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Published in Agron. J. 97:391-398 (2005).
© American Society of Agronomy
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Symposium Papers

Challenging Approaches to Nitrogen Fertilizer Recommendations in Continuous Cropping Systems in the Great Plains

Alan J. Schlegela,*, Cynthia A. Grantb and John L. Havlinc

a Southwest Res.-Ext. Cent., Kansas State Univ., Tribune, KS 67879
b Agric. and Agri-Food Canada, Brandon Res. Cent., Brandon, MB
c Dep. of Soil Sci., North Carolina State Univ., Raleigh, NC

* Corresponding author (schlegel{at}ksu.edu)

Received for publication April 12, 2004.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 OPTIMAL NITROGEN RATES
 NITROGEN RECOMMENDATION MODELS
 NITROGEN FERTILIZER REQUIREMENTS
 WHY NITROGEN FERTILIZER...
 WHY NITROGEN FERTILIZER...
 ADVERSE IMPACT OF NITROGEN
 NEW TECHNOLOGY
 CONCLUSIONS
 REFERENCES
 
Cropping systems in the Great Plains have evolved over the past two decades from reliance on summer fallowing to continuous cropping under reduced or no-tillage. Most N recommendation models were developed in fallow systems under conventional tillage and were based on average yield goal, with adjustments for soil profile N content. The objective of this review is to examine the impact of continuous cropping on N requirements. With high-residue continuous cropping systems, N requirements may increase because of increased annualized production, reduced contribution of N mineralization, and increased immobilization and volatilization potential of surface-applied fertilizer N. Mitigating these effects on N availability and supplemental N requirements are the reduction in yield per crop, reduced nitrate (NO3) leaching potential, increased N use efficiency (NUE), and increased rates of N mineralization due to higher soil organic matter (OM) content. Unfortunately, increased year-to-year yield variability with continuous cropping increases the difficulty in accurately estimating yield goals. Also, reducing the frequency and duration of fallow may reduce the usefulness of the preplant soil N tests in estimating N availability. Recent research has evaluated the use of optical sensors during the growing season to assess N stress and to estimate crop N requirements. If proved feasible for many crops, this would provide a drastic change for determining N recommendations. In the absence of a reasonable yield goal and known residual soil N content, a fertilizer N rate near 70 kg N ha–1 or less was generally sufficient to optimize small-grain or oilseed yields in several continuous cropping studies.

Abbreviations: CC, continuous corn • NDVI, normalized difference vegetation index • NUE, nitrogen use efficiency • OM, organic matter • PSNT, presidedress nitrate test • SF, grain sorghum–fallow • SS, continuous grain sorghum • WCF, wheat–corn–fallow • WF, wheat–fallow • WSF, wheat–grain sorghum–fallow • WW, continuous wheat


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
OPTIMAL NITROGEN RATES
 NITROGEN RECOMMENDATION MODELS
 NITROGEN FERTILIZER REQUIREMENTS
 WHY NITROGEN FERTILIZER...
 WHY NITROGEN FERTILIZER...
 ADVERSE IMPACT OF NITROGEN
 NEW TECHNOLOGY
 CONCLUSIONS
 REFERENCES
 
WIDESPREAD ADOPTION of reduced-tillage systems over the past two decades has increased precipitation storage efficiency. Moisture conservation with reduced tillage enhances the use of low and variable growing season rainfall in the Great Plains. Increased moisture availability under reduced tillage enables the transition from rotations that include high proportions of monoculture cereals and summer fallow to more intensified and diversified cropping rotations (Lafond et al., 1992; Peterson et al., 1996). Intensified crop rotations more efficiently utilize summer precipitation and the additional water retained with reduced tillage. Similarly, movement from cereal-dominated rotations to rotations with a greater proportion of pulse crops, oilseeds, and forages can also help synchronize crop water requirements with moisture supply.

Intensification and diversification of dryland cropping systems will enhance crop productivity (Peterson et al., 1996). This also reduces the risk of nutrient leaching (Eck and Jones, 1992) and salinization (Black et al., 1981) because of increased water use during the growing season. In addition, increasing crop production will increase N removal, plant biomass production, and surface residue cover, which will impact N cycling and fertilizer N management, likely increasing fertilizer N inputs.

The objective of this review is to examine the impact that continuous cropping in traditional summer fallow regions has on N requirements. For the purposes of this paper, it is assumed that continuous cropping will be done with very limited or no-tillage systems to best utilize limited precipitation.


   OPTIMAL NITROGEN RATES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 OPTIMAL NITROGEN RATES
NITROGEN RECOMMENDATION MODELS
 NITROGEN FERTILIZER REQUIREMENTS
 WHY NITROGEN FERTILIZER...
 WHY NITROGEN FERTILIZER...
 ADVERSE IMPACT OF NITROGEN
 NEW TECHNOLOGY
 CONCLUSIONS
 REFERENCES
 
Optimal N application rates enhance the economic return to the producer while minimizing environmental risks associated with N use. Crop yield response to N application often follows a diminishing return response (Bole and Pittman, 1980; Soper, 1971). For example, in a spring wheat (Triticum aestivum L.)–winter wheat (T. aestivum L.)–sunflower (Helianthus annuus L.) rotation in North Dakota, winter wheat yields were increased approximately 6% (109 kg ha–1) by increasing N rates from 34 to 67 kg N ha–1 but only an additional 2% by increasing N rates to 101 kg N ha–1 (Halvorson et al., 1999a). In the same study, spring wheat was slightly more responsive to added N with more than a 10% yield increase from increasing N rates from 34 to 67 kg N ha–1 and an additional 6% yield increase by increasing N rates to 101 kg N ha–1 (Halvorson et al., 2000). Sunflower yields increased 100 kg ha–1 for each additional increment of N (1390, 1490, and 1590 kg ha–1 for 34, 67, and 101 kg N ha–1, respectively) (Halvorson et al., 1999b). Averaged across the entire rotation, yields were increased < 5% (<76 kg ha–1 seed yield) by increasing N rates above 67 kg N ha–1 with considerable variation among years. Similarly, Halvorson and Reule (1994) found that the optimal fertilizer N rate was 67 kg N ha–1 for a spring barley (Hordeum vulgare L.)–corn (Zea mays L.)–winter wheat rotation in Colorado.

In Saskatchewan, the N requirement for wheat in continuous wheat (WW) or wheat–lentil (Lens culinaris Medik.) rotation varied from 5 to 60 kg N ha–1 (Fig. 1) (Zentner et al., 2001). Averaged across a 19-yr period, the optimal N rate was about 40 kg N ha–1 for WW compared with 30 kg N ha–1 for wheat following lentil. The increase in available N following lentil reduced the N rate required to optimize wheat yield. In contrast, N requirements may be higher to optimize corn yield. Maximum corn yields were obtained with 202 kg N ha–1 of fertilizer N plus soil NO3–N in western Nebraska (Blumenthal et al., 2003). Similarly, 170 kg N ha–1 of available N (soil plus fertilizer) was reported to optimize yields with annual cropping of spring barley–corn–winter wheat in Colorado (Halvorson and Reule, 1994).



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  		Fig. 1. Fertilizer N requirement for wheat in continuous wheat (WW) and wheat–lentil (W-L) cropping systems at Swift Current, SK (Zentner et al., 2001).

 
Under dryland conditions, maximum yield, N required to reach maximum yield, and the magnitude of the yield response to N are generally determined by the relationship between available moisture and N. Therefore, predicting available N is important in determining optimal N application rate. As the magnitude of the crop response to applied N decreases, fertilizer NUE and profitability decreases with each additional unit of N applied. Also, as fertilizer NUE decreases, the environmental risk associated with excess N increases. Therefore, prediction and application of optimal N rates is important both economically and environmentally.


   NITROGEN RECOMMENDATION MODELS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 OPTIMAL NITROGEN RATES
 NITROGEN RECOMMENDATION MODELS
NITROGEN FERTILIZER REQUIREMENTS
 WHY NITROGEN FERTILIZER...
 WHY NITROGEN FERTILIZER...
 ADVERSE IMPACT OF NITROGEN
 NEW TECHNOLOGY
 CONCLUSIONS
 REFERENCES
 
Most N recommendation models used in the Great Plains are based on estimating crop yield potential with N credits for residual soil profile N content (determined by standard soil-sampling and analysis methods), potential mineralizable N from native soil OM, previous manure applications, and/or previous legume crops. Other factors that may be considered are previous crop (other than legume), tillage practices, time of soil sampling, and N cost/grain price ratio. The magnitude of the crop N response depends on water availability. Nitrogen response curves that account for available moisture have been developed for a number of crops (Cowell and Doyle, 1993), including wheat (Gehl et al., 1990), barley (Bole and Pittman, 1980), and canola (Brassica napus) (Henry and MacDonald, 1978). Available soil moisture is included as a factor in N recommendation models used by some soil-testing laboratories.

An example of a typical N recommendation model used by Kansas State University includes yield goal, residual soil profile N, and soil OM to estimate N fertilizer requirement, along with credits for previous legume crops or manure applications where:

where Nrec = fertilizer N recommendation (kg ha–1), CF = crop factor, YG = yield goal (Mg ha–1), SOM = adjustment for soil OM content, and PN = profile (0 to 60 cm) soil NO3–N (kg ha–1) (Leikam et al., 2003). The model also provides credits for previous legume crops or manure applications. In some cases, an adjustment is made for previous crops other than legumes. A major limitation with N recommendation models that rely on yield goal is uncertainty in determining an accurate average yield goal, especially in regions with high variation in year-to-year moisture supply and yield potential.


   NITROGEN FERTILIZER REQUIREMENTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 OPTIMAL NITROGEN RATES
 NITROGEN RECOMMENDATION MODELS
 NITROGEN FERTILIZER REQUIREMENTS
WHY NITROGEN FERTILIZER...
 WHY NITROGEN FERTILIZER...
 ADVERSE IMPACT OF NITROGEN
 NEW TECHNOLOGY
 CONCLUSIONS
 REFERENCES
 
Crops vary in their relative N requirements (Table 1). Accurately determining N requirements is more critical for crops with high N requirements as the potential economic and environmental impact of N fertilization generally increases as rate of application increases. High-N-requiring crops generally exhibit a large response to N fertilizer, so applying too little N will reduce yield more for these crops than for less-N-responsive crops. Conversely, these crops provide the most opportunity to overapply N fertilizer because of overall higher N rates. With diversified continuous cropping, there is greater opportunity to utilize crops that vary in N requirements.


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  		Table 1. Relative N fertilizer requirements of various crops.

 
Nitrogen management for continuous cropping utilizes the same principles as with fallow systems. The goal is to supply the correct amount of N when the crop needs it and where the crop can access it while minimizing losses to the environment. The best management practices available to meet this goal, such as N rate, source, time of application, and placement, are similar for both systems. However, differences exist in N dynamics and management between conventionally tilled fallow systems and continuous cropping using reduced tillage (Grant et al., 2002). Continuous cropping often utilizes little or no tillage, so unless producers adopt low-disturbance fertilizer placement techniques or band their N during seeding, surface N applications may be more common than in conventional-till fallow systems. Therefore, potential ammonia (NH3) volatilization losses may be greater with continuous cropping while leaching losses will be of less concern. The relative importance of stored NO3–N compared with N mineralization during the growing season may be reduced under continuous cropping compared with fallow systems since the time period for nitrate accumulation between crops is reduced as cropping frequency is increased..


   WHY NITROGEN FERTILIZER REQUIREMENTS MAY INCREASE WITH CONTINUOUS CROPPING
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 OPTIMAL NITROGEN RATES
 NITROGEN RECOMMENDATION MODELS
 NITROGEN FERTILIZER REQUIREMENTS
 WHY NITROGEN FERTILIZER...
WHY NITROGEN FERTILIZER...
 ADVERSE IMPACT OF NITROGEN
 NEW TECHNOLOGY
 CONCLUSIONS
 REFERENCES
 
Crop N requirements will be affected by cropping frequency and increased crop diversification. Diversified crop rotations can increase yield potential by influencing plant diseases, weeds, root distribution, moisture utilization, and nutrient availability (Campbell et al., 1990). Ideally, crop diversification involves the inclusion of crop species with different input requirements {e.g., a high-N-using crop such as corn combined with a low-N-using or an N-fixing crop such as soybean [Glycine max (L.) Merr.] or lentil}. Copeland et al. (1993) and Copeland and Crookston (1992) reported that yields of first-year corn or soybean in rotation were 13% greater than when they were grown in monoculture, even when grown with high fertility. Similar results were obtained by Sahs and Lesoing (1985) in Nebraska. Rotational benefits also were seen in spring wheat grown after fababean (Vicia faba L.), lentil, and pea (Pisum sativum L.) over a range of N rates in studies conducted in Manitoba (Table 2) (Soper and Grenier, 1987). Crop responses to N fertilization may increase with increased cropping intensity, if crop yield potential is increased.


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  		Table 2. Impact of preceding crop and N management on grain yield of spring wheat in Manitoba (Soper and Grenier, 1987).

 
Continuous cropping increases total production, thereby increasing N removal. Campbell et al. (1990) found that annualized spring wheat production increased 230 to 580 kg ha–1 when cropping intensity increased from wheat–fallow (WF) to WW on five soil groups in Canada (Fig. 2). This represents an increase in production of 15 to 47%. Similarly, annualized winter wheat yields in Texas were 47% greater with WW than WF (Jones and Popham, 1997). Although of lower magnitude, annualized wheat yields were 14% greater for WW than WF in Kansas (Schlegel et al., 1999). In a long-term study in Saskatchewan, Campbell and Zentner (1993) reported total N export in wheat grain was 35% greater with continuous spring wheat than WF (646 kg ha–1 for WF compared with 874 kg ha–1 for WW from 1967 to 1990).



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  		Fig. 2. Annualized grain yield for continuous and fallow wheat on five soil types in Canada (adapted from Campbell et al., 1990). WF indicates wheat–fallow rotation, and WW indicates continuous wheat.

 
The increase in annualized grain production is also affected by crop species. In Texas, annualized grain production with continuous grain sorghum [Sorghum bicolor (L.) Moench] was 25% greater than a wheat–sorghum–fallow (WSF) rotation, 240% greater than WW, and 320% greater than WF (Jones and Popham, 1997). In general, they found grain production was twice as great with grain sorghum as with winter wheat. Annualized yield was 27% more for continuous no-till corn (CC) than for no-till WF in Colorado (Halvorson et al., 2002a). They also found that residue production was 24% greater for CC compared with WF.

Under dryland culture, grain yields may be more variable with continuous cropping compared with fallow systems. While annualized wheat yields averaged 47% greater for WW than WF in Texas, this difference ranged from –67 to +97% (Jones and Popham, 1997). Similarly, in Kansas, annualized wheat yields averaged 15% greater with WW than WF but ranged from –100 to +45% (Norwood et al., 1990). In the same study, annualized sorghum yields averaged 4% greater with continuous grain sorghum (SS) than grain sorghum–fallow (SF), but the range was from –100 to +127%. Increased variability in annual production can cause significant errors in estimating yield goals under continuous cropping.

With continuous cropping, the fallow period between crops is shortened, allowing less time for N mineralization and soil water accumulation. Also, with surface N applications, increased surface residue cover with continuous cropping increases N immobilization and NH3 volatilization potentials. McInnes et al. (1986) found that NH3 volatilization losses were 7.6 to 16.6% from urea ammonium nitrate (UAN) broadcast-applied to soil with wheat straw cover. Fowler and Brydon (1989) reported that volatilization losses could exceed 50% from urea broadcast to no-till wheat in the early fall. All of these factors can increase N fertilizer requirements. In Kansas, winter wheat no-till planted after grain sorghum required 21 kg ha–1 more N than following soybean (Staggenborg et al., 2003). They attributed this to higher residue production by sorghum, which increased immobilization of available N.

The N requirement may be greater during the period of transition between tilled and no-tillage systems. If soil OM increases with conversion to no-till, the N immobilized in the OM may reduce N available for crop growth. Kolberg et al. (1996) found that N requirements for winter wheat and corn in Colorado were higher in the initial years of no-till after converting from conventional tillage.


   WHY NITROGEN FERTILIZER REQUIREMENTS MAY DECREASE UNDER CONTINUOUS CROPPING
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 OPTIMAL NITROGEN RATES
 NITROGEN RECOMMENDATION MODELS
 NITROGEN FERTILIZER REQUIREMENTS
 WHY NITROGEN FERTILIZER...
 WHY NITROGEN FERTILIZER...
ADVERSE IMPACT OF NITROGEN
 NEW TECHNOLOGY
 CONCLUSIONS
 REFERENCES
 
Continuous cropping may increase annualized grain production, but yield per crop under moisture-limited conditions may be less, sometimes much less, than with fallow systems. Averaged across a 10-yr period in Texas, wheat yields were 26% less for WW than WF (1130 vs. 1540 kg ha–1, both grown no-till) (Jones and Popham, 1997). Similarly, in Kansas, no-till WW produced 43% less than no-till WF or WSF (Schlegel et al., 1999). Also, in Kansas, averaged across a 15-yr period, WW had 57% lower yields than wheat in a reduced-tillage WSF or WF rotation (Norwood et al., 1990). In the same study, grain sorghum yields were 48% less for SS compared with SF. With the shorter fallow period, available soil water at planting is decreased. Continuous wheat had 67% less available soil water at planting than reduced-tillage WSF or WF while SS had 38% less available soil water at planting than SF. Available soil water at wheat planting was 52% less in an annual cropping system [wheat–corn–proso millet (Panicum miliacium L.)] compared with WF in Colorado (Nielsen et al., 2002). This reduction in stored soil water reduced wheat yields by 51%. As yield potential is reduced, annual N requirements for optimum crop yield will decline.

While cropping intensification may increase crop removal of nutrients, it also enhances the amount of organic residues returned to the system. Total crop residue returned to the soil surface across a 12-yr period was 99% greater with annual cropping (spring wheat–winter wheat–sunflower) than WF (both no-till production) in North Dakota (Halvorson et al., 2002b). In the same study, the total amount of residue N was 121% greater with annual cropping than in WF, which would increase the potential for N mineralization from plant residues, particularly in fertilized systems. In studies conducted at Indian Head, SK, continuous cropping, compared with fallow systems, increased average yield, organic N, and initial potential rate of N mineralization after 34 yr, particularly where N and P fertilizers were applied (Table 3) (Campbell et al., 1996).


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  		Table 3. Average annual rate of change in yield trends over 34 yr and organic N, available P, and potential N-supplying capacity of soil in a thin Black Chernozem at Indian Head, SK (Campbell et al., 1996).

 
Nitrogen mineralization rates at Mandan, ND, were increased after 10 yr by reducing tillage intensity and by annual cropping compared with a crop–fallow system (Wienhold and Halvorson, 1999). In eastern Colorado, potential N mineralization capacity and N mineralization in the surface 5 cm of soil were increased after 3.5 yr by increasing the cropping intensity from a WF to a wheat–corn–fallow (WCF) rotation (Wood et al., 1990). Similarly, in a 9-yr study of continuous no-till spring wheat (Campbell et al., 1993b), N-supplying capacity of the soil was improved by a combination of fertilizing, reducing tillage, and cropping more frequently. Although continuous cropping will reduce the amount of residual mineral N in the soil, in the long-term, it may increase N mineralization potential. The amount of N mineralization during the growing season may be more important with continuous cropping than fallow systems since continuous cropping systems accumulate less stored NO3–N because of the limited time for N mineralization between crops. In contrast, fallow systems can accumulate significant amounts of NO3–N in the soil profile from mineralization during the extended fallow period. Therefore, the effectiveness of a soil NO3–N test for prediction of crop N requirements may be somewhat less effective under continuous cropping compared with fallow systems, unless combined with an estimate of in-season N mineralization.

However, if soil and environmental conditions result in a high probability of soil NO3–N present at planting, then measuring soil NO3–N may enhance NUE. In coarse grains, assessing soil NO3–N during early vegetative growth has been used to quantify in-season N rates (Magdoff et al., 1984). The presidedress nitrate test (PSNT) requires analysis of soil samples (0–30 cm) collected at V4 to V6 corn growth stage. Critical soil NO3–N concentration below which N applications are recommended is {approx}25 mg kg–1, which varies between regions and crops. Semiarid regions establish lower PSNT critical levels (13–15 mg kg–1) due to greater soil NO3–N accumulation (Spellman et al., 1996). Most studies show PSNT explains 60 to 85% of the variation in crop response to in-season N (Havlin, 2004). Measuring both NH4–N + NO3–N in the early growth period may be a better predictor of N sufficiency regardless of N source (Vaughan et al., 1990).

Continuous cropping also provides greater opportunity to incorporate pulse crops into the rotation. Grain crops following pulse crops may utilize symbiotically fixed N and require less N fertilizer and/or produce greater yields. In Saskatchewan, durum wheat (Triticum turgidum L.) grown following pulse crops [chickpea (Cicer arietinum L.), lentil, and dry pea] yielded 7% more than following spring wheat and had 11% higher grain protein content (Gan et al., 2003). Also, durum wheat yield was 18% greater and grain protein concentration 20% greater following 2 yr of pulse crops compared with WW. However, only a small fraction of the yield increase (28% at one site and less than 3% at the other four sites) was attributed to enhanced residual soil NO3–N or increases in plant available soil water following pulse crops. They speculated that the poor relationship was probably due to fall soil sampling not giving an accurate representation of spring water status and the possibility of increased disease pressure with continuous wheat.

Campbell et al. (1992) found that there was a cumulative enhancement of the N-supplying power of the soil and a gradual reduction in fertilizer N requirements in a wheat–lentil rotation in Saskatchewan compared with WW. A wheat–lentil rotation also reduced deep-leached NO3–N compared with well-fertilized continuous wheat (Campbell et al., 1993a). This was attributed to better synchrony of N uptake from decomposition of lentil residue compared with wheat residue.

Fertilizer NUE can be greater with more intensified cropping. In Colorado, fertilizer NUE averaged 27% for WF compared with 43% for WCF with grain N removal 75% greater with WCF than WF (Peterson et al., 1993).

Leaching losses of NO3–N may be less with continuous cropping than fallow systems. Campbell et al. (1984) reported that planting fall rye (Secale cereale L.) reduced leaching compared with WF in Canada. Increasing cropping intensity from WF to WCF reduced root zone soil NO3–N (hence, leaching potential) in Colorado despite a 73% increase in fertilizer N (Peterson and Westfall, 1994). Soil NO3–N levels and the probability of leaching were reported to be less with continuous cropping (SS or WW) than WF in Texas (Eck and Jones, 1992).


   ADVERSE IMPACT OF NITROGEN
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 OPTIMAL NITROGEN RATES
 NITROGEN RECOMMENDATION MODELS
 NITROGEN FERTILIZER REQUIREMENTS
 WHY NITROGEN FERTILIZER...
 WHY NITROGEN FERTILIZER...
 ADVERSE IMPACT OF NITROGEN
NEW TECHNOLOGY
 CONCLUSIONS
 REFERENCES
 
High N fertility levels can be detrimental to crop yields under conditions of severe water stress. Since the fallow period is reduced in continuous cropping, the frequency and severity of water stress increases. Nielsen and Halvorson (1991) reported that 112 kg N ha–1 applied to winter wheat in Colorado under moderate to severe water stress (evapotranspiration rates < 62% of potential evapotranspiration) increased water stress and reduced yields by 15%. Severe water stress has also been shown to reduce fertilizer NUE in spring wheat (Campbell et al., 1993b). Angus and van Herwaarden (2001) in a review of 13 studies evaluating water use and wheat yield in Australia found that high N status during vegetative growth could cause reductions in grain yield. They attributed this to N stimulating vegetative growth, which required carbohydrates to be used for structural tissue rather than as soluble carbohydrates that could be retranslocated to grain after anthesis.

Excess N application can also lead to negative environmental impacts (Grant et al., 2002). Losses of N as NO3, NH3, or nitrous oxides (NOx) can adversely affect air and/or water quality. Conversely, underestimating N requirements can reduce crop productivity and residue return to the soil, leading to a long-term reduction in SOM (Campbell and Zentner, 1993) Therefore, ensuring that the rate and timing of N supply is closely matched to N requirements for crop growth is critical both to optimize crop production and to reduce the potential for negative environmental effects.


   NEW TECHNOLOGY
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 OPTIMAL NITROGEN RATES
 NITROGEN RECOMMENDATION MODELS
 NITROGEN FERTILIZER REQUIREMENTS
 WHY NITROGEN FERTILIZER...
 WHY NITROGEN FERTILIZER...
 ADVERSE IMPACT OF NITROGEN
 NEW TECHNOLOGY
CONCLUSIONS
 REFERENCES
 
Conventional N management practices rely on N recommendations derived from yield goals and soil profile N content where a uniform N rate is applied. Current precision agriculture practices have used this same concept, except sampled more intensively, either in a grid pattern, by soil types, landscape position, or some other method to divide a field into more homogenous units (Pierce and Sadler, 1997). Within each of these homogenous units, a single rate of N is applied.

Another approach has been developed to estimate N requirements and is based on optical sensing crop N status during the early growing season. Use of chlorophyll meters to assess N status of growing crops is well established. Leaf chlorophyll and percentage N content are highly correlated over the range of yield response to fertilizer N. Increasing N rate increases grain yield and leaf N, but chlorophyll readings do not increase with N applied above that required for optimum yield (Schepers et al., 1992). For example, in winter wheat, in-season N is recommended with a chlorophyll reading at or below approximately 44 (Follett et al., 1992).

Remote-sensing applications in agriculture have advanced rapidly with uses in almost every area of soil and crop management (Moran et al., 1997). Many studies document use of visible and near infrared (NIR) spectral response from plant canopies to detect N stress (Ma et al., 1996). High correlation exists between crop N uptake and normalized difference vegetation index (NDVI) obtained with ground-based sensors (Stone et al., 1996). Since NDVI is highly correlated with plant N status, in-season remote sensing can be used for in-season N applications. Farrer et al. (2003) used red–green reflectance at wheat tillering to estimate in-season N requirement. Lukina et al. (2001) used late-tillering NDVI divided by growing degree days from planting to NDVI measurement to predict in-season N requirements. Raun et al. (2002) showed that prediction of wheat response to topdress N by remote sensing was positively correlated to measured N response and increased NUE by 28% and net return by 20%.

Another method for estimating in-season N requirements involves use of remote sensing to determine plant biomass (Weisz et al., 2001). Mid- and late-tillering are critical growth stages for N management to maximize wheat yield and fertilizer NUE. If tiller density is <540 tillers m–2, N application at Growth Stage (GS) 25 improves grain yield. When density was >540 tillers m–2, N requirements are based on tissue N analysis. Tiller number is measured using aerial photography. Aerial color infrared photographs at GS 25 also can be used to estimate optimum N rates applied at GS 30. Remote sensing has also been used to estimate in-season N requirements at pretassel in corn where Sripada et al. (2003) demonstrated a 35% reduction in N rate and 50% increase in fertilizer NUE.

These sensor-based systems usually require a non-N-limiting test strip in each field to allow for an in-season estimate of N response and a production system that allows for in-season application of fertilizer N. They also require specialized equipment, but commercial application equipment is available and being used for topdressing wheat on a limited scale. Improvements in prediction of N mineralization potential under continuous cropping would also improve N recommendations, enhancing both economic and environmental sustainability of N management.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 OPTIMAL NITROGEN RATES
 NITROGEN RECOMMENDATION MODELS
 NITROGEN FERTILIZER REQUIREMENTS
 WHY NITROGEN FERTILIZER...
 WHY NITROGEN FERTILIZER...
 ADVERSE IMPACT OF NITROGEN
 NEW TECHNOLOGY
 CONCLUSIONS
REFERENCES
 
Cropping systems in the Great Plains have evolved over the past two decades to increase crop productivity and profitability. The widespread adoption of reduced- and no-tillage systems has increased precipitation capture and utilization, enabling more intensive and diversified cropping. With these advances, continuous cropping is now feasible in semiarid regions where extensive fallowing has been traditionally practiced. Nitrogen is the most frequently limiting nutrient and is applied in the greatest amounts in the Great Plains. Fallow systems relied on N mineralized from the soil OM to provide N for crop production in the following year. With the reduction or elimination of fallow with continuous cropping, N availability can be increased or decreased depending on the situation.

Most N recommendation models used in the Great Plains were developed under conventional tillage in fallow systems and generally based on yield goal and crop N requirements with credits commonly given for nonfertilizer N availability (residual soil N, manure application, N-fixing crops, etc.). With high-residue continuous cropping, N requirements may increase because of increased annualized production, reduced accumulation of mineralized N (from shorter fallow periods), and increased immobilization and NH3 volatilization (from high residue conditions) of surface-applied N fertilizers. Conversely, several factors may mitigate the increase in N requirements. Shorter fallow periods result in less stored soil water, reducing yield per crop and leaching potential. Greater cropping intensity enhances NUE. Increased residue production increases soil OM content and potential N mineralization during crop growth. While continuous cropping increases annualized grain production, year-to-year variability in yield increases because of reduced water stored by fallowing. So establishing the proper yield goal, already inherently uncertain in dryland agriculture, is more difficult under continuous cropping systems. Although preplant soil N tests have proved useful in fallow systems, they may not be as valuable with continuous cropping since N mineralization during the growing season may be of greater importance than nitrate accumulated between crops. Improved prediction of potential N mineralization may improve N recommendations for continuous cropping. The challenge lies in development of models that will effectively predict mineralization in the highly variable environment inherent in dryland cropping systems.

Another proposed method to improve estimates of N requirements is to use optical sensors to estimate crop N status in-season and then variably apply the appropriate amount of fertilizer N. However, this approach is limited to crop production systems that readily allow for in-season N applications and requires specialized application equipment.

Based on recent research, optimal N rates for multiple crops throughout the Great Plains were approximately 70 kg N ha–1 or less. In the absence of specific site information (reasonable yield goal, soil profile N, etc.), limiting N applications to this amount may be a viable option with continuous cropping.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 OPTIMAL NITROGEN RATES
 NITROGEN RECOMMENDATION MODELS
 NITROGEN FERTILIZER REQUIREMENTS
 WHY NITROGEN FERTILIZER...
 WHY NITROGEN FERTILIZER...
 ADVERSE IMPACT OF NITROGEN
 NEW TECHNOLOGY
 CONCLUSIONS
 REFERENCES
 
Contribution no. 04-334-J from the Kansas Agricultural Experiment Station.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 OPTIMAL NITROGEN RATES
 NITROGEN RECOMMENDATION MODELS
 NITROGEN FERTILIZER REQUIREMENTS
 WHY NITROGEN FERTILIZER...
 WHY NITROGEN FERTILIZER...
 ADVERSE IMPACT OF NITROGEN
 NEW TECHNOLOGY
 CONCLUSIONS
 REFERENCES
 




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