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Published in Agron J 99:1009-1017 (2007)
DOI: 10.2134/agronj2006.0240
© 2007 American Society of Agronomy
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Wheat

Placement of Preplant Liquid Nitrogen and Phosphorus Fertilizer and Nitrogen Rate Affects No-Till Wheat Following Different Summer Crops

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

Kansas State Univ., Southeast Agric. Res. Center, P.O. Box 316, Parsons, KS 67357

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

Received for publication August 23, 2006.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Because of improved equipment technology, many producers in the eastern Great Plains are planting winter wheat (Triticum aestivum L.) no-till (NT) into previous crop residues, but management of fertilizer N and P remains critical. This field study was conducted from 1998 through 2003 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 [corn, Zea mays L.; grain sorghum, Sorghum bicolor (L.); and soybean, Glycine max (L.) Merr.], preplant placement method of liquid N–P fertilizer [subsurface-knife (KN), surface-band (SB), and surface-broadcast (BC)], and fertilizer N rate (22, 45, 90, and 134 kg N ha–1) on NT winter wheat yield, yield components, and nutrient uptake in a 2-yr cropping rotation. Wheat yields averaged 3.73, 3.56, and 2.97 Mg ha–1 following soybean, corn, and grain sorghum, respectively. However, as fertilizer N rate increased, yield differences between previous crops decreased. Grain yields also were influenced by placement of N–P fertilizer, averaging 3.68 Mg ha–1 for KN, 3.40 Mg ha–1 for SB, and 3.19 Mg ha–1 for BC. Plant and grain N 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 corn, and to better utilization of KN N–P than surface-applied. Fertilizing with greater N rates applied as a subsurface band, especially if following grain sorghum, may be necessary to maximize NT wheat yield potential in the eastern Great Plains.

Abbreviations: BC, surface-broadcast • KN, subsurface-knife • NT, no-till • SB, surface-band • UAN, urea-ammonium nitrate solution


   INTRODUCTION
TOP
 NOTES
 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, or grain sorghum, to diversify cropping systems. Improved equipment technology has made NT planting more feasible in high-residue conditions. Possible benefits associated with planting NT include less soil erosion, reduced input costs, greater soil moisture, and improved soil quality (Unger and McCalla, 1980). But, the adaptation of conservation tillage systems changes the physical, chemical, and biological nature of soil (Doran, 1980).

Leaving previous crop residues near the soil surface, such as in NT, affects fertilizer N management for winter wheat (Westfall et al., 1996). 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 much of those results to greater immobilization of N after grain sorghum (Hargrove et al., 1983; Sanford and Hairston, 1984; Staggenborg et al., 2003).

Placement method of fertilizer N in NT cropping systems also influences N efficiency. When urea-containing fertilizers such as urea-ammonium nitrate solution (UAN) are broadcast-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 reduces 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 top-dressing UAN in the winter or spring. In the eastern Great Plains, Kelley and Sweeney (2005) showed that wheat grain yields were significantly greater with a preplant KN application of UAN compared with BC in both reduced and NT systems when wheat followed grain sorghum or soybean. Banding of liquid UAN fertilizer in strips of varying width on the soil surface has been shown to be intermediate between subsurface-banding and BC in terms of nutrient efficiency (Lohry, 1998). Deibert et al. (1985) reported SB application of UAN fertilizer to be as efficient for NT spring wheat as deep-band placement. Improved performance of UAN in SB application compared with BC has been attributed to increased nutrient concentration and diminished volatilization of ammonia from urea in solution.

In the eastern Great Plains region, soil tests often show that additional P fertilizer also is needed to optimize wheat grain yields (Fixen, 2006). Similar to N, P placement method in NT wheat has been a concern because of the relative immobility of soil P. In addition, soil test P level has been shown to have a marked effect on wheat response to placement method (Peterson et al., 1981; Randall and Hoeft, 1988). Banding of low rates of P fertilizer near the seed on soils testing low in available P has been shown to be more effective than broadcast applications of P fertilizer at the same rate (Fiedler et al., 1989). However, on soils containing medium to high levels of available P, the method of P application in NT wheat has been shown to be less important (Halvorson and Havlin, 1992). Applying both N and P fertilizer together in subsurface bands, referred to as "dual placement," has been shown to increase P uptake compared with separate fertilizer applications, and P efficiency from dual knife applications has been shown to be approximately equal to band application at seeding (Leikam et al., 1983; Sander et al., 1991). Stecker et al. (1988) found no significant interaction of tillage system and P placement method on winter wheat response to P application rates in western Nebraska.

Many wheat producers in the eastern Great Plains prefer to preplant apply both fertilizer N (all or a portion of the total N requirement) and fertilizer P together in one field operation to facilitate more rapid and timely planting. Additional research is needed in this region to determine the optimum placement method for applying preplant fertilizer N–P for NT wheat following different summer crops as well as the optimum fertilizer N rate. The objectives of this research were to evaluate effects of the previous summer crop, preplant placement method of liquid N–P fertilizer, and fertilizer N rate on NT wheat grain yield, yield components, plant nutrient uptake of N and P, and total grain N.


   MATERIALS AND METHODS
TOP
 NOTES
 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, 95° W) from 1998 through 2003. The soil was a Parsons silt loam (fine, mixed, active, thermic Mollic Albaqualf). Initial chemical characteristics of the surface (0- to 15-cm depth) were pH = 6.5 (1:1 soil/water), Bray P-1 = 17 mg kg–1, exchangeable K (1 M ammonium acetate extract) = 94 mg kg–1, and organic matter = 27 g kg–1. All soils were analyzed according to North Central recommended procedures (Brown, 1998). The previous crops before study establishment were wheat and double-crop soybean.

The experimental design was a split-plot arrangement of a randomized complete block with four replications. Main plots consisted of three previous crops (corn, grain sorghum, and soybean). Subplots (3.0 by 12.2 m) consisted of a 3 by 4 factorial arrangement of three fertilizer N–P placement methods (BC, SB, and KN) and four fertilizer N rates (22, 45, 90, and 134 kg N ha–1) in addition to two unfertilized control plots (subsurface-knifed and no knife). Phosphorus was applied with fertilizer N at a constant rate of 34 kg P ha–1, except for control plots. 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.

Corn, grain sorghum, and soybean crops were planted in 76-cm row spacing in the spring of 1998, 2000, and 2002, using conventional tillage (disc–chisel–disc–field cultivate). Corn and grain sorghum were fertilized preplant with 112 kg N ha–1 as urea. All previous crops also received 26 kg P ha–1 and 56 kg K ha–1 as a preplant broadcast application. Fertilizer for previous crops was incorporated with tillage before planting. Weed control in previous crops followed recommended herbicide rates. Corn was harvested in late August, grain sorghum was harvested in mid-September, and soybean was harvested in late September. After corn and grain sorghum harvest, stubble was rotary mowed to facilitate preplant fertilizer applications. In addition, all 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) and liquid ammonium polyphosphate (100 g N kg–1 and 150 g P kg–1) were the N and P fertilizer sources for wheat. Surface-broadcast N–P fertilizer treatments were applied with a tractor-mounted sprayer equipped with a positive displacement pump and flat-fan nozzles on 40-cm spacing. Surface-band N–P treatments were applied with the same sprayer as broadcast treatments, but drop-tubes and a flow regulator orifice was installed on each nozzle outlet to deliver liquid N–P fertilizer in a narrow band on 40-cm spacing. Subsurface-knifed N–P treatments also were banded on 40-cm centers at a depth of 10 to 15 cm with a pull-type, coulter-knifed applicator equipped with a ground-driven piston pump. Potassium (112 kg K ha–1 as potassium chloride) was broadcast applied to all plots, including control plots. Fertilizer treatments were applied in mid-October before wheat planting. Hard-red winter wheat (‘Jagger’) was planted with a Great Plains NT drill (model 1005NT, Great Plains Mfg., Salina, KS) on 24 Oct. 1998, 13 Oct. 2000, and 17 Oct. 2002 at a seeding rate of 112 kg ha–1.

Before fall fertilizer applications, soil samples (0- to 15-cm and 15- to 30-cm depth) were randomly collected from each main plot for residual nitrate-N and soil P content. Residual soil nitrate-N was analyzed colorimetrically (Alpkem Corporation, 1986) using a 1 M 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 and P concentration using a Technicon autoanalyzer (Technicon Industrial Systems, 1977). Whole-plant N and P uptake were calculated by multiplying plant N and P concentrations by dry matter weight.

At physiological maturity, a 1.8- by 10.3-m area from the center of each wheat 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 grain moisture, 1000-kernel weight, and grain N. Grain from a subsample was ground with a cyclone mill (Udy Corp., Fort Collins, CO) and analyzed for N concentration using the same procedure described for whole-plant samples. Total grain N was calculated by multiplying grain N concentration by grain yield. Kernels per head were determined on 20 randomly collected heads. Head density was determined by counting the number of head-bearing tillers from a 0.25-m2 area.

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). Grain yield response functions to fertilizer N–P were generated using the PROC REG procedure for linear and quadratic regressions.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Growing Conditions
Rainfall and temperature varied yearly during the 3-yr period (Table 1), which influenced grain and plant responses to previous crop and fertilizer treatments. In Year 1 (1998–1999), above-normal precipitation in October after planting and from April through June resulted in waterlogged soils that reduced grain yield potential. In Year 2 (2000–2001), rainfall was above average in October, drier and colder than normal conditions occurred in November and December, rainfall was above-normal in January and February, but spring growing conditions were drier than normal, which resulted in high grain yields. In Year 3 (2002–2003), precipitation was well below normal from October through February, but spring rainfall was timely during the reproductive stage, resulting in high grain yield similar to 2001. The variable rainfall and temperature patterns experienced during the study are typical for this wheat-producing area of the eastern Great Plains.


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  		Table 1. Total monthly precipitation and average monthly air temperatures at Parsons, KS, for winter wheat growing seasons.

 
Grain Yield Responses
The year interaction of previous crop with fertilizer N–P placement and fertilizer N rate was significant for grain yield (Table 2). Thus, yield responses to previous crop, fertilizer N–P placement, and fertilizer N rate are shown individually for each year in Fig. 1 . Nearly all regression equations were significant at P = 0.05, with R2 ≥ 0.94 (Table 3).


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  		Table 2. Analysis of variance for various parameters.

 

Figure 1
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  		Fig. 1. Influence of previous crop, N and P fertilizer placement method [subsurface-knife (KN), surface-band (SB), and surface-broadcast (BC)], and N rate on no-till wheat grain yield in 1999, 2001, and 2003 at Parsons, KS.

 

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  		Table 3. Relationship of fertilizer N rate (X) to grain yield of wheat within previous crop and fertilizer N–P placement treatments.

 
In 1999, grain yields across all fertilizer treatments averaged nearly 1 Mg ha–1 greater when wheat followed either corn or soybean compared with wheat following grain sorghum. Regardless of previous crop, wheat yields were highest when N–P fertilizer was applied as KN, intermediate for SB, and lowest for BC. Grain yields also increased with increasing fertilizer N rate, although the rate of increase varied with previous crop and fertilizer placement method. When wheat followed soybean, only a small yield increase occurred above the 90 kg N ha–1 rate when applied as KN.

In 2001, wheat yields following soybean averaged 4.38 Mg ha–1 compared with 3.55 Mg ha–1 following either corn or grain sorghum. However, at the highest fertilizer N rate, grain yield differences between previous crops were small, especially when applied as KN. Wheat yields following feed-grain crops increased with increasing N rate, regardless of fertilizer placement method, which was similar to 1999. But, wheat yields following corn and/or grain sorghum were highest when N–P fertilizer was applied as KN, averaging nearly 0.5 Mg ha–1 greater than when BC. Grain yields for wheat following soybean were influenced only slightly by fertilizer placement, and yield response to fertilizer N was small above the 90 kg N ha–1 rate when applied as SB or BC and declined at the highest fertilizer N rate when applied as KN.

In 2003, wheat yields averaged 4.40 Mg ha–1 following corn, 4.05 Mg ha–1 following soybean, and 3.62 Mg ha–1 following grain sorghum. Similar to 2001, grain yield differences between previous crops were small at the highest N rate when applied as KN. When wheat followed corn, grain yields showed little yield increase above the 90 kg N ha–1 rate when applied as SB or KN. Similar to 2001, wheat yield following soybean also declined with >90 kg N ha–1 when applied as KN, although yields continued to increase with increasing N rate when applied as SB or BC. When wheat followed grain sorghum, grain yields continued to increase with increasing N rate, regardless of fertilizer N–P placement method. Similar to 1999 and 2001, grain yields following grain sorghum were highest when N–P fertilizer was applied as KN, intermediate when SB, and lowest when BC.

The variable grain response to previous crop and fertilizer treatments was influenced by differences in residual soil nitrate-N levels between previous crops in some years (Table 4). In all years, residual soil nitrate-N levels in the 0- to 30-cm depth were significantly greater following corn and/or soybean than following grain sorghum. But, in the fall of 2000, soil nitrate-N was especially greater following soybean compared with other previous crops. In 2000, soybean yields were very low (<0.5 Mg ha–1) because of low rainfall during the reproductive stage of plant development. Because of low yields, more residual nitrate-N (nearly 50 kg N ha–1 in the 0- to 30-cm depth) was available before wheat planting. Thus, in 2001, wheat yields following soybean at the high-N rate were reduced when applied as KN 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. Scharf and Alley (1994) also reported that considerable wheat yield depression can occur in humid wheat production regions where residual soil nitrate-N levels are above normal and with the application of additional fertilizer N.


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  		Table 4. Soil P, nitrate-N, and organic matter (OM) content at two soil depths following different previous crops before no-till planted winter wheat at Parsons, KS.

 
Because of the residual soil nitrate-N influence on grain yields in some years, relative grain yields also were expressed as a function of soil plus fertilizer N to normalize the response to N fertilization (Fig. 2 ). For relative yield determination, all yields were expressed as a percentage of the N treatment with the highest yield in a given year. Regression curves showed that with KN application, a soil plus fertilizer N level of nearly 150 kg N ha–1 was needed to achieve a 95% or greater yield potential for wheat following corn and soybean. However, with surface-applied fertilizer, nearly 200 kg N ha–1 was needed to achieve >90% yield potential. In this study, the highest N rate (134 kg N ha–1) likely was insufficient for maximum yield potential when wheat followed grain sorghum, especially when fertilizer N–P was surface-applied. Results agree with other research conducted in Kansas, which showed that the fertilizer N requirement for wheat following grain sorghum was at least 20 kg N ha–1 greater than for wheat following soybean (Staggenborg et al., 2003).


Figure 2
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  		Fig. 2. Relative no-till wheat grain yield following corn, grain sorghum, and soybean as a function of soil + fertilizer N for the years 1999, 2001, and 2003 at Parsons, KS. Regression equations and significance for wheat following corn: [subsurface-knife (KN): y = 27.5 + 0.899x – 0.00286x2 (R2 = 0.96, P = 0.001); surface-band (SB): y = 29.8 + 0.746x – 0.0022x2 (R2 = 0.83, P = 0.001); surface-broadcast (BC): y = 31.4 + 0.617x – 0.0016x2 (R2 = 0.83, P = 0.001)]. For wheat following grain sorghum: [KN: y = 27.8 + 0.703x – 0.00167x2 (R2 = 0.91, P = 0.001); SB: y = 22.0 + 0.51x – 0.000518x2 (R2 = 0.83, P = 0. 001); BC: y = 24.9 + 0.317x – 0.000321x2 (R2 = 0.79, P = 0.01)]. For wheat following soybean: [KN: y = 16.7 + 1.07x – 0.00352x2 (R2 = 0.94, P = 0.001); SB: y = 35.7 + 0.575x – 0.00131x2 (R2 = 0.94, P = 0.001); BC: y = 42.1 + 0.417x – 0.000801x2 (R2 = 0.84, P = 0.001)].

 
Results in Fig. 2 showed that wheat yield potential was highest, regardless of previous crop, when N–P fertilizer was applied as KN, but application costs and other factors may influence fertilizer management decisions. On the basis of custom fertilizer application costs in Kansas (Beaton et al., 2003) during the study period (U.S.$13.60 ha–1 for subsurface and $8.65 ha–1 for broadcast), the 0.5 Mg ha–1 3-yr average yield increase ($73 ha–1 with winter wheat valued at $146 Mg–1) from KN-N compared with BC-N would more than offset the increased application cost. The land area that could be fertilized during the workday, however, would be considerably less for KN compared with BC application because of typical differences in applicator widths and speeds. Thus, time constraints and additional labor costs likely would need to be included for subsurface N–P application. In NT systems, subsurface fertilizer placement also reduces the potential for N loss from surface runoff, which would provide significant, but less direct environmental benefit.

Even though wheat yields often were highest when N–P fertilizer was applied as KN, SB consistently resulted in significantly greater yield than BC (Fig. 2). On average, grain yields for SB treatments were 5% greater than BC when wheat followed either corn or soybean and 10% greater when wheat followed grain sorghum. Other wheat research also reported greater grain yields when applied as SB compared with BC when adequate precipitation was received soon after application (Deibert et al., 1985). Compared with BC, SB of N–P fertilizer results in higher concentrations of both N and P in the soil, which improves overall nutrient efficiency (Lohry, 1998).

Yield Component Responses
Grain yield component responses to main treatment effects for each year are shown in Table 5. Grain yields were correlated with number of heads per square meter (r = 0.92, P = 0.001) and number of kernels per head (r = 0.90, P = 0.001). In 1999, when spring rainfall was above-normal, both head density and kernel number were significantly lower than in 2001 and 2003 when growing conditions were more favorable during the reproductive stage of grain development. In 1999, head density and kernel number were greater for wheat following corn and/or soybean than wheat following grain sorghum; whereas in 2001, values were significantly greater for wheat following soybean compared with wheat following either corn or grain sorghum. In 2003, wheat head density was highest following corn and lowest following grain sorghum. In all years, head density and kernel number were highest for N–P applied as KN, intermediate for SB, and lowest for BC, regardless of previous crop. But, differences in yield components between fertilizer N–P placement methods were not always significant. In addition, head density and kernel number increased as fertilizer N increased, regardless of previous crop. Kernel weight varied with year and fertilizer N–P treatments, suggesting yield compensation among individual components.


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  		Table 5. Main effects of previous crop, fertilizer N–P placement, and N rate on wheat yield components, 1999 to 2003.

 
It has been shown that the primary yield component affected by N–P fertilizer is tillering and the number of heads per unit area (Black, 1982). Sander et al. (1991) also showed that knifed N–P or P banded with seed resulted in increased head number compared with broadcast P, which was the major reason for increased yields. However, when planting date is delayed, fertilizer P should be applied with the seed for maximum effectiveness (Sander and Eghball, 1999). Other studies have reported that tillering in NT winter wheat was increased by manipulating spring N applications (Rasmussen et al., 1997) and that GS-25 tiller density was a critical determinant in defining the impact of N topdress timing on tiller increase, head density, and grain yield of soft red winter wheat in the Coastal Plains soils of the Southeast (Weisz et al., 2001). They suggested that early or split N applications to NT wheat are recommended when GS-25 tiller densities are <550 tillers m–2. In our study, where all the N–P fertilizer was applied preplant, a head density of 500 to 600 m–2 was sufficient to produce excellent grain yields in 2001 and 2003.

Plant Nitrogen and Phosphorus Uptake and Total Grain Nitrogen Responses
Wheat grain yields were strongly correlated with plant N uptake (r = 0.93, P = 0.001), plant P uptake (r = 0.86, P = 0.001), and total grain N (r = 0.93, P = 0.001). Plant N and P uptake and total grain N responses to main treatment effects for each year are shown in Table 6. Regression response curves in Fig. 3 show that for a yield goal of 4 to 5 Mg ha–1, the required levels of whole-plant N and P uptake were 80 to 100 kg N ha–1 and 15 to 20 kg P ha–1, as well as 60 to 90 kg N ha–1 of total grain N.


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  		Table 6. Main effects of previous crop, fertilizer N–P placement, and N rate on plant N and P uptake and total grain N, 1999 to 2003.

 

Figure 3
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  		Fig. 3. Influence of whole-plant N and P uptake and total grain N on no-till wheat grain yield for the years 1999, 2001, and 2003 at Parsons, KS. Regression equations and significance for plant N uptake:[corn: y = –0.202 + 0.0775x – 0.00029x2 (R2 = 0.93, P = 0.001); grain sorghum: y = –0.504 + 0.0864x – 0.000326x2 (R2 = 0.91, P = 0.001); soybean: y = –0.605 + 0.0851x – 0.000336x2 (R2 = 0.92, P = 0.001)]. For plant P uptake: [corn: y = –0.912 + 0.454x – 0.0088x2 (R2 = 0.88, P = 0.001); grain sorghum: y = –0.81 + 0.422x – 0.00723x2 (R2 = 0.83, P = 0.001); soybean: y = –1.10 + 0.485x – 0.00977x2 (R2 = 0.81, P = 0.001)]. For total grain N: [corn: y = –0.42 + 0.093x – 0.00035x2 (R2 = 0.88, P = 0.001); grain sorghum: y = –0.72 + 0.107x – 0.000458x2 (R2 = 0.95, P = 0.001); soybean: y = –2.078 + 0.144x – 0.000753x2 (R2 = 0.89, P = 0.001)].

 
Plant N and P uptake and total grain N varied with year and previous crop (Table 6). In 1999, plant nutrient uptake and total grain N were similar for wheat following either soybean or corn, but values were significantly lower following grain sorghum. In 2001, when residual soil nitrate-N level was relatively high following soybean, plant N and P uptake and total grain N also were substantially greater when wheat followed soybean, whereas when wheat followed either corn or grain sorghum, differences were small. In 2003, plant N and P uptake and total grain N were highest following corn, intermediate following soybean, and lowest following grain sorghum.

The greater N availability for wheat following either soybean or corn compared with grain sorghum likely was influenced both by decomposition of plant residues as well as mineralization of soil organic N. Several studies have evaluated residue decomposition effects on N availability to corn. Green and Blackmer (1995) reported that net amounts of N immobilization induced by soybean residue were approximately equal to those induced by corn residue, but the soybean-induced immobilization was much more rapid, which was also demonstrated by Power et al. (1986). Broder and Wagner (1988) reported that soybean residue has a higher proportion of soluble components and decomposes more rapidly than corn residue. Researchers concluded that because soybean produces less residue and decomposes more rapidly under field conditions compared with corn, less immobilization of both soil and fertilizer N likely occurs following soybean. However, in our study, the effects of corn and soybean residue on N immobilization were more variable. Because corn was harvested in late August and soybean in late September, some decomposition of crop residues as well as mineralization of organic soil N likely occurred when soil temperatures were still warm in late summer and early fall. However, research has shown that complete mineralization of N in both soybean and corn residue does not normally occur until the following spring (Power et al., 1986). Thus, current fertilizer recommendations in Kansas for wheat following soybean do not consider N credits from soybean on the subsequent wheat crop (Leikam et al., 2003).

But, an explanation for the greater plant and grain N uptake for wheat following corn compared with wheat following grain sorghum, however, is unclear because after-harvest plant residue is likely similar. A more plausible explanation of the differences in N availability between corn and grain sorghum likely is influenced by sorghum's secondary tiller development after harvest, resulting in greater immobilization of both soil and fertilizer N. Other researchers also have shown that the 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), which is late enough to have minimal impact on the subsequent wheat crop.

Since double-crop soybean followed wheat in all cropping systems, recycling of residual fertilizer N and mineralized soil N also likely occurred as a result of the double-crop soybean. However, an earlier research study at this same site evaluated rate and time of fertilizer N application for wheat following different crops, and soil analyses showed that double-crop soybean was an effective scavenger of any residual soil nitrate-N (Kelley, 1995). In addition, in this study, soil analyses also showed no differences in organic matter content between cropping systems (Table 4). Thus, plant and grain N responses appeared to be primarily influenced by crop residues immediately preceding wheat.

Plant N and P uptake and total grain N values were highest each year when N–P fertilizer was applied as KN and lowest when BC (Table 6). Plant and grain responses between the two control plots (knifed versus no knife) were similar for each previous crop (data not shown), indicating that differences between placement methods were the result of nutrient placement rather than any physical effect from the coulter-knife. Research has shown that placement of fertilizer N below the surface reduces immobilization of N by soil organisms involved in the decomposition of crop residues (Doran, 1980) and also effectively removes ammonia volatilization as a major contributor to N loss for urea-based fertilizer, such as UAN (Rao and Dao, 1996).

In our study, the influence of fertilizer P placement on nutrient uptake cannot be separated individually from the fertilizer N effect because at the P rate applied (34 kg P ha–1), 22 kg N ha–1 also was applied. But, plant P uptake was strongly correlated with both plant N uptake (r = 0.84, P = 0.001) and total grain N (r = 0.81, P = 0.001). The dual placement of fertilizer N–P as a KN application may have contributed to greater plant P uptake compared with surface-applied fertilizer, which was previously shown in Kansas research (Leikam et al., 1983). However, because soil P tested in the medium range (Table 4), greater nutrient uptake from KN application may have been due more to the placement of N rather the dual N–P effect.

Nitrogen and P uptake and total grain N increased with increasing fertilizer N rate (Table 6), although the magnitude of response varied with previous crop and N–P fertilizer placement method (interaction data not shown). Results agree with previous NT wheat research in Kansas which showed that, unlike grain yield, plant and grain N uptake increased with increasing fertilizer N rate (Kelley and Sweeney, 2005).

Results from this study indicate that both plant and grain responses were influenced by a combination of factors, including the decomposition of previous crop residues, mineralization of soil organic N, and addition of N as inorganic fertilizer. It was beyond the scope of this study to determine the relative contribution of the different N sources to plant nutrient availability and subsequent effect on grain yield. Additional research is needed to further evaluate the influence of fertilizer N placement as well as time of N application on nutrient availability where wheat is planted NT into existing crop residues.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
No-till winter wheat grain yield, yield components, plant N and P uptake, and total grain N were strongly influenced by previous crop, fertilizer N–P placement method, and fertilizer N rate. Wheat planted after soybean and/or corn has a higher yield potential and a lower fertilizer N requirement compared with wheat following grain sorghum. Plant N and P uptake and total grain N 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 corn and to better utilization of N–P applied as KN than surface-applied fertilizer. The primary yield components affecting grain yield differences between previous crops and N–P fertilizer treatments were heads per unit area and number of kernels per head. Results also suggest that fertilizer N recommendations for NT wheat should take into account the placement method for applying fertilizer N. The fertilizer N requirement was at least 40 kg N ha–1 lower when KN than for SB or BC, especially when wheat followed soybean. Fertilizer application costs would be greater for injected than for broadcast, but the increase in yields and lower optimal N application rates should more than offset the increased costs, resulting in greater net returns from injected than broadcast N.


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


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Kansas Agric. Exp. Stn. Contribution no 07-49-J.


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





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