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Published online 27 April 2005
Published in Agron J 97:690-697 (2005)
DOI: 10.2134/agronj2004.0156
© 2005 American Society of Agronomy
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Production Papers

Tillage and Urea Ammonium Nitrate Fertilizer Rate and Placement Affects Winter Wheat following Grain Sorghum and Soybean

K. W. Kelley* and D. W. Sweeney

Kansas State Univ., Southeast Agric. Res. Cent., P.O. Box 316, Parsons, KS 67357. Kansas Agric. Exp. Stn. Contribution no 040397-J

* Corresponding author (kkelley{at}oznet.ksu.edu)

Received for publication June 10, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
In the eastern Great Plains, winter wheat (Triticum aestivum L.) is often rotated with other crops to diversify cropping systems. In these multicropping systems, wheat typically is planted with conservation tillage methods, but previous crop residues influence fertilizer N management. This field study was conducted from 1992 through 2001 in southeastern Kansas on a Parsons silt loam soil (fine, mixed, thermic, Mollic Albaqualf). The objectives were to determine effects and interactions of previous crop {grain sorghum [Sorghum bicolor (L.) Moench] and soybean [Glycine max (L.) Merr.]}, tillage system [reduced tillage (RT) and no-tillage (NT)], N rate (67 and 134 kg ha–1), and preplant placement (surface-broadcast and subsurface-knife) of urea ammonium nitrate solution (UAN, 280 g kg–1) on wheat grain yield, yield components, and plant N uptake in a 2-yr cropping rotation. Wheat yields averaged 3.39 Mg ha–1 following soybean compared with 2.90 Mg ha–1 following grain sorghum. Tillage effects on grain yield were smaller than other treatment factors, averaging 3.23 Mg ha–1 for RT and 3.06 Mg ha–1 for NT. Grain yields were greatest in all cropping systems for the high-N-rate subsurface-knife treatment. Plant N uptake responses indicated that grain yield differences were primarily related to greater immobilization of both fertilizer and soil N following grain sorghum, compared with soybean, and to better utilization of subsurface-knifed N than surface-broadcast N. Results indicate that wheat yield potential is more strongly influenced by previous crop, fertilizer N rate, and N placement method than tillage system.

Abbreviations: NT, no-tillage • RT, reduced tillage • UAN, urea ammonium nitrate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN THE EASTERN GREAT PLAINS, winter wheat often is planted after a summer crop, such as soybean, corn (Zea mays L.), or grain sorghum to diversify cropping systems. Previous crop residues typically are incorporated with RT before wheat planting. Improved equipment technology, however, has made NT planting more feasible in high-residue conditions. In the Midwest, nearly 50% of fall-seeded winter wheat acreage is planted with RT or NT (CTIC, 2002). Possible benefits associated with planting wheat NT include less soil erosion, improved soil quality, greater soil moisture, and reduced input costs (Casady and Massey, 2000).

Leaving previous crop residues near the soil surface, such as in RT and NT cropping systems, affects fertilizer N management for winter wheat (Westfall et al., 1996). Several mechanisms have been shown to influence fertilizer N efficiency in high-residue cropping systems. Decomposing crop residues with wide C/N ratios can immobilize significant amounts of surface-applied fertilizer N and/or slow mineralization of soil N (Rice and Smith, 1984; Knowles et al., 1993; Smith and Sharpley, 1993). Several researchers have reported lower wheat yields and a greater fertilizer N requirement for wheat planted after a high-residue crop, such as grain sorghum, compared with wheat planted after soybean and have attributed those results to greater immobilization of N after grain sorghum (Hargrove et al., 1983; Sanford and Hairston, 1984; Staggenborg et al., 2003).

When urea-containing fertilizers, such as UAN, are applied to the soil surface of high-residue cropping systems, ammonia (NH3) volatilization losses can occur (McInnes et al., 1986). Placing fertilizer N below the soil surface avoids potential N volatilization losses and increases fertilizer efficiency (Rao and Dao, 1996). In the central Great Plains, Schlegel et al. (2003) reported that fall or spring subsurface placement of UAN with a point-injector applicator was more profitable than topdressing UAN in the winter or spring. However, subsurface N placement of fertilizer N in conservation tillage systems requires more specialized fertilizer equipment than surface broadcast N applications.

Other dynamics of conservation tillage systems have been reported in various wheat-producing regions. In Canada, Soon et al. (2001) showed that N fertilizer recommendations should allow for greater mineralization of organic N under NT than conventional tillage when wheat follows a legume crop. In the Rolling Plains of Texas, dryland and irrigated wheat yields were less in RT compared with conventional tillage systems because of decreased plant populations and N deficiency caused by immobilization (Bordovsky et al., 1998). In the southeastern Coastal Plain soils, Frederick and Bauer (1996) found that using conservation tillage for wheat production has the potential to increase yields if prolonged periods of drought stress occur before flowering. In the southern Great Plains, however, tillage had little affect on wheat yields in a dryland winter wheat–grain sorghum–fallow rotation (Unger, 1994). In the Pacific Northwest, Camara et al. (2003) demonstrated that despite beneficial effects on soil properties, conservation tillage was less productive than conventional moldboard plowing because of the lack of grassy weed control in conservation tillage systems. In a wheat–fallow monoculture in the Great Plains, Carr et al. (2003) showed that cultivar recommendations may be extended from conventional tillage to RT and NT systems when hard red spring wheat is grown. In Kansas, Roth et al. (2000) evaluated allelopathic effects of sorghum on winter wheat under several tillage systems and found that the effect of sorghum residue on the following wheat crop depends on the degree of plant residue decomposition.

Although numerous studies have reported on the influence of previous crop, tillage, or fertilizer N on wheat yields, additional information is needed for the conservation tillage and cropping systems of the more humid eastern Great Plains wheat-producing region. In addition, few studies in this region have evaluated subsurface placement of fertilizer N in multicropping systems. The objectives of this research were to evaluate the effects of previous crop, tillage method, fertilizer N rate, and preplant placement of UAN on wheat grain yield, yield components, and plant N uptake. Also, because of likely interactions with environment, treatment effects were evaluated over an extended period of five complete cropping cycles.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
This field study was conducted at the Southeast Agricultural Research Center of Kansas State University near Parsons (37° N lat, 95° W long) from 1992 through 2001. The soil was a Parsons silt loam (fine, mixed, thermic, Mollic Albaqualf). Initial chemical characteristics of the surface soil (0- to 15-cm depth) were pH, 6.8 (1:1 soil/water); Bray P1, 40 kg P ha–1; exchangeable K (1 M ammonium acetate extract), 180 kg K ha–1; and organic matter, 20 g kg–1. All soils were analyzed according to North Central recommended procedures (Brown, 1998). The previous crop before study establishment was soybean.

The experimental design was a split-plot arrangement of a randomized complete block with four replications. Main plots consisted of a 2 x 2 factorial combination of two previous crops (grain sorghum and soybean) and two tillage methods [RT (tandem disk and/or field cultivator) and NT]. A 3-m border strip separated each of the four main-plot treatments to prevent possible residue contamination between plots. Subplots (3.0 by 12.2 m) consisted of a 2 x 2 factorial arrangement of two N fertilizer rates [67 and 134 kg N ha–1] and two application methods (surface-broadcast and subsurface-knife) in addition to a zero-N control. Treatments were imposed on the same plots each year of the study. Wheat followed each of the previous cropping systems, with double-crop soybean following wheat in all cropping systems, giving three crops in a 2-yr rotation.

Grain sorghum and soybean crops were planted in 76-cm row spacing in the spring of 1992, 1994, 1996, 1998, and 2000, using either conventional tillage (disc–chisel–disc–field cultivate) or NT. Grain sorghum was fertilized with 112 kg N ha–1 as a subsurface-knifed application. Grain sorghum and soybean also received 26 kg P ha–1 and 62 kg K ha–1 as a broadcast application. Weed control in previous crops followed recommended herbicide rates. Grain sorghum was harvested in mid-September, and soybean was harvested in early October. After grain sorghum harvest, stubble was rotary-mowed to facilitate preplant fertilizer N applications. Previous crop residues in RT systems were incorporated with disc tillage before fertilizer applications. In addition, all NT plots received 0.56 kg a.i. ha–1 of glyphosate [(N-phosphonomethyl)glycine] before wheat fertilizer treatments to control emerged winter annual weeds.

Urea ammonium nitrate solution (280 g N kg–1) was the fertilizer N source for wheat. Knifed N treatments were banded on 38-cm centers at a depth of 10 to 15 cm with a pull-type, coulter-knifed applicator equipped with seven shanks and a ground-driven piston pump. Broadcast N treatments were applied with a tractor-mounted compressed air sprayer with six flat-fan nozzles on 50-cm spacing. Phosphorus (26 kg P ha–1 as triple superphosphate) and K (62 kg K ha–1 as muriate of potash) were broadcast-applied. Fertilizer treatments were applied in mid-October before wheat planting. In RT treatments, broadcast N was shallow-incorporated with a tandem disk in grain sorghum plots or a field cultivator in soybean plots. In NT systems, broadcast N was not incorporated.

An early maturing hard winter wheat (‘Karl’, 1993; ‘Karl 92’, 1995 and 1997; ‘Jagger’, 1999 and 2001) was planted near mid-October in 19-cm rows with a NT drill at a seeding rate of 112 kg ha–1. Although fall plant populations were not determined, plant stands seemed good to excellent each year. Weed control, when needed (1995 and 1997), was achieved with an early-spring application (0.84 kg a.i. ha–1) of bromoxynil octanoate ester (3,5-dibromo-4-hydrozybenzonitrile) plus MCPA isooctyl ester [(4-chloro-2-methylphenoxy) acetic acid] (Bayer Crop Sci., Kansas City, MO).

Before fall tillage operations, soil samples (0- to 15- and 15- to 30-cm depth) were randomly collected from each plot. Samples were dried, ground, and analyzed colorimetrically (Alpkem Corp., 1986) for nitrate N using a 1M KCL extraction. Plant samples were collected from a 0.25-m2 area located in the seven center rows 1 wk after flowering stage (Feekes' 10.5). Samples were dried at 60°C, weighed for dry matter determination, ground to pass a 1-mm screen, and digested using a sulfuric acid–hydrogen peroxide digest (Isaac, 1977) to determine N concentration. Whole-plant N uptake was calculated by multiplying plant N concentration by dry matter weight.

At physiological maturity a 1.8- by 9.1-m area from the center of each plot was harvested with a research plot combine. Yield values were adjusted to 125 g kg–1 moisture. A subsample of grain was taken to determine 1000-kernel weight and grain moisture. Kernels per head were determined on 20 randomly collected heads. Head density was determined by counting the number of head-bearing tillers from a 0.5- by 0.5-m area of each plot.

After wheat harvest, double-crop soybean (Maturity Group IV) was planted into previous wheat stubble by using either RT (disking) or NT. Weeds in double-crop soybean were controlled effectively with pre-emergent and postemergent herbicides.

Treatment effects were analyzed across years and by individual years with the MIXED procedure of SAS (Littell et al., 1996). All factors except rep were considered fixed. Year was treated as a strip-plot effect so that across years, the data were analyzed as a strip-split plot. Treatment means were compared by using Fisher's protected LSD (0.05).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Rainfall varied yearly over the 5-yr period, which influenced grain yield and plant N responses. Above-normal precipitation in late spring of 1993, 1995, and 1999 (Table 1) resulted in waterlogged soils that reduced grain yield potential. In 1997 and 2001, rainfall was more favorable for wheat growth and development. The variable rainfall patterns experienced during the study are typical for this wheat-producing area of the eastern Great Plains.


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  		Table 1. Monthly precipitation totals for Parsons, KS, for winter wheat growing seasons.

 
Wheat grain yield, yield components, and plant N uptake were influenced by previous crop, tillage system, fertilizer N rate, N placement method, and by various year interactions for the different treatment factors (Table 2). These year x treatment interactions were primarily influenced by the magnitude of response and the variability of the 2001 wheat crop. However, because of the potential impact of long-term data sets, the previous crop x tillage system x N rate x N placement interactions obtained for yield and plant N uptake when analyzed across years also can clarify the archetypal response that would be used for recommendations to producers.


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  		Table 2. Analysis of variance across years for the effect of previous crop, tillage system, and N fertilizer rate and placement method on grain yield, plant N uptake, and yield components of winter wheat.

 
Grain Yield Responses
Grain yields for wheat following soybean were greater than wheat following grain sorghum, except for 2001 (Fig. 1). In fall of 2000, soybean yields were very low (less than 0.5 Mg ha–1) because of low rainfall during the reproductive stage of plant development. Because of low soybean yields, more residual soil nitrate N was available before wheat planting (Table 3). In 2001, wheat yields following soybean at the high N rate were reduced significantly because the combined effect of high fertilizer N and greater-than-normal soil nitrate N resulted in more plant lodging (data not shown), which lowered overall yield compared with wheat following grain sorghum (interaction data not shown). Wheat yields for the 5-yr period averaged 3.39 Mg ha–1 following the low-residue soybean crop compared with 2.90 Mg ha–1 for wheat following the high-residue grain sorghum crop. Results agree with previously reported research conducted in eastern Kansas (Kelley, 1995; Staggenborg et al., 2003).



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  		Fig. 1. Effects of previous crop [grain sorghum (GS) and soybean (Soy)], tillage [reduced tillage (RT) and no-tillage (NT)], N rate [67 kg N ha–1 (67 N) and 134 kg N ha–1 (134 N)], and N placement [surface-broadcast (BC) and subsurface-knife (KN] on wheat yield for 1993, 1995, 1997, 1999, 2001, and 5-yr mean. Within each year, bars with the same letter are not statistically different at the 0.05 level according the protected LSD test.

 

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  		Table 3. Effects of previous crop and tillage on preplant residual soil nitrate N content, Parsons, KS, 1992–2000.

 
Even though results showed that grain yields were greater for wheat following soybean than wheat following grain sorghum, many producers in the eastern Great Plains still plant a portion of their wheat acreage after feed-grain crops because these crops typically are harvested in August and September, whereas full-season soybean mature in early to mid-October (35° to 40° N lat). Thus, producers have more time for tillage and preplant fertilizer applications before wheat planting.

Tillage effects on grain yield generally were of a smaller magnitude than other treatment effects. In 4 of 5 yr, grain yields were greater with RT than NT (Fig. 1). In 2001, yield differences between tillage systems were not consistent across previous crops for some fertilizer N treatments (interaction data not shown). For the 5-yr period, grain yields averaged 3.23 Mg ha–1 for RT and 3.06 Mg ha–1 for NT. Since grain yield differences between tillage systems were small when averaged across years (Fig. 2), the positive economic aspects of eliminating tillage operations, along with potentially less soil erosion, make NT planting of wheat a viable management practice for the multicropping systems of the eastern Great Plains.



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  		Fig. 2. Interactive effects of previous crop (grain sorghum and soybean), tillage (reduced till and no-till), N rate [67 kg N ha–1 (67 N) and 134 kg N ha–1 (134 N)], and N placement [surface-broadcast (BC) and subsurface-knife (KN)] on wheat yield averaged over 5 yr.

 
Fertilizer N rate influenced wheat yield in all years (Fig. 1). Grain yields averaged 2.82 and 3.47 Mg ha–1 for the low (67 kg N ha–1) and high (134 kg N ha–1) N rate, respectively. Averaged across years, at the lesser N rate, wheat following grain sorghum was more N deficient and yielded significantly less in both RT and NT systems compared with wheat following soybean (Fig. 2). At the greater N rate, grain yield differences between RT and NT were smaller than at the lesser N rate. In Texas, Knowles et al. (1993) showed similar wheat yield responses to fertilizer N in NT and conventional tillage systems involving grain sorghum and wheat residues.

Placement of UAN fertilizer affected grain yields in 4 of 5 yr, averaging 3.37 Mg ha–1 for subsurface-knife compared with 2.92 Mg ha–1 for surface-broadcast (Fig. 1). Averaged across years, grain yields were greatest in all cropping systems for the 134 kg N ha–1 subsurface knife treatment, averaging between 3.5 and 4.0 Mg ha–1, which was nearly 0.5 Mg ha–1 greater than for surface-broadcast at the greater N rate (Fig. 2).

Although results clearly showed that subsurface placement of N fertilizer has the potential to significantly increase wheat yields in both RT and NT systems when compared with surface-applied N, application costs and other factors may influence fertilizer management decisions. However, based on custom fertilizer application costs in Kansas (Beaton et al., 2003) during the study period ($13.60 ha–1 for subsurface and $8.65 ha–1 for broadcast), the 0.5 Mg ha–1 average yield increase ($58 ha–1 with winter wheat valued at $116 Mg–1) from subsurface-knifed N compared with surface-broadcast would more than offset the increased application cost. But, the land area that could be fertilized during the workday would be considerably less for subsurface-knifed compared with surface-broadcast because of typical differences in applicator widths. Thus, time constraints and additional labor costs likely would need to be included for subsurface N application. In NT systems, however, subsurface N placement also reduces the potential for N loss from surface runoff, which would provide significant environmental benefit.

Yield Component Responses
Grain yields were correlated with number of heads per square meter (r = 0.79, P = 0.05) and number of kernels per head (r = 0.78, P = 0.05). The number of heads per square meter was greater for wheat following soybean than for wheat following grain sorghum, except in 2001 (Table 4). Previous crop and fertilizer N rate had a greater effect on head density than did tillage. The number of heads per square meter increased as N rate increased and was greater for the subsurface, banded-knife application than for surface-broadcast N. The number of kernels per head increased with increased rate of N, but kernel weight often decreased, indicating yield compensation among individual components.


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  		Table 4. Main effects of previous crop, tillage, and N fertilization rate and method on wheat yield components, 1993–2001.

 
Plant and Soil Nitrogen Responses
Plant N uptake was significantly greater in all years for wheat following soybean compared with wheat following grain sorghum (Fig. 3). Over the 5-yr period, N uptake averaged 74 kg N ha–1 following grain sorghum compared with 93 kg N ha–1 following soybean. Plant N responses suggest that the main factor affecting yield differences between previous crops was related to greater N availability for wheat following soybean compared with wheat following grain sorghum.



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  		Fig. 3. Effects of previous crop [grain sorghum (GS) and soybean (Soy)], tillage [reduced tillage (RT) and no-tillage (NT)], N rate [67 kg N ha–1 (67 N) and 134 kg N ha–1 (134 N)], and N placement [surface-broadcast (BC) and subsurface-knife (KN)] on plant N uptake for 1993, 1995, 1997, 1999, 2001, and 5-yr mean. Within each year, bars with the same letter are not statistically different at the 0.05 level according the protected LSD test.

 
Tillage affected plant N uptake less than previous crop. Plant N uptake was greater with RT than NT, except for 2001 (Fig. 3). In 2001, plant N uptake was greater with NT than RT, especially for wheat following grain sorghum (interaction data not shown). For the 5-yr period, plant N uptake averaged 86 kg N ha–1 for RT and 81 kg N ha–1 for NT (Fig. 3). Except for 2001, tillage effects on plant N uptake were consistent across previous crops, averaging 96 kg N ha–1 following soybean with RT, 91 kg N ha–1 following soybean with NT, 77 kg N ha–1 following grain sorghum with RT, and 70 kg N ha–1 following grain sorghum with NT.

Plant N uptake increased with increasing N rate in all years, averaging 67 kg N ha–1 for the low N rate compared with 100 kg N ha–1 at the high N rate (Fig. 3). Although the optimum N rate for wheat following different cropping systems could not be determined from only two N rates, overall plant N responses suggest that the fertilizer N requirement was greater for wheat following grain sorghum than for wheat following soybean, especially when planted NT (Fig. 4). Other research in Kansas showed that wheat following grain sorghum required 21 kg ha–1 more N to maximize grain yields compared with wheat planted after soybean (Staggenborg et al., 2003). Kansas fertilizer N recommendations also are increased 34 kg N ha–1 for wheat following grain sorghum and 22 kg N ha–1 when planted NT (Leikam et al., 2003).



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  		Fig. 4. Interactive effects of previous crop (grain sorghum and soybean), tillage (reduced till and no-till), N rate [67 kg N ha–1 (67 N) and 134 kg N ha–1 (134 N)], and N placement [surface-broadcast (BC) and subsurface-knife (KN)] on plant N uptake averaged over 5 yr.

 
Similar to grain yield responses, plant N uptake in all years was greater for subsurface-knife application of UAN than for surface-broadcast N (Fig. 3), indicating greater utilization of applied N. In 1997, it was unclear why the differences in N uptake between application methods were greater than in other years. Plant N uptake for the 5-yr period averaged 72 kg N ha–1 for surface-broadcast and 95 kg N ha–1 for subsurface-knife. Plant N uptake in all cropping systems was greatest for the 134 kg N ha–1 subsurface-knife application (Fig. 4). When wheat followed grain sorghum, plant N uptake for the 67 kg N ha–1 subsurface-knife N application averaged nearly 70 kg N ha–1, whereas N uptake for the 134 kg N ha–1 surface-broadcast application averaged only 74 kg N ha–1. When wheat followed soybean, broadcasting N at 134 kg ha–1 resulted in about 15 kg ha–1 greater N uptake than knifing at 67 kg N ha–1. These results suggest that, in the high-residue cropping rotation, subsurface N was nearly twice as efficient as surface-broadcast N for N uptake by wheat.

Fall soil samples showed that NO3–N concentrations were less following grain sorghum than soybean (Table 3). Residual soil nitrate N concentrations for the 0- to 30-cm depth averaged nearly 21 kg N ha–1 following soybean compared with 8 kg N ha–1 following grain sorghum. Other researchers have shown that recovery of any mineralized N from sorghum residues is greater for the crop being grown the following summer (Wagger et al., 1985; Vigil et al., 1991). Because soybean was harvested in early October, some mineralization of N in soybean residue as well as N in soil organic matter probably occurred during the fall when soil temperatures were still warm. But complete mineralization of N in soybean residue likely did not occur until after wheat had matured (Power et al., 1986). In Kansas, current fertilizer N recommendations for wheat following soybean do not consider N credits to soybean (Leikam et al., 2003).

In high-residue conditions, ammonia volatilization losses can occur when urea-containing fertilizers, such as UAN, are surface-broadcast. In this study, ammonia loss likely was small in RT because UAN was incorporated by tillage after application. In the NT system, some volatilization may have occurred. However, loss most likely was relatively small because cooler air temperatures at the time of UAN application in early October, as well as normal or above-normal precipitation shortly after wheat planting, likely reduced possible NH3 volatilization losses.

Plant N uptake at Feekes' Growth Stage 10.5 was correlated (r = 0.67, P = 0.05) with grain yield, suggesting that the main factor affecting yield differences between various treatment factors was N availability. However, allelopathy effects from previous crop residues (Ben-Hammouda et al., 1995), such as grain sorghum, cannot be ruled out. In Kansas, Roth et al. (2000) showed that tilled sorghum residue often delayed development of the following wheat crop but did not affect grain yields whereas NT sorghum had little effect on stand establishment but frequently reduced grain yields of wheat. They felt that prompt tillage and adequate soil moisture before wheat planting would likely reduce harmful effects of allelopathic compounds from sorghum stubble. In our study, allelopathic effects were likely minimal because precipitation in September and early October probably stimulated early degradation of any allelopathic compound.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Wheat yields were strongly influenced by previous crop, averaging 3.39 Mg ha–1 following soybean compared with 2.90 Mg ha–1 following grain sorghum. But differences in grain yield due to tillage were small, averaging 3.23 Mg ha–1 for RT and 3.06 Mg ha–1 for NT. Thus, potential economic and environmental benefits should still make NT planting of wheat a viable management practice in multicropping systems. Fertilizer N rate and N placement method were important management factors, regardless of previous crop or tillage system. Grain yields generally increased with increasing N rate, averaging 2.82 and 3.47 Mg ha–1 for the low (67 kg N ha–1) and high (134 kg N ha–1) N rate, respectively. Grain yields were greatest in all cropping systems for the 134 kg N ha–1 subsurface-knife treatment, averaging between 3.5 and 4.0 Mg ha–1, which was nearly 0.5 Mg ha–1 greater than for surface-broadcast at the greater N rate. Plant N uptake by wheat at the flowering stage suggested that grain yield differences were primarily related to greater immobilization of both fertilizer and soil N following grain sorghum, compared with soybean, and to better utilization of subsurface-knifed N than of surface-broadcast N. Compared with surface-applied N, subsurface placement of N fertilizer has the potential to significantly increase wheat yields in both RT and NT systems while reducing the potential for N loss from surface runoff, especially in NT.


   ACKNOWLEDGMENTS
 
The authors express appreciation to Michael Dean, Bobby Myers, and David Kerley for their technical assistance during the experiment.


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




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K. W. Kelley and D. W. Sweeney
Placement of Preplant Liquid Nitrogen and Phosphorus Fertilizer and Nitrogen Rate Affects No-Till Wheat Following Different Summer Crops
Agron. J., June 5, 2007; 99(4): 1009 - 1017.
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