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Agronomy Journal 92:1176-1183 (2000)
© 2000 American Society of Agronomy

SOIL FERTILITY

Evaluation of the Presidedress Soil Nitrogen Test for No-Tillage Maize Fertilized at Planting

Hernán Sainz Rozas, Hernán E. Echeverría, Guillermo A. Studdert and Germán Domínguez

Facultad de Ciencias Agrarias (U.N.M.P.)-Estación Experimental Agropecuaria Balcarce (I.N.T.A.), Unidad Integrada Balcarce, C.C. 276, (7620) Balcarce, Buenos Aires, Argentina

hecheverr{at}balcarce.inta.gov.ar


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Soil N transformations under no-tillage (NT) could affect the utility of the presidedress soil nitrate test (PSNT), particularly where early season growing conditions are cool as in Balcarce, Argentina. The PSNT was evaluated for irrigated NT maize (Zea mays L.) with different N rates (Exp. 1) and for rainfed maize under NT and conventional tillage (CT) with different preceding crops and N rates (Exp. 2). In both experiments, N was surface broadcast as (NH2)2CO at planting. The reliability of the PSNT was evaluated when NH+4–N was determined and when samples were taken up to a 60-cm depth in the first experiment. In this experiment, the relative yield (RY) was highly correlated with soil NO-3–N concentration (0–30 cm) at the six-leaf stage (V6). The reliability (R2) did not improve when NH+4–N was determined or when sampling was done up to a 60-cm depth. In the second experiment, there was a good relationship between the RY and NO-3–N concentration (0–30 cm) at V6 , and the R2 value increased when the preceding crop was wheat (Triticum aestivum L.). Soil NO-3–N critical concentrations ranged between 17 and 27 mg kg-1 and were associated with a RY of 0.92 or higher in both experiments. The results of this study show that the PSNT can be used to evaluate preplant N applications as a complementary method to N budget in maize under different management practices.

Abbreviations: CT, conventional tillage • LRP, linear response-and-plateau model • NT, no-tillage • PSNT, presidedress soil nitrate test • RY, relative yield • V6, six-leaf stage


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
NO-TILLAGE AFFECTS SOIL AND FERTILIZER N dynamics, decreasing N availability for crops due to potentially greater losses and immobilization (Fox and Bandel, 1986). Consequently, fertilizer N requirement of maize under NT is greater than under CT (Meisinger et al., 1985), and N fertilization is a key management practice to equal the yields under CT. Despite this constraint, approximately 7000000 ha are being cropped with NT in Argentina (Derpsch, 1999).

Nitrogen fertilization at V6 (Ritchie and Hanway, 1982) is more efficient than the application at planting, particularly under NT (Wells and Bitzer, 1984; Fox et al., 1986; Wells et al., 1992; Sainz Rozas et al., 1999). However, Argentinean farmers generally surface broadcast (NH2)2CO at planting, and N rates are commonly determined using the aboveground N balance approach (Meisinger, 1984) according to the following equation:

where Nf is fertilizer N input, Nch = crop grain N, Ncr = crop residue N, Nmin = estimated soil N mineralization, Nsin = estimated soil inorganic N, ef = fraction of Nf in the aboveground biomass, es = fraction of Nsin in the aboveground biomass, and em = fraction of Nmin in the aboveground biomass. In the above equation, Nsin is determined at planting, and Nmin is estimated from organic N content multiplied by a mineralization coefficient (1–3%). This methodology can be subject to considerable error because a variable fraction of soil mineral N at planting in humid climates can be lost before plant uptake (Hergert, 1987). Besides, the magnitude of N mineralization depends on organic matter content, substrate quality and accessibility, soil moisture, and temperature (Rice and Havlin, 1994).

Recent studies using the PSNT have shown a good correlation between maize grain yield and surface soil NO-3–N content when plants are at V4 to V6 (Magdoff et al., 1984; Fox et al., 1989; Binford et al., 1992; Meisinger et al., 1992b; Sims et al., 1995). Soil NO-3–N content at that stage represents the net balance between production (mineralization from soil organic matter, manure, and/or fertilizers) and loss (leaching, denitrification, and immobilization) because little or no N uptake occurs prior to that stage (Meisinger et al., 1992b). Magdoff et al. (1984) have proposed the PSNT as an index of N-mineralization intensity from soil organic matter, assuming that most N should be applied at V4 to V6. However, the PSNT can also be used as a tool to evaluate preplant N applications and to indicate if additional N is needed at V6 (Binford et al., 1992).

The PSNT can have some limitations. For example, it could be less useful in: (i) cool springs that can affect mineralization and nitrification of N from soil organic matter or fertilizers, (ii) highly leachable nonstructured soils, and (iii) circumstances where denitrification or immobilization can affect the soil NO-3–N pool (Meisinger et al., 1992b). No-tillage changes the soil environment with respect to CT, and consequently can affect some soil-N transformation processes such as mineralization, nitrification, immobilization, denitrification, and/or leaching (Fox and Bandel, 1986).

In Balcarce, mean soil temperature at a 20-cm depth is 14.5 and 17.5°C for October and November (period between planting and V6), respectively (A. Della Maggiora, personal communication, 1985), and these temperatures are lower in soils under NT than in soils under CT (Griffith et al., 1977; Creus et al., 1998). Optimum temperature for soil N mineralization and (NH2)2CO hydrolysis is 40°C (Fox and Bandel, 1986; Kissel and Cabrera, 1988). However, (NH2)2CO and organic-matter decomposers have a range of growth temperatures that oscillate between 0 and 40°C (Alexander, 1977). On the contrary, the optimum temperature range of nitrifiers oscillates between 30 and 35°C (Alexander, 1977), being more sensitive to low and high temperatures (Haynes, 1986). Therefore, it is probable that N mineralization from soil and (NH2)2CO from planting to V6 represents the available N for maize, and the determination of NH+4–N beside NO-3–N in the soil sample could improve the predictability of the PSNT under NT.

On the other hand, N immobilization and denitrification losses can be important in soils under NT (Rice and Smith, 1982, 1984; Linn and Doran, 1984; Kitur et al., 1984). If these processes are of great magnitude after V6, the utility of the PSNT could be affected in soils under NT. However after V6, maize starts its most active growth and substantially increases its N and water consumption. Small denitrification losses and N immobilization have also been observed after that stage in maize crops under CT (Duxbury and McConnaughey, 1986; Bronson et al., 1992; Qian et al., 1997; Jokela and Randall, 1997).

Nitrate leaching losses from sandy soils can be greater under NT than under CT because of the higher moisture content (Thomas et al., 1973). On the contrary, Meisinger et al. (1992b) reported that NO-3–N leaching is not a highly efficient process in structured soils due to preferential flow through macropores, which are increased under NT (Thomas et al., 1989). However, NT generally shows a greater infiltration capacity than CT because of continuous macropores that are open at the soil surface (Unger and McCalla, 1980). Therefore, initial leaching losses of surface-applied N could be rapid under NT if heavy rainfall occurred soon after fertilizer application, probably resulting in a significant movement of water and NO-3–N through macropores (Tyler and Thomas, 1977). Conversely, fertilizer that had time to diffuse into aggregate micropores would be protected from subsequent leaching because of the higher proportion of water flowing through the macropores system under NT (Cameron and Haynes, 1986). Therefore, the predictability of the PSNT would not be improved by sampling deeper than 30 cm under NT.

No-tillage maize is becoming a very important crop in Balcarce, where climate is temperate-humid with cool springs, and soils present high organic-matter content (5–7%). Mineralized N from soils of this area with optimum water availability during maize growing season under CT oscillates between 150 and 240 kg N ha-1, with the highest value mentioned for maize occurring after pasture (Echeverría and Bergonzi, 1995). Information about N mineralization under NT is scarce, but N fertilization is needed either under NT or CT to obtain high yields. In order to improve N-use efficiency and minimize environment quality degradation, it is necessary to study the usefulness of the PSNT as an indicator of soil N availability.

The objective of this research was to evaluate the usefulness of the PSNT at Balcarce, Argentina to predict N availability for NT-irrigated continuous maize while sampling at different depths, with and without the determination of NH+4–N concentration in the soil sample. An additional objective was to evaluate the usefulness of the PSNT for maize under rainfed conditions with different preceding crops and tillage systems.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
This study used two N-response experiments that were carried out at the Estacin Experimental Agropecuaria of INTA-Balcarce, Argentina (37°45' S, 58°18' W; 130 m above sea level; 870-mm mean annual rainfall; 13.7°C mean annual temperature) on a soil complex of a fine, mixed, thermic Typic Argiudoll and a fine, illitic, thermic Petrocalcic Paleudoll (petrocalcic horizon was below 0.7 m). In the first experiment, NT irrigated maize was grown continuously from 1994–1995 to 1997–1998 growing seasons. Average soil surface (0- to 20-cm depth) organic-matter content and pH were 58.4 g kg-1 and 5.85, respectively. In the second experiment, maize was grown under rainfed conditions with different preceding crops and tillage systems (NT and CT) during the 1997–1998 growing season. Soil surface (0- to 20-cm depth) pH was 6.0 and organic matter content was 66.7 and 51.7 g kg-1 when preceding crops were a 3-yr-old pasture and wheat (Triticum aestivum L.), respectively. The pasture included grasses and legumes usually employed in the area [orchardgrass (Dactylis glomerata L.), perennial ryegrass (Lolium perenne L.), white clover (Trifolium repens L.), and red clover (Trifolium pratense L.)]. The plots with wheat as preceding crop had been under continuous cropping since 1976 and included wheat, potato (Solanum tuberosum L.), maize, sunflower (Helianthus annuus L.), and soybean [Glycine max (L.) Merr.]. Soil surface horizon (20 cm) of both experiments showed soil P concentrations that oscillated between 14 and 18 mg kg-1 (Bray and Kurtz, 1945). Nevertheless, in both experiments 100 kg ha-1 of triple superphosphate (0–46–0) was banded 5 cm below the seeds. Weeds and pests were adequately controlled.

In the first experiment, two single cross-maize hybrids were used: Dekalb 636 in the first two growing seasons and Dekalb 639 in the last two. Maize was planted in 70-cm row spacing on 15, 18, 20, and 23 October for 1994, 1995, 1996, and 1997, respectively. Plant stands at harvest were of 63700, 75000, 79000, and 74273 plants ha-1 for the first, second, third, and fourth growing seasons, respectively. Plots were 12-m long and four rows wide (2.8 m, 33.6 m2) in the first two growing seasons and five rows wide (3.5 m, 42.0 m2) in the others. The experimental design was a randomized complete block with a factorial treatment arrangement and a control treatment without N (0-N) with three replications in the four growing seasons. In the first growing season, the factorial arrangement was 3 x 2: Three N rates of 35, 70, and 140 kg N ha-1 with and without NBPT [N-(n-butyl) thiophosphoric triamide]. In the second growing season, the factorial arrangement was 4 x 2 with an additional N rate of 210 kg N ha-1. In the third growing season, only the 70- and 210-kg N ha-1 rates were evaluated (factorial arrangement of 2 x 2). In the last growing season, the experimental design was a randomized complete block with five N rates of 0, 70, 140, 210, and 280 kg N ha-1. In all cases, N was applied as surface broadcast (NH2)2CO at planting. The NBPT used was dissolved in CH3OH and sprayed on the (NH2)2CO at a rate of 0.25% (w/w). The NBPT did not significantly affect maize yields, even though it had been demonstrated in a previous publication (Sainz Rozas et al., 1999). Treatments with the inhibitor were included just to evaluate the PSNT in a greater number of situations, increasing the PSNT reliability.

For the second experiment, the design was a randomized complete block with a split split-plot treatment arrangement and three replications. Preceding crops (pasture and wheat) were considered main plots, tillage systems (NT and CT) as subplots, and N rates of 0, 60, 120, and 180 kg N ha-1 were assigned to the sub-subplots (131.3 m2). Conventional tillage consisted of moldboard plowing, disking, and harrowing. Tillage operations (CT) and chemical fallow (NT) were started 3 mo before planting. Nitrogen was applied as surface-broadcast (NH2)2CO at planting. Maize (Dekalb 639) was planted at 70-cm row spacing on 23 October. Plant stand at harvest was of 71400 plants ha-1.

Soil sampling was done when maize plants were at the V5–V6 stage (Ritchie and Hanway, 1982) by randomly collecting eight or nine 2-cm diam. cores between rows per experimental unit from the 0- to 30-cm soil depth every year, and also from the 30- to 60-cm soil depth in 1995 and 1996. Cores were composited for their analysis. In the years 1994, 1995, and 1996, NH+4–N and NO-3–N contents were determined by steam microdistillation in the first experiment (Bremner, 1965). In 1997, NO-3–N content was determined by colorimetry with the phenol-disulfonic acid method in both experiments (Bremner, 1965).

At maturity, 7.15 m of the two center rows of the experimental units were hand-harvested to determine grain yield. All reported yields were corrected to a 140 g kg-1 grain moisture content. Also, in both experiments 10 maize plants per experimental unit were collected for determination of aboveground dry-matter accumulation at physiological maturity (black layer). Plants were cut at ground level, separated into leaf blades (stalk+sheaths+tassel+husks) and grain, and oven dried, weighed, and milled (1-mm mesh). Reduced N was determined in each fraction without salicylic acid [C6H4(OH)(COOH)] modification (Method A) as reported by Nelson and Sommers (1973). Total N accumulated in each fraction was calculated as the product of its N concentration (dry wt basis) by its dry weight.

A linear response-and-plateau model (LRP) was used to describe the relationship between RY and soil NO-3–N (or NO-3–N NH+4–N) at the V5–V6 stage. Both LRPs and lineal models were fitted using the nonlineal (NLIN) and regression (REG) procedures of the Statistical Analysis System (SAS Inst., 1985). Soil NO-3–N (or NO-3–N NH+4–N) concentration at the intersection of the two lines of LRP was defined as the critical concentration. In both experiments, yield observed from each treatment factor combination was divided by average yield of the treatment with the highest N rate to determine the RY.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
Water availability did not limit maize yield in the first three growing seasons because rainfall and irrigation (Table 1) met evapotranspiration demands (530 mm) calculated for Balcarce by Andrade and Gardiol (1995). However, in the last growing season, the reduced water availability during grain filling (Table 1) could have limited crop yield slightly in the first experiment (Table 2) and markedly in the second experiment (Table 3) . In this experiment, soil water availability up to a 100-cm depth after the critical period for kernel set (1–2 wk before to 2–3 wk after silking) was lower than 50% of the maximum soil-available water, particularly in the fertilized treatments (Dominguez, 1999). Mean air temperature and incident radiation were slightly different among growing season (Table 1). The lower yield observed in the first experiment in 1996 to 1997 (Table 3) was caused by the effect of Maize Rough Dwarf virus disease (< 10% of the plants affected).


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  		Table 1 Rainfall and irrigation, monthly means of air mean temperature and incident photosynthetically active radiation for the 1994–1995, 1995–1996, 1996–1997, and 1997–1998 growing seasons

 

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  		Table 2 Analysis of variance of grain yield of maize at physiological maturity for different N rates with and without NBPT in the 1994–1995, 1995–1996, 1996–1997, and 1997–1998 growing seasons (Exp. 1)

 

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  		Table 3 Analysis of variance of grain yield of maize at physiological maturity for different preceding crops, tillage system, and N rates in the 1997–1998 growing season (Exp. 2)

 
In the first experiment, N fertilization significantly increased grain yield every year. On the contrary, grain yield was not significantly increased by NBP in any growing season (Table 2). This was attributed to the small volatilization losses observed for (NH2)2CO broadcast at planting (2.02 and 8.20% of N applied) in both growing seasons (Sainz Rozas et al., 1999). Rainfall events (>10 mm) shortly after fertilization (4 d) only occurred in 1995 to 1996. However, if rainfall events had not occurred, the magnitude of losses would not have been greater than those observed since loss rates had diminished notably after 3rd d of fertilization (Sainz Rozas et al., 1999). In the second experiment, there was significant interaction between the preceding crop and tillage system, preceding crop and N rate, and tillage system and N rate (Table 3). The lower water availability up to a 100-cm depth during grain filling in the fertilized treatments, and the presence of discontinuous layers of petrocalcic horizon at a 70-cm depth, could explain the decrease in grain yield observed at the higher N rates (Table 3). No-tillage diminished grain yield when the preceding crop was wheat (Table 3) and that would indicate the greater N availability generated by the pasture counteracted the lower N availability observed under NT. Compared with wheat, pasture produced a greater maize grain yield only when N was not applied (Table 3). Maize grain yield under NT was lower than under CT only in the treatments without N (Table 3), indicating that N mineralization was lower or that N losses were greater under NT than under CT. These results agree with those found by Meisinger et al. (1985).

High relationships between RY and NO-3–N concentration in the surface soil (0–30 cm) were observed every growing season in the first experiment (Fig. 1) . These results agree with previously reported research (Magdoff et al., 1984; Fox et al., 1989; Meisinger et al., 1992b; Binford et al., 1992; Bundy and Andraski, 1995; and Sims et al., 1995). These results indicate that under NT with relative low soil temperatures before V6, PSNT can also be used as an index of N mineralization intensity from soil organic matter before V6 and of N nitrified from (NH2)2CO applied at planting, both of which denote the potential N supply to the crop. The small number of points falling in the upper left quadrant confirm this idea (Fig. 1). On the other hand, the few points falling in the lower right quadrant indicate that leaching out of the root zone, denitrification losses, and immobilization of NO-3–N after V6 were not important under the conditions of this experiment. Figure 1 also shows that the points corresponding to (NH2)2CO with and without NBPT were equally spread over the plot. This indicates that mineral N availability at the V6 stage was not affected by NBPT due to the small volatilization losses and to the short duration of hydrolysis inhibition (Sainz Rozas et al., 1999).



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  		Fig. 1 Relationship between relative yields (RY) and NO-3–N concentrations in the surface 30 cm of soil at V6 in no-tillage irrigated continuous maize (Exp. 1) in the 1994–1995, 1995–1996, 1996–1997, and 1997–1998 growing seasons. 0-N= control; 35, 70, 140, 210, and 280= kg N ha-1; I= urea with NBP

 
Figure 2 shows the relationship between NO-3–N content up to a 30- to 60-cm depth and the N applied in 1995 and 1996. The slopes of the relationship for 0 to 60 cm were not significantly greater (P>0.05) than the slopes of the relationship for 0 to 30 cm in both growing seasons. This indicates that, despite some leaching occurred from the 0- to 30-cm layer to the 30- to 60-cm layer as suggested by a greater slope, the intensity of the process was low. In well structured soils, mineralized and nitrified N is most likely in the small-pore space where it can be partially protected from moving down through large pores (Meisinger et al., 1992b). This prevented NO-3–N leaching under NT, despite the 265-mm rainfall in October and November 1995 and 1996 (Table 1). Therefore, the reliability of the PSNT was only slightly improved in 1996 by sampling up to a 60-cm depth instead of a 30-cm depth (Table 4) . Binford et al. (1992) reported similar results under CT on similar soils in Iowa.



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  		Fig. 2 Relationship between NO-3–N content at V6 in 30- and 60-cm soil depths and N applied (NA) in the years (a) 1995 and (b) 1996

 

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  		Table 4 Parameters of fitting a linear response- and plateau-model (LRP) to maize relative yields (RY) as a function of NO-3–N concentrations up to a 30- and 60-cm depth (Part A) and as a function of NH+4–N + NO-3–N concentrations and of NO-3–N concentrations alone in a 30-cm depth (Part B) in Exp. 1

 
The relationship between maize RY and NH+4–N plus NO-3–N concentration in the 0 to 30-cm layer did not improve the reliability of the PSNT compared with the NO-3–N concentration alone (Table 4). This was due to the low soil NH+4–N content observed at the V6 stage for N fertilizations at planting (Sainz Rozas et al., 1999). These results agree with Blackmer et al. (1989) and Binford et al. (1992), who reported that the R2 did not improve when NH+4–N plus NO-3–N were determined in the soil sample compared with NO-3–N alone. On the contrary, Meisinger et al. (1992b) reported that the relationship between RY and soil mineral N (NO3-–N NH+4–N) was better than for NO-3–N alone. In this experiment, manure and composted sludge were used and soil was sampled within 6 wk after application, which may have caused that N was not completely nitrified at PSNT sampling. Rapid nitrification under NT with relatively low soil temperatures before V6 can be due to the high population of nitrifiers in the soil under study (Navarro et al., 1980). Likewise, since indigenous nitrifiers have temperature optima adapted to their climatic regions (Mahendrappa et al., 1966), those lower soil temperatures under NT before V6 were probably close to the optimum for nitrifier activity in these soils.

When maize was grown under rainfed conditions with different preceding crops and tillage systems in the second experiment, the R2 value of the relationship between RY and NO-3–N concentration was smaller than those observed in the first experiment (Fig. 3a) . However, the PSNT reflected different N availability generated by preceding crops, tillage systems, and N rates rather well. The greater error of the PSNT observed in this experiment was originated by plots that showed a low RY with NO-3–N concentrations greater than the critical concentration (lower right quadrant) and high RY with NO-3–N concentrations smaller than critical concentration (upper left quadrant) (Fig. 3a). The first situation could be caused by events of drought during grain filling (Dominguez, 1999) that affected maize growth and yield erratically because this soil presents discontinuous layers of petrocalcic horizon at 0.7 m that limits its water-storage capacity. Bundy and Andraski (1995) reported that soils of shallow root zone increased the error of the PSNT because other variables such as water availability affected crop yield.



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  		Fig. 3 Relationship between relative yields (RY) and NO-3–N concentrations in the surface 30 cm of soil at V6 in maize under no-tillage (NT) and conventional tillage (CT) with (a) pasture and wheat as preceding crops, and (b) maize under no-tillage (NT) and conventional tillage (CT) with wheat as preceding crop in the 1997–1998 growing season. Past= maize after a 3-yr pasture; W= maize after wheat

 
On the other hand, most of the plots with high grain yield and NO-3–N concentration lower than the critical concentration (upper left quadrant) had pasture as the preceding crop. The reliability of the PSNT grew only when the data of maize after wheat were analyzed (Fig. 3b), indicating that the PSNT had a greater error when pasture was the preceding crop. Recovery of labile soil organic-C fractions in this soil is quick and thorough under pasture (Studdert et al., 1997) and fraction is highly associated with potentially mineralizable N (Curtin and Wen, 1999). Echeverría and Bergonzi (1995) reported values of potential N mineralization of 200 to 250 kg N ha-1 during the maize growing season for similar soils under pasture. Therefore, when the preceding crop was a pasture, the organic N pool was greater than when wheat was the preceding crop. However, the N effectively mineralized by the time of sampling could have been low, and/or the greater amount of C returned to the soil with the pasture could have generated a more intense immobilization before V6. Similar results have been reported by Bundy and Andraski (1993).

Critical NO-3–N concentrations varied among growing seasons and experiments according to the maximum yield observed. A high relationship between these two variables was observed ( , Fig. 4) . In the first experiment, where maize yields ranged between 10948 and 14065 kg ha-1 (Table 2), critical concentrations ranged between 21 and 27 mg kg-1 (Fig. 1), which agree with previously reported research (Fox et al., 1989; Meisinger et al., 1992b; Binford et al., 1992; Bundy and Andraski, 1995; Jemison and Lytle, 1996). However, for maize grown under rainfed conditions in the second experiment, maximum grain yields ranged between 8000 and 8700 kg ha-1 (Table 3) and response to N fertilizer would not be expected if the soil NO-3–N concentration at V6 were > 17 mg kg-1 (Fig. 3). This result agrees with those reported by Sims et al. (1995), who determined a critical concentration of 17 mg kg-1 for similar grain yields. In both experiments, the N accumulated by maize at R6 was highly associated with lower-than-critical soil NO-3–N concentrations at V6 (Fig. 5) . Therefore, when the PSNT results were lower than the critical concentration, it could be used as an aid for making accurate N fertilization recommendations.



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  		Fig. 4 Relationship between maximum grain yield (GY) and NO-3–N concentration at V6 for no-tillage (NT) irrigated continuous maize (CM) in the 1994–1995, 1995–1996, 1996–1997, and 1997–1998 growing seasons, and no-tillage and conventional tillage (CT) maize grown under rainfed conditions after wheat and pasture in the 1997–1998 growing season (Exp. 2, 1997–1998). Maximum GY is the average of the highest N rate for each experiment in each growing season. NCC= critical concentration of NO-3–N

 


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  		Fig. 5 Relationship between accumulated nitrogen (AN) by maize at R6 and below-critical-concentration soil NO-3–N concentration at V6 for no-tillage irrigated continuous maize (CM) in the 1994–1995, 1995–1996, 1996–1997, and 1997–1998 growing seasons, and no-tillage (NT) and conventional tillage (CT) maize grown under rainfed conditions with pasture and wheat as preceding crops in the 1997–1998 growing season. Past= maize after 3-yr pasture; W= maize after wheat

 
Under the conditions of this study, the results show that the PSNT adequately reflects soil N availability for NT maize, even when some N had been applied at planting, and can be used as a complementary method to the aboveground N-balance approach. However, when the preceding crop was a pasture, the PSNT would have a greater error and would be less useful to predict maize N requirements. There was a slight advantage of sampling up to 60 cm instead of up to 30 cm, but this advantage was not great enough to justify it. When NH+4–N exchangeable was included in soil sample, the predictability of the PSNT was not improved.Meisinger Bandel Angle O'Keefe Reynolds 1992; SAS Institute 1985


   ACKNOWLEDGMENTS
 
This study was made possible with financial support of the Instituto Nacional de Tecnología Agropecuaria and the projects PICT-97 08-00000-00089 from the Agencia Nacional de Promoción Científica y Tecnológica and 15/A107 from the Universidad Nacional de Mar del Plata.

Received for publication October 18, 1999.
   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 




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The SCI Journals Crop Science Vadose Zone Journal
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome