| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Section for Animal Production Systems, School of Veterinary Medicine, Univ. of Pennsylvania, 382 West Street Rd., Kennett Square, PA 19348
* Corresponding author (jdtoth{at}vet.upenn.edu)
Received for publication December 23, 2005.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Abbreviations: STP, soil test phosphorus • GS, growing season • NGS, non-growing season • DPS, degree of phosphorus saturation
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
The environmental risks of nitrate losses to groundwater have been understood for many years, and have been the subject of intensive studies (Follett and Walker, 1989). Agricultural best management practices for N losses have been widely implemented, such as tillage modification, soil testing programs, N crediting from legumes and manures, nutrient application timing, crop rotations, and others (Drury et al., 1993; Power et al., 2001). Conscientious implementation of nutrient management strategies can reduce the risk that soil water NO3–N will exceed the USEPA drinking water standard of 10 mg L–1.
Optimal management for N implies basing manure application rates on supplying the crop N requirement, which had been the standard procedure in agriculture for many years (Sims, 1995). However, this approach tends to oversupply the soil system with P. Animal manures as excreted have N/P ratios of 3 to 5:1. Ammonia volatilization losses and delayed mineralization of organic N constituents of manure typically reduce the available N/P ratio to 2:1 (Beegle et al., 1996), whereas harvested portions of most agronomic crops have an N/P ratio of 5 to 6:1. Unlike N, P is relatively immobile in the soil system, and P not taken up by plants will accumulate in soil, potentially to levels far in excess of amounts needed for optimal crop growth. Soils high in P are associated with elevated P losses in runoff (Sharpley et al., 1994) and leaching in well structured, macroporous soils (Brye et al., 2002).
In recent years, the emphasis in nutrient management has largely shifted from N to P. Key to the development of strategies to reduce P loading in agricultural soils has been the understanding of relationships between soil test P (STP) levels and the potential for various chemical forms of P to be lost into the environment (Sharpley and Moyer, 2000), site-specific factors accelerating P loss (Gburek and Sharpley, 1998), and development of effective, flexible models to assess P loss potential (Lemunyon and Gilbert, 1993). There are now management guidelines available to help producers reduce the risk of P loss into the environment and minimize associated water quality degradation.
There are a range of designs of soil water sampling devices now available, all of which have positive features and liabilities, which should be matched to the experimental material. Fixed-tension, passive capillary wick samplers were employed in this study due to the advantages of low cost for a large sampler array, ease of construction, high efficiency of leachate collection (Zhu et al., 2002), and relatively little effect on leachate solute analytes (Knutson and Selker, 1996). However, it has been suggested that capillary wick samplers oversample leachate when the soil water potential is high (Holder et al., 1991) and alter chemical forms and concentrations of analytes in dilute soil solutions (Goyne et al., 2000).
At present, few direct comparisons have been made of the effects of N- and P-based manure management on N and P movement in the soil-crop-soil water system. An experiment was initiated in 1998 in which three crops common in the Mid-Atlantic region of the U.S. received dairy manure at rates based on expected crop N uptake (N-based manure management) or crop P uptake (P-based manure management). The objectives of this study were: (1) determine if there were differences in concentrations of nitrate and soluble P in leachate and in harvested crops as a function of nutrient inputs; and (2) monitor changes in soil test P under two contrasting manure application strategies. The hypothesis tested was: There were not differences in leachate nitrate and total P concentrations, crop N and P uptake, or soil test P accumulation as a function of basing nutrient inputs to three agronomic crops on crop N requirements vs. P requirements.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
The experimental design was a split-plot randomized complete block with three crop species and four nutrient input treatments. The main plots were nine crop strips of 80.2 x 15.8 m with each of the crops randomly allocated within three field block replications. Subplots, with dimensions of 15.8 x 20.1 m, were nutrient input treatments. There were a total of 36 plots.
Precipitation
Thirty-year (1971 through 2000) mean annual precipitation (Fig. 1) was 107 cm (Pennsylvania State Climatologist, 2005), with 58 cm falling in the growing season (GS, April through September as defined here), and 49 cm in the non-growing season (NGS, October through March).
|
The manure, consisting of mainly feces plus urine and some wood shavings as bedding, was collected from the floor of a free-stall barn housing lactating Holstein (Bos taurus L.) cows. Application was performed using a calibrated side-delivery manure spreader. Manure applied to the corn strips was incorporated immediately before planting by moldboard plowing in 1998 and by chisel-disk tillage from 1999 through 2001, and surface-applied three times annually to the forages before spring regrowth in early April and following two forage harvests, typically mid-July and late September. Manure N input rates were based on Pennsylvania Department of Environmental Protection recommendations (Beegle et al., 1996), and were calculated by modifying manure N contents by availability factors for moisture content, ammonium N and total N concentrations, and surface application or time to incorporation. In the first year of the study, nutrient rates applied to alfalfa and orchardgrass were reduced by one-third from recommended values due to the expected lower biomass production in the establishment year.
Sampler Design and Construction
Passive capillary wick samplers were constructed based on an adaptation of the design of Holder et al. (1991). Fiberglass wicks of 0.9-cm diameter (item no. 1381, Pepperell Braiding, Pepperell, MA) were prepared by heating wick material in a muffle furnace at 350°C for 4 h to volatilize manufacturing residues. Wick sections were soaked in P-free citrate-based detergent solution (2%) followed by 10% HCl solution with rinsing steps between each soaking. In a final cleaning step, wicks were leached vertically with 2 L of deionized water passing through each wick section followed by oven-drying. Fiberglass strands from a bundle of six wick sections were spread across the 30 x 30 cm plexiglass collecting surface. The wick bundle was enclosed in flexible vinyl drainage hose and directed to a 25-L collecting jug. The vertical distance from the collecting surface to the end of the wick bundle was set at 50 cm, thus providing a tension of 50 cm on the hanging water column. Direct matching of wick tension with soil hydraulic conductivity was not performed; the 50 cm tension was based on measured conductivity of a similar silt loam soil's B horizon.
To collect leachate moving below the crop root zone, samplers were installed in all 36 plots in access pits 1.2 x 1.2 x 2.1 m in depth with framed, pressure-treated plywood walls and a trapdoor. Precipitation falling on the trapdoor was diverted away from the immediate area of the sampler with a rain gutter and drainage pipe to a sump. A cavity was excavated through a cutout in one sidewall of the pit to a length to accommodate the sampler, plus a buffer of 30 cm of undisturbed soil. The ceiling of the cavity was carefully smoothed with a steel spatula to remove the smear layer created during excavation, and the sampler collecting surface was brought into contact with the ceiling using turnbuckle supports. Samplers were installed at a depth of 95 cm near the bottom of the B horizon.
Sample Collection and Analyses
Sample collection years extended from 1 April through the following 31 March; thus, the duration of the study was April 1998 through March 2002. Leachate samples were collected following rainfall events sufficient to generate leaching. Total leachate volumes from each sampler were measured, and a 30-mL subsample taken for nitrate and phosphorus analysis. Samples were stored at –20°C before analysis, and analyzed within 1 wk of collection. Thawed leachate samples were filtered through Whatman 42 paper then analyzed for NO3–N by the cadmium reduction method (American Public Health Association, 1995b) on an autoanalyzer (Autoanalyzer II, Bran + Luebbe, Buffalo Grove, IL), and for total P by inductively coupled plasma atomic emission spectroscopy (ICP; Thermo Jarrell Ash ICAP 61E, Thermo Electron, Waltham, MA).
Samples of dairy manure were obtained from each load of manure spread on the field. Manure samples were analyzed for dry matter, ammonium N, total Kjeldahl N (TKN), total P, and total K by a commercial laboratory. Average analyses of manures (n = 62) were (means in g kg–1, with standard deviations in parentheses): dry matter = 174 (23); NH4–N = 8.68 (3.02); TKN = 27.12 (5.46); total P = 5.15 (1.12); and total K = 11.42 (3.11).
Soil test P was monitored annually in November following the final forage harvest of the season and again in late March or early April before regrowth of the active crop. Eight 1.5-cm diameter soil cores were collected from each plot to a 20-cm depth, separated into 0- to 5-, 5- to 10-, 10- to 15-, and 15- to 20-cm increments, and composited by depth. Soil samples were air-dried and ground to pass a 2-mm sieve in a hammer mill. Three-gram subsamples of soils were extracted in 30 mL of Mehlich-3 (Mehlich, 1984) solution for 15 min with agitation on a reciprocal shaker at 180 rpm. Extracts were filtered with a vacuum through Whatman 42 paper and STP was determined colorimetrically on a spectrophotometer (Spectronic 1001, Bausch & Lomb (Milton Roy), Rochester, NY) using the Murphy and Riley (1962) procedure.
Degree of P saturation (DPS) was determined on archived ground soil samples (collected in spring 1998 and 2002) with Mehlich-3 extracts and analyzed for total P, Al, and Fe by ICP. Degree of P saturation was calculated as the molar ratio (P/[Al+Fe])/100 with units of mmol kg–1 (Kleinman and Sharpley, 2002).
Alfalfa and orchardgrass were sampled by collecting vegetation from two to four 0.125-m2 quadrats from representative areas of each plot and composited by plot. Plants were cut at an above-ground height of approximately 2.5 cm with a battery-powered hand clipper to simulate mechanical crop harvest. Forage samples were dried at 65°C for 2 to 4 d and ground in a blade mill to pass a 2-mm sieve.
Corn was sampled by selecting six representative plants per plot, cutting them at an above-ground height of approximately 10 cm, then chopping them in a commercial chipper-shredder to simulate silage. Silage samples were dried and ground using the same procedures as for the forages.
To measure total N and P uptake, forage and silage samples were acid-digested using a microwave system (MARS 5, CEM, Matthews, NC). For TKN analysis, 0.25 g of dried, ground plant material were digested for 20 min in 15 mL of concentrated H2SO4, 7.5 mL 30% H2O2 solution was added, then the samples were redigested for an additional 20 min. For total P analysis, 0.25 g of dried, ground plant material were digested for 20 min in 9 mL concentrated HNO3 and 3 mL HCl. Diluted digests were analyzed colorimetrically for TKN using the phenate method (American Public Health Association, 1995a), and for total P by ICP.
Measurement of atmospheric N deposition, denitrification, and runoff losses of N and P were outside the scope of this study; therefore data should not be used in a strict mass-balance approach.
Data Analysis
Statistical testing was by a two-way analysis of variance. Statistical analyses were performed in SAS (SAS Institute, 1999) using the General Linear Models procedure. A TEST statement was included to employ the correct error terms for main and subplot effects. Separation of means was performed using the Tukey-Kramer test. Additional analyses by pairwise t tests were performed when applicable. A probability level of 0.05 was used for all statistical analyses.
|
RESULTS AND DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Four-year cumulative leachate depths collected during the GS (sum of all samples collected April through September) were greater in the samplers installed beneath corn, (75 cm), than in those under alfalfa (51 cm) or orchardgrass (52 cm), although the difference was not statistically significant. The average planting date for corn was 13 May, with full crop canopy cover developing over a month later, whereas active regrowth of the alfalfa and orchardgrass took place typically in the first half of April. Over a period of several weeks in spring, therefore, there was little evapotranspiration occurring in the corn crop compared to the two forage crops.
In the NGS (October through March) cumulative 4-yr leachate depths did not differ by crop (146, 162, and 149 cm for alfalfa, corn, and orchardgrass, respectively). Corn harvest and small-grain cover crop planting was generally completed by mid-October, and active growth of the forages was halted by cold temperatures in November or December. Averaged across all crops, 28% of precipitation in the growing season was collected by the samplers, and 93% in the NGS. However, in another indication of the contribution of water run-on to the plots to percolate, 11 of the 36 samplers (six of them in replication 1) had leachate totals >105% of NGS precipitation. Sheet flow of runoff across the relatively sparsely vegetated field during heavy over-winter rainfall and snowmelt events was observed on a number of occasions.
Two factors may have confounded interpretation of leaching loss data in this study. The 50-cm fixed tension of the samplers, based on measured hydraulic conductivity of a similar silt-loam soil, may have oversampled percolate, especially in periods of high soil water status when most leaching occurs. This would effectively dilute samples collected at those times. Variable- or equilibrium-tension samplers (e.g., Brye et al., 2002), which have tension on the soil water column adjusted for temporal changes in soil water potential, are able to some extent to overcome limitations in the fixed-tension design. Variable-tension sampler technology was in the development stages when the present study was being planned. Second, the low concentrations of total P in leachate may have been altered by the fiberglass wick material, as was suggested by Goyne et al. (2000). Given these limitations, interpretation of leaching data in the present study should be done carefully with attention paid to possible factors affecting volumes and analyte concentrations.
Crop Uptake of Nitrogen and Phosphorus
Mean annual N and P inputs in inorganic fertilizer, dairy manure, and crop uptake of total N and P are presented in Table 1. Target N input rates were 280, 235, and 225 kg N ha–1 for alfalfa, corn, and orchardgrass, respectively. After the establishment year in 1998, N input rates of the manure treatments were adjusted for residual N carryover. Target P rates were 37, 57, and 33 kg P ha–1 for alfalfa, corn, and orchardgrass, respectively. An error in the calculation for expected P uptake by crops led to oversupply of P in the P-based manure treatment in corn in all 4 yr of the study. The available N/P ratio of manure for corn was higher (2.2:1) than the forage crops (1.2:1) because of greater N availability with the immediate incorporation of manure after application to corn, compared to manure surface-applied to the forages.
|
Mean annual P uptake by the crops ranged from 14 to 37 kg P ha–1. Total P uptake tended to be greater in the manured treatments for all three crop species, although the differences were not significant at the 5% level. Actual P uptake matched the calculated expected uptake (Pennsylvania State University, 1999) only in alfalfa; for corn and orchardgrass actual uptakes were 65 to 75% of the book values used to calculate the P-based manure rate. Late GS water deficits in 3 yr of the study (Fig. 1) may have been responsible for reduced yields and therefore reduced crop P uptake. Corn and orchardgrass are relatively shallow-rooted compared to alfalfa, and may have been more seriously affected by low soil water status in the growing season.
Nitrate and Phosphorus Losses in Leachate
Nitrate Losses
Leachate sample nitrate N concentrations varied widely both over the duration of the study and within replications of crop x nutrient treatment combinations at individual sampling times. Of the 986 samples collected during the 4-yr period, the range was from below detection limit (0.02 mg L–1) to a maximum of 86 mg L–1, with 49% of the samples >10 mg L–1. Individual highest leachate nitrate concentrations tended to occur in the late fall and early winter periods. Four-year mean flow-weighted NO3–N concentrations from control plots receiving no N inputs of the three crops were 7.6, 7.0, and 7.1 mg L–1 for alfalfa, corn, and orchardgrass, respectively. However, treatments receiving N in inorganic fertilizer or manure generally had NO3–N concentrations >10 mg L–1, with a 4-yr mean flow-weighted concentration of 13.7 mg L–1 over all crops and N input treatments.
Leachate nitrate N concentrations did not differ across all crops and nutrient input treatments (Table 2). For the corn crop, mean flow-weighted NO3–N concentrations were 7.0, 13.4, 10.7, and 21.0 mg L–1 for the control, inorganic fertilizer, and N- and P-based manure treatments, respectively. Leachate nitrate N concentrations did not differ in the treatments receiving N inputs in manure or inorganic fertilizer. The reasons for the high nitrate N concentrations in the P-based manure treatment are unclear. In two periods of the 4-yr study (fall 1999 through early winter 1999–2000, and fall 2000 and early winter 2000–2001), leachate nitrate from two of the plots in the P-based treatment were higher than other treatments with many individual samples exceeding 30 mg NO3–N L–1. It can be speculated that more N became available more rapidly in the P-based manure treatment (in which half of the N inputs to the P-based manure treatment were as N fertilizer, 111 of 219 kg N ha–1 yr–1) than in the N-based manure, which had much of the N in slowly mineralized organic forms.
|
For the alfalfa crop, mean flow-weighted NO3–N concentrations were 7.6, 20.0, and 14.6 mg L–1 for the control, N- and P-based manure treatments, respectively, which did not differ statistically. Previous studies suggest relatively low nitrate in leachate from alfalfa when no N was applied. For example, Randall et al. (1997) reported a mean concentration of 3 mg L–1 over 6 yr, and Toth and Fox (1998) found a 3-yr mean leachate concentration of 4 mg L–1 for alfalfa receiving no N applications. However, information is scarce on nitrate losses from the root zone when an alfalfa crop is used for disposal of or as a repository for animal manures. Daliparthy et al. (1994), using ceramic cup samplers installed in an alfalfa field, found NO3–N concentrations from alfalfa receiving a single annual dairy manure application of 112 kg N ha–1 did not differ from the control (2.5 and 3.1 mg L–1, respectively). However, leachate nitrate concentrations from alfalfa receiving a higher manure rate (336 kg N ha–1) were significantly greater over at least part of the sampling period, reaching 32 mg NO3–N L–1 in fall of the second year of the study.
Nitrate-N concentrations in leachate from orchardgrass tended to be somewhat lower than for the other two crops in this experiment, averaging 7.1, 8.7, 11.4, and 9.4 mg L–1 for the control, inorganic fertilizer, N- and P-based manure treatments, respectively; which did not differ statistically. Orchardgrass is a densely-rooted perennial and apparently is more efficient in nutrient uptake than tap-rooted crops such as alfalfa or annual row crops such as corn. Also, N inputs to orchardgrass were lower than the other two crops since nutrient input rates were based on expected nutrient uptake. Drury et al. (1993) included bluegrass (Poa pratensis L.) in their study of nitrate losses as a function of crop and tillage and found leachate NO3–N from the grass treatment, which received 189 kg N ha–1, to be 1.2 and 2.6 mg L–1 in 2 yr.
Annual mass of NO3–N lost in leachate over all crops ranged from 28 to 117 kg ha–1, and did not differ by nutrient treatment over all crops (Table 2). Leachate nitrate N losses accounted for 10 to 35% of the total inorganic fertilizer and manure N inputs over all crops receiving fertilizer or manure applications except the P-based manure treatment in alfalfa. The latter had a disproportionately high value (94%), although uptake of symbiotically fixed N was a confounding factor. For corn, the 4-yr mean annual mass of nitrate N lost in leachate was 64 kg ha–1 for the inorganic fertilizer treatment, and 66 and 117 kg ha–1 for the N- and P-based manure treatments, which did not differ statistically. Randall and Iragavarapu (1995) reported 43 kg ha–1 yr–1 lost in tile drainage from continuous inorganically fertilized (200 kg N ha–1) corn in Minnesota over 11 yr. In a second study described by Randall et al. (1997), annual mass of NO3–N in leachate averaged 55 kg ha–1 from continuous corn receiving 35 to 180 kg N ha–1 fertilizer N. Brye et al. (2001), in a study of carbon and nitrogen losses from corn and prairie in Wisconsin measured with equilibrium tension lysimeters, found annual mean losses of inorganic N in leachate under chisel-tilled corn to be 45 kg ha–1 over 4 yr. For alfalfa, the annual mean of 30 kg ha–1 NO3–N in leachate from the control in the present study was substantially higher than previous reports, 1 kg ha–1 yr–1 over 6 yr by Randall et al. (1997) or the 9 kg ha–1 yr–1 in 3 yr of an alfalfa–corn rotation study by Toth and Fox (1998).
Over the course of the present study, the concentrations of NO3–N in leachate were generally greater in the GS samples than NGS, but differences were small and were not significant (Table 3). On a mass basis, nitrate losses were lower in the GSs due to smaller volumes of leaching water. Owens et al. (1995) observed a time lag in nitrate movement through the soil pore system below a corn–soybean rotation, therefore the highest leachate nitrate concentrations were in the late dormant/early GSs (February through July). In the present study, the greatest loads of nitrate lost in leachate took place during two NGS periods (Fig. 2). Both were in the fall and winter months of the 2 yr when precipitation exceeded the 30-yr average. During those two periods, maximum rates of nitrate loss in leachate were 0.89 kg NO3–N d–1 from September 1999 through April 2000 for the P-based manure treatment in corn and 1.15 kg NO3–N d–1 from December 2000 through March 2001 for the N-based manure treatment in alfalfa.
|
|
Another interesting observation was a surge of nitrate leaching loss in the first few months following the moldboard plowing of the field at the beginning of the project. Nitrate-N concentrations exceeded 25 mg NO3–N L–1 in leachate from all treatments including the controls (data not shown). This was likely a function of stimulated mineralization of manure and soil organic matter following tillage, combined with low nitrate uptake during the period of crop establishment.
Phosphorus Losses
Concentrations of total P in leachate in individual samples (n = 969) ranged from below the detection limit of 0.001 mg L–1 to a maximum of 0.91 mg L–1, with 34% of the samples exceeding 0.10 mg L–1, the USEPA level of concern for surface water (USEPA, 1986). Four-year flow-weighted treatment mean total P concentrations in leachate ranged from 0.06 to 0.19 mg L–1 yr–1 and did not differ across crops and nutrient treatments (Table 2). The highest annual mean concentrations, 0.18 and 0.19 mg P L–1, were found in leachate beneath manured treatments.
In a comprehensive review of existing research on P leaching, Sims et al. (1998) summarized that in most studies of P leaching from agronomic crops grown on mineral soils, soluble P concentrations were generally well below the USEPA surface water level of concern of 0.10 mg P L–1. Brye et al. (2002) reported total P concentrations in leachate collected by tension samplers under corn receiving 10 kg P ha –1 in starter fertilizer ranged from 0.01 to 0.08 mg L–1; most of their comparisons of P concentrations did not differ between chisel-plow or no-tillage and N fertilization treatments. In a study involving one-time or split annual liquid dairy manure applications to corn (60 to 116 kg P ha–1) and orchardgrass (50 to 73 kg P ha–1), van Es et al. (2004) found high P concentrations in subsurface drain water in a clay-loam soil in New York. Three-year mean soil water total P concentrations exceeded 0.19 mg P L–1 for all treatments in their study. The authors suggested that preferential flow in the well-structured clay loam soil might have allowed rapid movement of P in leachate. Partitioning of leachate into preferential and matrix flow cannot be definitely determined by the data collected in the present study.
Mass of total P lost in leachate in the present study averaged 0.5 kg ha–1 yr–1 and did not differ by crop or nutrient treatment. Brye et al. (2002) reported 0.2 to 3.4 kg P ha–1 lost by leaching in their 8-mo study.
A closer examination of data for leachate collected during the 30 d following manure application events indicated that nitrate and P concentrations in leachate immediately following manure spreading did not differ from other times of the year (data not presented).
Soil Test Phosphorus and Degree of Phosphorus Saturation
Before the experiment (spring 1998), mean STP concentrations in the surface 20 cm were 76, 70, and 64 mg P kg–1 for the alfalfa, corn, and orchardgrass strips, respectively. Soil samples for individual plots were not available for spring 1998 (i.e., before establishment of experimental treatments), therefore, samples from fall 1998 after the first season's treatment applications were used for comparison with those from the end of the 4-yr study, collected in spring 2002 (Table 4).
|
Degree of phosphorus saturation, a measure of the extent to which soil P sorption sites have been filled, was determined on samples collected in spring 1998 and spring 2002. Degree of P saturation (Table 5) in the 0- to 20-cm soil samples declined in four of the five crop x nutrient input treatment combinations which received no P inputs (control and fertilizer treatments) and increased in the N-based manure treatment in orchardgrass. Averaged over all crops, DPS in the N-based manure treatment increased from 0.070 to 0.084 in the 0- to 20-cm soil layer after 4 yr. However, in the surface 5 cm of the N-based manure treatment in 2002, DPS was 0.181 and 0.155 in alfalfa and orchardgrass (data not shown). Degree of P saturation can be used as a predictor of the likelihood of increased P losses in leachate. Using Mehlich-3 extracts of soils from the Mid-Atlantic region, Sims et al. (2002), and Butler and Coale (2005) suggested the critical DPS point is in the range of 0.25 to 0.30.
|
|
CONCLUSIONS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
After 4 yr of manure applications at N-based rates, STP in the surface 5 cm increased from 85 to 125 mg STP kg–1, or by 47% over all crops. Generally, no change in STP was found in soils receiving the P-based manure rate. Degree of P saturation in the surface 5 cm increased from 0.08 and 0.07 to 0.16 and 0.18 in alfalfa and orchardgrass, respectively, after 4 yr of manure applications. Degree of P saturation was still well below levels (0.25 to 0.30) suggested to lead to increased risk of P leaching losses, but increased with time. Basing dairy manure applications on crop N requirements compared to P requirements did not lead to greater nitrate or total P losses in leachate, but the trend of increase in soil test P and degree of P saturation over a few years of N-based manure applications indicates the potential for P loss into the environment if manure is applied at N-based rates. Ancillary issues such as the need for increased monitoring of soil nutrients, nutrient management planning and the greater land area required for P-based manure applications need to be addressed and resolved to support a sustainable dairy agriculture sector.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Crop Science | |||
| Vadose Zone Journal | Journal of Plant Registrations | ||||
| Journal of Natural Resources and Life Sciences Education |
Soil Science Society of America Journal |
