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Published online 4 April 2007
Published in Agron J 99:707-714 (2007)
DOI: 10.2134/agronj2006.0200
© 2007 American Society of Agronomy
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Nitrogen Management

Response of Coastal Bermudagrass Yield and Nutrient Uptake Efficiency to Nitrogen Sources

Maria L. Silveiraa,*, Vincent A. Habyb and Allen T. Leonardb

a Univ. of Florida, Range Cattle Research and Education Center, 3401 Experiment Station, Ona, FL 33865
b Texas Agricultural Experiment Station, Texas A&M Univ. System, P.O. Box 200, Overton, TX 75684-0200

* Corresponding author (mlas{at}ufl.edu)

Received for publication July 7, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Nitrogen is an important agronomic input for bermudagrass production in the southern USA. Fertilizers that can efficiently provide N to grass pastures and hay meadows are an important issue because of increasing costs and environmental problems associated with N losses. This experiment was designed to determine the effectiveness of various N sources on ‘Coastal’ bermudagrass [Cynodon dactylon (L.) Pers.] production and N uptake efficiency. Nitrogen was applied at 0, 45, 90, and 135 kg ha–1 harvest–1 as urea–ammonium nitrate (UAN), urea, ammonium nitrate (AN) and ammonium sulfate (AS) on Gallime (Glossic Paleudalf) and Lilbert (Plinthic Paleudult) soils. Mixtures of S with UAN and of Ca and B with urea were also evaluated. Bermudagrass was periodically harvested and subsampled for total N analysis. At termination of the study soil samples were collected for pH and extractable NO3–N analyses. Bermudagrass yield responses to N sources were significant only in the Gallime soil. In this soil, AN and AS increased yields and resulted in greater N uptake compared to urea and UAN. Lilbert soil showed no effect of N sources on dry matter (DM) production. There was a yield response to N rates and maximum bermudagrass production was generally achieved at the 90 kg ha–1 N rate regrowth–1. Fertilizer efficiency declined as the N rate was increased. Soil acidity increased in response to N application, particularly for the AS treatments. Selection of N sources and rates should be carefully planned to avoid detrimental effects on soil acidity and, consequently, fertilizer efficiency.

Abbreviations: AN, ammonium nitrate • AS, ammonium sulfate • ATS, ammonium thiosulfate • DM, dry matter • UAN, urea-ammonium nitrate solution


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
COASTAL BERMUDAGRASS is a high-yielding warm-season perennial grass, widely cultivated in the southern USA for pasture and hay production (Burton, 1954). This grass requires relatively moderate amounts of N for optimum production and can efficiently respond to inorganic fertilizer application (Day and Parker, 1985; Haby, 2002; Wilkinson and Langdale, 1974). Nitrogen fertilizer can increase both yield and nutritive value of bermudagrass pastures (Burton and Hanna, 1995), particularly in low fertility Coastal Plain soils, where N is often the most limiting nutrient for forage production (Robinson, 1996).

Although N is an important agronomic input for productive bermudagrass pastures, the increasing costs of commercial fertilizers and environmental problems associated with improper fertilization management have prompted the need to re-examine optimum rates and efficient sources to supply pastures with N. Studies have determined that the amounts of N required by Coastal bermudagrass vary according to the production system (grazing vs. haying), cultivar, soil type, and environmental factors (Taliaferro et al., 2004). In the early 1950s, research demonstrated Coastal bermudagrass hay yields of 22 and 24 Mg ha–1 in response to 448 and 896 kg N ha–1 (Burton and DeVane, 1952). Haby et al. (1979) found that Coastal bemudagrass yield on a Glossarenic Plinthic Paleudalf responded to application of 310 kg N ha–1. Adeli et al. (2005) reported increases in DM yield and crude protein with application of up to 450 kg N ha–1 on a silty clay soil in Mississippi. Despite the benefits of increasing N rates on bermudagrass yields, N recovery is usually decreased as N rates increase (Osborne et al., 1999; Prine and Burton, 1956). Mathias et al. (1973) indicated that N recovery was highest at 240 kg N ha–1. Eichhorn (1989) reported that maximum economic yields of bermudagrass are obtained with N rates of 135 kg ha–1. Efficient N application and optimization of N rates are keys for more sustainable pasture production systems.

Ammonium nitrate has been a major N source used on pastures in the USA. Because of rising costs of natural gas used in N fertilizer synthesis, fuel for N transport, and potential explosive properties, AN has become a scarcer N source for forage production. Alternatively, urea utilization has increased because of its relative lower costs of production and transport, and represents a more economical alternative to supply pastures with N than AN. Fluid fertilizers (i.e., UAN) have also been frequently used in forage grass systems. However, different N sources may not be equally effective when applied to established Coastal bermudagrass pastures. Burton and Hanna (1995) reported that Coastal bermudagrass responded similarly to anhydrous ammonia, AS, and AN in terms of percentage crude protein, N recovery, and yields, while urea was less effective. Sloan and Anderson (2001) observed that urea effectiveness depended on soil characteristics. These latter authors indicated that urea was as effective as AN on a calcareous Ships clay (Chromic Hapludert), but on an acidic Lufkin fine sandy loam (Oxyaquic Vertic Paleustalf), yield of bermudagrass pastures fertilized with urea were lower than when fertilized with AN. In a greenhouse study, Picchioni and Quiroga-Garza (1999) observed no significant differences from urea, AS, and AN on bermudagrass DM production. Osborne et al. (1999) indicated that at N rates up to 240 kg ha–1 N recovery by bermudagrass was usually greater for AN (88% N recovery) compared with urea (34% N recovery).

When surface-applied, urea can release significant amounts of N by volatilization of NH3. Increased temperature and soil CaCO3 concentrations have been shown to increase NH3 volatilization (Fenn and Kissel, 1974). Several compounds have been proposed to inhibit NH3 volatilization. For instance, ammonium thiosulfate (ATS) and calcium chloride may reduce N loss from urea fertilizers (Sloan and Anderson, 2001). Goos (1985) indicated that the addition of ATS (1–10% vol/vol) to liquid UAN significantly reduced nitrification and retarded urea hydrolysis under laboratory conditions. According to Goos and Fairlie (1988), efficiency of ATS in inhibiting urea hydrolysis depends on the method of fertilizer application (i.e., droplet size), soil type, and environmental conditions after application. Fenn et al. (1981) suggested that Ca–N fertilizer mixtures may reduce ammonia volatilization due to precipitation of CaCO3, and subsequent retention of ammonium by the soil cation exchange sites.

Although extensive research has documented response of bermudagrass to high N rates, the effects of fertilizer sources on N recovery are not well understood. In addition, soil type and environmental factors may play an important role in N uptake efficiency and subsequent forage production. The objective of this study was to investigate the effectiveness of various N sources and rates relative to Coastal bermudagrass DM production and N uptake efficiency.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The studies were conducted at the Texas A&M University Agricultural Research and Extension Center at Overton on established Coastal bermudagrass pastures on two soils: Lilbert loamy fine sand (loamy, siliceous, thermic Arenic Plinthic Paleudult, 32°19'08'' N; 94°58'14'' W) and Gallime fine sandy loam (fine-loamy, siliceous, thermic Glossic Paleudalf, 32°17'28'' N; 95°02'20'' W). Initial soil samples were analyzed for pH in a 1:2 (v/v) soil: water suspension (van Lierop, 1990; Table 1). Extractable nutrient cations were determined using ammonium acetate–ethylenediaminetetraacetic acid (NH4Oac–EDTA; Hons et al., 1990) followed by analysis on a Perkin Elmer model 1100B atomic absorption spectrophotometer (Norwalk, CT). Phosphorus was determined colorimetrically using stannous chloride reduction and analysis on a Bausch and Lomb model 2000 spectrophotometer (Rochester, NY). Hot-water extractable B was determined using the azomethine–H method (John et al., 1975) on the same colorimeter. Major variations between the two soils were in cation exchange capacity (CEC), Mg, Fe, Cu, and B concentrations.


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  		Table 1. Pre-treatment soil chemical attributes in the 0- to 15-cm depth.

 
Limestone [effective calcium carbonate equivalence (ECCE) of 60%] was surface-applied at 1.5 Mg ha–1 to the Lilbert soil before initiation of the study. Both soils received annual applications of approximately 50 kg P ha–1 as triple superphosphate during the 3-yr study. Potassium was applied at 335, 186, and 372 kg K ha–1 in Years 1, 2, and 3, respectively. All plots received 5 kg Zn ha–1 and 0.1 kg Mo ha–1, except for the treatment denoted as "–Zn,–Mo". Simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] and paraquat [1,1'-dimethyl-(4,4'-bipyridinium)] were used for weed control and diazinon 14G [2-chloro-4,6bis-(ethylamino)-S-triazine] for fire ant control.

The treatments consisted of the following N sources and combinations with S, B, and Ca: (i) UAN, (ii) UAN without Zn and Mo (UAN, –Zn, –Mo), (iii) UAN + ATS, (iv) UAN + AS, (v) urea + Ca, (vi) UAN + B, (vii) urea, (viii) AN, and (ix) AS. Sulfur was applied with UAN–N in an N/S ratio of 2.8. Treatment 5 was a combination of Ca and urea–N at a 0.34:1 ratio. Boron was applied with UAN at a rate of 0.45 kg ha–1.

Granular fertilizers were surface-applied and fluid fertilizer treatments were surface dibble-banded at 36-cm spacing between bands. Nitrogen rates of 0, 45, 90, and 135 kg ha–1 (0, 40, 80, and 120 lb N acre–1, respectively) were applied for each bermudagrass regrowth. Individual plot size was 3 by 6 m, and a 2-m alley was placed between blocks. Plots were clipped with a Hege 211B forage plot harvester (Waldenburg, Germany) with a 1.5-m cutter bar. Forage plots were cut at 5-cm stubble height at approximately 4 to 5 wk of regrowth during this 3-yr study. Dry matter yields were recorded and subsampled for chemical analysis.

Plant tissue was analyzed for total N by microKjeldahl digestion followed by analysis using a Technicon Autoanalyzer II (Technicon Instruments Corp., Tarrytown, NY). Nitrogen uptake by Coastal bermudagrass was used to determine the effectiveness of various N sources and rates. Soil samples were collected from depths of 0 to 5, 5 to 15, 15 to 30, 30 to 45, 45 to 60, and 60 to 90 cm at termination of the study. Samples were dried at 60°C for 48 h, screened to 0.85 mm, and analyzed for pH in 1:2 (v/v) soil/water suspension (van Lierop, 1990). Soil NO3–N was determined using 2 M KCl as the extractant at a soil/solution ratio of 1:4 (w/v) and shaking for 30 min on a reciprocating shaker (Bremmer and Keeney, 1965). Filtered extracts were analyzed using colorimetric auto analyzer techniques (Technicon Industrial Systems, 1977a, 1977b). Nitrogen uptake efficiency was calculated according to the following equation.

Formula 1[1]
Average monthly rainfall collected at 32°17'41'' N; 94°58'33'' W across the 3-yr study is presented in Fig. 1 . Average monthly temperature ranged from 8.7°C (December) to 27.9°C (July). Frosting temperature generally occurs in mid-November and occasionally extends until April.


Figure 1
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  		Fig. 1. Monthly rainfall distribution. Line represents the 38-yr average rainfall.

 
Statistical Analysis
Treatments were applied in triplicate in a two-way factorial arrangement using a randomized complete block design. The main factors were N sources (n = 9), N rates (n = 4), and soil type (n = 2). All data were analyzed using the PROC MIXED procedure of SAS (SAS Institute, 2001). Nitrogen rates and sources were fixed variables and field replicates (n = 3) were random variables. Year was a subplot tested using repeated measures (Littell, 1989). Main effects and the interactions between N source and rates for each soil were evaluated. Mean separation was performed using the SAS least square means test (PDIFF) at P ≤ 0.1, unless indicated otherwise. The effect of N rates on DM production was evaluated using orthogonal polynomial contrasts.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Dry Matter Yields
The main effects affecting bermudagrass DM yields are presented in Table 2. Bermudagrass yields increased from Year 1 to 3. Rainfall in Year 1 was 15% lower than the 38-yr average (Fig. 1) and only three harvests were obtained. In Years 2 and 3, normal rainfall conditions allowed four and five harvests, respectively. Unfertilized control plots on the Gallime soil produced 4117, 5854, and 7933 kg DM ha–1 in Years 1, 2, and 3, respectively. Yields from control plots increased from 3669 kg DM ha–1 in Year 1 to 6685 and 7718 kg DM ha–1 in Years 2 and 3, respectively on the Lilbert soil. That indicated an increase of 93 to 110% in yields from Year 1 to Year 3 in the Gallime and Lilbert soils, respectively. Although Coastal bermudagrass has been suggested to be drought tolerant, lack of rainfall at the initiation of the study reduced yields considerably at both sites. Greater yields in Years 2 and 3 also could be due to more frequent harvesting, which may favor bermudagrass production. Clapp et al. (1965) showed that increasing frequency of defoliation up to 23 cuttings per season resulted in greatest Coastal bermudagrass yields. Since no undesirable effects on stand were observed when bermudagrass plots were subjected to more frequent harvesting in Year 3, our results indicated a favorable root reserve to support plant regrowth.


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  		Table 2. Analysis of variance for the effects of year, soil, source, and rate of N on bermudagrass dry matter yields during a 3-yr period.

 
The interaction between N source, N rate, and soil was not significant (Table 2), so only the main factors are presented. Nitrogen rates significantly increased DM production at both experimental sites (Fig. 2 ). Compared to unfertilized control plots, application of 45 kg N ha–1 for each regrowth increased bermudagrass yields from 2 to 2.7 times in the Gallime soil, and from 1.6 to 2.2 times in the Lilbert soil. Orthogonal contrasts showed a quadratic trend (P ≤ 0.0001) in DM production with increasing N rates (Fig. 2). Maximum bermudagrass yields were, in general, obtained at the 90 kg N ha–1 rate. In Year 2, however, yields on the Lilbert soil showed a linear response to N rate and maximum bermudagrass production was achieved at the 135 kg N ha–1 rate. Decreasing yield response to high N application rates suggested that bermudagrass was not efficiently using additional N. Increased N level may affect morphological characteristics of Coastal bermudagrass (Prine and Burton, 1956). Frequent clipping associated with high N rates may decrease root yields (Carreker et al., 1978) and, consequently, reduce bermudagrass response to N fertilizer application. Our results suggested that the root system was not able to supply adequate nutrients and moisture for additional top growth at high N rates. The lack of yield response to high N rates may also be related to increased soil acidity due to high N fertilizer application rates, particularly for the AS treatments.


Figure 2
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  		Fig. 2. Coastal bermudagrass dry matter (DM) yield response to N rates for Gallime and Lilbert soils.

 
The interaction "N source x year x soil" was significant (Table 2), suggesting that the effectiveness of various N sources was largely influenced by soil type and rainfall distribution across the 3-yr study. Responses of Coastal bermudagrass to N sources were significant on the Gallime soil in Years 1 and 2 (Table 3). Conversely, bermudagrass production on the Lilbert soil did not respond significantly to the various N sources. Osborne et al. (1999) also observed that yield responses to N sources were site-dependent. Anderson (1987) observed no significant differences in bermudagrass yield response to N sources on an acid (pH = 4.9) sandy soil in Texas. Overall yields were generally greater on the Gallime soil compared to those on the Lilbert soil. Possibly, the greater CEC in the Gallime soil (Table 1) resulted in increased soil moisture and N retention and, consequently, higher bermudagrass yields.


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  		Table 3. Response of Coastal bermudagrass dry matter yields to various N sources on Gallime and Lilbert soils.

 
On the Gallime soil, AN or AS generally produced greater DM yields compared with UAN and urea. Although urea produced DM yields similar to AN and AS in Year 1, bermudagrass production in Year 2 was significantly lower with urea than with AN and AS. Compared to UAN and urea, cumulative yields across the 3-yr study were approximately 5% greater for AS and 9% for AN (Table 3). Because of the daily high temperatures (up to 34°C in August) during the growing season, ammonia volatilization from urea could potentially be significant in our study sites. Crop response to N source suggested that loss of ammonia may have occurred to some extent in the Gallime soil. Our results are consistent with a previous study by Burton and Jackson (1962), who observed that AN, AS, UAN, and urea gave relative Coastal bermudagrass hay yields of 100, 96, 92, and 82%, respectively. Westerman et al. (1983) reported that bermudagrass yields were generally 4 to 15% lower with urea than with AS and UAN. Osborne et al. (1999) observed larger differences in bermudagrass production with N sources in a silt loam soil in Oklahoma, and also found that bermudagrass yields were 23 to 29% greater when fertilized with AN than those with urea.

Addition of S to UAN, as AS and ATS had no effect on DM yields, indicating that S concentrations were probably adequate in both soils for 3 yr of bermudagrass production. Similarly, Ca and B added to urea showed no effect on bermudagrass production compared to urea alone. This suggested that Ca addition to urea at equivalent ratios of 0.34:1 (Ca/urea–N) failed to improve fertilizer efficiency. Similarly, Anderson (1987) found no significant yield differences between urea + Ca and urea alone. Hot-water soluble B levels of 0.39 and 0.18 mg kg–1 in the Gallime and Lilbert soils (Table 1), respectively, appear to be adequate for bermudagrass production. No significant yield differences were reported for Coastal bermudagrass when using urea or UAN solution, indicating that the efficiency of liquid fertilizer was comparable to the solid form. However, as observed by Walker et al. (1979), yields with UAN were lower than those fertilized with AN and AS.

Forage Nitrogen
Nitrogen concentrations in Coastal bermudagrass were increased by higher N rates (Fig. 3 ). For all sources, highest plant N concentrations occurred with the highest yields at the 135 kg ha–1 N rate, suggesting that plant N was not greatly diluted by the increased DM production. At the 45 kg ha–1 N rate per cutting, UAN and UAN + ATS resulted in lower N concentrations in plant tissue in Year 1 (Fig. 3a), while in Year 2, UAN + B showed the lowest N concentration (Fig. 3b). At the highest N rate (135 kg ha–1 regrowth–1), lowest plant N concentrations were, in general, observed for urea treatments.


Figure 3
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  		Fig. 3. Plant N concentrations in Coastal bermudagrass fertilized with various N sources for Gallime and Lilbert soils combined. Horizontal line indicates plant N concentration for the unfertilized control plots. UAN = urea–ammonium nitrate solution, ATS = ammonium thiosulfate, AS = ammonium sulfate, and AN = ammonium nitrate. Bars indicate standard deviation.

 
Nitrogen uptake was significantly affected by N rate and by source (Table 4). Nitrogen uptake response to N sources was quite variable across years. Ammonium nitrate and AS generally resulted in greater N uptake, however depending on the year and rate, other N sources showed similar N uptake. For instance, in Years 1 and 3, UAN blended with AS resulted in similar N uptake as AN and AS. In contrast, at the highest N rate (135 kg ha–1) urea always exhibited the lowest N uptake. Regardless of the source, lowest N removal was observed in Year 1 mainly because only three bermudagrass cuttings were obtained. Over the same period in Years 2 and 3, more frequent harvesting favored greater N uptake.


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  		Table 4. Nitrogen uptake of Coastal bermudagrass fertilized with various N sources at rates of 0, 45, 90, and 135 kg N ha–1.

 
Relative Efficiency of Nitrogen Sources
Because both sites showed similar trends in N uptake efficiency, only results from the Gallime soil are discussed. There was a significant interaction of year and N source on N uptake efficiency. Regardless of the year, AN and AS showed greater N uptake efficiency (Fig. 4 ). Similarly, in Years 1 and 3, UAN + AS resulted in N uptake efficiency comparable with the latter two N sources. In general, UAN and urea showed the lowest efficiency of N uptake. This is consistent, but greater than N uptake efficiencies reported by Osborne et al. (1999), who showed N recoveries by bermudagrass varying from 29 to 45% for AN and from 16 to 27% for urea. Matocha et al. (1973) also observed N recovery percentages by Coastal bermudagrass increased in the following order: AS > AN > urea. However, N recoveries up to 88% observed in our study were much greater than reported by others. For instance, Westerman et al. (1983) observed relatively lower efficiency of N uptake with AS (37–48%), UAN (45–36%), and urea (31–38%). According to Osborne et al. (1999), repeated (4–5) harvesting of bermudagrass before anthesis may lead to high plant N recoveries and improved forage N use efficiency.


Figure 4
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  		Fig. 4. Relative N uptake efficiency for Coastal bermudagrass fertilized with various N sources for Gallime and Lilbert soils combined. UAN = urea–ammonium nitrate solution, ATS = ammonium thiosulfate, AS = ammonium sulfate, and AN = ammonium nitrate. Same letters are not significantly different at P ≤ 0.05.

 
There were no significant differences in N efficiency between UAN and urea treatments either applied alone or blended with secondary nutrients. Addition of Ca to urea did not increased bermudagrass N uptake. Although ATS was reported to reduce nitrification of UAN by as much as 65% (Goos, 1985), our results showed no significant effect of UAN + ATS on N recovery. These differing results may be related to the controlled-conditions laboratory study on neutral to alkaline soils (Goos, 1985) and field research on acid soils.

For any given source, N uptake efficiency was greater in Years 1 and 2, and was significantly reduced in Year 3. The highest N efficiency was observed in Year 2, in which AN and AS resulted in 80 and 88% N recovery, respectively.

Estimated N uptake efficiency exceeded 61, 71, and 53% at the 45-kg ha–1 N rate in Years 1, 2, and 3, respectively (Fig. 5 ). However, percentage recovery of N decreased as N rates increased for all sources. In Year 1, although the highest N rate gave lower N recovery, there were no statistical differences between the 45- and 135-kg ha–1 N rate (Fig. 5). Differences in N uptake efficiency among N rates were more evident in Years 2 and 3.


Figure 5
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  		Fig. 5. Nitrogen uptake efficiency as a function of N application rate. Same letters within years are not significantly different at P ≤ 0.05.

 
For each kilogram of applied N, AN and AS produced greater yields (kg DM kg–1 N) than UAN and urea at the 45- and 90-kg ha–1 N application rates (Table 5). However, at the 135-kg N ha–1 rate, N sources showed similar responses, except for AS, which exhibited the lowest production efficiency. That is probably due to excessive soil acidity caused by repeated application of AS as the sole source of N. At lower rates (~45 kg ha–1), AN and AS produced better yields than the UAN and urea treatments. At increasing N rates, our results indicated that differences in production efficiency among the N sources became less evident.


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  		Table 5. Production efficiency of various N sources applied at different rates to Coastal bermudagrass plots.

 
Soil Acidity and Nitrate-Nitrogen Concentrations
Significant changes in soil pH associated with N treatments were observed at the completion of the study. Soil pH was significantly affected by N sources and rates at both sites. By increasing N rates, soil pH was markedly decreased, especially at soil depths >15 cm (Fig. 6 ). Averaged across soil depths, at completion of the experiment soil pH at the highest N level was 5.4, while unfertilized plots exhibited pH near 5.9. Yields and nutrient uptake efficiency may have been limited by low soil pH (<5.5), especially at the high N rates. According to Burton and Hanna (1995) and Haby et al. (1979), soils with pH below 5.5 should be limed for adequate Coastal bermudagrass production.


Figure 6
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  		Fig. 6. Changes in soil pH in response to N fertilizer application rates of 1, 45, 90, 135 kg N ha–1 harvest–1. Three harvests were made in Year 1, four in Year 2, and five in Year 3.

 
Strongest soil acidity at the 0- to 45-cm depths was caused by the AS treatments (Tables 6 and 7). Especially at the 0- to 15-cm soil depth for the Gallime soil (Table 6) and at the 0- to 60-cm for the Lilbert soil, pH for the AS treatments were ≤5.0, and to some extent, may have restricted forage growth. No differences in soil pH due to different N sources were found at depths >45 cm for the Gallime soil (Table 6). On the Lilbert soil that had a lower CEC, the entire soil profile was affected by AS applications (Table 7). In general, addition of Ca to urea in our field studies did not decrease soil pH as observed by Fenn et al. (1981) in laboratory studies. Contrary to the results reported by Belesky and Wilkinson (1983), urea did not produce a significant lasting effect on soil pH.


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  		Table 6. Effect of N sources on soil pH at the 0- to 90-cm depth on a Gallime soil.

 

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  		Table 7. Effect of N sources on soil pH at the 0- to 90-cm depth on a Lilbert soil.

 
Although increasing N fertilizer rates is expected to increase residual N, NO3–N concentrations at the 0- to 90-cm soil depth did not respond to N sources and rates (data not shown). On average, soil NO3–N levels varied from 0.2 to 0.4 mg kg–1 for the control treatments to 0.2 to 0.3 mg kg–1 for the highest N rate. At the current production level, N demand by Coastal bermudagrass is likely limiting NO3–N accumulation in the soil. Nitrate–N concentrations across the 0- to 90-cm soil depth showed no evidence of accumulating to a potentially environmental threatening level in response to N fertilization.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Bermudagrass yield response to N sources likely varies according to the soil characteristics and rainfall distribution. In one of our study sites, AN and AS resulted in increased yields, N uptake, and fertilizer efficiency compared with urea and UAN. In the other experimental site however, UAN and urea were as effective as AS and AN. Addition of S to UAN as either ATS or AS did not significantly affect forage yields and N uptake by Coastal bermudagrass on these two soils. Similarly, Ca blended with urea did not increase fertilizer efficiency compared with urea alone. For all sources, increasing N rates from 45 to 135 kg ha–1 resulted in greater yields, except in Year 3, which showed no significant differences between 90 and 135 kg N ha–1 rates. Although N fertilizer efficiency was reduced as N rate was increased, it appeared that the history of previous applications and soil acidity were the main factors controlling plant N recovery. Our results showed that as much as 88% of the applied N can be potentially taken up by bermudagrass. Soil acidity should be carefully monitored, especially when annually applying N sources such as AS that can significantly reduce soil pH. Selection of N source and rates should be carefully planned to avoid detrimental effects on soil acidity and, consequently, fertilizer efficiency. On low-buffer capacity, acid sandy soils, use of AS as the sole source of N throughout the growing season should be limited. In addition on these soils where high rates of N are applied, a diligent liming program is needed to help maintain soil pH in a favorable range for Coastal bermudagrass production.


   ACKNOWLEDGMENTS
 
Partial funding support for this research was provided by the Fluid Fertilizer Foundation 2805 Claflin Road, Suite 200, Manhattan, KS 66502.


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





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