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Published online 16 May 2007
Published in Soil Sci Soc Am J 71:1038-1044 (2007)
DOI: 10.2136/sssaj2006.0170
© 2007 Soil Science Society of America
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NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Changes in Soil pH, Organic Carbon, and Extractable Aluminum from Crop Rotation and Tillage

Chad B. Godseya,*, Gary M. Pierzynskib, David B. Mengelb and Ray E. Lamondb

a Oklahoma State Univ., Stillwater, OK 74078
b Dep. of Agronomy, Kansas State Univ., Manhattan, KS 66506

* Corresponding author (chad.godsey{at}okstate.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Recent attention has focused on management of soil acidity in no-till (NT) soils due to the limited movement of surface-applied lime in these systems. Interactions of exchangeable Al and organic matter have been recognized for many years, but limited data exist investigating how these interactions should affect management decisions for NT soils. This study was conducted to identify effects of rotation and tillage on soil pH and soil organic carbon (OC) content and to determine the influence of soil pH and OC on KCl and CuCl2 extractable-Al (AlKCl and AlCuCl2, respectively). Soil samples were collected to a depth of 15 cm, in 2.5-cm increments, from a long-term rotation and tillage study near Manhattan, KS. Soil pH and OC concentrations were influenced by rotation and tillage, especially in the surface 2.5 cm. Organic C concentrations were on average 2.3 g kg–1 greater with NT than with conventional tillage in the surface 15 cm of soil. Aluminum extracted with KCl and AlCuCl2 increased exponentially with decreasing soil pH. Copper chloride extractable-Al values were on average 8% greater than AlKCl values. When using a regression model to predict the difference between AlCuCl2 and AlKCl, inclusion of OC explained only 4% more variability compared with inclusion of only soil pH in the model. A change in OC concentrations of 2.3 g kg–1, as observed in this study, after reducing tillage would likely not alleviate Al toxicity if pH became very acidic (pH < 5).

Abbreviations: AlCuCl2, copper chloride extractable-aluminum • AlKCl, potassium chloride extractable-aluminum • CEC, cation exchange capacity • CT, conventional tillage • NT, no-till • OC, organic carbon • OM, organic matter • SO, grain sorghum • SY, soybean • WH, winter wheat


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The increased use of no-till (NT) systems in crop production systems has raised questions on how acid soils should be managed under these conditions. Increases in organic carbon (OC) in the surface 5 to 7.5 cm from NT soils have been well documented. Dick (1983) observed an increase of OC in the surface 7.5 cm of soil with NT corn when compared with intensive tillage (moldboard plow). Ismail et al. (1994) reported that OC was significantly higher in the surface 5 cm of soil in NT soils when compared with plowed soils after 20 yr of continuous corn. Many researchers have hypothesized that the surface 5 cm of soil in NT systems will be less sensitive to decreases in pH caused by surface application of N fertilizers because of increased OC (Blevins et al., 1978; Hargrove and Thomas, 1981). As early as the 1930s, scientists were studying the nature of soils in relation to Al toxicity and recognized that plants growing in soils high in organic matter (OM) did not exhibit symptoms of Al toxicity compared with soils with similar pH but less OM content (Mattson, 1931).

A large body of evidence suggests that in temperate climates the soil solution Al is largely associated with OM for organic and mineral soils (Hoyt and Turner, 1975; Bloom et al., 1979). Soil OM is a complex and heterogeneous mixture of compounds that is poorly understood (Lofts et al., 2001). A significant portion of cation exchange capacity (CEC) in a soil is often attributed to OM. For the organic fraction, the charge arises largely from ionization of COOH groups, although some contribution from phenolic OH and NH groups may exist (Stevenson, 1982). Unlike clay minerals, OM does not have a fixed capacity to bind cations; rather, the CEC of OM increases markedly with increasing pH. The results from Helling et al. (1964) show that for each unit change in pH, the change in CEC contributed from OM was several times greater compared with CEC contributions from clay for several Wisconsin soils. At pH 8.0, 45% of the CEC in several grassland soils was contributed by OM, whereas only 19% of the CEC was attributed to OM at pH 2.5. Kamprath and Welch (1962) estimated that total CEC of OM ranged from 62 to 279 cmol kg–1 in 18 Coastal Plain soils at pH 7. Organic matter of these soils ranged from 5 to 184 g kg–1.

Trivalent cations are more competitive in binding with OM than divalent cations, which, in turn, are more effective in binding with OM than monovalent cations (Stevenson, 1982). Evidence exists for the complexing of divalent and trivalent cations by humic and fulvic acids, a fraction of soil OM, including the inability of K+ and other monovalent cations to replace sorbed micronutrients from mineral and organic soils (Coleman and Thomas, 1967; Stevenson, 1982; Hue et al., 1986). The effect of OM on AlKCl is evident in the data shown by Coleman and Thomas (1967), who found that AlKCl as a fraction of CEC was less for surface horizons than for subsurface horizons for soils in Virginia. Hue et al. (1986) observed that Al-organic acid complexes accounted for 93 and 76% of the total solution Al concentrations in two coastal plain soils in Alabama. Thomas (1975) showed that AlKCl was less at any given pH as OM increased in a Maury silt loam.

Abundant work on reactions of Al with soil OM has underscored the importance of OM in controlling Al equilibria (Bloom, 1979; Bloom et al., 1979; Hargrove and Thomas, 1981; Berggren and Mulder, 1995). Results from these studies show that Al forms relatively stable complexes with soil OM by interaction with carboxyl groups and, to a lesser extent, with phenolic hydroxyl groups (Hargrove and Thomas, 1984). The amount of complexed Al is dependent on pH and Al concentration in soil solution. Berggren and Mulder (1995) proposed that Al in solution is controlled by equilibrium reactions involving soil OM in organic soils. They based this on Al3+ activity at equilibrium depending on the amount of reactive Al, and this phenomenon would not likely occur where mineral phases [e.g., Al(OH)3] control Al solubility.

Several researchers have found CuCl2 to be the strongest extractant for Al bound to soil OM (Juo and Kamprath, 1979; Oates and Kamprath, 1983; Hargrove and Thomas, 1984). Juo and Kamprath (1979) used CuCl2 to estimate the potentially reactive Al pool in acid soils and found that 0.5 M CuCl2 extracted 1.6 to 12 times more Al than did 1 M KCl. Similarly, Oates and Kamprath (1983) found that the order of effective displacement of Al from OM was Cu2+ > La3+ > K+ for several soils. Hargrove and Thomas (1984) stated that Cu2+ was an exception to the rule that as valence increases so does the ability to displace Al3+, due to the tendency of Cu to form inner-sphere complexes with OM. They also found that La3+ was more highly correlated with titratable acidity. As a result, they proposed that Al extraction with La3+ may be more appropriate as an Al3+ extractant because it was highly correlated with titratable acidity and thus closely related to lime requirements of soils. For determining lime requirements, La3+ may be a better choice because it is closely correlated to titratable acidity; however, Cu2+ provides a better estimate of Al associated with OM due to the tendency to form inner-sphere complexes with OM.

Several researchers have investigated the effect of OM addition to acid soils to reduce Al toxicity (Hoyt and Turner, 1975; Hargrove and Thomas, 1981). Hoyt and Turner (1975) observed a decrease in soluble Al in acid soils to which fresh alfalfa meal had been added. The decrease in soluble Al was temporary, and exchangeable Al returned to the original concentrations after 6 mo of incubation. Hargrove and Thomas (1981) also observed that exchangeable Al was decreased at any given pH with the addition of OC (as peat). Plant and root growth was also increased with the addition of organic material to a very acid soil (pH 4.2) compared with the nonamended soil.

Evidence suggests that OM can greatly influence the Al toxicity of plants, but little research has focused on how this relationship affects crop growth and management strategies for acid soils in agricultural production systems. The objectives of this experiment were to (i) identify effects of rotation and tillage on soil pH and soil OC content, (ii) to determine the influence of these changes on AlKCl and AlCuCl2, and (iii) to determine if changes in OC may affect management decisions regarding soil acidity.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The experimental site was established in 1974 on a Reading silt loam soil (fine-silty, mixed, superactive, mesic Pachic Argiudoll) and Muir silt loam (fine-silty, mixed, mesic Pachic Haplustolls) at the Kansas State University Agronomy Farm located at Ashland Bottoms (Riley County; 39°07' N, 96°37' W). The site was established to evaluate the effects and feasibility of conservation tillage systems in eastern Kansas. A composite soil sample (0–15 cm) collected in 2004 from the entire plot area that represented all tillage and rotations treatments indicated a Mehlich3 P (Frank et al., 1998) concentration of 52 mg kg–1; CEC (Warncke and Brown, 1998) of 20.3 cmol kg–1; and concentrations of sand and clay (Bouyoucos, 1962) of 140 and 280 g kg–1, respectively.

The experimental design is considered a split plot, with rotation as the whole-plot treatment and tillage as the subplot treatments. Rotation treatments were not truly randomized in that they were applied continuous (straight through the field) through the site and not randomized within each replication. This was done to ease planting operations and because of limited space. Tillage treatments were replicated four times and were completely randomized within each whole plot (rotation). The subplots were 6.1 m wide by 18.3 m long.

Rotations included three crops: soybean (SY) (Glycine max (L.) Merrill), grain sorghum (SO) (Sorghum bicolor (L.) Moench), and winter wheat (WH) (Triticum aestivium L.) in five rotation combinations (WHSY, SYSO, SOSO, SYSY, and WHWH). A urea-diammonium phosphate mixture was broadcast at 112 kg N ha–1 yr–1 for all rotations before planting each crop. After application, the fertilizer was incorporated into the conventional tillage (CT) plots and left on the surface of the NT plots. When the study was established, the investigators wanted to reduce the impact of rotations receiving different amount of fertilizers, especially N, so all rotations received the same amount and rate of fertilizer before planting.

Tillage treatments consisted of CT and NT. Conventional tillage consisted of the incorporation of crop residue and inorganic fertilizer to an approximate depth of 10 cm between harvest and seedbed preparation by using a disk, chisel plow, and rotary tiller as needed. The NT treatment consisted of leaving crop residue on the surface and planting directly into the residue.

Soil Sampling and Analysis
Soil samples were collected in fall 2004. Samples were taken to a depth of 15 cm and separated into 2.5-cm increments. Fourteen 2.5-cm soil cores were taken in each plot and homogenized into a single sample for each plot and depth. Samples were air dried, ground to pass a 2-mm sieve, and stored in an air-tight container until analysis. Samples were analyzed for pH (1:1, soil/water) with an AS-3000 Dual pH Analyser (Labfit Pty Ltd, Burswood, Western Australia) (Watson and Brown, 1998), 1.0 M AlKCl (Bertsch and Bloom, 1996), 0.5 M AlCuCl2 (Bertsch and Bloom, 1996), and OC by dry combustion using a LECO CN-2000 (LECO, St. Joseph, MI) (Nelson and Sommers, 1996). Aluminum concentrations in both extracts were determined by inductively coupled plasma atomic emissions spectrometry with a Fison Model Accu-141 ICP (Fison Instruments, Dearborn, MI).

To determine the amount of Al extracted by CuCl2 at pH values above 6.0, various amounts of CaO were added to randomly selected soils (pH < 6.0) and mixed with 100 g of soil. Amounts of CaO were added to increase pH from 6.0 to 7.5. Incubation lasted for 6 d. Samples were mixed every 48 h, and soil moisture was maintained near field capacity. Soils were air dried and ground to pass a 2-mm sieve. Samples were analyzed for AlCuCl2 as described previously.

Statistical Analysis
For statistical purposes, the experimental design was considered a split plot, even though we had pseudo-replication with the rotation treatments. Data were analyzed according to the PROC MIXED procedure in SAS (SAS, 1998). Pair-wise comparison was used along with PROC MIXED to compare individual treatment means. Sampling depth was treated as a repeated measure. An unstructured covariance structure was used to model the correlations between pairs of depths. Contrasts were used to determine individual treatment differences at a probability level of 0.05. The PROC REG procedure (SAS, 1998) was used to analyze the effect of pH and OC on extractable Al. Regression analysis (SAS, 1998) was used to evaluate exchangeable Al data. The model that provided the lowest residual sum of squares was used.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Chemical Properties
Significant tillage by rotation and depth by rotation interactions were observed for soil pH (Table 1). For both tillage treatments, the pH of the WHWH rotation was significantly lower than all other treatments (Table 2). The pH of the WHWH rotation was 0.4 to 0.6 lower than the pH of other rotations in the NT plots and 0.3 to 0.5 lower in the CT plots compared with the pH of all other rotations. In addition, the pH of the SOSO rotation in NT plots was 0.2 lower compared with the pH of the WHSY and SYSY rotations. Average soil pH of <5.0 and wheat grain yields (2660 kg ha–1) in the NT WHWH rotation having been 941 kg ha–1 lower, on average, since 1995, compared with the CT WHWH rotation (data not shown), indicates that Al toxicity may be limiting wheat grain yield at this site. The rotation by depth interaction for soil pH is shown in Fig. 1. The WHWH rotation consistently had significantly lower pH compared with other rotations at all depths. This may be partly explained by the application of N fertilizer increasing salt concentration in the soil solution approximately 1 mo before soil sample collection, whereas other rotations received fertilizer 6 mo before sampling. Differences in salt concentrations in soil often cause temporal variability in soil pH measurements (Kissel and Vendrell, 2006). The WHSY treatment had an average pH of 0.7 higher than the WHWH rotation. Evidently, the inclusion of SY into the rotation reduced the rate of soil acidification even though the same amount of N fertilizer was applied to all rotations. Also, the SYSO, SOSO, and SYSY rotations had a significantly lower average pH, compared with the WHSY rotation, in the uppermost 7.5 cm. A partial explanation of decreased pH in the WHWH rotation and NT plots may be from the higher concentrations of OM present compared with other rotations (see OC discussion later in text). Soil OM can be a potential source of soil acidity (Coleman and Thomas, 1967). Upon microbial decomposition of OM, CO2 is produced and quickly reacts with H2O to produce H+ and HCO3– (Havlin et al., 2005). In addition, soil OM contains reactive carboxylic and phenolic groups, which behave as weak acids, releasing H+ when pH is below 7. However, the contribution of OM to overall soil acidity is minimal in mineral soils with relatively low OM concentrations compared with its contribution in organic soils, so this only partially explains the decreased pH in the WHWH rotation.


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  		Table 1. Analysis of variance results for pH, organic carbon (OC), and KCl and CuCl2 extractable-Al.

 

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  		Table 2. Tillage by rotation interaction for pH (averaged over depth) after 30 yr of crop rotation on a Reading and Muir silt loam.

 

Figure 1
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  		Fig. 1. The interaction of depth and rotation for pH after 30 yr of crop rotation on a Reading and Muir silt loam. An asterisk indicates significant differences between rotations for the same depth ({alpha} = 0.05). Key to rotations: SOSO = continuous grain sorghum; SYSO = soybean-grain sorghum; SYSY = continuous soybean; WHSY = wheat-soybean; WHWH = continuous wheat.

 
Significant rotation and tillage main effects and a depth by tillage by rotation interaction were found for OC (see Table 1). Means for rotation main effects are shown in Table 3. Rotations that included WH or SO generally had greater concentrations of OC. This is probably a result of greater biomass production with WH and SO compared with SY. No direct measurements were taken for biomass production, but an indication of greater biomass production from wheat is suggested by considering average harvest index and grain yield for soybean (1861 kg ha–1) and wheat (2750 kg ha–1). Harvest index (grain yield/total plant mass) values for these two crops are similar, and, with wheat having a higher average yield, this would suggest greater biomass production with wheat compared with soybean (Spaeth et al., 1984; Sinclair, 1998). The SYSY rotation had significantly lower OC concentrations compared with all other rotations. Organic C concentrations of the SYSY rotation were 2.3 to 3.2 g kg–1 lower than the other rotations (Table 3). This agrees with Havlin et al. (1990), who found that including SO in a rotation, rather than growing continuous SY, increased OC and total N in the soil. Averaged over depth, NT (14.0 g kg–1) significantly increased OC by 2.3 g kg–1, compared with CT (11.7 g kg–1). Similarly, Saffigna et al. (1989) in Australia reported that soil OC in the surface 10 cm was 7% greater under NT than under CT (disked twice and cultivated three times) in sorghum. They attributed the increase in OC to a slower rate of decomposition of OM, a different distribution of OM in the profile, and a decrease in erosion of surface soil under NT.


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  		Table 3. The effect of crop rotation on organic carbon (averaged over tillage and depth) after 30 yr of production on a Reading and Muir silt loam.

 
The depth by tillage by rotation interaction for OC is shown in Fig. 2. In the surface 2.5 cm of soil, NT plots had greater concentrations of OC compared with the rotations in CT (Pr > T = 0.01). Average OC concentrations for NT plots were 9.6 g kg–1 higher compared with the average OC of CT plots in the surface 2.5 cm of soil. The NT WHWH rotation had the greatest OC concentrations compared with all other tillage and rotation combinations in the surface 2.5 cm of soil. Concentrations of the NT WHWH rotation increased OC by a minimum of 2.6 g kg–1 and a maximum of 16.2 g kg–1 compared with all other treatment combinations. Organic C contents were generally lower at each depth for the SYSY rotation than for all other rotations. This is probably a result of soybean producing less biomass than the other crops. The NT SYSY rotation had less OC, compared with other NT rotations, in the surface 5 cm of soil. Similarly, the CT SYSY rotation had lower OC concentrations, compared with other CT rotations, in the surface 10 cm and had lower OC concentrations, compared with all possible treatment combinations, in the surface 7.5 cm of soil. In the 2.5- to 5-cm depth, NT rotations continued to have higher OC concentrations than the CT rotations, except SYSY, which had OC concentrations similar to those of the CT rotations (Fig. 2). The NT WHSY and SYSO rotations had greater OC concentrations, compared with all CT rotations, in the 2.5- to 5-cm depth. Differences in OC concentrations in the 5- to 7.5-cm depth were less obvious (Fig. 2), but NT WHSY and SYSO rotations had greater OC concentrations, compared with all other possible treatment combinations, in the 5- to 7.5-cm depth.


Figure 2
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  		Fig. 2. The interaction of depth, tillage, and rotation for organic carbon after 30 yr of crop rotation on a Reading and Muir silt loam.

 
Significant tillage by rotation and depth by tillage by rotation interactions were observed for AlKCl (Table 1). In general, NT and CT plots in the WHWH rotation had greater concentrations of AlKCl than all other rotations, reflecting the lower soil pH of WHWH (Fig. 3). No significant difference was found between NT plots and CT plots in AlKCl, except for WHWH.


Figure 3
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  		Fig. 3. The interaction of depth, tillage, and rotation for KCl extractable-Al after 30 yr of crop rotation on a Reading and Muir silt loam.

 
Significant tillage by rotation and depth by tillage by rotation interactions were observed for AlCuCl2 (Table 1). Similar to AlKCl, AlCuCl2 was significantly greater for the WHWH rotation in the NT and CT plots (Fig. 4). Copper chloride extractable-Al in the NT WHWH rotation was greater than with other NT rotations by a minimum of 1.2 cmol kg–1 and a maximum of 1.6 cmol kg–1, whereas the AlCuCl2 in the CT WHWH rotation was greater than in other CT rotations by a minimum of 0.9 cmol kg–1 and a maximum of 1.2 cmol kg–1. Analysis of the tillage by rotation interaction revealed that the WHWH rotation for both tillage treatments consistently had greater concentrations of AlCuCl2, compared with all other possible treatment combinations, in the 0- to 10-cm depth (Fig. 4). This is probably due to the lower pH observed with the WHWH rotation compared with the other rotations (Table 2). In some instances, NT had greater amounts of AlCuCl2 compared with the same rotation in the CT treatment, but, due to the interaction, we cannot tell if these differences are due to pH or higher OC concentrations in NT plots. It seems that pH has the greatest influence because the SYSY rotation often had significantly less OC than other rotations (Table 3) and because AlCuCl2 values with this rotation for NT and CT are often similar to the WHSY, SYSO, and SOSO rotations (Fig. 4), indicating that lower pH is likely the cause of increased AlCuCl2 in NT soil.


Figure 4
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  		Fig. 4. The interaction of depth, tillage, and rotation for CuCl2 extractable-Al after 30 yr of crop rotation on a Reading and Muir silt loam.

 
Influence of Organic Matter on Extractable Aluminum
Potassium chloride was used to estimate the active Al (readily exchangeable Al); CuCl2 was used to measure exchangeable Al and the amount of Al associated with OM because CuCl2 is a much stronger extractant (Juo and Kamprath, 1979; Hargrove and Thomas, 1981).

A relationship between AlKCl and soil pH for all samples collected from the site indicated that AlKCl increased exponentially with decreasing pH (Fig. 5). Based on this relationship, when pH was below 5.1, AlKCl extractable-Al was in excess of 0.3 cmol kg–1, which corresponds to the critical level for AlKCl determined by Unruh (1988) for winter wheat in Kansas in field research plots. Unruh (1988) also determined, after analyzing 193 soils for AlKCl from south-central Kansas, that the pH corresponding to an AlKCl concentration of 0.3 cmol kg–1 was 5.1, which is similar to what we observed. Aluminum from KCl extracts approach our detection limit (0.02 cmol kg–1) at a pH of 5.5 (Fig. 5). Assuming AlKCl is a good measure of plant available Al, then we would not expect Al to inhibit plant growth above pH 5.5 for crops that are relatively insensitive to Al, such as corn and wheat (Havlin et al., 2005). Aluminum measured with KCl is believed to be only trivalent (Al3+) because studies have shown that Al that is exchanged by neutral salts is trivalent and that aqueous hydrolysis species of Al are not exchangeable by neutral, unbuffered salts (Rich, 1960; Thomas, 1960).


Figure 5
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  		Fig. 5. Potassium chloride (1 M) extractable-Al as a function of pH for all sample depths.

 
A strong relationship between AlCuCl2 and soil pH was observed (Fig. 6). For all samples taken, AlCuCl2 increased exponentially with decreasing pH. As expected, CuCl2 extracted significantly more Al compared with KCl extracts (Pr > T = 0.01). Copper chloride extracted 1 to 55 times as much Al for samples that had measurable concentrations (>0.02 cmol kg–1) of AlKCl. When all samples were averaged together, CuCl2 extracted 8 times as much Al as KCl. This average agrees with Juo and Kamprath (1979), who found that 0.5 M CuCl2 extracted 1.6 to 12 times more Al than did 1 M KCl on soils from North Carolina. The greater amount of Al extracted by CuCl2 is from CuCl2 being able to remove Al bound to OM and to partly extract hydrolysis species of Al from interlayers and surfaces of clay minerals, in addition to the easily exchangeable Al and Al in soil solution that is extracted by KCl (Juo and Kamprath, 1979; Hargrove and Thomas, 1981). Copper chloride is also believed to extract aqueous hydrolysis species of Al in addition to Al3+ in solution (Juo and Kamprath, 1979). The majority of the Al extracted from the OM is believed to be mononuclear Al. Berggren and Mulder (1995) determined that CuCl2 has a high degree of specificity to extract only mononuclear Al bound to OM, at least in the pH range of 3 to 5. At a pH of 6.0, CuCl2 was still extracting rather large quantities of Al (1.6 cmol kg–1) (Fig. 6). In an attempt to determine the behavior of AlCuCl2 at pH values above 6.0, we increased the pH of selected samples by adding various amounts of CaO and incubating for 6 d. One might anticipate further declines in CuCl2 extractable-Al as pH increases if Al-bound to OM is released and then precipitates as insoluble Al oxy(hydr)oxides. Conversely, the ability of OM to complex Al should increase as pH increases because additional functional groups deprotonate. The change in AlCuCl2 at higher pH was minimal compared with values at pH 6.0 (data not shown). This suggests that Al associated with OM is tightly held or that Al release by OM is slow and may not happen within the 6-d incubation time. This seems to agree with what others have found regarding OM controlling Al equilibria (Bloom, 1979; Bloom et al., 1979; Hargrove and Thomas, 1981; Berggren and Mulder, 1995). Palmer and Wesolowski (1992) estimated the log KS to be 7.74 when accounting for Al complexation by organic ligands in pH buffers, which may help explain why Al was still being extracted from incubated soils that had pH of 6.0 to 7.2.


Figure 6
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  		Fig. 6. Copper chloride (0.5 M) extractable-Al as a function of pH for all sample depths.

 
Figure 7 illustrates the difference between AlCuCl2 and AlKCl for all samples. For every 1 unit decrease in pH, CuCl2 extractable-Al increased 1.17 times as much as Al. Assuming that this difference is mainly Al associated with OM, the increase is important because Al associated with OM is increasing along with Al saturation of exchange sites associated with OM. This has been shown by Posner (1966) and Hargrove and Thomas (1982), who observed that the effective CEC of OM is inversely proportional to the amount of Al present. The slope of the relationship seems to agree with Hargrove and Thomas (1984) in that the amount of complexed Al is dependent on pH and Al concentration in soil solution.


Figure 7
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  		Fig. 7. The difference between 0.5 M CuCl2 and 1 M KCl extractable-Al as a function of pH.

 
To determine the effect of OC on plant available Al, a regression analysis was used. In the model, pH was the independent variable (Pr > T = <0.01), and the difference between AlCuCl2 and AlKCl (assumed to be mainly Al associated with OM) for all depths, tillage practices, and rotations was used as the dependent variable, resulting in an r2 of 0.65. Including OC (Pr > T = <0.01) in the regression model improved the R2 to 0.70. Although significant, increase in explanation (r2) when including OC in the regression model was minimal, leading us to believe the observed differences in OC due to tillage and rotation were not greatly influencing plant available Al at this site. In addition, regression analysis was performed only on soil samples from the surface 2.5 cm from both tillage treatments. This depth represents the greatest difference (9.6 g kg–1) in OC between tillage treatments, and any difference in OC that might influence the degree of Al toxicity could likely be seen at this depth. The r2 increased from 0.60 (Pr > T = <0.01) to 0.68 (Pr > T = <0.01) when OC (Pr > T = <0.01) was included with pH.

Increases in OC concentrations in surface soil of NT systems have been well documented (Dick, 1983; Saffigna et al., 1989; Ismail et al., 1994). From these data, however, it seems unlikely that increases in OC from tillage management practices would significantly reduce the risk of Al toxicity (in these soils at present pH of 4.5–6.0) because changes in OC, although statistically significant, are relatively small (~3.0 g kg–1 for the15-cm profile). This is in contrast to what others have hypothesized (Blevins et al., 1978; Hargrove and Thomas, 1981) that increases in OM from reduced tillage may alleviate Al toxicity and allow crops to grow satisfactorily in more acidic soils compared with CT at similar pH. Increases in OM may increase the buffer capacity of soils but may not greatly influence active Al.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Tillage and rotation influenced soil pH and OC content. No-till plots had a lower average pH and greater OC concentration at the surface compared with CT plots. No-till plots had OC concentrations 2.3 g kg–1 greater than CT plots. Concentrations of AlKCl and AlCuCl2 were highly correlated with pH. Both AlKCl and AlCuCl2 increased exponentially as pH decreased.

The difference between AlKCl and AlCuCl2, which many researchers have attributed to Al associated with OM, increased linearly with decreasing pH. The increased Al associated with OM as pH decreases indicates that Al saturation of OM is increasing, thereby decreasing the effective CEC of the soil at lower soil pH. This potentially active Al pool that is extracted by CuCl2 contained, on average, 8 times more Al compared with what was extracted by KCl in the Reading and Muir silt loams analyzed in this study. Organic matter may be important in controlling active (KCl extractable) Al, but changes in OM concentrations in the surface 7.5 cm of soil, often observed after reducing tillage, would likely not alleviate Al toxicity if pH became very acidic (pH <5). Therefore, pH critical values (pH at which yield may start being reduced from Al toxicity) for liming acid soils in NT production systems should not be adjusted based on OM content for typical agricultural soils (OM = 15–35 g kg–1).


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication April 25, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome