The chlorophyll meter is a promising tool to monitor the level of nitrogen (N) through the greenness index (VI). The hypothesis was that the estimates obtained from the use of the chlorophyllometer will be more linked to the N harvested in the dry matter of wheat (Triticum aestivumL.) than to the grain yield since this will be affected by the climatic variation. The objective was i) to determine the sensitivity of the IV to predict the dry matter (MST) and grain, protein content and accumulation of N in the crop and ii) to evaluate the availability of N at the time of planting to establish reference plots with sufficiency. of N, through the use of a chlorophyllometer in different stages of the wheat crop, comparing field trials with another in pots with controlled water. In 2010, a fertilization trial with N in pots was carried out, while in 2011 and 2012 field trials were carried out in two sites in the Southwest of Buenos Aires (SOB). In two (Z22 and Z40) and three stages (Z22, Z40 and Z70) of the wheat crop, chlorophyllometer readings were taken for field and pot trials, respectively.2 =0.11-0.29). In pots, the prediction of N absorbed with readings in the three stages increased, with no differences between the slopes and obtaining a high adjustment (R 2 =0.79). The chlorophyllometer would be a promising indicator of N absorbed, however, in these regions it would not be an accurate tool to predict yield or protein content in wheat (variety ACA 303) at field level. It would also be useful to estimate the necessary available N and also to know the nutritional status of the plant.
Keywords. Greenness index; ACA 303; Sub-humid region.
ASSESSING NITROGEN REQUIREMENTS OF WHEAT CROPS WITH A CHLOROPHYLL METER IN THE SOUTHWESTERN PAMPAS
ABSTRACT
The chlorophyll meter is a promising tool for monitoring the N status through the green index (IV). The hypothesis is that estimates from the use of a chlorophyll meter is more closely linked to the N harvested from dry matter than to wheat (Triticum aestivumL.) grain yield, which is affected by climate variability. The objectives of this study were to i) determine IV sensitivity to predict total dry matter (MST) and grain yield, protein content and N-uptake, and ii) evaluate N-availability at planting in order to establish reference plots for using a chlorophyll meter during different stages of wheat. In 2010, a nitrogen fertilization trial was conducted in pots, while in 2011 and 2012, the trials were conducted on two sites in the Southwest of the Pampas (SOB) under no-tillage. In two (Z22 and Z40) and three (Z22, Z40 and Z70) stages of the crop, chlorophyll meter readings were taken for the field and pot trials, respectively. The field prediction of MST and grain yield, protein content and N uptake with chlorophyll meter readings showed significant regressions with low goodness of fit (R2 = 0.11 to 0.29). In the pot experiment, the prediction of N-uptake for the three sampled stages increased, without differences in slopes and with a high goodness of fit (R 2 =0.79). The chlorophyll meter would be a promising indicator of N-uptake by the crop; in sub-humid regions, however, it would not be an accurate tool to predict grain yield or protein content in wheat crops. Also, a chlorophyll meter would be useful to estimate the available N needed by the wheat crop and to determine the nutritional status of the plant.
Key words. Green index; ACA 303; Sub-humid Region.
INTRODUCTION
The cultivation of wheat is the base of the productive systems in a wide sector of the Southwest of Buenos Aires (SOB). Its yields are influenced by climatic conditions and soil properties, requiring an efficient use of water and nitrogen (Martínez et al., 2012). In semi-arid and sub-humid regions, optimization in the application of fertilizers is difficult, due to the erratic availability of water from rainfall (Galantini et al., 2000 ). In addition to said variation and scarcity, much of the rainfall occurs during the fallow period, which is why most of it is not used by crops, limiting yield and response to fertilization (Quiroga et al. ,2005). At present, the most widespread methodology for the diagnosis of N for the wheat crop is based on the measurement of the nitrate content in soils (0-60 cm), a practice that brings with it the inconvenience of the costs of the analyzes that can be and the operating time from sample collection to obtaining the analytical data. That is why diagnostic tools are being sought to reduce costs and increase the efficiency of N use. The chlorophyll meter (for example, the Spad ®) is a promising tool to monitor the nitrogenous status through the IV of the crop, which is expressed in reading units (Spad, US units). Specifically, it allows the relative concentration of chlorophyll to be measured by means of light transmitted through the leaf at 650 nm, which is the photosynthetically active wavelength and at 940 nm. The transmittance difference between these two wavelengths results in the VI that can be associated with the extractable chlorophyll content (Waskomet al., 1996). The green color intensity is related to the concentration of chlorophyll and N in the leaf (Wood et al., 1993), since the chloroplasts contain approximately 70% of the N of the leaf (Madakadze et al., 1999). This methodology allows characterizing the nitrogenous state in a non-destructive way (Sainz Rosas & Echeverría, 1998), quickly and easily, allowing frequent sampling and exploring the variability in a site in greater detail compared to other known methods (Gandrup et al. , 2004). In addition, it has the advantage of allowing correction in the short term since the data is obtained “in situ”.However, since the VI is affected by numerous factors such as genotypes, growth stages, other nutrients, diseases or insect attacks, and environmental conditions at the time of measurement, it is advisable to perform calibrations of the readings taking these into account. mentioned factors. One of the ways to generalize the IV readings to reduce the influence of the variables that can affect it is through calibration with reference plots that present excess N (Gandrup et al., 2004 ) . This makes it possible to determine the N sufficiency index (ISN), which results from the ratio between the US of each treatment with respect to the US of the reference treatment without N limitations (Gandrup et al., 2004 ) .
Due to the scarce information on the use of the chlorophyllometer in the SOB, its use requires a calibration that considers the environment and the different varieties of wheat, since these are important factors that affect the results obtained (Bavec & Bavec, 2001). . Regarding the effect of the variety sown, Cox et al. (1985) confirmed differences in terms of absorption and remobilization of the vegetative parts towards the grains.
The use of the chlorophyllometer will allow its use as a tool for the diagnosis of N deficiencies during the wheat cycle and in the event that it is necessary to carry out deferred applications of N. Arregui et al.(2006) have shown IV variations generated by different genotypes of the same species in different growth stages due to different water availability. Therefore, they concluded that it is not possible to establish a single critical IV value that indicates N sufficiency in all crops, sites, years, and environmental conditions. Consequently, for the conditions proposed in the sub-humid region of the SOB, it was hypothesized that the estimates obtained from the use of the chlorophyllometer will be more linked to the amount of N harvested in the wheat dry matter than to grain yield. since this will be affected by climatic variation.
The objective of this work was i) to determine the sensitivity of the IV to predict the yield of both MST and wheat grain, protein content and N accumulation in the crop and ii) to evaluate the availability of N at the time of sowing to establish reference plots with a sufficient amount of N. To this end, the comparison of IV values obtained with a chlorophyllometer in two stages of the crop of wheat grown in field trials and in pots with controlled irrigation was carried out.
MATERIALS AND METHODS
During the years 2010, 2011 and 2012 wheat fertilization trials with N were carried out. In 2010 a preliminary trial was carried out in pots, while in 2011 and 2012 field trials were carried out.
field trials
They were located in two different SOB sites: Hogar Funke (2011) and La Casilda (2012), both located in the Tornquist district. The soils, whose edaphic characteristics are detailed in Table 1, were taxonomically classified as typical Ar-giudol (Hogar Funke) (38°10’15.6″ S; 62°01’50.1″ W) and ustic Argiudol ( La Casilda) (38°19’25.4″ S; 61°44’21.4″ W). In both cases, the petrocalcic horizons (tosca) are located below a meter of effective depth and therefore do not constitute a limiting factor for crop development. The tillage system was direct sowing for both trials.
The experimental design was randomized complete blocks, with three replications during both years. The treatments consisted of 6 doses of N (0, 25, 50, 100, 150 and 200 kg N ha -1 ) applied at planting, in the form of urea (granulated, 46-0-0) and broadcast. The plots covered an area of 36 m 2 (9 m long and 4 m wide). Wheat sowing was carried out on June 22 and July 15, 2011 and 2012, respectively. The wheat variety used was ACA 303 in both cases. To ensure phosphorus (P) sufficiency in the soil, a rate of 20 kg P ha -1 was applied as triple superphosphate (granulated, 0-46-0, equivalent grade) during planting. The predecessor crops were sunflower(Helianthus annus L) and wheat, in the years 2011 and 2012, respectively.
Twenty measurements (readings) were made per plot with the Spad Minolta 502 chlorophyllometer in the wheat crop in two growth stages following the Zadoks et al. scale. (1974): tillering (Z22) and embuchada spike (Z40). The readings are They were carried out on the upper third of the last fully expanded leaf, in the center of the leaf between the margin and the central vein, avoiding said vein as well as chlorotic or damaged areas, if any (detailed recommendations in the equipment instructions). Simultaneously, at the same sampling times, 1 linear m of plant material was collected to quantify the production of MST and the content of total N (Nt) in the leaves (Bremner, 1996). At physiological maturity (Z90), samples of plant material were taken for the determination of MST and grain yield. On the plant material, the Nt content was determined both in grain and straw (Bremner, 1996). The protein was determined by multiplying the Nt of the grain by the factor 5.75 (Novoa & Loomis, 1981). From now on to MST, grain,
Because the soil properties were similar at both sites and no differences were found in the granulometric fractions (Table 1), in the statistical study the sites were taken as replicates and the years as a variable due to the climatic variability that characterizes the SOB.
The average yields in MST and grain of the treatments N 0 to N 200 of each trial were expressed as Relative Yield (RR), dividing its value by the average yield reached by the N 200 treatment:
RR= Performance N 0 to N 200 / Average performance N 200 (1)
The critical levels of the US and ISN variables were determined using the Cate & Nelson (1971) graphic method, setting a critical RR of 0.90.
pot test
The trial in pots was carried out on the premises of the Department of Agronomy of the Universidad Nacional del Sur, located in Bahía Blanca (38°41’48.2″ S; 62°15’0.17″ W) during the year 2010. The same was carried out with the pots outdoors (not under cover or greenhouse), in order to reproduce the field conditions in the most approximate way, but without water limitations, since weekly complementary irrigations were carried out with distilled water. A total of 225 plastic pots were used, with the following measurements: height (h)= 0.14 m, minor diameter (d)=0.103 m, major diameter (D)= 0.16 m, which resulted in a volume of 1.899 L and a total surface area of the pot of 0.020 m -2. The soil used was extracted from the depth of 0-20 cm, obtained from the same site where the pot trial was established. The analytical data of the sampled soil are detailed in Table 1.
Table 1. Edaphic characteristics of the soil (0-20 and 0-60 cm) of the selected sites and the trial in pots (0-20 cm). Table 1. Soil properties of sites (0-20 and 0-60 cm) and pot experiment (0-20 cm).
The pots were filled with 1600 g of soil each and sowing was done manually on July 16. Twelve seeds were placed per pot with the purpose of thinning once germination had occurred, leaving an average of 4 plants per pot. The wheat variety used was ACA 303, the same as in the field trial. A completely randomized experimental design with 15 repetitions was used. The evaluated treatments were 0, 25, 50 and 100 kg N ha -1 , applied in the form of urea (granulated, 46-0-0), on the surface of the pot. In addition, P was applied to the soil at the time of planting at a rate of 20 kg P ha -1, in the form of triple superphosphate (granulated, 0-46-0, equivalent grade). The IV measurements were made at three different times: tillering, embedded spike and watery grain (Z22, Z40 and Z70, respectively). At each sampling moment, ten readings were randomly taken with the chlorophyllometer, from which an average value per treatment was obtained. Immediately after the readings, the plants were harvested in three pots per treatment in the stages of Z22 (20%) Z41 (20%) and, the rest (60%), in aqueous grain (Z70). These samples were taken to an oven and dried at 60°C until reaching constant mass to quantify the dry matter (MST). Likewise, Nt was determined by the kjeldahl method (Bremner, 1996) and with these data the average kg N m -2 was obtained.. These data were then converted to kg per ha -1 . The RR was determined in MST, which was the relative yield of the plants at Z70 and was estimated in the same way as with the field method.
Chlorophyll readings both in the field trial and in the pots were relativized using the Nitrogen Sufficiency Index (ISN).
Statistic analysis
For the field trial, double ANOVA was performed to differentiate effects of doses and years. The evaluation of means was carried out by Fisher’s DMS (p<0.05). ANAVA was also carried out to evaluate the effect of N doses on US in the different stages, both in the field trial and in the pot trial. Simple linear regressions were performed both for the prediction of the MST, grain, protein and N absorbed yield by means of the US and for the determination of the N absorbed at harvest with the ISN. ANCOVA was performed to statistically verify the equality of slopes of the regressions between the ISN and the N absorbed. The statistical analysis was performed with the Infostat software (Di Rienzo et al., 2011).
RESULTS AND DISCUSSION
field trials
Total annual rainfall was 776 and 912 mm for 2011 and 2012, respectively. Despite the differences, both years were characterized by severe droughts in the spring months (Fig. 1), these being yield determinants according to Miranda & Jorquera (1994). In addition, taking into account the theoretical need for water (Nc) of wheat proposed by Paoloni & Vázquez (1985), a winter water deficit was observed for the year 2012.
Significant differences (p<0.01) were detected for the production of MST between the studied sites, in the moments of tillering (Z22) and spike embuchada (Z40). In addition, in the first sampling, significant effects (p<0.05) of the dose and dose x year interaction were observed (Table 2). When analyzed by year, for 2011 no effects of the dose were found (p>0.05) for the production of MST, while the differences were significant in 2012 (p<0.01), obtaining the highest productions in the dose of 100 kg N ha -1 . In Z40, differences were found between years but no effect of the doses, being the MST in 2011 > 2012, with mean values of 3561 and 2771 kg MST ha -1 , respectively.
At physiological maturity (Z90), no significant interaction (p>0.05) was observed between year and dose on MST, grain, and protein, despite the differences in the initial supply of inorganic N at planting time for both. years. In addition, there was no evidence of the effect of the year on any of the parameters evaluated. On the MST and the grain production, the effect of the doses (p<0.05) of N applied was evidenced (Table 2), as with the protein, increasing the level of significance (p<0.001) (Table 2).
Figure 1. Rainfall recorded during the years 2011 and 2012 and theoretical need for wheat water (Nc).
Figure 1. Rainfall during sampled years and crop water requirement (Nc).
Table 2. Total aerial dry matter of wheat in tillering (Z22), embedded spike (Z40) and physiological maturity (Z90), grain and protein yield according to nitrogen doses applied for both years. Table 2. Total wheat dry matter at tillering (Z22), booting (Z40) and physiological maturity (Z90) stages, grain yield and protein for both years and nitrogen application rates.
In relation to the N absorbed by the crop, no interaction between doses per year was determined. In stages Z22 and Z90, highly significant effects of N fertilization (p<0.01) were found for the average of both years (Fig. 2a). In Z40 only significant differences (p<0.001) were found between years (data not shown) as well as in stage Z22 (p<0.01), with higher mean values of 94 and 60% in 2012 compared to 2011. for Z22 and Z40, respectively. In addition, a highly significant relationship (p<0.001) was observed between the N absorbed in Z90 and the available N with an adjustment of 58% (Fig. 2c).
The US readings presented differences in the two wheat stages for both years, with no dose x year interaction (Table 3), despite the different initial availability of inorganic N (Table 1). only observed
decreases in US between the two moments analyzed for the control treatment, in accordance with what was reported by Sainz Rozas & Echeverría (1998), Falótico et al. (1999) and Gandrup et al. (2004). In Z22, no significant differences (p>0.05) were observed between the N doses applied. This is due to the fact that the maximum N absorption rate of wheat occurs from the first node (Barbieri et al., 2009). On the other hand, in Z40, highly significant differences were obtained between doses (p<0.001) with respect to the treatment without N application. In SOB soils, IV values between 46-47 US measured in spike would ensure an adequate supply of N, although determined on wheat varieties other than ACA 303(Loewy & Ron, 2008). In our study, with the exception of the control treatment, all the other doses would indicate a good supply of N in the spike and for both years. Lopez-Bellido et al. (2004) found close relationships (R 2 =0.90) between US and N doses in the flag leaf stage. This value would be similar (R 2 =0.92) to that found in our study for the mean values in stage Z40 (data not shown).
Figure 2. Amount of nitrogen absorbed a) average of both years for the field trial and b) in pots, by wheat stage (kg N ha -1 ) and available N level in the soil c) at depth sowing from 0-60 cm in the field trials and d) for 0-20 cm in the pot trial. Different letters indicate significant differences between doses with p<0.05.
Figure 2. Absolute amount of nitrogen uptake (kg N ha -1 ) for a) average of two years from field and b) pot experiments, for each wheat growth stage; and available soil nitrogen at c) sowing in field experiment at the 0-60 cm-depth and d) in pot experiment at the 0-20 cm-depth. Different letters indicate significant differences at p<0.05.
pot test
The yield of MST in milky grain (Z70), final stage of the crop showed significant effects (p<0.05)between doses (Table 4). The N absorbed showed significant differences between the doses in the three evaluated stages, being significant (p<0.05) for Z40 and highly significant (p<0.001) for Z22 and Z70 (Fig. 2b). In Z70, a highly significant close association (p<0.001) was observed between the N absorbed by wheat and the N available at sowing (Fig. 2d). A significant effect of N doses on US was determined in all stages. Unlike the field trial, lower values were detected in the US in the three stages analyzed, with a slight increase between Z22 and Z40 and a decrease between Z40 and Z70 (Table 4). These results would indicate that when water availability was not limiting, US would be a good estimator of the N content absorbed by the wheat crop.-1 were below the threshold reported by Loewy & Ron (2008), who working on SOB soils and with other wheat varieties (Buck Sureño, Buck Guapo and Buck Farol), reported values of 4346 US in spike for a correct supply of N. However, these values coincide with what was found in previous works (Martínez et al., 2012) for a soil from the Tornquist district and the Buck Poncho wheat variety, not finding large differences between the US in Z22 and Z40 .
Prediction of the parameters evaluated: comparison of trials in the field and in pots
Under field conditions, the prediction of MST, grain, protein and N absorbed at physiological maturity by US showed that although the regressions were mostly significant (p<0.05) (except protein with US in Z22), the fits were low (R 2 <0.29) (Table 5). Reeves et al. (1993) stated that the chlorophyllometer could be used as a good predictor of wheat grain yield in Z22. However, Vidal et al. (1999) failed to establish a significant relationship between US and wheat yield in Z30, coinciding with our study. Denuit et al. (2002) and Arregui et al.(2006), for their part, suggested that the stadiums closest to Z40 would be better predictors of grain yield.
Table 4. Greenness index (US) of wheat as a function of growth stages and total dry matter (MST) in Z70, according to nitrogen dose for the pot trial.
Table 4. Wheat green index (US) at different stages and total dry matter (MST) at Z70 by nitrogen rate and statistical analysis (ANOVA) per treatment in pots experiment.
| greenness index | MST | |||
| Dose N (kg N ha -1 ) | Z22 | Z40 | Z70 | Z70 |
| (US) | (kg ha -1 ) | |||
| 0 | 35.8 to | 37.3 to | 32.6 to | 1318 to |
| 25 | 42.3b | 37.9 to | 39.4 b | 1698 BC |
| fifty | 43.6 BC | 46.9 b | 42.2 BC | 1491 ab |
| 100 | 45.2c | 48.0b | 45.8c | 1819c |
| Dose (D) | *** | ** | ** | * |
ns: not significant; (*): p<0.05; (**): p<0.01; (***): p<0.001
Different letters indicate significant differences between doses with p<0.05.
Different letters indicate significant differences at p<0.05.
In the experiment carried out in pots, significant regressions (p<0.05) were observed between the US and the MST for Z22 and Z70 (R 2 = 0.41; 0.43), while for the absorbed N the predictions increased considerably. in the three evaluated stages. In the three moments, highly significant regressions were established (p<0.001) with values of linear adjustments greater than 0.76 (Table 5). The elevated R 2obtained in the experiment in pots, even in early phenological stages such as Z22, would show that the chlorophyllometer would be a good tool to determine the nutritional status of wheat and thus be able to early correct eventual N deficiencies when water conditions are not limitations. These differences between the field experimentand that of pots, would show that when additional irrigation was not carried out, the water factor turned out to be of great importance for these sites with water deficit and erratic rainfall. Echeverría & Studdert (2001), working on soils with less water restrictions and with the wheat variety ProInta Oasis, showed that the IV measured in advanced stages would be a promising estimator of nitrogen nutrition in the wheat crop. Nevertheless, the fit achieved by other authors was greater than that reported in our work, since values of R 2 =0.89 are quoted in the regression between protein and US in Z73 (Echeverría & Studdert 2001) and R 2 = 0.69 and 0.62 for the prediction in Z22 and Z40 in the Buck Ponchoen varietySOB soils (Martínez et al., 2012).
When the relationship between the ISN and the N absorbed at harvest by wheat was evaluated, important differences were observed between the field and pot trials. For the field experiment, significant relationships (p<0.05) were found for both stages, however, low degrees of association were observed (Fig. 3a) and the slopes of the regressions between ISN and N absorbed for both stages were different. (p=0.0084). In Z22 it was observed that in most of the data, the ISN was above the 95% proposed by Falótico et al. (1999) as a sufficient level of N, which indicates that in this stage the crops presented nitrogenous nutrition without deficiencies. This could be due to the fact that during the initial stages of the crop cycle the N needs are not great and are covered by the supply of said nutrient from the soil.
Figure 3. Relationship between the nitrogen sufficiency index (ISN) and the absorbed nitrogen in a) field trial and b) average of the three stages in pots.
Figure 3. Relationship between nitrogen sufficiency index and nitrogen uptake at different stages for a) the field experiment and b) the average of three stages in pots experiment.
In the experiment in pots, no differences were found in the slopes (p=0.3047) in the relationships of the ISN with the US in Z22, Z40 and Z70. On average of the three stages, a highly significant linear relationship was found with an adjustment of R 2 =0.79 with the N absorbed at physiological maturity (Fig. 3b). This trend coincides with what was observed with the US (Table 5). Gandroup et al. (2004) found similar results in both the ISN and the US in two sites in the Southeast of Buenos Aires without water limitations, for which the additional irrigation carried out in the pots demonstrated the importance of an adequate water supply in the absorption and remobilization of N. (Papakosta & Gagianas, 1991). According to LeBail et al.(2005) the remobilization of N stored in plant organs at the time of flowering represents approximately 70% of the N absorbed in grain. This high degree of explanation between the variables at all times in the experiment in pots, even in early phenological stages such as tillering, demonstrated that the use of the sufficiency index, as well as the US, allowed determining the nutritional status of wheat under conditions without water limitations. However, in field conditions these results differed due to the climatic conditions registered in the region, as already mentioned.
Spad Unit Sensitivity and Nitrogen Sufficiency Index
In the field trial, through the calibration of Cate & Nelson (1971), critical values of 45 and 49 US were detected below which there would be a response to the application of N in Z22 and Z40, respectively. The error in the calibration was 28% in both stages studied (Fig. 4a and b). In addition, low degrees of fit were observed in the linear relationships between both parameters (R 2 = 0.16; 0.20). In Balcarce, Sainz Rozas & Echeverría (1998), they reported close relationships between the RR of maize and the US working in different stages.
In the experiment in pots, critical values lower than those obtained in the field trial and with fewer errors (16 and 25%) were found, with values of 43.5 and 39 US for Z22 and Z40, respectively. In addition, better adjustments of the RR of MST were observed with the US with values that oscillated between 0.41 for Z22 and 0.27 for Z40 (Fig. 4c and d). On the other hand, for Z70, a better adjustment was found (R 2 =0.43) and a critical range that oscillated between 39 and 43 US, and an error of 25%.
When the calibration between ISN and N available at sowing was carried out, comparing the experiments both in the field and in pots, at both sampling moments, contrasting results were found. For the field trial with the ISN at 95%, in Z22 no critical values of N available below the threshold were observed. This was possibly due to the fact that in this crop the maximum N uptake rate began at the one-node stage (Barbieri et al., 2009), while in Z40 a critical value of 160 kg available N ha -1 was observed (Fig. . 5). In contrast, in the pot trial, critical ranges of available N were obtained that were similar in both stages, ranging between 50 and 80 kg N ha -1. This confirms what was previously described about the water stress suffered by the crops in the field, while in the pots this could be reversed with additional irrigation, confirming a better remobilization of N in the crop (Fig. 2d). It has been reported that remobilization from vegetative organs to grains and its efficiency are dependent on climatic conditions and genotype (Barbottin et al.,2005). In summary, the production factor that most limits production in these soils is humidity due to the particular conditions of the SOB, characterized by the interannual variability of rainfall both in distribution and quantity. Water deficits, produced especially in critical periods of the crop, such as the grain filling period, are of great importance in the remobilization of N to the grain (Angus & Fisher, 1991).
Other authors (Calviño et al, 2002; Barbieri et al., 2009) working on soils with less water restrictions, reported values for which the wheat crop did not respond to fertilization, varying between 100 and 150 kg N available ha -1 for the depth of 0-60 cm. These values were similar to those found in this study, however, they were determined for yields much higher than those obtained locally, so this lower efficiency would be the result of water scarcity in certain critical periods of the crop.
CONCLUSIONS
Based on the results obtained, the chlorophyll meter would be a promising indicator of N absorbed; however, in the SOB, due to climatic variability, it would not be an accurate tool to predict yield or protein content in the field with the variety of wheat ACA 303. It would also be useful to estimate the available N necessary for wheat sowing and also to know the nutritional status of the plant.
The readings carried out at more advanced moments allowed us to predict that the N absorbed in physiological maturity>MST>grain, the accuracy of the prediction being lower as the water factor had a greater influence on the parameters evaluated.
Figure 5. Relationship between the nitrogen sufficiency index and available nitrogen for a) Z22 and b) Z40 for the depths studied in the field and pot trials.
Figure 5. Relationship between nitrogen sufficiency index and soil available N at a)Z22 and b)Z40 wheat growth stages for field and pot experiments at their respective depths.
The results found confirm the consequences of water deficits that generally occur in critical crop periods in these environments, placing them as the main production factor in these sub-humid regions. These results could be useful for other sites with similar climatic characteristics, where severe water stress prevents a response to nitrogen fertilization in most years.
Thanks
The authors want to thank Profertil SA and the producers of the Bahía Blanca Regional of AAPRESID, particularly the Hogar Funke and La Casilda establishments where the trials were carried out.
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