal equations presented above.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil and Manure Properties
Soil test P and K for this soil were near 100% sufficiency (Table 1) (Johnson et al., 2000). Soil pH was low (pH = 4.3) and Al saturation was high (Alsat = 29.5%). Kamprath (1984) indicated that Alsat < 10% was needed for maximum yields of wheat and soybeans on highly weathered oxisols and ultisols while Johnson et al. (1997) referred to Oklahoma soils with <16% Alsat as lower levels of Al potential phytotoxocity.

Poultry litter contained greater concentrations of base cations and nutrients, which is typical of this type of manure (Zhang, 2005). The C/N ratio of the PL was less than the C/N ratio of FM (Table 2)."

Effect of Amendments on Soil pH, Aluminum Concentrations, and Wheat Biomass
All three amendments (PL, FM, and urea) significantly increased (p < 0.05) soil pH as compared with the unamended control (Table 3). Soil pH was positively related (p < 0.001, r2 > 0.85) to the addition of PL, FM, and urea (Fig. 2A , 2B). The pH increase rate was statistically similar (p > 0.05) between the PL and FM treatments. The rate of pH increase by urea was approximately 20-fold greater (p < 0.05) than that of PL and FM. Poultry litter increased pH by 0.040 units per g kg–1 addition and FM raised pH by 0.042 units per g kg–1 addition (Fig. 2A) while urea raised pH by 0.80 units per g kg–1 addition (Fig. 2B). However, one must be aware that much lower levels of urea were used to supply the same amount of N as compared with manure, thus slope comparisons between the urea and manure treatments may be misleading. The pH increase due to manure application is consistent with results reported by several other researchers (Hue, 1992; Cooper and Warman, 1997, Whalen et al., 2000; Materechera and Mkhabela, 2002; Mokolobate and Haynes, 2002a, 2002b; Parham et al., 2002). Manures contain humic and fulvic substances. Functional groups such as carboxyl, which can consume or release protons depending on the pH of soil, are present in manures (Wong and Swift, 2003). Thus, the pH increase by the addition of manure to acidic soils may be partially explained by proton exchange between the soil and the added manure (Tyson and Cabrera, 1993; Wong et al., 1998; Wong and Swift, 2003). The increase in pH with the addition of urea was probably due to the consumption of protons to form bicarbonate during the hydrolysis of urea to form ammonium (Kissel et al., 1988).


Figure 2
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  		Fig. 2. The relationship between soil pH and (A) application rate of poultry litter and feedlot manure (B) application rate of urea. ***p < 0.001.

 
The unamended study soil contained 6.93 mg kg–1 of AlCaCl2 and 128 mg kg–1 of AlKCl (Table 3). Calcium chloride (0.01 mol L–1) is a commonly used extractant (Sparks, 1996) that removes the fraction of Al closely related to plant response (Baligar et al., 1992; Hoyt and Nyborg, 1970; Machado and Gerzabek, 1993; Wright et al., 1989). The three different amendments significantly decreased (p < 0.05) concentrations of both AlCaCl2 and AlKCl as compared with the unamended soil (Table 3). The AlCaCl2 was exponentially and negatively related (p < 0.001, r2 > 0.85) to the addition of PL, FM, and urea (Fig. 3 ). The rate of decrease of AlCaCl2 was statistically similar for the PL and FM treatments with the rate of decrease being much greater for the urea treatment. Conversely, AlKCl was negatively linearly related (p < 0.001, r2 > 0.90) to the rate of addition of PL, FM, and urea (Fig. 4 ). The addition of 1.0 g kg–1 of FM decreased AlKCl 8.6 mg kg–1 while1.0 g kg–1 of PL decreased AlKCl by 12.1 mg kg–1 (Fig. 4A). The difference in reduction may be due to precipitation of Al by greater levels of P compounds present in PL as compared with FM. Exchangeable Al constitutes an important buffered reserve of labile Al that can be solubilized readily by other exchangeable cations. The decrease of AlKCl reduced the potential capacity of soil to supply soluble Al to soil solution. There were no significant differences (p > 0.05) at 30, 65, and 120 d for AlCaCl2 averaged across all treatments for both PL and FM (data not shown). Additionally, statistical differences did not exist for AlKCl measured at 30, 65, and 120 d. This suggests the amelioration effect of these manures on reducing Al toxicity could last for at least one growing season. Our results are similar to those of other researchers who have reported that the application of PL was effective in reducing AlKCl in acidic soils (Materechera and Mkhabela, 2002; Mokolobate and Haynes, 2002a, 2002b, 2003).


Figure 3
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  		Fig. 3. The relationships between 0.01 mol L–1 CaCl2 extractable Al and (A) poultry litter application rate, (B) feedlot manure application rate, and (C) urea application rate. ***p < 0.001.

 

Figure 4
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  		Fig. 4. The relationship between 1.0 mol L–1 KCl extractable Al and (A) application rate of poultry litter and feedlot manure (B) application rate of urea. ***p < 0.001.

 
Relationships Between Biomass and Application Rate
Addition of PL and FM significantly increased (p < 0.05) biomass as compared with the control but the addition of urea did not significantly increase (p > 0.05) the growth of wheat as compared with the control (Table 3). The results are similar to those of Mokolobate and Haynes (2003) who conducted a glasshouse potting study and reported that an application rate of 20 mg PL kg–1 soil greatly increased growth of maize in acidic Oxisol (i.e., pH = 4.2).

Significant relationships existed between wheat biomass and application rates (p < 0.001) for the PL treatment (r2 = 0.93) and the FM treatment (r2 = 0.92) (Fig. 5A ). However, due to the lack of response to treatments, significant relationships did not exist (p > 0.05, r2 = 0.38) between biomass and application rate of urea (Fig. 5B). The urea treatments were utilized as a positive control to evaluate if wheat was responding to a N fertilization effect. The lack of a growth response in wheat indicates that wheat did not respond to the addition of N during the experiment. Therefore, N added was not used as one of the independent variables for the path analysis model. Yet the fact that there was not a significant increase in wheat yield in response to urea addition despite an increase in soil pH indicate that factors other than pH were probably responsible for yield increases when PL and FM were added.


Figure 5
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  		Fig. 5. The relationships between wheat yield and (A) application rate of poultry litter and feedlot manure (B) application rate of urea. ***p < 0.001.

 
Path Analysis and Multiple Regression
Results for the path analysis are shown in Table 4 . Simple correlation coefficients (r) between OC added, P added, AlCaCl2, AlKCl, soil pH, and wheat biomass are presented for comparison with path analysis results. The uncorrelated residual value (U) was low (0.28) while the coefficient of determination (R2) was high 0.92 indicating the path analysis explained the majority of variation in wheat biomass. Significant correlation coefficients (p < 0.01) were found between OC added (r = 0.63), P2O5 added (r = 0.87), AlCaCl2 (r = –0.83), AlKCl (r = –0.87), soil pH (r = 0.76) and wheat biomass (Table 4).


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  		Table 4. Path analysis direct effects (diagonal, underlined) and indirect effects (off diagonal) of organic carbon (OC) added, P2O5 added, extractable Al, exchangeable Al, and soil pH on biomass for the Teller study soil.

 
Path analysis partitions each r value into one direct effect and four indirect effects. Partitioning by path analysis showed significant direct effects of OC added (r = 0.68) and P2O5 added (r = 0.94) on wheat biomass. However, the direct effects of AlCaCl2, AlKCl, and soil pH were not significant (p > 0.05). Examination by path analysis revealed that the indirect effects of OC added (r = –0.56) and P2O5 added (r = –0.62) were important contributors to the correlation between AlCaCl2 and biomass and between AlKCl (OC r = –0.57 and P2O5, r = –0.69) and biomass. Similarly, the indirect effects of OC added (r = 0.63) and P2O5 added (r = 0.48) contributed greatly to the correlation between soil pH and biomass.

Stepwise multiple regression identified a two-term model based on OC added and P2O5 added that explained 92% of the variation in wheat biomass (Table 5 ). The multiple stepwise regression agreed well with path analysis and identified the same two terms whose direct effects were identified as significant by path analysis


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  		Table 5. Multiple regression formulae describing the relationship between soil properties and biomass for the Teller study soil.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Poultry litter and FM significantly increased soil pH, reduced AlCaCl2 and AlKCl, and improved wheat growth. Soil pH was positively related to the addition of both PL and FM with the pH rate of increase being statistically similar between PL and FM. Extractable Al (AlCaCl2) was exponentially and negatively related to the addition of PL and FM and the rate of decrease was statistically equivalent between PL and FM. Although the decrease of AlKCl was linearly related for both PL and FM, the rate of decrease of AlKCl was greater for PL probably due to greater concentrations of P2O5 precipitating Al. The addition of both PL and FM increased yield while the addition of urea failed to increase wheat growing in a very acidic soil.

Several studies have suggested that an increase in soil pH due to the addition of manures is the most important property controlling the amelioration of Al toxicity. For our study, the amount of OC and P added through manure and soil pH were positively correlated with wheat biomass while other soil properties (AlCaCl2 and AlKCl) were negatively correlated with biomass. The results of our study clearly indicate that an increase in soil pH was not responsible (i.e., the direct effect was not significant) for an increase in wheat biomass. Rather, OC added complexed Al and P2O5 added precipitated Al and resulted in an increase in wheat biomass (significant direct effects). Additionally, path analysis showed that the direct effects of AlCaCl2 and AlKCl were not statistically significant and that these relationships were highly influenced by the indirect effects of OC and P2O5. Multiple regression also found that the combination of OC and P2O5 were the two most important properties related to wheat biomass. Animal manures have the potential to reduce Al toxicity in acidic soils but need to be evaluated at a field scale.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication January 4, 2007.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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