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a Dep. of Crop and Soil Sciences, Michigan State University, 286 Plant and Soil Sciences, East Lansing, MI 48824
b Dep. of Soil, Water, and Climate, University of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108
* Corresponding author (laboski{at}msu.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: MAP, monoammonium phosphate
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Injection of liquid manures into the soil provides a unique soil chemical-biochemical environment because the manure injection bands are undisturbed by subsequent tillage and thus impact only about one-tenth of the soil in a field (Schmitt et al., 1992). Schmitt et al. (1992) studied the unique chemical environment within a band of injected beef manure with respect to N. The behavior of P in manure injection bands has received limited study (Comfort et al., 1987; Motavalli et al., 1989; Sutton et al., 1982).
Many nutrient management practitioners assume 60 to 90% of total P in swine manure is plant available in the year of application. This is based on the fact that swine manure typically contains 10 to 30% organic P and 70 to 90% inorganic P (Gerritse and Zugec, 1977; Peperzak et al., 1959) and it is thought that the organic P fraction in manure is not immediately plant available. However, some research suggests that manure P may be equally available or more available than fertilizer P (Gale et al., 2000; Meek et al., 1979; Abbott and Tucker, 1973).
When manure and fertilizer P is applied together, a synergistic effect occurs whereby available P is increased more than the sum of the increase from either applied singly (Copeland and Merkle, 1941; Dalton et al., 1952; Toor and Bahl, 1997; Reddy et al., 1999). This may be explained by the fact that several anions of organic acids have been found to prevent P fixation and are able to replace P bound to the soil resulting in greater concentrations of available P (Swenson et al., 1949; Nagarajah et al., 1970; Kafkafi et al., 1988). Swenson et al. (1949) found that the organic acid's effectiveness in decreasing P sorption was dependent on the anion concentration. Struthers and Sieling (1950) discovered that at any pH found in agricultural soils, there were organic anions that could effectively prevent P sorption to Fe and Al and they were the same anions of organic acids that are produced in great quantity by microbial degradation of organic matter. Hue (1991) theorized that complex organic acids (containing phenolic rings) added to soil were more effective in increasing lettuce (Lactuca sativa L.) yield than short-chained aliphatic acids because the complex organic acids were more resistant to degradation and remained in the soil longer at concentrations that were effective in reducing P sorption. Anderson et al. (1974) found that inositol hexaphosphate (an organic P compound found in manure) was preferentially sorbed to the soil compared with orthophosphate such that treatment of the soil with inositol hexaphosphate released orthophosphate bound to the soil and reduced later sorption of orthophosphate.
In two separate studies, the concentration of P in organic amendments was found to impact the availability of P (Fuller et al., 1956; Singh and Jones, 1976). The critical concentration of P in organic amendments above which there is no net immobilization of P by soil microorganisms is 0.2 to 0.3% P.
In a 13-wk incubation, P was strongly immobilized over the entire time period when fresh solid beef or young dairy manure compost was amended to a sandy loam soil (Gagnon and Simard, 1999), while poultry litter application increased available P over the same time period. All manure composts had >0.9% P. They also found that manure management, during storage and composting, significantly impacted P release during the incubation. Gagnon and Simard (1999) found that materials with high P and humic substances contents and low C/P ratios released more P.
Manure P has not always been found to be more available than fertilizer P. Elias-Azar et al. (1980) found manure P, from fresh and composted dairy manure, was as available as KH2PO4–P in alkaline sandy soils. The average relative P availability of fresh manure was 0.71 and for composted manure was 0.65, in the other soils studied (textures ranging from sandy loam to silty clay and pH 4.6 to 8.2). Additionally, Elias-Azar et al. (1980) reported that the relative availability of manure P increased as pH and sand content increased. Overall, manure P was less available than KH2PO4–P.
Gracey (1984) amended ryegrass (Lolium spp.) turves with cattle (2.71 g L-1 P), pig (4.85 g L-1 P), and sheep (2.49 g L-1 P) manure, and monoammonium phosphate (MAP) at the same rates of total P. After 168 d of incubation, the order of P availability was MAP > pig > cattle > sheep. Using the mean available soil P level for the greatest P application rate given by Gracey (1984), the relative availability of manure P compared with MAP was 0.62, 0.47, and 0.36 for pig, cattle, and sheep, respectively.
Sharpley and Sisak (1997) found that poultry litter leachate P was less available than KH2PO4 when incubated with 193 soils for 7 d. When soils were broken into three groups, calcareous, slightly weathered, and highly weathered, the relative availability of litter leachate P compared with KH2PO4–P was 0.61, 0.58, and 0.52, respectively.
The objectives of this study were designed to obtain a better understanding of manure P availability in manure injection bands by (i) determining how the availability of soil P changes over time when soils are amended with manure or fertilizer, (ii) evaluating the relative availability of manure P compared with fertilizer P, and (iii) determining if previous manure applications impact the P availability of subsequent manure or fertilizer applications.
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MATERIALS AND METHODS |
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Manure was applied at rates of 0, 120, and 240 mL manure kg-1 soil. This was equivalent to 0, 144, and 288 mg P kg-1. These manure application rates are equivalent to common field applications of 37 300 and 74 500 L manure ha-1 when injected into the soil. To obtain the injected application rates, as opposed to broadcast-incorporated rates, several assumptions and calculations were required. First, it was necessary to estimate the manure's zone of influence in the soil. It was assumed that the manure impacted an area of soil 0.15 m in diameter and the soil had a bulk density of 1.3 Mg m-3. From this it was determined that a knifed-in manure application with 0.76-m knife spacing would impact 310 Mg ha-1 soil assuming no disruption of the impacted soil by secondary tillage (Schmitt et al., 1992). Thus, manure applications of 37 300 and 74 500 L ha-1 that are injected into the soil would be equivalent to 120 and 240 mL manure kg-1 soil, respectively. Laboratory grade KH2PO4 (fertilizer) was applied to each soil at the same rates of total P that were applied with manure.
For each soil, 500 g of soil was weighed into 18 different resealable plastic bags. The bags were then randomly assigned a treatment. The treatments were arranged in a full factorial design composed of P source (manure or fertilizer) and P application rate (0, 144, and 288 mg P kg-1) with three replications. Phosphorus rates of 0, 144, and 288 mg P kg-1 are designated as M-0, M-1x, and M-2x, respectively, for manure treatments and F-0, F-1x, and F-2x, respectively, for fertilizer treatments. Treatments were applied and mixed thoroughly into the soil. Distilled water was added to the soil so that all treatments received the same volume of liquid initially. The liquid volume in the treatments exceeded the water-holding capacity of Sanburn and Verndale soils. In these soils half the full treatment rate was applied at the experimental starting point and the other half 1 wk later.
After treatments were applied and mixed into the soil, the bags were closed with a small opening left for air exchange. They were placed in a dark growth chamber where a replication was placed on each of three shelves. To accommodate for differences in airflow and temperature, each replication was moved to a different shelf every 7 to 10 d. The temperature was maintained at 25 ± 2°C.
The water-holding capacity of each soil was estimated using the equations developed by Rawls et al. (1982). Moisture in each bag was maintained at 70 to 90% of field capacity by adding distilled water when necessary. Watering occurred every 7 d for the first month and every 14 to 21 d for the remainder of the incubation period.
A 50 g (dry weight basis) subsample of soil was obtained from each bag at 1, 2, 3, 6, and 9 mo. Soil samples were air dried and ground to pass a 2-mm sieve. All samples were analyzed for Bray-1 P and pH. Barnes 1 and Barnes 2 soil samples were analyzed also for Olsen P. Soil test P and pH were measured on all incubation samples after the incubation was complete.
Traditionally, the term available P refers to the amount of soil P available to plants. Even though no plants were grown in this study, the term available P will be used to refer to the extractable-P concentration as measured by the Bray-1 or Olsen soil tests.
Statistical Analysis
To determine if incubation duration had an effect on available P, available P was regressed on sampling date for each soil, P source, and rate of P applied using the REG procedure in SAS (SAS Institute, 2000). With this analysis, a significant regression indicated that available P changed over time.
The rate of increase in available P for each unit of P applied was determined for each soil by regressing soil test P on the amount of P added as manure or fertilizer. The relative availability of manure P to raise soil test levels compared with fertilizer P was determined by testing the hypothesis that the slopes of the regressions were the same. Relative availability is reported as the slope of manure P versus P applied line divided by the slope of fertilizer P versus P applied line. The relative availability of manure P to fertilizer P was tested for the 1 and 9 mo samples.
The impact of manure history on the rate of increase in available P for each unit of P applied was also tested. Here relative availability is defined as the slope of manure P, on a soil with a manure application history, versus P applied line divided by the slope of manure P, on a soil with no manure application history, versus P applied line. Slopes were compared using the REG procedure in SAS to test the hypothesis that the two slopes were the same.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Because one of the goals of this research was to determine the effect that time has on available P concentrations, available P in the 144 and 288 mg P kg-1 treatments were adjusted for the amount of available P in the controls on each date. This was done to reduce the impact that background P mineralization had on the data, so that manure and fertilizer treatment effects on available P over time could be more easily assessed.
To determine if length of incubation played a significant role in changing soil test P concentration, adjusted soil test P was regressed on date of sampling for each soil, P rate, and P source. The results of the regression are given in Table 4. A significant regression indicates that available P did change over the duration of the incubation. The effect of time was significant (P <0.05) more often in the fertilizer treated (14 of 30 soil by rate combinations) compared with manure treated (4 of 30 soil by rate combinations) soils. The Sanburn soils with no prior manure history (Sa1) was the only soil when treated with fertilizer at both the 144 and 288 mg P kg-1 rates, that had a significant (P < 0.05) increase in available P over time. In all other cases where the regression was significant, available P decreased over time when fertilizer was added. The reduction in available P over time when fertilizer was applied can be expected. As time of P contact with the soil increases, P becomes irreversibly sorbed to Fe and Al through the formation of stable multidentate ligands (Hingston et al., 1974). Available P also decreases with time as more stable (and less soluble) Ca–P complexes are formed (Olsen and Khasawneh, 1980). It is unknown why available P increased over time in the Sanburn soil.
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Where soil test P significantly changed over time for both application rates of a given source of P on a given soil, comparisons were made between the slopes of the P application rates with time. The slopes were only significantly different (P < 0.05) for the manure treated Ba2 soil. This indicates that soil test P concentration decreased more at the larger rate of P applied.
Both manure and fertilizer treated soils experienced a reduction in pH over the incubation period from 1 to 9 mo (Table 3). At either 1 or 9 mo, the pH was not different for different rates of fertilizer applied to a given soil. For manure treated soils 9 mo after incubation, there was a general trend where as manure application rate increased from 0 to 288 mg P kg-1, pH decreased. There was no distinct trend in pH for manure application rates at 1 mo. These data suggest that the cause of the pH change in fertilizer treated soils was not related to the rate of fertilizer added but to general chemical reactions and biochemical processes that occurred over time; perhaps partly caused by the rewetting of dry soil and resultant stimulation of soil organisms. However, changes in the pH of manure treated soils suggest that manure was stimulating biochemical processes which resulted in the generation of hydrogen ions.
Phosphorus Availability from Manure and Fertilizer Sources
The relative effectiveness of manure and fertilizer to increase available P was determined. Available P was regressed on the rate of P applied for each soil and P source at 1 and 9 mo of incubation. The results of these regressions are given in Table 5. Examples of the data are shown for Port Byron, Verndale, and Ves soils in Fig. 2, 3, and 4
, respectively. The regressions are significant (P < 0.01) for all soil, P source, and date combinations (Table 5). The relative availability of manure to increase soil test P compared with fertilizer was evaluated by comparing slopes from Table 5. For example, the following hypothesis was tested: the slope of the regression of PB1 with manure applied equals the slope of the regression of PB1 with fertilizer applied, both at 1 mo after incubation. Relative availability of manure compared with fertilizer on a given soil at 1 and 9 mo after incubation, along with the P values for the hypothesis test, are given in Table 6.
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Generally the relative availability of manure P compared with fertilizer P increased from 1 to 9 mo of incubation. This is caused mostly by a reduction in fertilizer P availability over time (Table 4) and somewhat by a nonsignificant increase in manure P availability over time (Table 4).
Manure P may be more available than an equivalent amount of fertilizer P for several reasons. The organic P fraction in manure is composed of mostly high molecular weight compounds and probably consists of DNA complexes with polyphosphates (Gerritse and Zugec, 1977) and to a much lesser extent inositol hexaphosphate (IHP) (Peperzak et al., 1959). The IHP in the manure may have preferentially sorbed to the soil resulting in the release of inorganic P bound to the soil (Anderson et al., 1974). The organic fraction of P found in high molecular weight compounds would be expected to have no impact on P sorption and subsequent P availability (Ohno and Crannell, 1996). It is also possible that organic acids produced during microbial degradation of manure caused the increased P availability from manure (Swenson et al., 1949; Struthers and Sieling, 1950). Additionally, Gerritse and Zugec (1977) reported that the biological P reaction cycle in pig slurry has a turn over time of 10 to 20 wk, regardless of storage conditions. It is possible that on-farm storage of the swine manure before freezing was long enough that at least one reaction cycle was completed. If the microbially mediated cycling of P in manure also produces organic acids, then sufficient organic acids may have been added to the soil as part of the manure. These organic acids could out compete manure P for sorption sites on the soil, causing manure P to be more available. Greater manure P availability could have been maintained because organic acids have been found to form stable chelates with Fe and Al, thus preventing P sorption at those sites in the future (Nagarajah et al., 1970). Also the P content of the manure added was 2.33% P, large enough that microbial immobilization of P would not be expected (Singh and Jones, 1976; Fuller et al., 1956).
It is acknowledged that soil test P was not measured before 1 mo into the incubation. It would be expected that immediately after addition of manure or fertilizer, that fertilizer P would be more available than manure P because manure does contain a portion of P in organic forms that would not be immediately available. However, the reactions that occurred to make manure P more available occurred within one month's time. It is also acknowledged that manure injection zones in a field setting would not remain undisturbed for 9 mo. However, it is likely that they may persist for a month. These data suggest that in a field setting there may be a period of time after manure injection when there is greater risk of P loss to runoff or leaching.
Manure History Effects on Phosphorus Availability
To understand the effects of previous manure application history on available P, changes in STP caused by a new application of manure or fertilizer were compared between soils with and without previous manure application histories. Comparisons were made within a soil-mapping unit. Table 7 shows the effect that manure history had on subsequent applications of manure or fertilizer. The hypothesis, that the relative availability of manure P or fertilizer P applied to a previously manured soil compared with that applied to a previously nonmanured soil was equal to one, was tested; P values are given in Table 7.
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When a manure history effect was significant, it usually resulted in decreased availability of P from later applications of manure or fertilizer. Thus, for a soil that had previously received manure, it would take larger P applications (from manure or fertilizer) to raise soil test P concentration by 1 mg P kg-1 soil compared with manure or fertilizer application on a soil that had no prior manure history. Manure application may decrease the P availability of later applications of manure or fertilizer by creating new P binding sites through the addition of organic matter (Sample et al., 1980). This mechanism, however, is unsubstantiated for these soils.
A significant manure history effect was more prominent in the clay loam soils. For the calcareous Ves soil, prior manure application increased the available P from recent applications of manure and fertilizer. These results confirm the findings of Abbott and Tucker (1973), where manure P increased soil test P because the P remained more soluble as a result of enhanced microbial activity of manure amended calcareous soils. Because application of manure has been shown to increase soil test P concentration and maintain it at high levels for several years in a calcareous soil (Meek et al., 1979), it might be expected that subsequent application of P would result in greater soil test P levels. The results reported here for the field manured Ves with later fertilizer P application might also reflect the positive synergistic effect of manure plus fertilizer addition on increasing soil test P concentration as reported by Reddy et al. (1999). However, this was not the case for the calcareous Barnes soil where manure history effect was either not significant or reduced fertilizer P availability.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Phosphorus from liquid swine manure is more available than P from fertilizer when applied at high rates that exist in the zone of soil impacted by vertical injection of manure. The greater relative availability of manure P continues from 1 to 9 mo after application and is greater 9 mo after application compared with 1 mo for most soils.
Prior manure history may impact the availability of P from subsequent applications of manure or fertilizer. At various times, five of the seven soil series had a decrease in fertilizer-P or manure-P availability when applied to a soil with a manure history compared with one without a manure history. One soil series showed increased manure-P and fertilizer-P availability when applied to a soil with a manure history compared with one without. In general soils that have a history of manure application may respond differently when fertilizer or manure is applied in the future compared with soils without a manure history.
Injection of liquid manures is considered a best management practice with regard to efficient N capture and reduced environmental degradation. This study suggests that manure injection may create bands of soil with high P availability. Further studies need to be conducted in the field to determine the effect of manure injection on the spatial and temporal variability of available P in the soil and determine how manure application methods may impact plant uptake of P and P loss to runoff or leaching.
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ACKNOWLEDGMENTS |
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Received for publication November 6, 2001.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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