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Published in Agron. J. 96:18-25 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA

PRODUCTION PAPER

Evaluation of Soil Nitrate as a Predictor of Nitrogen Requirement for Sugar Beet Grown in a Mediterranean Climate

Marcelino Bilbaoa, Juan J. Martíneza and Antonio Delgado*,b

a AIMCRA, Apdo. 4210, 41080 Sevilla, Spain
b Dpto. Ciencias Agroforestales, EUITA, Universidad de Sevilla, Ctra. de Utrera Km 1, 41013 Sevilla, Spain

* Corresponding author (adelgado{at}us.es).

Received for publication October 31, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Rational N fertilization is important to reduce environmental impact of fertilization and to increase profitability in crop production. An evaluation of preplant soil nitrate concentration in soil to estimate optimum N rate for autumn-sown sugar beet (Beta vulgaris L.) was done by performing 33 N response experiments in different locations between 1989 and 2000. For each location and year, the N rate for maximum beet and sucrose production and the economic optimum N rate for adjusted beet production (yield adjusted for sucrose concentration) were estimated. Nitrate N concentration in the soil (0–30 cm deep) before fertilization ranged between 6 and 156 mg kg–1. This concentration allows one to distinguish between N responsive and nonresponsive sites when no particular leaching (tile-drained soils in rainy winters—more than 470 mm from October to January—or furrow irrigated) or water stress limitations exist (rainfed locations with less than 350 mm annual rainfall—less than 170 mm during spring). A critical value for the preplant soil nitrate was determined, above which no response to N fertilizer can be expected. This value was 39 mg kg–1 for beet production and 33 mg kg–1 for adjusted beet production. Preplant soil nitrate also provided an accurate estimation of the optimum N rate for adjusted beet production under the same conditions (R2 = 0.93). It is concluded that soil nitrate before planting can be a useful method for assessing N fertilizer rate in production of sugar beet under a Mediterranean climate in soils where leaching is not favored by tile draining or where yield potential is not excessively limited by water supply.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
OPTIMUM MANAGEMENT OF N is necessary to reduce environmental impact of agricultural practices and to increase profitability in crop production. In the sugar beet (Beta vulgaris L.), N determines not only crop development, but also crop quality for sucrose production. Nitrogen is especially important for vegetative development (Pocock et al., 1988; Scott and Jaggard, 1993), but excessive or late N applications may result in decreased quality (increase in amino N; Pocock et al., 1988) and sucrose yield (Hills et al., 1982). Thus, N recommendations for sugar beet may involve a detailed profit evaluation related to the quality factors affected by excessive N applications.

Soil testing can improve the accuracy of N recommendations for different crops such as corn (Zea mays L.) (Meisinger et al., 1992; Bundy and Andraski, 1995), grain sorghum [Sorghum bicolor (L.) Moench] (Frank and Roeth, 1996), winter wheat (Triticum aestivum L.) (Wehrmann et al., 1988), and spring-sown sugar beet (Carter et al., 1974; Giles et al., 1975). Soil mineral N before planting (essentially nitrate in soils not amended with manure; Sims et al., 1995; Schröder et al., 2000) and mineralization of organic matter are considered the sources of soil N to plants. However, in soils with low organic matter content receiving high rates of N fertilizers, fertilizer and residual N are supposed to be the majority of N supply to crops, and thus soil tests for N recommendations can be based on nitrate analysis (Black, 1993; Frank and Roeth, 1996). These tests [preplant soil nitrate test (PPNT) or presidedress soil nitrate test (PSNT)] allow one to establish critical soil nitrate concentrations, above which there is low probability of a yield response to N application. However, these tests do not always provide an accurate estimation of the optimum N rate below the critical value (Meisinger et al., 1992; Schmitt and Randall, 1994). Acceptable N recommendations can usually be obtained for corn (Rehm et al., 1993; Penas et al., 1994) and grain sorghum (Sander and Frank, 1980) if mineralization of soil organic matter and other N sources (i.e., previous legume crop) are also considered as sources of available N.

Crop analysis (total N in leaves and nitrate concentration in stalks or petioles) has been also used to formulate recommendations on N use since the plant is considered to integrate factors such as the presence of soil mineral N, the availability of this N, the weather, and the crop management (Binford et al., 1992a). However, N estimation based on crop analyses at early stages, particularly nitrate in petioles, are unable to fully reflect the N supply by the soil bulk yet to be explored by roots or when soil water deficits limits plant uptake (Schröder et al., 2000). Fox et al. (1989) considered that the cause for the relatively poor prediction of soil N availability for corn by the stalk nitrate was the sensitivity of nitrate concentration in stalk to solar radiation and soil moisture.

The usual method of assessing the N rate for the spring-sown sugar beet in most European countries is the Nmin method (Gordo and Bilbao, 1999). The optimum N rate is estimated based on the amount of inorganic N present in the soil before planting (Black, 1993; Neeteson, 1995). According to the Nmin method, the optimum N rate is estimated as a linear function of residual inorganic N (Y = A – B x inorganic N), independently of beet yield or other N sources. According to Neeteson (1995), this method can be used under conditions where residual N is available at the beginning of the growing season and at least partially available during the period of maximum development.

Under Mediterranean climate conditions, temperate winters allow sugar beet to be sown in autumn and harvested in summer. Sowing in October to November and harvesting in June to August is typical in southern Europe and northern Africa. A longer growing season contributes to higher yields in relation to spring-sown sugar beet. A limitation in the use of the Nmin method or a preplant soil nitrate test in assessing optimum N rates could be climatic conditions since under a Mediterranean climate, a majority of the precipitation occurs during winter when rates of evapotranspiration and N extraction by crops are low. However, leaching water in soil is usually limited, except in rainy winters or in sandy soils (not usual for sugar beet in southern Spain); thus, significant NO3–N loses by leaching during winter are not probable under these conditions.

Sampling depth is another important factor. In most cases, nitrate analyses for samples taken at 0 to 90 cm provide the best estimation of critical soil nitrate values and also the best estimation of optimum N rates (Bundy and Andraski, 1995; Ehrhardt and Bundy, 1995). However, deep sampling could pose a limitation for practical use by farmers in assessing N fertilization. For this reason, some recommendations for the use of nitrate soil tests are based on a 30-cm sampling depth (Magdoff et al., 1984; Magdoff, 1991; Binford et al., 1992b), supported by the evidence of close relationships between the soil mineral N supply in the upper 30 cm and the supply in deeper layers (Sims et al., 1995; Schröder et al., 2000).

The main purpose of this work is to study the accuracy of preplant soil nitrate as a sole guide for fertilizing autumn-sown sugar beet grown in a Mediterranean climate.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experiments to determine the effects of soil NO3–N on sugar beet response to N fertilizer were performed in different locations in the Guadalquivir Valley (southwest Spain) to include typical soils from Mediterranean areas (Inceptisols, Alfisols, Vertisols, and Aridisols). Locations, main properties of soils, rain, and irrigation during the experiments are shown in Table 1. These experiments were conducted in commercial farms by personnel from the Research Association for the Improvement of Sugar Beet Crop of Spain (AIMCRA, Sevilla, Spain). No organic amendment was applied to the soil in the 4 yr preceding the experiment.


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  		Table 1. Description of experimental sites, water supply, and soil properties of topsoil (0–30 cm).

 
Thirty-three experiments were conducted from 1989 to 2000. (Table 1 shows experiments performed in each year and location.) For each site, a randomized complete block design with six replications was performed. Treatments included eight N fertilizer rates: 0, 40, 80, 120, 160, 200, 240, and 300 kg N ha–1. Nitrogen was applied broadcast by hand as ammonium nitrate. One-third of the corresponding rate was applied preplant and two-thirds sidedress (one-third with plants at four true leaves and one-third with plants at 8 to 12 true leaves). The first sidedress application was done from the middle of December to the middle of January and the second between the middle of February and the middle of March, depending on the year. Although most of the sites showed high available levels of P and K, 65 kg P ha–1 and 61 kg K ha–1 were applied before planting to avoid possible deficiencies. After preplant application, fertilizer was incorporated in the soil by plowing. Around the experimental plot, fertilizer was also applied by hand to avoid interferences.

The plots were 5 m (ten 50-cm rows) wide by 6 m long. The experiments were planted between 10 October and 15 November. Sugar beet cultivar Pamela was sown and hand-thinned to a population of 80000 plants per hectare. Plants from 7.5 m2 in the middle of the plot (5-m length of the three center rows) were collected (between 70 and 80 plants) between 15 June and 15 July to measure root yield and quality parameters (sucrose and {alpha}-amino N concentration in roots). Weeds, insects, and diseases were controlled using standard commercial procedures.

Soil samples were obtained by taking 20 soil cores to a 30-cm depth in the experimental site just before treatment. Soil cores were mixed and maintained at 4°C until processing. Processing involved drying at 35°C in a forced-draft dryer, milling, and grinding to pass a 2-mm screen. Texture (pipette method according to Gee and Bauder, 1986), pH (1:2.5 soil/water extract), carbonates (weight loss after addition of 3 M HCl), organic matter (Walkley and Black, 1934), and nitrate were determined in soil samples. Nitrates were determined in 2 M KCl extracts (1:10 soil/extractant ratio) by the Cd-reduction and nitrite determination methods as described by Mulvaney (1996).

At harvest, roots were washed and weighted, and a subsample of the roots was taken with circular saws. A subsample of brei (macerated root material) was used to measure sucrose content by the polarimeter method (it was assumed that optically active substances correspond essentially to sucrose; Halvorson et al., 1978), and another subsample was used to determine the content in {alpha}-amino N according to Last et al. (1976). Sucrose yield was estimated by multiplying the root yield per the sucrose concentration as determined by the polarimeter method.

Statistical analysis was performed using analysis of variance for each experiment. When the effect of the N rate was significant (P < 0.1), root and sucrose yield responses to applied N were fitted to the model with the highest R2 values (quadratic in all the cases). If the effect of N rate was not significant, N rate for maximum production was considered 0 kg N ha–1.

For profit evaluation, it must be considered that the price paid to farmers per metric ton of sugar beet depends on the sucrose content. In most European countries, the price that farmers receive for sugar beet is a function of the sucrose concentration of the beet. The adjusted beet yield was estimated as the root yield per the price correction factor used by the Spanish sugar company Azucareras Ebro Agrícolas S.L. (Madrid, Spain) in such a way that returns were normalized for 16% sucrose concentration:

where a% is the concentration (%) of sucrose in the harvested beet and pricea% and price16% are prices per metric ton of beet with a% of sucrose content and 16% of sucrose content, respectively.

Adjusted beet yield was also subjected to analysis of variance, and yield response to applied N rate was fitted using the best model (quadratic in all the cases) when the effect of fertilizer was significant (P < 0.1).

The optimum N rate for maximizing profit was estimated as the N rate at which the tangent of the crop value curve (quadratic) has the same slope as the cost of fertilization function (US$ 0.3 kg–1 applied N) (Black, 1993). The crop value curve was estimated as the adjusted beet yield response curve using the price of US $40 per metric ton for beet with 16% sucrose.

Relative beet, sucrose, and adjusted beet yield were determined at each site, as the ratio of the control mean yield and the non-N-limited yield. The non-N-limited yield was estimated as the maximum value (according to the quadratic model used to fit yield response) for each experiment. When the data did not fit a model, the non-N-limited yield was considered the average of the highest-yielding treatments that were not different based on the protected LSD analysis (P < 0.1). According to Meisinger et al. (1992), the expression of yields on a relative basis allows the evaluation of results across experiments, sites, and years. To estimate the critical NO3–N value in soil for yield response to N application, relative yield (beet, sucrose, and adjusted beet) vs. soil nitrate was fitted to a linear-plateau model. This model fit a linear response line for nitrate values lower than the critical value and a plateau line for soil nitrate values higher than critical value. The critical value was considered the intersection of linear and plateau lines. The linear-plateau model was also used to fit the optimum N rate vs. initial soil nitrate. All statistical analyses, including analysis of variance and fittings to quadratic or linear-plateau models were performed using the SAS program (SAS Inst., 1989).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Response to Applied Nitrogen: Critical Nitrate Values
Residual nitrate from the previous crop ranged from 6 to 156 mg of NO3–N and thus sometimes represented a significant N supply to sugar beet. In general, high levels of residual nitrate can be expected after cotton (Gossypium hirsutum L.), corn, and horticultural crops such as tomato (Lycopersicum esculentum Mill.) (Table 1), which are crops that usually receive high N rates (usually 350– 400 kg N ha–1). There were locations where the response was significant, with very high initial NO3–N concentration in soil (156 mg kg–1 in Hato Ratón in 1997–1998, 73 mg kg–1 in 2052, and 72 mg kg–1 in Hato Ratón in 1998–1999), and others where no response was observed, with low initial NO3–N (Verdiblanca) (Table 2). An accurate evaluation of the response to applied N must consider particular circumstances of each location. Three groups can be established to describe the response to applied N fertilizer: (i) locations with an important risk of nitrate leaching (tile-drained soils with furrow irrigation and/or rainy winters—more than 470 mm from October to January, 80% of the annual mean rainfall), (ii) locations with an important restriction in water supply (rainfed locations in drought year, less than 350 mm rain during the year and less than 170 mm during the spring), and (ii) others with no important leaching or water supply limitations.


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  		Table 2. Effect of N rates on yield parameters [beet production, sucrose concentration in beet, and adjusted beet production (AB)] and N rates for maximum beet and sucrose production and for adjusted beet production (maximum and economic optimum).

 
In drained soils, nitrate leaching due to preferential flow may be important in rainy periods or with high irrigation rates (Martin et al., 1991; Andreu et al., 1999). Nitrate losses in these soils may be particularly important if furrow irrigation is used (Benjamin et al., 1998). Thus, yield response to applied N can be expected (independently of the initial residual nitrate) in locations under these conditions (San Carlos, 2089, Hato Ratón, and 2052) because a significant fraction of N (residual or applied) is lost by leaching.

In locations under rainfed management and drought years, water is limiting for crops and for N uptake, and thus an irregular response to applied N can be expected: No significant response was observed at Santa and Verdiblanca, and significant effect of N was observed at Motilla and Sevillana (Table 2).

When no leaching or water supply limitations are expected, a critical value for preplant NO3–N in soil may be estimated, allowing the distinction of N responsive and nonresponsive sites. Figure 1 shows the relative yield (beet, sucrose, and adjusted beet) in relation to NO3–N in soil before planting and critical values for locations with no special limitations (drainage or water supply). For beet and sucrose production at these sites, a critical value of 39 mg kg–1 NO3–N was estimated. For adjusted beet yield, the critical value was 33 mg kg–1. Maximum beet production was obtained with N rates higher than those needed for maximum adjusted beet production as Table 2 shows. A greater optimum N for beet production than for adjusted beet production can be the consequence of the effect of excessive N uptake on: (i) quality parameters, especially sucrose concentration in the root, which is negatively related to N rate in most locations (Table 3), and (ii) the increase in the shoot/root ratio in plants as the N rate increases (Halvorson et al., 1978; Halvorson and Hartman, 1980).



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  		Fig. 1. Relation of (a) relative beet yield, (b) relative sucrose yield, and (c) relative adjusted beet yield to preplant soil NO3–N. Tile-drained soils with high risk of leaching (furrow irrigation or winter rain higher than 470 mm—80% of the annual mean precipitation in the studied area) and rainfed locations in drought years (less than 350 mm year–1—less than 170 mm during spring) are excluded.

 

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  		Table 3. Linear regressions relating sucrose (polarimeter method) and {alpha}-amino N content in roots with N fertilizer rate.

 
In most locations, there was a significant negative effect of N fertilization on sucrose concentration in roots and a positive effect of N fertilization on {alpha}-amino N content (Tables 2 and 3). This determines the crop profitability since the payment to growers is related to sucrose concentration (in some European countries, price is also affected by the {alpha}-amino N content). The effect of N on sucrose content is well documented (Halvorson et al., 1978; Akeson et al., 1979; Carter and Traveller, 1981) although in the locations studied, sucrose concentration in roots was also affected by other factors. Water stress, especially at the end of the crop season, can result in a higher sucrose content in the root. This is a physiological response to low soil water potential, which explains why locations with high water restriction showed high sucrose content in roots (Verdiblanca, Motilla, and Sevillana; Tables 2 and 3). In this sense, when all of the locations were considered, the mean sucrose concentration in roots at zero-N rate was negatively related to total water supply during the crop cycle (Y = 179 – 0.02X; R2 = 0.18, P < 0.05).

When locations with a large risk of nitrate leaching were excluded, the mean {alpha}-amino N content at zero-N rate was related to initial soil nitrate and negatively related to total water supply (Y = 39 + 0.63 x NO3–N – 0.03 x Water; R2 = 0.34, P < 0.01), indicating water stress could contribute to increase the {alpha}-amino N content, negatively affecting the industrial quality. In locations with substantial nitrate leaching, the content of these substances was generally low (Tables 2 and 3), indicating that excessive N uptake by crops that determine high {alpha}-amino N content was limited by leaching conditions. In locations with no leaching or water stress limitation, the mean {alpha}-amino N in root at zero-N rate was related to initial nitrate levels in the soil (Y = 4.2 + 1.01 x NO3–N; R2 = 0.44, P < 0.01). Thus, high levels of residual nitrates in soils could negatively affect the industrial quality of the crop. Above the critical value of 33 mg kg–1 NO3–N for adjusted beet production, the level of {alpha}-amino N content tends to be greater than 30 mmol kg–1 (limit for reasonable industrial quality).

Optimum Nitrogen Fertilizer Rate
Table 2 shows the N rate for maximum beet, sucrose, and adjusted beet production and the optimum economic N rate for adjusted beet production, which is the most important variable for evaluating profitability. As discussed above, an accurate study of preplant soil NO3–N for estimating optimum N rate must consider particular conditions of each location, particularly leaching risk in tile-drained soils and water stress in rainfed locations. Only when these locations are excluded can an accurate relationship between optimum N rate and preplant soil NO3–N be determined. Under these conditions, preplant soil NO3–N explained 93% of the variation in the optimum N rate for adjusted beet production (Fig. 2) and provided an accurate estimation of the optimum N rate in half of the locations. Except for two of the locations, the deviation of the optimum N rate for adjusted beet production from the estimated rate using soil nitrate did not imply a reduction in profitability greater than 1%. According to our evaluation, no N should be applied at NO3–N values greater than 38 mg kg–1. The y-axis intercept of the regression represents the optimum N rate with no residual N in soil (no N supply by soil). This is an approximation of the amount of total N necessary for optimum production of the crop: 268 kg N ha–1 for adjusted beet yield. For locations where drought stress and/or leaching are not significant, N rate for maximum beet or sucrose production can also be reasonably estimated using soil nitrate (Fig. 2).



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  		Fig. 2. Relation of N rate for maximum beet (a) and sugar production (b) and optimum N rate for adjusted beet production (c) to preplant soil NO3–N. Tile drained soils with high risk of leaching (furrow irrigation or winter rain higher than 470 mm–triangles) and rainfed locations in drought years (less than 350 mm–circles) are excluded from the regressions.

 
In locations with no particular limitations (leaching or high water stress), no better estimation of optimum N rate can be expected considering other soil or climate factors (organic matter and water supply) or expected yield (beet, sucrose, or adjusted beet) besides the NO3–N concentration in soil before sowing. Under Mediterranean conditions, mineralization of organic matter has low contribution to the available pool of N due to: low levels of organic matter in soil and (ii) unfavorable environmental conditions for mineralization (soil water content and soil pH). López-Bellido and López-Bellido (2001) observed that the amount of N mineralized in similar soils from southwest Spain usually ranged from 15 to 35 kg ha–1 during a crop season (from 6–13% of the y-axis intercept in Fig. 2c), similar to observed denitrification losses in irrigated crops of the zone (Sánchez et al., 2001). This contributes to explain why organic matter content does not improve the estimation of optimum N rate.

In drained locations with a high leaching risk (San Carlos, 2089, Hato Ratón, and 2052), optimum N rate for the different yield parameters was not related to preplant soil nitrate, apparently because of N leaching. This may have also contributed to the strong relationship between optimum N rate for adjusted beet yield and winter rain in these locations (Y = 66 + 0.31X; R2 = 0.96, P < 0.001). In locations with high water deficiency (rainfed locations with total water supply lower than 350 mm during the year), an irregular relationship between optimum N rate and preplant soil nitrate was observed. In two cases (Santa and Verdiblanca), no response to applied N (optimum rate = 0) was observed (Table 2). In two other locations (Motilla and Sevillana), there was a significant response to applied N.

Distinction between soils with different yield potential can improve the accuracy of N recommendation based on soil nitrate (Bundy and Andraski, 1995; Andraski and Bundy, 2002). In the studied soils (in Mediterranean agriculture in general), main limitation to yield potential was water supply. No especial chemical or physical limitations, such as soil depth, drainage, water-holding capacity, or length of growing season (properties used to distinguish locations with different yield potentials; Andraski and Bundy, 2002), are usual for growing sugar beet in southern Spain. In studied locations, water supply in spring (from February to harvest) explained 47% of the variation in mean beet yield (Y = 33 + 0.09X; R2 = 0.47, P < 0.001). Thus, distinction based on water supply should constitute, in fact, a separation between locations with different yield potential. However, when only nonlimited locations (no leaching and no high water deficiency) were considered, the variation in mean beet yield explained by water supply in spring was still 27% (P < 0.01). In these locations, water supply in spring did not contribute to explain optimum N rates, probably because the lower the water supply is, the lower the N extraction by crop (lower beet yield) but also the lower the efficiency in N use by crop (Black, 1993).

Shock et al. (2000) observed that the petiole NO3–N concentration could be used to estimate N fertilizer requirements of sugar beet. However, the prediction of N requirement based on this analysis may fail when the residual N remaining from the preceding crop is very high (Shock et al., 2000; Schröder et al., 2000). Petiole NO3–N concentration decreases during crop season (Gordo and Bilbao, 1999), and it is affected by environmental factors such as radiation or soil moisture levels (Marschner, 1995). Thus, for petiole nitrate analysis, sampling procedure must take in account the precise development stage and even the hour of the day and radiation of previous days. For routine recommendations, sampling procedures could limit the application by growers, comparing to the simplicity of preplant soil sampling. However, major limitation for using petiole analysis for N recommendation for sugar beet in Mediterranean areas could be the decreased N uptake under water deficiency in dry winters before irrigation season or in rainfed locations. Under these conditions, N uptake by plants may be limited, decreasing the nitrate levels in petiole (Gordo and Bilbao, 1999), which may not reflect the N supply by soil in future steps (Schröder et al., 2000). In contrast, under rainy winters, the nitrate movement to deeper soil horizons could determine a decreased nitrate concentration in petioles at sampling time (end of winter for presidedress application), which may not predict future N supply by deep horizons. These reasons may explain the nonsignificant relationship between petiole nitrate at the end of the winter and N response in autumn-sown sugar beet in Spain obtained in preliminary studies by AIMCRA (unpublished results).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Preplant soil nitrate allows the distinction of N responsive and nonresponsive sites for sugar beet and provides an accurate estimation of the optimum N rate for adjusted beet production when drainage and water supply limitations in soils are considered. The method is useful in nondrained irrigated soils (without leaching or water stress limitations), which represents most of sugar beet soils in southwest Spain, a representative zone of Mediterranean agriculture. In tile-drained soils, it is useful when irrigation system (furrow) or winter rain do not promote an excessive N leaching. Thus, it can be used in tile-drained soils under sprinkler or drip irrigation, but recommendations must be revised after rainy winters (rainfall between October and January >80% of the annual mean rainfall in the zone) to assess an accurate sidedress fertilization. The method has been performed in nonorganically amended soils without legumes in crop rotation, usual management in soils used for sugar beet grow in southwest Spain. If manure is used or legumes introduced in the crop rotation, other N credits must be considered, limiting the usefulness of this recommendation method.


   ACKNOWLEDGMENTS
 
This work was funded by AIMCRA (Research Association for the Improvement of Sugar Beet Crop of Spain), a nonprofit institution financed by the Spanish Sugar Trust, with complementary funding from Spain's National R + D Plan (Project AGF99-0574-CO2-01). The authors thank Dr. José María Urbano for invaluable aid in statistics analysis.


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





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