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Published in Agron J 100:477-483 (2008)
DOI: 10.2134/agronj2007.0162
© 2008 American Society of Agronomy
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BIOSOLIDS

Effects of Repeated Application of Municipal Sewage Sludge on Soil Fertility, Cotton Yield, and Nitrate Leaching

Vasilios Samarasa,*, Christos D. Tsadilasa and Stamatis Stamatiadisb

a National Agricultural Research Foundation, Institute of Soil Classification and Mapping, 1 Theophrastos St., 41335 Larissa, Greece
b Soil Ecology and Biotechnology Lab., Gaia Environmental Research and Education Center, Goulandris Natural History Museum, 13 Levidou St., 145 62 Kifissia, Greece

* Corresponding author (vsamaras{at}nagref.gr, tsadilas{at}lar.forthnet.gr).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The effects of sludge application on soil properties and cotton (Gossypium hirsutum L.) response were investigated in an Inseptisol in central Greece. Digested sludge was incorporated in the 0 to 15 cm soil depth at rates of 10, 30, and 50 Mg ha–1, and repeated for four consecutive years. Sludge treatments were compared to an inorganic fertilizer application and an untreated control in a completely randomized design with four replications. Sludge application increased soil organic matter, associated nutrients and improved physical properties. However, soil electrical conductivity increased with increasing sludge application to levels that may affect growth of salt-sensitive crops and warns against long-term application that may impair essential soil functions. The multifold increase of Olsen P and nitrate N beyond crop needs is a reason of concern for surface runoff and nitrate leaching below the root zone at the higher sludge application rates. Sludge application of 10 Mg ha–1 was sufficient to improve soil chemical properties with less risk of water contamination. Cotton responded to sludge application by increased nutrient uptake and yield, which indicated that sludge could replace inorganic fertilizer needs even at the lower application rate. However, fluctuations of nutrient uptake and yield between growing seasons were of greater magnitude than those caused by sludge application. Multiple regression analysis revealed that P uptake was the major limiting factor for determining cotton yield.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.

Received for publication May 17, 2007.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IMPROVEMENT OF SOIL PHYSICAL and chemical properties is among the benefits of sewage sludge application to agricultural land. As a soil conditioner, sludge reduces bulk density and increases porosity, improves structural stability, and enriches soil with organic carbon (Pagliai et al., 1981; Metzger and Yaron, 1987; Tester, 1990; Sort and Alcaniz, 1999; Marinari et al., 2000). These changes result generally in increased water retention capacity in coarse-textured soils and, in the long-term, in enhanced water transmission properties and resistance to soil erosion (Khalee et al., 1981; Metzger and Yaron, 1987).

In association with its high organic matter content, sludge also contains appreciable amounts of N and P with significant fertilizer replacement value, although its K content is low for meeting crop requirements (Smith, 1996; Warman and Termeer, 2005). The pools of soluble nutrients in sludge are initially small, and plant uptake must await mineralization of organic constituents (Petersen et al., 2003). The extent and temporal dynamics of nutrient release and uptake are less variable than those of livestock manures, but will still depend on sludge characteristics, the method and timing of application, soil type and properties, and environmental conditions (Smith, 1996; Smith et al., 1998). Crop response to soil amendment with sewage sludge results in yields often equal to or higher than those resulting from recommended fertilizer applications (Epstein, 2003) unless sludge has a high C to N ratio, excess metals, high soluble salts or is applied at extremely high rates (Warman and Termeer, 2005).

These beneficial effects make sludge application an attractive option for eroded soils of dry Mediterranean climates that often have low organic matter content. Relatively large quantities of sludge, of the order of 30 Mg ha–1, are generally required to raise soil N content significantly and have a measurable effect on soil physical properties (Hall and Coker, 1983; Metzger and Yaron, 1987). Comparable quantities of sludge cakes are commonly applied to degraded soils of olive orchards in Spain (Gasco and Lobo, 2006) or by certain recycling operations in England (Smith et al., 1998). These rates, however, exceed crop N requirements and may cause undesirable changes in soil chemical properties leading to environmental contamination. Such effects include ammonia volatilization and denitrification, excessive soil acidification from nitrification of ammonia, accumulation of nitrates in sludge-treated profiles, and increased nitrate leaching from susceptible loamy soils (Powlesland and Frost, 1990; Smith and Doran, 1996). Even when sludge is applied at rates consistent with the N requirement of the crop, P is supplied in excess because the P requirement of crops is only about 10 to 25% that of N while sludges generally contain half as much P as they do N (Smith, 1996). Accumulation of P in sludge-amended soil can result in benefits for P nutrition of future crops but also potentially impact water bodies through surface runoff and leaching (Sui et al., 1999; Maguire et al., 2000; Penn and Sims, 2002).

In this study, the effects of repeated sludge application were investigated at rates that are reportedly required for a measurable improvement of soil fertility in a fine loamy soil of the semiarid Mediterranean region. The cultivated crop, cotton, requires high levels of N with measured uptakes as much as 230 kg N ha–1 under irrigation (Constable and Rochester, 1988). Higher rates of sludge application than those applied in this study produced cotton yields that were comparable to those obtained from fertilizer addition in the semiarid southern Arizona (Watson et al., 1985). The specific objectives were the evaluation of the effects of digested sludge application on (i) improvement of soil physical and chemical properties, (ii) its potential to replace inorganic fertilizer for cultivation of cotton, and (iii) environmental contamination in terms of nutrient loss potential.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Design and Sampling
The experimental field was located 1 km west of the village of Rizomilos and 20 km NW of the city of Volos, central Greece (39°26'04.04'' N, 22°42'49.64'' E). The soil is classified as a Typic Xerochrept that is a clay loam with a high cation exchange capacity (33 cmol 100 kg–1). Part of the field (30 by 37 m) was divided into 20 plots (5 by 5 m) with a 2-m spacing between them. Each plot was randomly assigned to receive one of five treatments as follows: three rates of sewage sludge (10, 30, and 50 Mg dry solids ha–1 yr–1), one rate of inorganic fertilizer (160 kg NH4–N ha–1 yr–1 and 80 kg P2O5 ha–1 yr–1) and an unfertilized control with no organic or inorganic amendments. The complete randomized block design had four replicates (plots) for each treatment.

The digested sludge was derived from the municipal treatment plant of the city of Volos after treatment of sewage in aerobic tanks and dewatering. Phosphorus was chemically precipitated in the tanks by addition of FeClSO4.

Sludge was distributed manually and incorporated to approximately 15 cm by rotovation and fertilizer was sprinkled uniformly on the soil surface 2 wk before sowing (beginning of April) for four consecutive years from 1996 to 1999.

Planting of cotton took place in the middle of April with 22 kg seed ha–1. Seed variety was not the same each year, but all seed varieties were selected for early maturity characteristics and high yield potential. The domestic variety Korina 01256 (Cotton and Industrial Plant Institute, National Agricultural Research Foundation) was planted in 1996, DP 50 (Delta & Pine Land Co.) in 1997 and 1998, and ST 453 (Stoneville) in 1999. The following pesticides and herbicides were applied to the crop in all plots: phorate to the seed (10 kg ha–1), prometryne (10 kg ha–1) on the soil surface after sowing, endosulfan (3 kg ha–1) at first bloom on 20 July and two to three sprays of pyrethrine in combination with acaricides thereafter. Groundwater was supplied to the plants by drip irrigation 11 times during the growing season up to 25 August with an average of 370 m3 ha–1 time–1.

Leaf and soil samples were taken from the middle rows of each plot. Leaf samples were taken at full bloom (end of July) by removing well-developed leaves from the middle of the canopy (Sabbe and Zelinski, 1990). Composite soil samples were taken at the same time from 0- to 25- and 25- to 50-cm depth for the determination of chemical and physical properties (pH, EC, extractable P and K, organic matter, and total N). Selected soil physical properties were only measured the third year of the experiment. Additional composite soil samples were taken at the beginning (early June) and end (mid-September) of the growing season in the last 3 yr to assess the vertical distribution of nitrate N at four soil depths (0–25, 25–50, 50–75, 75–100 cm). Cotton yield (lint + seed dry weight) was measured by harvesting each 5 by 5 m plot at the end of each growing season.

Soil samples sealed in plastic bags, and leaf samples in paper bags were transported to the laboratory in a portable cooler. Soil samples were mixed and passed through a 2-mm sieve. Soil analysis included pH (1:5 soil to water ratio), electrical conductivity (1:5 soil to water ratio), nitrate content in soil extracts of 2 M KCl using a colorimeter (Keeney and Nelson, 1982). Carbonate content was determined using the Bernard method (Nelson, 1982), organic matter by the Walkley–Black method of wet oxidation (Nelson and Sommers, 1996) and total N by the Kjeldahl wet oxidation method (Bremner and Mulvaney, 1982). Soil K was extracted with 1 nM ammonium acetate at pH 7 (Knudsen et al., 1984), while P was extracted from soil with 0.5 M sodium bicarbonate at pH 8.5 as proposed for calcareous soils (Olsen and Sommers, 1984). Bulk density, gravimetric water content and infiltration were measured in the third year (1998) of the experiment using a portable soil quality kit (Liebig et al., 1996). Leaf samples were ground to a fine powder and heated at 500°C for 5 h. Ash was digested with 1 M HCl. Metal concentrations were determined using an atomic absorption spectrophotometer and K and P concentrations with a flame photometer (Benton Jones et al., 1991). Leaf N content was determined by the Kjeldahl wet oxidation method (Bremner and Mulvaney, 1982).

The soil methods were used for sludge analysis in the case of pH, EC, carbonates, organic matter and total N content. Total P, K, and Na were determined by heating sludge in a furnace at 500°C for 4 h, dissolution of ash with aqua regia and measurement with a spectrophotometer (Benton Jones et al., 1991). The chemical composition of sludge remained more or less constant throughout the 4 yr of the experiment (Table 1 ). Based on an average N content of 3.25%, annual N inputs were approximately 325, 975, and 1625 kg ha–1 for each sludge treatment, respectively. Assuming 40% N mineralization during the first year of application (Watson et al., 1985), the lower application rate provided approximately 130 kg N ha–1 which is less than the reported maximum N uptake by cotton under irrigation (230 kg N ha–1, Constable and Rochester, 1988).


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  		Table 1. Annual variation of the chemical composition of sewage sludge of the city of Volos.

 
Statistical Analysis
Data analysis included analysis of variance (general linear models), correlation analysis, and multiple regression. The linear mixed model for soil variables was a special case of split plot in time that used restricted maximum likelihood (REML) for the estimation of fixed and random effects. Treatment was the whole-plot fixed effect while year and soil depth were treated as split-plot fixed effects relative to treatment. The random effects of the model were plot, year, and soil depth that were all nested within treatment. The LSD test was used to detect differences between means of the fixed effects at P < 0.05 and to compute standard errors using their root mean square errors. The same model was used for the analysis of plant variables, but with the absence of the depth effect. In stepwise multiple regression, the independent variables of the model were selected by allowing at least 10 degrees of freedom for the estimation of the error term. The regression model was checked by appropriate diagnostic procedures (collinearity and influence diagnostics). Data analysis was conducted using Statistical Analysis System software, version 6 (SAS Institute, 1990).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Properties
Soil properties were strongly affected by treatment and soil depth (Table 2 ). The unamended soil (control) was slightly alkaline (Table 3 , 0–25 cm) with 378 mg kg–1 of exchangeable K that is sufficient for crop growth (Allan and Killorn, 1996). Extractable P was relatively low (7 mg kg–1) in that a P fertilizer application would probably cause a yield response (Olsen and Sommers, 1984; Allan and Killorn, 1996). The application of ammonium fertilizer did not result in any changes of soil properties in comparison to the control despite the fact that nitrification effects, such as acidification and a rise in EC, are known to occur after inorganic fertilization of agricultural soils (Patriquin et al., 1993; Smith and Doran, 1996).


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  		Table 2. Analysis of variance of soil properties for the sludge experiment conducted in Greece from 1996 to 1999.

 

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  		Table 3. Effect of sludge and fertilizer application on selected soil properties 4 mo after application (July). Means are averaged across years.

 
Sludge application decreased soil pH and increased all other measured chemical properties except soil K (Table 3). The values of soil EC, organic matter, extractable P and K were higher in the surface 0 to 25 cm than in the 25 to 50 cm, but the rate of change due to sludge application was similar in both depths (Table 3). The reduction of soil pH with increasing sludge application may be partly attributed to the lower pH of sludge, but also to microbial nitrification of ammonium contained in sludge (Patriquin et al., 1993; Smith and Doran, 1996). Reduction of soil pH was in the order of 0.4 units in the high rate of sludge application in comparison to the control over the course of the 4 yr of the experiment. This pH reduction toward more neutral values appears to improve soil fertility. However, long-term sludge application may lead to excessive soil acidification and reduction of crop yields in other soils of low buffering capacity (Smith and Doran, 1996). Soil EC increased with increasing rate of sludge application and reached 0.30 mS cm–1 (1:5 soil-to-water mixture) in the highest sludge application rate (Table 3). This value equates to 1.5 mS cm–1 for a 1:1 soil-to-water mixture. The rise of EC was caused by the high NaCl content of sludge (Table 1). Possible sources of NaCl in sewage are the fish preservation industry in the area, the use of water softeners and the intrusion of sea water from leakage of pipes. Cotton is a salt-tolerant crop with soil threshold values that exceed 4 mS cm–1 (Smith and Doran, 1996). However, a fine textured soil with an EC value of 1.5 mS cm–1 (1:1) is considered slightly saline for salt sensitive species such as bean (Phaseolus vulgaris L.), most legumes, maize (Zea mays L.), pepper (Capsicum annuum L.), and tomato (Lycopersicon esculentum Mill.). Furthermore, such EC values may have adverse effects on microbial mineralization processes and increase ammonia volatilization (Smith and Doran, 1996).

On the contrary, the increase of organic matter, total N, and extractable P content indicated the beneficial effects of sludge application on soil fertility. A significant increase in both organic matter and total N content appeared in the treatments of 30 and 50 Mg ha–1. These results agree with suggested rates of 20 to 30 Mg ha–1 to have a measurable effect on soil organic matter and physical properties (Hall and Coker, 1983; Metzger and Yaron, 1987; Smith, 1996). However, surface Olsen P increased to 60 mg kg–1 in the 50 Mg ha–1 treatment in the last year of the experiment (Fig. 1 ). This value is four times above those considered high for application of P fertilizers in Minnesota soils (12–16 mg kg–1; Allan and Killorn, 1996). Such high P values after repeated sludge application are posing risks for dissolved phosphates in runoff. A similar increase in extractable P occurred in the 25- to 50-cm depth (Table 3, Fig. 1) and indicated significant leaching despite the general immobility of P due to fixation and precipitation processes in soil (Smith 1996; Petersen et al., 2003). An increase of total P was also found at the 5- to 25-cm depth after 6 yr of continuous biosolids application to poplars (Sui et al., 1999). The low content of K in sludge did not result in any differences between treatments.


Figure 1
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  		Fig. 1. Mean annual concentration of extractable P (±SE) as affected by treatment in two soil depths.

 
Significant treatment effects were also detected in soil physical properties as measured in the third year of the experiment (Table 4 ). Soil bulk density decreased and field water holding capacity increased with increasing rate of sludge application. Even though the water infiltration rate doubled in the 50 Mg ha–1 sludge application in comparison to the control, it was still very low and made this soil susceptible to surface erosion. Even the 10 Mg ha–1 treatment brought about significant improvement in soil bulk density and water holding capacity (Table 4). These results confirm findings of previous studies about improvement of soil physical properties on long-term sludge application (Smith, 1996). Several authors (Reganold and Palmer, 1995; Ellert and Bettany, 1995; Doran and Parkin, 1996) caution against gravimetric comparisons between soil management systems of differing bulk densities because their biological or ecological relevance may be misrepresented. In our study, the reduction of bulk density due to sludge application reduced the magnitude of differences between treatments, but the reduction was not great enough to alter the statistically significant differences between means (Table 4).


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  		Table 4. Physical soil properties and conversion of gravimetric to volumetric units for chemical soil properties measured at the end of July 1998 (0- to 25-cm depth).

 
Soil Nitrates
Additional sampling at the beginning and end of the growing season provided evidence of nitrate leaching during the last 3 yr of the experiment. The nitrate N content of the control soil early in the growing season (Table 5 , 0–25 cm) appeared adequate for growing crops such as corn without the need of N amendments (Bundy and Meisinger, 1994). Sludge or fertilizer application increased soil nitrate N to excessive levels in the surface soil. Nitrate levels increased with increasing rate of sludge application up to six times the level of the control in the 50 Mg ha–1 sludge application (50 Mg ha–1, Table 5). An expected pattern of increased nitrate N with year of sludge application was disrupted in 1998. Compared to June of 1997 and 1999, nitrate levels in 1998 were reduced in the surface soil (0–25-cm depth) and increased in greater depths (Fig. 2 ). This was indicative of increased nitrate leaching and was explained by unusually high precipitation in May of that year (119 mm) as recorded by a meteorological station 45 km away. Nitrate concentration reached a maximum in the last year of the experiment (0–50 cm, Fig. 2) being three to four times higher in the 50 Mg ha–1 treatment than sufficiency levels for corn during early growing season (20–25 ppm, Bundy and Meisinger, 1994). These high concentrations in the top soil did not decline at the end of the growing season (Fig. 2), thus posing a threat for surface runoff and water contamination given the low infiltration rates of this soil (Table 4). Ojeda et al. (2006) detected relatively high concentrations of mineral N in the runoff 5 mo after sludge application in a loamy soil under similar climatic conditions characterized by irregular and intense rainfall events.


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  		Table 5. Nitrate-N levels (mg kg–1) at the beginning and the end of the growing season.

 

Figure 2
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  		Fig. 2. Vertical distribution of soil nitrates within and after the growing season. Annual means ( ± SE) are averaged across treatments.

 
Nitrate-N content declined rapidly with depth but its concentration in the high sludge applications (30 and 50 Mg ha–1) was higher than that of the other treatments even at depths considered to be below the root zone (75–100 cm, Table 5). The elevated nitrate levels in this depth provide further evidence of leaching and reason for concern of groundwater contamination. Residual nitrates at the end of the growing season were generally lower than those in June (Table 5), but their concentrations remained excessive and similar to those in June in the last year of the experiment (Fig. 2). The low sludge application rate (10 Mg ha–1) had, on average, nitrate concentrations similar to those of the inorganic fertilizer at all depths and similar to those of the control at depths below 50 cm (Table 5). However, nitrates accumulated to excessive levels in the surface soil the final year of the experiment. Therefore, the 10 Mg ha–1 application appears to be acceptable from an environmental point of view provided that repeated annual applications are avoided.

Leaf Nutrients and Yield
Compared to most soil properties, nutrient uptake and yield were influenced to a greater extent by growing season than by sludge application (Table 6 ). Leaf N and P content followed strong annual patterns (Table 7 ). Increased leaf P content occurred in the 1996 and 1999 growing seasons in all treatments and was closely followed by yield. Differences between cotton varieties were not expected to be the cause of these large annual variations because all varieties were selected for their high yield potential and early maturity characteristics. Similar annual differences in yield were also obtained with a single cotton cultivar in sludge-amended plots in southern Arizona (Watson et al., 1985). It is possible that the increased uptake of P was caused by increased soil P availability as a result of higher rainfall in March and April of these years. Air temperature could not be implicated in annual differences of nutrient uptake since summer monthly temperatures showed a remarkable similarity between years (data not shown). Nitrogen uptake did not exactly follow the annual pattern of P and yield in that a great reduction of leaf N content occurred only in 1997 in the sludge-treated plots (Table 7).


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  		Table 6. Analysis of variance of leaf nutrients and yield for the sludge experiment conducted in Greece from 1996 to 1999.

 

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  		Table 7. Effect of growing seasons on leaf nutrients and yield.

 
Despite the strong effect of growing seasons, cotton yield increased with sludge and fertilizer application, but not by more than 1.2 times that of the untreated control (Table 8 ). Differences in yield between sludge and inorganic fertilizer treatments were not evident (Table 8) indicating that sludge application, even at the lower rate of 10 Mg ha–1, can successfully replace inorganic fertilizer needs from the first year of application. In a previous 3-yr field experiment with cotton, Watson et al. (1985) used higher rates of repeated sludge application (20, 40, and 60 Mg ha–1) on a clay loam in southern Arizona. Otherwise, their results were similar to ours in terms of (i) the proportional yield increase in sludge-amended plots relative to the untreated control and (ii) the nonsignificant difference in yield between sludge treatments and the fertilized control. Leaf nutrient content increased with increasing rate of sludge application and even above the level of the inorganic fertilizer in the case of N (Table 8). Leaf K differences between treatments were relatively small reflecting the lack of differences in soil K content (Table 3). In any case, leaf K concentration was within the sufficiency range between peak bloom and first open boll (Reuter et al., 1997; Mengel, 2007).


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  		Table 8. The effect of treatment on leaf nutrients and yield over four growing seasons.

 
Multiple regression analysis indicated that P uptake was the major limiting factor for determining cotton yield when soil properties and leaf nutrients were used as independent variables of the model. Leaf P concentration accounted for 64% of yield variation in the control and sludge treatments and resulted in a positive linear relationship (Fig. 3 ). The increased P uptake was partly explained by the high concentration of available soil P in the sludge-amended plots (Table 3). Although relatively weak, the correlation between leaf P and Olsen P in the root zone (25–50-cm depth) was significant (r = 0.50, n = 60). Positive correlations between NaHCO3–extractable P and uptake of P by cotton and other crops have been obtained in the past (Olsen and Sommers, 1984). However, even at the excessive levels of extractable soil P in the higher rate of 50 Mg ha–1, leaf P content remained below the sufficiency limit of 0.3% at flowering as reported by Reuter et al. (1997) and Sanchez (2007) for cotton. Soil nitrate levels ranged from adequate to excessive during the growing season (Fig. 2). Consequently, leaf N concentrations were generally above the critical deficiency level for this growth stage (leaf N >3%, Reuter et al., 1997) and fell below that only during the second growing season (Table 7). Therefore, N uptake was not a major factor in determining yield response as evidenced by their weak correlation (Fig. 3).


Figure 3
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  		Fig. 3. Relationship between leaf nutrients and yield as affected by sludge application.

 

   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Repeated sludge application over four growing seasons improved soil fertility by means of increased soil organic matter, associated nutrient content (except K) and improvement of soil physical properties. However, soil EC increased with increasing sludge application to levels that may affect growth of salt-sensitive crops and warns against long-term application that may impair essential soil functions. In addition, the multifold increase of available soil P and nitrate N beyond crop needs is a reason of concern for surface runoff and leaching below the depth of the root zone. These processes are likely to result in surface- and ground-water contamination during and after the growing season due to high sludge application rates. The lower rate of sludge application (10 Mg ha–1) was sufficient to improve soil chemical properties with less risk of phosphate runoff and nitrate leaching below the depth of the root zone. This rate of application appears to be an acceptable practice from an environmental point of view provided that it is not repeated annually.

Cotton responded to sludge application by increased nutrient uptake (N and P) and yield and indicated that sludge can replace inorganic fertilizer needs even at the lower application rate. However, fluctuations of nutrient uptake and yield between growing seasons were of greater magnitude than those caused by sludge application. Multiple regression analysis of all data revealed that P uptake was the major limiting factor for determining cotton yield.


   ACKNOWLEDGMENTS
 
This project was funded by the Municipal Wastewater Treatment of Magnesia. Special thanks are extended to Prof. Kent Eskridge (University of Nebraska) for assistance in the statistical analysis of the data.

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


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




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