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a Agriculture and Agri-Food Canada, Research Centre, 5403 First Avenue South, Lethbridge, Alberta, Canada T1J 4B1
b Agrium Inc., 11751 River Road, Fort Saskatchewan, Alberta, Canada T8L 4J1
* Corresponding author (larney{at}agr.gc.ca)
Received for publication March 4, 2005.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AN, available nitrogen • DM, dry matter • EC, electrical conductivity • NORM, naturally occurring radioactive material • PAN, percent available nitrogen • PAS, percent available sulfur • PG, phosphogypsum • TC, total carbon • TN, total nitrogen • TP, total phosphorus • TS, total sulfur • WC, water content
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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However, N losses during composting of feedlot manure have been reported in southern Alberta. A recent study by Hao et al. (2004) showed that up to 42% of initial TN was lost during composting of straw-bedded manure. Lower losses (12% of initial TN) were measured in the same study for wood chip–bedded manure. Most (approximately 95%) of the N loss occurred as ammonia (NH3) volatilization with the remainder emitted as nitrous oxide, a greenhouse gas. Eghball et al. (1997) reported gaseous N losses amounting to 19 to 42% of initial feedlot manure N during composting in Nebraska, with ammonia volatilization accounting for more than 90% of the N loss. As well as contributing to the greenhouse effect, N losses also reduce the N content of the final composted product, which affects its market value.
Phosphogypsum is the principal waste by-product of P fertilizer production from mined phosphate rock (USEPA, 1993). It is primarily (>90%) calcium sulfate dihydrate or gypsum (CaSO4·2H2O) resulting from the mixing of phosphate rock with sulfuric acid to dissolve phosphorus. For every 1 Mg of phosphate rock processed, approximately 1.5 Mg of PG is produced, which translates to 5 Mg of PG per 1 Mg of P2O5 (United Nations Environmental Program, 1998). Currently, there are large stockpiles (>40 x 106 Mg) of PG in Alberta (Arocena et al., 1995), with 1.3 x 106 Mg added annually (Stantec Consulting, 2003). There are more than 100 x 106 Mg of PG stockpiled in Canada (Alcordo and Rechcigl, 1993) and at least 8 x 109 Mg in the United States (USEPA, 1993). Global production is about 300 x 106 Mg per year (Degetto et al., 1999).
Studies have shown that PG reduces ammonia volatilization losses from soil-applied urea (Alcordo and Rechcigl, 1993, 1995; Bayrakli, 1990) and greenhouse gas emissions during cattle manure composting (Hao et al., 2005). Phosphogypsum has also been used to supply sulfur (S) and calcium (Ca) for crops; ameliorate aluminum (Al) toxicity and soil dispersion; and modify Mg to Ca ratios of soils (Alcordo and Rechcigl, 1993, 1995; Ritchey et al., 2000). Currently, a feedlot manure compost–PG blend, Re-Claim (EcoAg Initiatives, High River, AB, Canada), is commercially available in Canada for reclamation of salt spills and improvement of saline and sodic soils.
There is evidence to suggest that PG addition to manure reduces the amounts of N lost during the composting process. Prochnow et al. (1995) reported that PG addition at a rate of 100 kg Mg–1 decreased volatilization of ammonia from chicken and cattle manure. In a laboratory experiment, Kohut and Babiak (2000) found that PG reduced NH3 volatilization by 55 to 67% from cattle manure and by 35 to 68% from chicken litter. The mechanism appeared to be via the trapping of volatile ammonia by gypsum and/or a pH reduction since PG is acidic. However, field scale experiments on the potential role of PG addition in increasing N retention during composting have not been conducted in a feedlot setting. For the practice to be adopted, there is the need to establish how much PG to add to manure, and if more N is retained during subsequent composting to make PG addition economical. Reduced N losses are linked to decreased odor emissions during composting as well as increased nutrient value of the final composted product.
Because PG contains trace concentrations of radium (226Ra), a naturally occurring radioactive material (NORM), its release to the environment depends on radioactivity (Miller, 1995). Much of the PG stockpiled in central Alberta has been produced using sedimentary phosphate rock from Togo and Florida. Phosphogypsum derived from these rocks typically has NORM in the range 0.4 to 0.7 Bq g–1. For unconditional release to the environment, Canadian NORM guidelines require that radioactivity of the PG does not exceed 0.3 Bq g–1 (Health Canada, 2000). Materials containing >0.3 Bq g–1 can be released without further consideration if the proposed use results in personal exposures of <30% of the annual public dose limit. Detailed risk assessments are therefore required for any proposed use, but all agricultural and industrial uses studied to date indicate that the risks are far below natural background exposures (<10% of the annual public dose limit). In the United States, the Code of Federal Regulations (1996) permits use of PG for agricultural purposes if the 226Ra concentration of the material is <0.37 Bq g–1.
Co-composting PG with manure will obviously result in a final product with significantly lower specific radioactivity due to a dilution effect. In addition to potential nutrient benefits from PG addition, widespread diversion of PG to manure composting could alleviate the accumulation of PG in large stacks.
The objective of this study was to determine the effects of different rates of PG addition to fresh feedlot manure on chemical and physical changes during composting.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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spectrometry (Becquerel Laboratories, Mississauga, ON, Canada). Water content, total carbon (TC), pH, and electrical conductivity (EC) were analyzed using the methods described for manure–PG co-composts below.
Windrow Establishment and Sampling
The co-composting experiment was performed during July through October 1999 at the Agriculture and Agri-Food Canada Research Centre, Lethbridge, AB (49°43' N, 112°48' W). Precipitation and air temperature were monitored at an automated weather station located approximately 0.5 km from the compost site. On 23 July 1999, manure was removed using a loader and truck from open feedlot pens which had been bedded with barley (Hordeum vulgare L.) straw or wood chips. The wood chips were a mixture of sawdust and bark peelings derived from 80% lodgepole pine (Pinus contorta var. latifolia Engelm.) and 20% white spruce [Picea glauca (Moench) Voss]. Initial samples of each type of manure were obtained for water content (WC) determination. This allowed the estimation of PG addition rates on a dry weight basis. Each truckload of wet manure was weighed and manure was sampled for WC as it was being deposited into open-air compost windrows to arrive at a dry weight of manure in each windrow.
At windrow formation, PG was added and mixed with manure at target rates of 0, 50, 100, and 200 kg PG Mg–1 manure plus PG (dry weight) using a tractor and front-end loader. The actual rates, corrected for WC of manure and PG at the start of composting, were 0, 40, 70, and 140 kg Mg–1 (dry weight). This resulted in eight treatments (two bedding types x four rates of PG addition) which were replicated three times to give 24 individual compost windrows. Each windrow was 8 m long, 2.5 m wide at the base, 2 m high, and roughly trapezoidal in shape. The windrows were turned with a tractor-pulled EarthSaver windrow turner (Fuel Harvesters Equipment, Midland, TX) at 7-d intervals during the first 28 d of composting and at 14-d intervals thereafter until Day 70. Consistent with thermophilic composting, windrow core temperature (thermocouples logged every 20 min and averaged to give mean daily values) of a straw-bedded windrow with 70 kg Mg–1 PG reached a maximum value of 72°C (Day 4) while a wood-bedded manure windrow with 40 kg Mg–1 PG attained a maximum value of 70.3°C (Day 13). Thermophilic composting was considered complete (windrow core temperature falling to within 10°C of ambient air temperature) on 30 Oct. 1999, or 99 d after the start of composting.
Compost samples were taken from each treatment for laboratory analysis at windrow establishment and when thermophilic composting was complete. On each sampling date, six samples were taken from each windrow by cutting the windrow in the center with a skid-steer loader and sampling the vertical faces (top, middle, and bottom locations) exposed on each side.
Compost Analysis
Samples were oven-dried at 60°C to a constant weight for WC determination and chemical analysis. Water content was expressed on a wet weight basis. Available N concentration (NO3–N + NO2–N + NH4–N) was determined using an AutoAnalyzer II (Technicon, Tarrytown, NY) following extraction of 10 g of wet sample with 200 mL of 2 M KCl. Finely ground samples (<0.15 mm) were analyzed for C, N, and S using an automated CNS analyzer (Carlo Erba, Milan, Italy). Available P was determined by the modified Kelowna method (Ashworth and Mrazek, 1995) in which 5 g of air-dried soil was extracted with 50 mL of solution (0.015 M NH4F, 1.0 M NH4OAc, and 0.5 M HOAc), and total phosphorus (TP) by colorimetric analysis after digestion with H2SO4 and H2O2. Electrical conductivity and pH were measured with a pH/conductivity meter (Accumet pH meter 50; Fisher Scientific, Hampton, NH) in extracts obtained using a 1:4 compost to water ratio. Ash content was determined by combustion in a muffle furnace at 650°C for 24 h. Mass balances [%, (initial – final)/(initial) x 100] for water, dry matter (DM), total mass (water + DM), N, and C were estimated by assuming that the ash mass remained constant during composting (Bernal et al., 1998). Multi-element concentration and radiological analyses were determined on the final composts as described for PG above.
Data Analysis
Statistical analysis was performed using the General Linear Models procedure (PROC GLM) of SAS (SAS Institute, 1999) with bedding type as the main treatment and PG rate as the sub-treatment in a split-plot layout. The LSMEANS statement in PROC GLM was used with the PDIFF option (SAS Institute, 1999) to separate treatment means. Treatment effects were considered significant at the 5% probability level.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Chemical and Radiological Analysis of Phosphogypsum
Phosphogypsum properties were: water, 0.11 kg kg–1 (wet weight basis); TC, 8.5 g kg–1 dry weight; pH, 4.9; and EC, 3.0 dS m–1. Essential nutrient concentrations (dry weight basis) were 216 g Ca kg–1, 160 g S kg–1, 1.6 g P kg–1, 0.10 g Mg kg–1, and 0.09 mg K kg–1, with no detectable N. The PG also contained small amounts of the micronutrients Fe (0.90 g kg–1), Co (2.0 mg kg–1), Mn (11.0 mg kg–1), Na (2400 mg kg–1), Ni (13.0 mg kg–1), and Zn (7.0 mg kg–1); the phytotoxic elements, F (3.60 g kg–1) and Al (1.3 g kg–1); and traces of Cr (3.0 mg kg–1) and Cd (1.0 mg kg–1). Radium (226Ra) activity of the PG was 0.56 Bq g–1.
Manure Properties
Before the start of the experiment, straw-bedded manure (0.66 kg kg–1 WC, wet weight) was wetter than wood chip–bedded manure (0.53 kg kg–1) (Table 1). Straw-bedded manure also tended to be higher in nutrient and salt content and in pH than wood chip–bedded manure (Table 1).
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Total N content in the straw-bedded treatments decreased from 20.3 g kg–1 with no PG addition to 15 g kg–1 at the 70 kg Mg–1 PG rate (Fig. 2) . Phosphogypsum application, however, did not significantly decrease TN in the wood chip–bedded treatments. It was expected that the addition of PG, which had no detectable N, would result in a decrease in TN concentration due to a dilution effect. The anomalous behavior of wood chip–bedded treatments could not be easily explained, but might have been due to variability in the measurements.
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Total Phosphorus
Main and interaction effects of bedding and PG rate were not significant for initial TP content of the manure–PG mixtures (Table 2). Phosphogypsum, despite the name, contains a lower amount of P (1.6 g kg–1) than manure (mean 4.2 g kg–1), and did not affect the TP content of the initial mixtures. There is likely a threshold where increases in P by addition of PG are masked by decreases in P due to the dilution effect. Nitrogen to phosphorus ratio in the mixtures (Table 2) was higher for straw-bedded than wood chip–bedded manure treatments, reflecting the higher TN concentration of the latter. The N to P ratio increased with PG addition up to the 70 kg Mg–1 rate, above which additional increments of PG had little effect.
pH and Electrical Conductivity
Initial pH of unamended manure (zero PG) was significantly higher for straw-bedded (8.3) than wood chip–bedded manure (7.7) at the start of the experiment. Across the PG rates, the pH was significantly higher for straw-bedded (7.8) than wood chip–bedded manure (7.3) (Table 2). Averaged over the bedding sources, initial pH decreased from 8.0 without PG addition to 7.5 with application of 40 kg Mg–1 PG. Phosphogypsum rates of >40 kg Mg–1, however, did not result in further pH decrease.
There was a significant bedding x PG rate interaction for initial compost EC (Table 2). Phosphogypsum application had no significant effect on the initial EC of straw-bedded manure–PG mixtures. For the wood chip–bedded manure–PG mixtures, the EC increased with PG application, but there was little EC change at PG rates above 40 kg Mg–1. The increase in EC with PG rate was expected since PG, which is primarily CaSO4·2H2O, results in higher solution Ca2+ and SO42+ concentrations on dissolution.
Final Properties of Co-Composts
Water Content
After 99 d of composting, there was no significant effect of bedding type or PG rate on the WC of the compost (Table 3). The WC across all treatments ranged from 0.19 to 0.26 kg kg–1, with a mean of 0.23 kg kg–1. Hao et al. (2004) reported similar WC values for cattle feedlot manure following 99 d of composting.
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Bedding type and PG rate effects were nonsignificant for TN content (mean 16.5 g kg–1) and C to N ratio (mean 14.8) of the final compost (Table 3). The rate effect was consistent with the nonsignificant response of ash content to PG addition.
There were no significant treatment effects on TP concentrations of the final compost (Table 3). The increase in TP content in the final (mean 5.4 g kg–1) compared to the initial compost (mean 4.2 g kg–1) was a concentration effect due to higher DM losses than P losses.
The N to P ratio was significantly higher for straw-bedded than wood chip–bedded treatments, but was not affected by PG addition (Table 3). Livestock manures have lower N to P ratios (1.8–4:1) than plants (4.5–6:1) and since manures are often applied on an N basis to meet crop N demand, P is over-applied (Eghball, 2002). Therefore, amendments which lower the N to P ratio of the final composted product would magnify the P over-application effect, but this was not the case with PG.
Total sulfur (TS) after 99 d of composting was significantly affected by PG rate but not by bedding material (mean 17.1 g kg–1; Table 3). Across the bedding materials, there was a highly significant linear response of TS to PG rate (r2 = 1; P < 0.001), with the TS increasing by 0.019% (0.19 g TS kg–1 co-compost dry weight) for each 1 kg Mg–1 increment in the PG rate. Based on the TS concentration of the PG, 1 kg PG Mg–1 would increase the TS concentration of the compost by 0.016%, assuming no TS losses. This calculated TS concentration is slightly lower than the measured increase in compost TS, which is an expected concentration effect due to DM loss during composting. Total S without PG addition averaged 5.4 g kg–1.
The TS concentration range measured in this study (5–32 g kg–1) is consistent with the 17 to 35 g kg–1 range reported in a similar study by Hao et al. (2005). The increase in S content with PG addition has implications for the use of the co-compost in S-deficient soils, especially for crops such as canola (Brassica spp.), which has a high S requirement. In Alberta, approximately 2.83 million ha or 29% of the total area of cropland is considered S-deficient for optimum canola production (Alberta Agriculture, Food and Rural Development, 2001).
Available Nutrients
Available nitrogen (NH4–N + NO3–N) after 99 d of composting was significantly higher for wood chip–bedded manure than for straw-bedded manure (Table 3). Averaged across the bedding materials, the final available nitrogen (AN) concentration of the composts increased from 1297 mg kg–1 without PG addition to 2017 mg kg–1 with addition of 40 kg Mg–1 PG. There was little change in AN at PG rates above 40 kg PG Mg–1.
Percent available nitrogen [PAN = (available N/TN) x 100] increased significantly with PG application, but PG rates above 40 kg Mg–1 resulted in smaller additional increases in the PAN (Table 3). The increase in PAN may be related to reductions in volatilization losses of NH3 as a result of lower pH with PG addition (Table 3). Bayrakli (1990) reported an 85% reduction in NH3 loss when PG was mixed with urea at a PG to urea ratio of 2.3:1, compared to urea applied alone. The author attributed this reduction in NH3 loss to the lower pH resulting from PG addition, which inhibited NH4+ transformation to NH3.
As with available N, mean PAN in our study was significantly higher for wood chip–bedded (13.5%) than straw-bedded manure compost (11.3%). This is consistent with the higher initial C to N ratio for wood chip–bedded compost, which would be expected to result in lower N losses through NH3 volatilization.
Available phosphorus (AP) concentration was not significantly affected by either bedding type or PG rate, even though PG contains small amounts of P (Table 3). Available P concentration, averaged over all treatments, increased during composting from 2430 mg kg–1 at the beginning of the composting to 3484 mg kg–1 in the final compost. The increase in AP concentration was likely a combination of mineralization of organic P and a concentration effect resulting from DM loss.
Available sulfur (SO4–S) and percent available sulfur [PAS = (available S/TS) x 100] after 99 d of composting were significantly affected by PG rate but not by bedding material (means 9227 mg kg–1, and 53.8%, respectively; Table 3). Response of SO4–S to PG rate differed between bedding materials, as indicated by the significant bedding x PG rate interaction (Table 3). Increasing the PG rate to 140 kg Mg–1 did not result in a further increase in SO4–S concentration for wood chip–bedded manure but resulted in a significant increase in the SO4–S concentration for straw-bedded manure (Fig. 3) . The difference in response between the two bedding materials could not be easily explained but appears to be related to the low SO4–S concentration measured at the 70 kg Mg–1 compared to the 40 kg Mg–1 PG rate for straw-bedded material, possibly due to sampling error.
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pH and Electrical Conductivity
There was a significant bedding x PG rate interaction effect on pH of the final compost (Table 3). For the straw-bedded compost, pH decreased sharply with the addition of 40 kg PG Mg–1, but additional rate increments had little effect on the pH (Fig. 4) . The decrease in pH was expected, since PG has an acidic pH (pH 4.9). It is not clear, however, why there was no significant rate effect for the wood chip–bedded compost treatments.
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Other Major and Trace Elements
Of the remaining macro- and micronutrients in the final compost, only Ca was significantly affected by PG addition, increasing linearly (P < 0.001) with PG rate (Table 4). The Ca response was expected since PG is primarily CaSO4·2H2O. Composts amended with PG can therefore be a major source of Ca for crops if salinity is not too high. Straw-bedded compost had significantly higher concentrations of Mg, K, Na, Mo, B, Zn, Cu, and Co than wood chip–bedded compost, likely reflecting fertilizer and herbicide–pesticide application effects on the barley straw, whereas Ca, Fe, Ni, and Mn concentrations were similar for the two materials (Table 4).
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Radioactivity
Radiological analysis at the end of the composting period indicated that the activity of 226Ra and its progeny in all treatments was below the limit of 0.3 Bq g–1 set by Health Canada (2000) for unrestricted release to the environment. Mean 226Ra activity was not significantly affected by PG rate or bedding material and ranged from <4 x 10–4 Bq g–1 in compost that received zero PG to 0.02 Bq g–1 with addition of 70 or 140 kg PG Mg–1. Since radionuclide content is a major factor limiting distribution of PG for agricultural use, co-composting of PG with manure may allow widespread use in agriculture and help minimize stockpiling of the by-product.
Water, Dry Matter, and Total Mass Losses
Main and interaction effects were significant for water loss during composting (Table 6). Straw-bedded manure compost lost more water (90.4% of initial) than wood chip–bedded manure compost (74.8%) without PG addition (Fig. 5)
. When PG was added, however, there were no significant differences in water loss between the two materials. For the straw-bedded manure compost, there was a significant decrease in water loss with successive PG rates up to 70 kg PG Mg–1. Further addition of PG, however, did not result in significant change in the water loss. Addition of 40 kg PG Mg–1 to the wood chip–bedded manure had no significant effect on water loss (79.7%) compared to the zero PG treatment (74.8%). Higher PG rates (70 and 140 kg Mg–1), however, significantly reduced water loss (mean 73.4%) compared to the 40 kg Mg–1 rate. Reduced water loss with PG addition may be an asset during summer composting where excessive losses may inhibit thermophilic microorganisms (Larney et al., 2000). Our water loss values were consistent with previously reported values for southern Alberta of 83% (Larney et al., 2000). Michel et al. (2004) reported water mass losses of up to 89% for dairy manure composting in Ohio.
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Total mass (water + DM) losses were significantly affected by bedding type and PG rate (Table 6). Straw-bedded manure compost lost an average of 53.6% while wood chip–bedded manure lost 46.6% of initial mass. This may be due to the wetter straw-bedded manure at the outset of the study since water loss accounted for a large proportion of the mass loss. The loss in total mass during composting was a linear function of PG rate, decreasing by 1.16% for each 1 kg Mg–1 increase in PG rate [mass loss (%) = 56.4 – 0.12PG (kg Mg–1); r2 = 0.74; P = 0.002]. Adding PG at 70 or 140 kg Mg–1 significantly reduced the total mass loss (42–43%) compared to the 40 kg Mg–1 PG rate (54%) or the zero PG rate (58%). This has implications for increased haulage requirements of manure–PG co-composts with >40 kg Mg–1 PG. The reduction in total mass measured in this study is consistent with previously reported losses of 30 to 50% (Dao and Cavigelli, 2003; DeLuca and DeLuca, 1997; Larney et al., 2000) and was a result of the combined loss of water and OM during composting.
Carbon Losses
Total C losses were significantly affected by PG rate but not by bedding type (mean 26.1%) (Table 6). The rate effect was due to the significant decrease in TC at the 70 kg Mg–1 PG rate compared to lower rates. Hao et al. (2005) reported that C loss through CO2 emission accounted for 34 to 46% of initial TC during composting of feedlot manure with PG, with PG addition significantly reducing methane (CH4) emission, which accounted for <6% of TC loss.
Nitrogen Losses
Overall TN losses during the entire composting period were significantly higher for straw-bedded manure compost (19.9%) than wood chip–bedded manure compost (4.5%) (Table 6). Eghball et al. (1997) reported N losses ranging from 19 to 42% during manure composting, with the losses highest at a C to N ratio of 12, and lowest at a C to N ratio of 17. In the current study, TN loss was higher from the straw-bedded treatment, which had an initial C to N ratio of 14.5, than the wood chip–bedded treatment, which had a C to N ratio of 20.8.
Total N loss during composting decreased linearly with PG rate, as indicated by the significant linear trend (P = 0.01). Averaged over the bedding materials, TN loss without PG addition was 19.3% of initial TN. Regression analysis of the pooled data indicated a 0.11% decrease in TN loss for each 1 kg Mg–1 increment in PG rate. The decrease in N loss may be related to the low PG pH (4.9), which resulted in the initial and final pH of the co-composts decreasing significantly with PG rate (Tables 2 and 3). There is a well-established link between NH3 volatilization and pH, with volatilization losses decreasing dramatically as pH decreases. Al-Kanani et al. (1992) reported daily ammonia volatilization values of <10 mg N kg–1 manure at pH 6.3 compared with values of 120 mg N kg–1 at pH 8 for liquid hog manure. At low pH, N may be stable in the NH4–N form rather than volatilizing from the windrows as NH3. Although Kohut and Babiak (2000) reported NH3 volatilization reductions of up to 68% 96 d after addition of PG to poultry manure in a laboratory study, Kithome et al. (1999) found that addition of gypsum (CaSO4) to poultry manure did not conserve NH3 under simulated laboratory composting.
The percent decrease in N to P ratio (Table 6) showed a significant linear decline with increasing PG rate, corroborating the N loss findings. For example, the zero PG rate resulted in a decline of 21.8% in N to P ratio, while the 140 kg Mg–1 PG rate led to a decline of 14.3% in N to P ratio, showing that less N was lost per unit mass.
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CONCLUSIONS |
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A potential drawback at higher PG rates is the increased compost mass resulting from addition of an inorganic material, which means larger quantities to handle and higher trucking and field application costs for the final product. Phosphogypsum rates investigated in this study, however, did not result in any significant increase in the ash content of the final compost.
Radionuclide levels have been a major factor restricting agricultural use of PG. This study demonstrates the significant dilution of radioactivity that occurs when PG is co-composted with cattle manure. This has important implications for the more widespread use of PG in agriculture, which would substantially minimize its stockpiling. Co-composting of manure with PG, therefore, presents an opportunity to combine two perceived "waste" products, and create a desirable end-product (containing, N, P, S, Ca, and trace elements) for use as a fertilizer supplement and soil amendment. Further research is needed on the effect of manure–PG co-composts on soil salinity.
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ACKNOWLEDGMENTS |
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NOTES |
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