| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
a Jr., Animal Sciences Dep., Univ. of Florida, IFAS, Gainesville, FL 32611
b Central Mississippi Research and Extension Center, MSU, Raymond, MS 39154
* Corresponding author (krw{at}mail.ifas.ufl.edu)
Received for publication June 30, 2002.
 |
ABSTRACT |
|---|
 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
concentration below the rooting zone for two perennial, sod-based, triple-cropping systems over four 12-mo cycles (1996–2000). The soil is an excessively drained Kershaw sand (thermic, uncoated Typic Quartzipsamment). Effluent N rates were 500, 690, and 910 kg ha-1 per cycle. Differences in N removal between a corn (Zea mays L.)–bermudagrass (Cynodon spp.)–rye (Secale cereale L.) system (CBR) and corn–perennial peanut (Arachis glabrata Benth.)–rye system (CPR) were primarily related to the performance of the perennial forages. Nitrogen removal of corn (125–170 kg ha-1) and rye (62–90 kg ha-1) was relatively stable between systems and among cycles. The greatest N removal was measured for CBR in the first cycle (408 kg ha-1), with the bermudagrass removing an average of 191 kg N ha-1. In later cycles, N removal for bermudagrass declined because dry matter (DM) yield declined. Yield and N removal of perennial peanut increased over the four cycles. Nitrate N concentrations below the rooting zone were lower for CBR than CPR in the first two cycles, but differences were inconsistent in the latter two. The CBR system maintained low NO-3–N leaching in the first cycle when the bermudagrass was the most productive; however, it was not a sustainable system for long-term prevention of NO-3–N leaching due to declining bermudagrass yield in subsequent cycles. For CPR, effluent N rates 
500 kg ha-1 yr-1 have the potential to negatively affect ground water quality.
Abbreviations: CBR, corn–bermudagrass–rye system • CPR, corn–perennial peanut–rye system • DM, dry matter
|
INTRODUCTION |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
In the LSRW, the types of soils and underlying hydrogeology make the UFA vulnerable to nitrate contamination. Most of the soils are excessively to moderately drained, deep, sandy Entisols. Also, the UFA is relatively close to the surface and layers above it are thin and unconfined (Andrews, 1992). These characteristics, coupled with relatively high annual rainfall (1310–1460 mm), increase the possibility that nitrate from inorganic and organic fertilizer sources will contaminate ground water. The Suwannee River is also vulnerable because during low flow, it is supplied with water from the UFA through springs and bottom seeps (Andrews, 1992).
Most of the dairies in the LSRW keep their milking herds in confinement lots. Typically, manure effluent is applied in sprayfields where forages are grown year-round. Several double and triple cropping systems with annual and perennial forages have been used, but the most common combination consists of two sequentially planted, summer-annual forages, such as corn and forage sorghum [Sorghum bicolor (L.) Moench], and one winter annual, such as rye. Some dairies use bermudagrass, a perennial that is grown during the entire warm season, followed by a winter-annual forage that is no-till planted into the dormant bermudagrass sod.
There is evidence that sod-based cropping systems may be better at recovering N in dairy sprayfields compared with systems made up solely of sequentially grown annual forages. In north-central Florida, French et al. (1994) evaluated a corn–forage sorghum–rye system and two sod-based systems under dairy effluent irrigation. The sod-based systems maintained lower NO-3–N concentrations in soil water below the primary rooting zone compared with the corn–forage sorghum–rye system. Woodard et al. (2002) reported that a bermudagrass–rye system was much more effective at preventing nitrate leaching than a corn–forage sorghum–rye rotation in dairy sprayfields in northern Florida.
In the present study, five year-round forage systems were evaluated under dairy effluent irrigation over four consecutive 12-mo cycles and with three loading rates of effluent N. Due to the large amount of data collected in the overall study, results from two of the forage systems are reported. Our objectives were to measure the capacities of the year-round forage systems and system components to remove N from the soil and prevent NO-3–N from leaching below the rooting zone.
|
MATERIALS AND METHODS |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
|
|
During a typical irrigation, 16 mm of water were applied. To determine application volume, six 20-L containers were placed in each mainplot. Dairy effluent was collected at two locations during an irrigation by placing two lab-cleaned, 20-L containers ahead of the pivot. After retrieval, five 125-mL samples of effluent were collected at each location, acidified (
2 pH), and placed in a cooler within 15 min. Freshwater samples were taken using similar procedures. Loading rates of N were calculated using concentrations of Kjeldahl N (unfiltered) plus nitrate N (filtered) in dairy effluent and freshwater, and application volumes.
The four-cycle average N loading rate was 500 kg ha-1 cycle-1 for the low, 690 kg ha-1 for the medium, and 910 kg ha-1 for the high (Table 1). During the 1996–1997 cycle, effluent was underapplied for the medium and high loading rates due to delays associated with a number of pivot repairs. During the 1998–1999 cycle, the measured N rate exceeded the target rate by an appreciable amount for all loadings. While effluent N was overapplied in that cycle, some of the difference between target and measured rates was due to the nitrate N applied in the many overall freshwater irrigations needed during the cycle that was the driest of the four study cycles (Fig. 2c).
|
In northern Florida, irrigated bermudagrass is generally harvested four to five times when grown during the entire warm season (April–October). Perennial peanut is harvested three to four times. Both perennial forages are dormant in winter. In the present study, the rye component was planted into the dormant bermudagrass and perennial peanut sods in late November to early December, with a no-till grain drill (20-cm row spacing). In the spring, corn was planted into the semidormant sods with a four-row, no-till corn planter (76-cm row spacing). The no-till planter was equipped with in-row chisels that penetrated to a soil depth of 30 cm. Planting and harvest dates are presented for the forage components of the two systems in Table 2. Corn was harvested in the early dent stage of kernel development and rye at full anthesis. The bermudagrass was harvested two times following corn harvest; perennial peanut was harvested once. Forage within an 8-m2 area was harvested in each subplot for yield determination. Bermudagrass, perennial peanut, and rye were harvested at a 3-cm cutting height. Corn stalks were cut 5 cm above the soil surface. The fresh biomass of each crop was weighed and a 1.6 ± 0.3-kg subsample was collected and dried at 65°C in a forced-air oven.
|
2 pH) and placed in a cooler within 15 min of extraction. Lysimeters were also installed nearby in an unaffected area characterized as an open woodland. Nitrate N concentration from these lysimeters was negligible during the sampling period (0.2 mg L-1).
Parameters and Methods of Analysis
For total N concentration in dairy effluent and freshwater, USEPA Method 351.2 was used (USEPA, 1983), which involves a standard Kjeldahl digestion followed by semiautomated colorimetry with a Technicon Auto Analyzer (Technicon Instruments Corp., Tarrytown, NY). For total ammoniacal N in dairy effluent, the same method was used but without Kjeldahl digestion. For nitrate plus nitrite N concentration (represented by NO-3–N), Method 353.2 (USEPA, 1983) was used (determination by automated colorimetry). The NO-3–N levels in effluent were <0.02 mg L-1 during the study.
For soil water samples from lysimeters, NO-3–N concentration was determined by USEPA Method 353.2 with a Flow IV, air-segmented, automated spectrophotometer (O-I Analytical, College Station, TX). For ammonia N determination, USEPA Method 350.1 (automated phenate colorimetry) was used. Ammonia N concentrations were negligible during the sampling period (mostly <0.2 mg L-1 but not >0.4 mg L-1).
Forage samples were ground in a Thomas-Wiley mill (1-mm screen). The N analysis involved a modification of the standard Kjeldahl procedure (Gallaher et al., 1975), followed by automated colorimetry (Hambleton, 1977) with a Technicon Auto Analyzer.
Statistical Analysis
Responses were analyzed by fitting mixed effects models (Littell et al., 1996) using the PROC MIXED procedure of SAS (SAS Institute, 1992). Effluent loading rates were main plots and forage systems were subplots. The 12-mo cycles were considered repeated measures. To make inferences about the forage components of the different systems, a term to describe crop nested within systems and its resultant interaction terms was added to the analysis. Crop (within system) was considered as a repeated measure. Covariance structures in the repeated measures were modeled using autoregressive order one (Littell et al., 1996). To determine the effect of effluent N rate on dry matter (DM) yield, forage N concentration, and N removal, contrasts for linear and quadratic effects were examined. To compare soil water NO-3–N concentration means among loading rates for each sampling date within each forage system and 12-mo cycle, a paired comparison of least squares means was made using pooled variances from the entire project (five systems). A predecided 0.10 significance level was chosen for treatment and interaction terms because of the variable nature of the experiment associated with (i) the variation in soil fertility within the sprayfield before treatment initiation, (ii) fluctuation of N concentration in dairy effluent over time, and (iii) variability often observed in soil water NO-3–N concentration data.
|
RESULTS AND DISCUSSION |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
0.02) and a rate x cycle interaction for N concentration (P = 0.06).
Forage System Comparison and Effect of Loading Rate
Dry biomass yields for CBR were superior to those of CPR in all cycles, though CBR yields were lower in Cycles 3 and 4 vs. Cycles 1 and 2 (Table 3). For CPR, yields were consistent over the four cycles. Nitrogen concentration was greater for CBR than CPR in Cycles 1 and 2 but differences did not occur in Cycle 4, thus resulting in a system x cycle interaction (Table 4). In Cycle 3, concentrations were greater for CBR at low and medium rates but systems did not differ at the high rate. For CPR, N concentration was higher in Cycles 3 and 4 (average = 16.4 g kg-1) than in Cycles 1 and 2 (average = 13.8 g kg-1).
|
|
Effluent rate generally had no effect on the system's biomass yield, though there were two exceptions (CBR in Cycle 2 and CPR in Cycle 1; Table 5). Nitrogen concentration in the biomass of both systems increased linearly as effluent rate increased in Cycles 3 and 4, but there was no effect of loading rate in Cycles 1 and 2, thus resulting in a cycle x rate interaction (Table 4). This reflects the greater effect of effluent rate on N concentration in some forages in later cycles (Table 6). In Cycle 1, N concentration in all forage components was unaffected by rate. This is probably the result of preexperimental N becoming available, which would tend to negate the effect of N rate. Nitrogen removal increased linearly as loading rate increased for CBR in Cycle 2 and for CPR in Cycles 2, 3, and 4 (Table 7).
|
|
|
Unlike the annual forages, major differences occurred between the perennial components. Dry matter yield and N removal remained higher for bermudagrass than perennial peanut in all cycles, though both parameters increased for perennial peanut over the four cycles and decreased for bermudagrass over the first three cycles. Bermudagrass showed a great deal of promise in the first cycle, with a mean N removal of 191 kg N ha-1. However, by Cycle 3, it had declined to 134 kg N ha-1, a 30% reduction. The main cause for this decline was a 48% reduction in bermudagrass yield. The full effect of the decrease in yield on N removal was partially offset by a 37% higher mean N concentration in Cycle 3 (Table 6). In Cycle 4, N concentration in bermudagrass was lower than in Cycle 3, but yield and N removal did not differ.
Though perennial peanut yield was lower than that of bermudagrass in all cycles, it increased over the four-cycle period (Table 5) while N concentration remained relatively stable (Table 6). Therefore, the increase in N removal was mainly the result of an increase in yield and not N concentration.
During a 2-yr period, French et al. (1997) evaluated the CPR system under dairy effluent irrigation with supplemental N from ammonium nitrate. The corn was planted into a well-established perennial peanut sod where dairy effluent had not been previously applied. Mean DM yields were 18.6 Mg ha-1 per cycle for corn, 4.6 Mg ha-1 for peanut, and 4.3 Mg ha-1 for rye. Average N removals were 225 kg ha-1 for corn, 106 kg ha-1 for peanut, and 84 kg ha-1 for rye. At Tifton, Georgia, Johnson et al. (1991) evaluated a corn, bermudagrass (‘Tifton 44’), and rye system under dairy effluent irrigation. Average DM yields were 17.5 Mg ha-1 for corn, 4.6 Mg ha-1 for bermudagrass, and 5.3 Mg ha-1 for rye. Mean N removals were 205 kg ha-1 for corn, 124 kg ha-1 for bermudagrass, and 201 kg ha-1 for rye.
Effect of Loading Rate on Forage Components
Yield, N concentration, and N removal of corn in both systems were mostly unaffected by effluent N rate, but there were exceptions. There was a linear increase in yield of CPR–corn with increasing effluent rate in Cycles 1 and 4. Nitrogen concentration increased linearly for CBR–corn in Cycle 3 while N removal increased linearly for CPR–corn in Cycle 4.
Rye yields were generally unaffected by loading rate except in Cycle 2, where they increased linearly in both systems. The greatest effect of loading rate was on N concentration. It increased linearly in CBR–rye in Cycles 2, 3, and 4 and in CPR–rye in Cycles 3 and 4. Nitrogen removal increased with loading rate for CBR–rye in Cycle 2 and for CPR–rye in the Cycles 2, 3, and 4.
Dry matter yield of bermudagrass in CBR was unaffected by loading rate except in Cycle 3, where it decreased as loading rate increased. As with rye, effluent N rate had a much greater effect on N concentration than yield in bermudagrass. In Cycles 2, 3, and 4, N concentration increased linearly (P < 0.01). Johnson et al. (2001) also observed increases in N concentration in full-season Tifton 85 bermudagrass with increasing fertilizer N rates (ammonium nitrate). In their study, concentrations increased from an average 22 g N kg-1 at a seasonal N rate of 390 kg ha-1 to 29 g N kg-1 at a N rate of 785 kg ha-1. In the present study, N removal for bermudagrass was mostly unaffected by loading rate, except in Cycle 3 where it declined with an increase in N rate (P < 0.10). The main cause was the decline in dry matter yield.
Dry matter yield, N concentration, and N removal of perennial peanut in CPR were unaffected by effluent N rate in all cycles. These results are in agreement with the findings of French et al. (1997). Results are also in general agreement with those of Venuto et al. (1998) and Redfearn et al. (2001). In both studies, fertilizer N was applied to full-season perennial (rhizoma) peanut at rates from 0 to 220 kg ha-1.
Overall Assessment of Annual and Perennial Components for Nitrogen Removal
The corn component of the two systems maintained the highest DM yields during the four-cycle period (9.9–14.5 Mg ha-1). Its primary shortcoming, which resulted in low N removals, was low forage N concentration (10.6–15.8 g kg-1). In addition, N concentration did not generally respond to increased effluent N rate. Conversely, rye had moderately high N concentration (12.6–29.1 g kg-1) that generally increased with loading rate. Its major shortcoming was low yield (2.0–5.9 Mg ha-1), which led to low N removal.
In the first two cycles, bermudagrass N removals were greater than those of corn, rye, and perennial peanut. The high N removals for bermudagrass resulted from moderately high DM yields (7.1–10.3 Mg ha-1) and N concentrations (20.1–26.1 g kg-1). Though N concentration increased as loading N rate increased in Cycles 2, 3, and 4, the major disadvantage of bermudagrass was the reduction in yield that occurred over the first three cycles. Also, the decline in yield was most evident in plots receiving the higher loading rates. Yield reductions from Cycle 2 to 3 were 1.6 Mg ha-1 for the low rate, 2.8 Mg ha-1 for the medium, and 3.7 Mg for the high. When paired comparisons for yield were made between the two cycles for each rate, computed P values were 0.18 for the low loading rate, 0.02 for the medium, and <0.01 for the high. Visual estimates of bermudagrass coverage were made on 8 September in Cycle 4. Plot coverage was 98% for the low loading rate, 92% for the medium, and 86% for the high.
The higher effluent N rates undoubtedly played a major role in the yield decline of Tifton 85 bermudagrass. For perennial grasses in general, excess N fertilization tends to decrease carbohydrate reserves (White, 1973) and increase winterkill (West and Prine, 1973). Also, Tifton 85 bermudagrass probably has a predisposition to winterkill because it has fewer rhizomes than ‘Coastal’ and Tifton 44 bermudagrasses (Burton et al., 1993). The more stoloniferous growth habit is probably linked to its parent ‘Tifton 68’ (Cynodon nlemfuensis Vanderyst), which is cold susceptible and has no rhizomes (Burton and Monson, 1984). Furthermore, the CBR system exerts a great deal of stress on the bermudagrass, which would accentuate problems with persistence. After corn harvest, the weakened bermudagrass must quickly reestablish, produce high yields, and allocate adequate energy reserves to belowground plant structures before the first winter frosts, if it is to maintain high N removals and persist over multiple seasons.
Dry matter yield of perennial peanut (Florigraze) in CPR continued to increase over the four-cycle period. This is a clear demonstration of its ability to persist under adverse conditions. Florigraze perennial peanut is known to be persistent, an attribute generally associated with its extensive rhizome system. However, there may be more specific features responsible for its survival in the CPR system. Research has shown Florigraze to be tolerant of moderate shading (Johnson et al., 1994). Also, Saldivar et al. (1992) found that during the cooler autumn months, photosynthate produced in the shoots of Florigraze is partitioned into rhizome growth.
Though perennial peanut was persistent in CPR, the primary disadvantages were that it was slow to establish and had relatively low DM yield. The moderately high N concentration in peanut forage (20.2–25.3 g kg-1) did not adequately compensate for the low yield in terms of N removal.
Nitrate Nitrogen Concentration below the Primary Rooting Zone
Effects for Cycle, Forage System, and Effluent Nitrogen Rate and Interaction Terms
With suction lysimeters, NO-3–N concentration was measured in soil water at 1.5 m below the surface from July 1996 to March 2000. Main effects were significant for cycle (P < 0.01), forage system (P < 0.01), and effluent N rate (P = 0.04). There were cycle x system and cycle x N rate interactions (P < 0.01). When NO-3–N concentration was analyzed within cycle, the effect for forage system in Cycles 1 and 2 was significant (P < 0.01), but the rate effect was not (P > 0.10). It should be noted that P for rate effect in Cycle 2 was 0.13. In both cycles, average 12-mo NO-3–N concentration (averaged across effluent rates) was higher for CPR than CBR (Table 8). The effects of forage system and rate were significant in Cycles 3 and 4, but there was a system x rate interaction (P < 0.01). In Cycle 3, systems did not differ for the low and medium rates but for the high rate, NO-3–N concentration was higher for CBR than CPR. In Cycle 4, levels were higher for CPR than CBR for the low and high rates but the systems did not differ for the medium rate.
|
In Cycle 1, the bermudagrass of CBR recovered quickly after corn harvest and removed an average of 191 kg N ha-1 from the soil during its growth period. For all three loading rates, NO-3–N concentration in the lysimeter water samples reflected the rapid recovery and high N removal. Concentrations began to increase following corn harvest but rapidly declined to near-zero levels, which were maintained throughout the bermudagrass growth period (Fig. 4a) . Over the next two cycles, however, the bermudagrass became successively less effective in its ability to curb NO-3–N leaching (Fig. 4b,c). Leaching patterns in Cycle 3 clearly show the effects of reduced bermudagrass yield and stand, most evident in plots receiving the highest loading rate. Following corn harvest, NO-3–N concentration increased sharply. In late September, peaks reached an average of 36 mg L-1 for the low rate, 52 mg L-1 for the medium rate, and 111 mg L-1 for the high rate.
|
|
500 kg ha-1 has a high risk of negatively affecting ground water quality. The CBR system was very effective at minimizing leaching in Cycle 1 when the Tifton 85 bermudagrass was the most productive; however, it did not exhibit long-term control of nitrate leaching, a trend most noticeable with the two higher loading rates. The lowest long-term risk of nitrate contamination of ground water would be for the 500 kg ha-1 N rate. At that level, bermudagrass stands and dry matter production can be maintained for a longer period. It should be noted that the decrease in bermudagrass yield and stand may be exclusive to Tifton 85. It is possible that bermudagrass cultivars that develop greater rhizome systems and are known to be persistent under adverse growing conditions may improve the long-term performance of the CBR system to reduce nitrate leaching.
Woodard et al. (2002) concluded that a bermudagrass–rye system receiving annual effluent N rates up to 700 kg ha-1 would result in low risk of nitrate contamination of ground water, if healthy bermudagrass stands were maintained. Also, the use of the corn–forage sorghum–rye system in dairy sprayfields with effluent N rates 500 kg ha-1 per cycle had a high risk of nitrate contamination of ground water. Previous work by French et al. (1994) evaluated CPR and corn–forage sorghum–rye systems under dairy manure effluent irrigation with supplemental N fertilization. Their study suggested that perennial peanut maintained lower nitrate concentrations below the rooting zone from the time of corn harvest to the start of the winter season, compared with forage sorghum.
Nitrate Load in Topsoil
Year-round leaching patterns suggest that rye is highly effective at controlling nitrate leaching. There was a sharp decrease in lysimeter NO-3–N concentration from very high peaks in early rye growth to almost zero levels during the latter half of rye growth in Cycle 3 for CBR (Fig. 4c) and in Cycles 1, 2, and 3 for CPR (Fig. 5a–c). Declining NO-3–N levels were measured during the same period in Cycles 1, 2, and 4 for CBR and Cycle 4 for CPR. These patterns suggest that large amounts of N and soil moisture were being extracted from the soil by the rye. This outcome was inconsistent with the low N removal of the rye.
The magnitude of the peaks during early rye growth was probably related to the amount of residual N remaining in the upper soil profile at the end of the warm season, which is directly related to the efficiency of the perennial sod components in removing N from the soil. The reduction of peak NO-3–N concentrations to low levels can be attributed in part to reduced rates of mineralization of effluent organic N and nitrification of effluent ammoniacal N during the cool season. About one-third of dairy effluent N was in organic form while the remainder was ammoniacal N. The 4-yr mean for ammoniacal N as a percentage of total effluent N was 68%. Due to the effect of seasonal temperatures on microbial processes that convert effluent N to NO-3–N (Das et al., 1995; Dou et al., 1997), the constant year-round surface loading of effluent N may not result in constant availability of NO-3–N in the rooting zone.
Our explanation for the steep drop in lysimeter NO-3–N concentration during rye growth in some cycles involves two events occurring simultaneously. As a wave of residual soil NO-3–N moved downward in early winter, concentrations in lysimeters increased to high peaks. While this was occurring, cooler temperatures reduced the rate of effluent N conversion to NO-3–N, thus reducing the NO-3–N load in the topsoil. The reduced load probably matched the low N removal capability of the rye. With the two events taking place during rye growth, the high peaks rapidly fell to near zero-levels.
There are practical implications of seasonal variation in NO-3–N availability in the upper soil profile. To control leaching year-round in dairy effluent sprayfields, warm-season forages will be required to remove more N from the soil relative to the amounts of effluent N applied to the soil surface during their growth periods. Therefore, forage selection should probably match seasonal changes in the nitrate load in the topsoil and not the constant loading of effluent N to the soil surface.
|
CONCLUSIONS |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Leaching patterns of the CBR system were consistent with the decline in performance of the bermudagrass. The lowest NO-3–N concentrations in soil water below the rooting zone occurred in the first cycle, but as bermudagrass yield declined in subsequent cycles, NO-3–N levels increased. Although yield and N removal increased over the four cycles for perennial peanut in CPR, consistent changes in leaching patterns were far less discernable. The CBR system maintained significantly lower NO-3–N concentrations than CPR in the first two cycles at all loading rates, but differences between systems in the last two cycles were not consistent.
With Florida's ground water NO-3–N standard of 10 mg L-1 as a reference point, the leaching pattern of the first cycle shows that the CBR system can maintain low NO-3–N leaching in dairy sprayfields. However, CBR was not a sustainable forage rotation for long-term control of nitrate leaching due to the decline in bermudagrass performance. For the CPR system, loading rates 500 kg N ha-1 have the potential to negatively affect ground water quality.
|
ACKNOWLEDGMENTS |
|---|
|
NOTES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
|
REFERENCES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
|---|
Related articles in JEQ:
This article has been cited by other articles:
|
|
 |
|
  J. C. B. Dubeux Jr., L. E. Sollenberger, B. W. Mathews, J. M. Scholberg, and H. Q. Santos Nutrient Cycling in Warm-Climate Grasslands Crop Sci., May 31, 2007; 47(3): 915 - 928. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. R. Woodard, L. E. Sollenberger, L. A. Sweat, D. A. Graetz, V. D. Nair, S. J. Rymph, L. Walker, and Y. Joo Phosphorus and Other Soil Components in a Dairy Effluent Sprayfield within the Central Florida Ridge J. Environ. Qual., May 25, 2007; 36(4): 1042 - 1049. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. M. Crohn Optimizing Organic Fertilizer Applications under Steady-State Conditions. J. Environ. Qual., March 1, 2006; 35(2): 658 - 669. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH |
