Services on DemandArticleIndicatorsCited by SciELO Related linksCited by Google Similars in Google ShareOn-line version ISSN Chilean J. Agric. Res. vol.70 no.1 Chillán Mar. 2010 http://dx.doi.org/10.-00015
Journal of Agricultural Research 70(1):142-149 (January-March 2010)
Index to Estimate Crop Nitrogen Status in Potato Processing Varieties
?ndice Verde como Estimador del Estatus de Nitrógeno
en Variedades de Papa para Industria
Claudia Marcela Giletto1*, Cecilia Díaz1,
Jorge Edgardo Rattín1, Hernán Eduardo Echeverría2, and
Daniel Osmar Caldiz3
1Universidad Nacional de Mar del Plata, Facultad de Ciencias Agrarias,
Ruta 226 km 73,5, Balcarce, Buenos Aires, Argentina. *Corresponding
author ().
2Instituto Nacional de Tecnología Agropecuaria (INTA), Ruta 226
km 73,5, Balcarce, Buenos Aires, Argentina.
3McCain Argentina S.A., División Agronomía, Ruta 226 km 61,5. Balcarce
(B7620EMA), Buenos Aires, Argentina.
Part of this work was presented in XX Argentinean Congress of Soil Science,
13 March 2009.
Accepted: 21
July 2009.
Nitrogen is the most important nutrient in growing potatoes (Solanum
tuberosum L.) because either a deficit or excess in its supply can affect
yield and quality, resulting in economic losses. To make a rational use of
this nutrient, it is necessary to monitor N status during crop development.
The green index (IV) could be a valuable method to assess N sufficiency. In
experiments with variable N supply, the IV was assessed during crop growth
and an N sufficiency index was derived (ISN-IV). The relationship between
tuber yield and the IV and ISN-IV was established, as well as the association
between the IV and ISN-IV and N concentration in the following varieties:
Innovator, Bannock Russet and Gem Russet. During the vegetative growth stage,
the IV and ISN-IV were respectively above 40 SPAD units and 93%; while N concentration
in the plant was above 4% in the three varieties, which suggests the N supply
was sufficient. During the tuber-bulking stage, the necessary IV and ISN-IV
thresholds to reach maximum yields were 40.6 and 35 SPAD units and 93.7 and
91% at 89 and 103 days after planting, respectively. We concluded that the
chlorophyll meter is an appropriate tool to determine the nutritional status
in potato crops. The IV and ISN-IV values showed that the crop achieved maximum
yields when N levels are above the threshold values found in this research.
Key words: nitrogen, chlorophyll meter, potato varieties, Solanum
tuberosum.
El N es uno de los nutrientes que más afecta al cultivo de papa (Solanum
tuberosum L.), puesto que su aporte en defecto o exceso ocasiona pérdidas
en rendimiento y calidad. Para realizar un uso racional del nutriente es necesario
monitorear el N en las distintas etapas fenoló el índice
verde (IV) puede resultar un método alternativo promisorio a tal efecto. En
ensayos con dosis de N variable, se midió el IV a lo largo del ciclo y se
relativizó el IV a un índice de suficiencia de N (ISN-IV). Se estableció la
relación del rendimiento de tubérculos con el IV e ISN-IV y la asociación
entre el IV e ISN-IV con la concentración de N en las variedades Innovator,
Bannock Russet y Gem Russet. Durante el crecimiento vegetativo IV e ISN-IV
fueron superiores a 40 SPAD y a 93%, respectivamente, y la concentración de
N en planta fue mayor al 4% en las tres variedades, lo que sugiere adecuada
nutrición con N. En el llenado de tubérculos, los umbrales necesarios para
alcanzar los máximos rendimientos para IV e ISN-IV fueron 40,6 y 35 SPAD y
93,7 y 91% para los 89 y 103 días después de la plantación, respectivamente.
Se concluye que el medidor de clorofila fue una herramienta apropiada para
determinar el estado nutricional en papa. Los valores de IV o ISN-IV mostraron
que el cultivo alcanzó el máximo rendimiento y niveles suficientes de N cuando
éstos fueron superiores a los umbrales mencionados.
Palabras clave: nitrógeno, medidor de clorofila, variedades de papa,
Solanum tuberosum.
INTRODUCTION
Nitrogen is the nutrient that has received the most attention in potato
cultivation because either a deficiency or an excess of the nutrient can negatively
affect the development of the crop in terms of yield and the quality of tubers
(Echeverría, 2005; Caldiz, 2006; Samborski et al., 2009). Consequently,
it is necessary to monitor N status to meet crop requirements in a timely
and adequate manner in order to maximize yield and quality by taking the best
advantage of N use (Echeverría, 2005).
Among the techniques for assessing N levels is notably the chlorophyll meter,
which measures the green index (IV) of the leaf. This index estimates the
chlorophyll content in the leaf and through this the N concentration in vegetal
tissue, based on the assumption that N and chlorophyll content have a strong
correlation (Haverkort and MacKerron, 2000). Chlorophyll meters have been
used to determine N deficiencies in various crops, such as wheat (Triticum
aestivum L.) (Vidal et al., 1999; Arregui et al., 2000),
rice (Oryza sativa L.) (Esfahani et al., 2008), potato (Gianquinto
et al., 2004) and corn (Zea mays L.) (Ziadi et al., 2008).
In potatoes IV is measured in the terminal leaflet of the fourth totally expanded
leaf on the stalk given that the higher leaves that are more exposed to solar
radiation tend to have higher N content than those lower down (Westermann,
1993; Gallardo, 2000; Westermann, 2005; Abbate and Andrade, 2005). Minotti
et al. (1994) and Giletto et al. (2006) concluded that the IV
is more affected by the intraspecific variation in the color of the leaves
than by N fertilizer additions and that the IV should be calibrated for each
The IV is affected by several factors, among them: varieties, growth states,
position of the leaves, nutritional deficiencies other than N, soil type,
radiation, foliar diseases and environmental conditions such as water availability
and temperature, independent of N availability (Sainz Rozas and Echeverría,
1998; Olivier et al., 2006). Consequently, to minimize the influence
of these factors it is recommended to relativize the IV by establishing reference
areas with excess N to obtain leaves with a maximum chlorophyll concentration.
Based on the IV readings from the reference areas without N limitations, an
N sufficiency index (ISN) is estimated from the quotient between the IV of
the sector of the plot being characterized and the IV of the sector without
N limitations (Schepers et al., 1992; Denuit et al., 2002).
Snapp et al. (2002) determined that the ISN should be higher than 96%
throughout the crop cycle. If the ISN is below this threshold, it is recommended
to correct the deficiency by fertilizing with N.
To use the chlorophyll meter as an estimator of the early nutritional state,
it is necessary to obtain acceptable correlations between IV or ISN with yield
in the early stage of the cycle, when correction of N deficiencies are still
feasible. Wood et al. (1992) determined good correlations between IV
and yield in cotton (Gossypium hirsutum L.) and Piekielek
et al. (1995) did the same with corn. However, close relationships between
these variables have still not been established in potato crops in order to
determine the threshold of the IV and the ISN-IV (Giletto, 2002; Giletto et
al., 2006).
The objective of this study was to determine the evolution of the IV under
different N doses in potato crops in order to establish the relation between
tuber yield with IV and INS-IV and between IV and ISN-IV and N concentration
at different stages in the crop cycle of the varieties Innovator, Gem Russet
and Bannock Russet.
MATERIALS AND METHODS
assay was carried out during the
agricultural season at the McCain
experimental field station in Balcarce (37?45' S; 58?18' W, 130 m.a.s.l.),
Buenos Aires Province, Argentina, in a typic Argiudol (Soil Survey Staff,
2008) without limits in its use capacity.
summarizes
some soil characteristics at the time of planting and crop management data.
The soil was prepared for planting with a disk harrow, chisel (Industrias
Erca, Armstrong, Argentina) and vibrocultivator (Nievas, Buenos Aires, Argentina)
and subsequently before the crop reached 100% coverage, furrows were hilled.
Pest and disease control were carried out with the application of chemicals
products specific for each case. Weed control was conducted with chemical
applications and mechanical methods. Irrigation was carried out with a Reinke
Electrogator II center pivot system (Reinke-Deshler, Nebraska, USA) from 40-45
d after planting (DDP) until the end of February. The experimental design
was randomized complete blocks divided into plots, with three replications.
The main plot was the sampling date and the sub-plot was the N treatment.
The N rates are shown in . The treatment with the
lowest N doses was applied at planting, while the intermediate doses were
divided between planting and beginning of tuber-filling stage, and for the
highest doses there was also a pre-planting application. The combination of
doses and timing of fertilization provided a wide range of N availability
throughout the crop cycle. The N source was granulated urea [(NH2)2CO].
1. Some soil characteristics prior to planting and crop management data.
2. Nitrogen application rates.
IV was determined with a SPAD-502 chlorophyll meter (Minolta, Spectrum Technologies,
Chicago, Illinois, USA). The terminal leaflet of the fourth expanded leaf
of the main stalk was used for the determination, with a total of 20 plants
per plot. Readings were taken every 15 d. The readings were taken at 47, 60,
75, 89 and 103 DDP. Plants in a 1-m-long line from randomly selected furrows
for each replication were harvested for sampling. Each sample was dried at
60 ?C for 72 h or until constant weight was reached and N concentration was
determined using the Kjeldahl method (Nelson and Sommers, 1973). When the
crop reached maturity, plants were manually harvested from 2 m of the five
central furrows of each plot and estimations of yield were made in fresh weight.
Relative yield (RR) was calculated as the ratio between yield of each treatment
and highest average yield of the assay. The ISN-IV was determined as the relation
between the reading of each experimental unit and the highest average reading
of the assay. RR was associated with the IV and ISN-IV and the IV and ISN-IVN
were associated with N concentration.
The results were analyzed with the Statistical Analysis Systems program (SAS
Institute, 1988). The measurements from each treatment were compared using
the least significant difference test (LSD) (p &#) when ANOVA was
significant. The linear functions (linear-plateau) and quadratic functions
(quadratic-plateau model) were obtained using the PROC N-LIN procedure of
the SAS system.
RESULTS AND DISCUSSION
ANOVA determined significant interaction between sampling date (main plot)
and fertilization treatment (sub-plot). The IV decreased (p & 0.05) throughout
the cycles of the three varieties, the decrease in Bannock Russet being the
least (). This difference was probably due to Bannock
Russet matures later than the other two varieties (Love et al., 2005).
Booij et al. (2000) suggested that the IV should be higher than 40-41
SPAD units in the first 7 to 8 wk of the crop and that values below this threshold
are indicators of N deficiency. In the first stage of the cycle (up to 60
DDP), the IV was 40 SPAD units in all the treatments of the three varieties.
Consequently, the crop did not manifest any N deficiency through this nutritional
indicator. This indicated that N availability in the soil was sufficient to
meet crop requirements, including the control. In the second half of the cycle
(after 60 DDP), IV tended to decrease as the crop cycle advanced and it fell
below the reference threshold. During this period the treatments were ordered
according to N availability. The decrease in the IV was because photoassimilates
and nutrients were being directed to the tubers and foliar senescence had
begun, first among plants that suffered N stress and then those that had more
access to the nutrient.
Table 3. Green index (IV) for each treatment and sampling
date for ?Innovator', ?Bannock Russet' and ?Gem Russet' potatoes.
To relate IV
to ISN-IV, in the first stage of the cycle (up to 60 DDP) it was determined
that this index was above 93% in the three varieties and did not vary (p &
0.05) with N application. Beginning at 73 DDP, the treatment that received
235 kg ha-1 (N3) presented higher ISN-IV values than the other
treatments, which were ordered according to N availability, demonstrating
that N deficiencies become evident in the treatments that had received less
N. The differences in ISN-IV among the different treatments in Innovator
were greater than in Bannock Russet and Gem Russet ().
1. Evolution of N sufficiency index of the green index (ISN IV) for each treatment
(N0, N1, N2 and N3) and variety (Innovator, Bannock Russet and Gem Russet).
No association
was established between relative yields and the IV and the ISN-IV in the three
varieties (p & 0.05) () at the vegetative stage
and the beginning of the tuber-filling stage (45-47 DDP and 60-62 DDP, respectively).
The IV was over 40 SPAD units, confirming that the crop did not present N
deficiencies, concurring thus with Booij et al. (2000). Zebarth et
al. (2002) demonstrated that the IV in corn is not consistent with the
N level in plants when the values are above 43.7 SPAD units. At the same time
the ISN-IV was above 93%, close to the value established by Snapp et al.
(2002) as a sufficient N level for potatoes. Falótico et al. (1999)
established that a relative SPAD value of 92-95% in wheat is indicative of
sufficient N levels. Given the above, the chlorophyll meter showed that the
potato plants had sufficient N levels in the early stages of the cycle.
2. Relation between relative yields (RR) and green index (IV) and N sufficiency
index of IV (ISN-IV) at the vegetative growth stage and the beginning of the
tuber-filling stage in the varieties Innovator (IN), Bannock Russet
(BR) and Gem Russet (GR).
Different thresholds
of N sufficiency were determined during the tuber-bulking stage, based on
those that obtained the highest yields. The IV and ISN-IV values were 40.6
and 35 SPAD units and 93.7 and 91% at 89 and 103 DDP, respectively (). Varvel et al. (1997) recommended N fertilization for corn to
correct deficiencies if the ISN at the eight-leaf stage is less than 95%.
Andrián et al. (2001) concluded that the association in wheat between
ISN and yield is closer beginning with anthesis. Consequently, the association
among the variables in the potato is also more precise in the second half
of the cycle.
3. Relation between relative yields (RR) and green index (IV), N sufficiency
index of IV (ISN-IV) and the N concentration in the tuber-bulking stage
in the varieties Innovator (IN), Bannock Russet (BR) and Gem Russet (GR).
relation was determined between IV and ISN-IV and N concentration during the
vegetative growth stage (40-47 DDP). The IV and ISN-IV values remained relatively
constant when N concentration in the plant was above 4%, concurring with what
was determined by Di Salvo (2001) in fescue (Festuca arundinacea Schreb.)
and tall wheat grass (Elytrigia elongata Dewey). This confirms that
the chlorophyll meter showed that in the early stages the crop had sufficient
N, including the control treatments. Subsequently (60-73 and 91-103 DDP),
the IV and ISN-IV tended to increase with increased N concentration in the
three varieties, with this association being closer between 91 and 103 DDP
4. Relation of green index (IV) and N sufficiency index (ISN IV) to the N
concentration at the different stages of the cycle in the varieties Innovator
(IN), Bannock Russet (BR) and Gem Russet (GR).
4. Value of the linear adjustment coefficients between the green index (IV)
and N concentration in Innovator, Bannock Russet and Gem Russet at 60-73 and
91-103 days after the planting.
shows the relationship between IV and ISN-IV and N concentration for
each variety throughout the cycle, establishing that both variables increased
(p & 0.05) with increased N concentration. The linear regression coefficients
between the IV and N concentration differed with the variety. IV values of
42.2, 43.3 and 37.6 for Innovator, Bannock Russet and Gem Russet, respectively,
are required to reach 4% N. These results confirm the differences in the intensity
of the green coloring among the varieties and the need to relativize the IV
(Schepers et al., 1992; Denuit et al., 2002). The regression
analysis, using a quadratic-plateau model, established that the maximum ISN-IV
values among the three varieties were reached when the N concentration was
5. Relation between green index (IV) and N sufficiency index (ISN-IV) and
N concentration in varieties Innovator, Bannock Russet and Gem Russet.
CONCLUSIONS
Based on the analysis presented in this work we can conclude that the
chlorophyll meter is an ideal instrument for determining the nitrogen status
of potato crops. The studied crop reached maximum yield when the IV index
was above 35-40 SPAD units and the ISN-IV was 91-93.7%. An N concentration
of 4% was sufficient for the crop.
LITERATURE CITED
Abbate, P.E.,
y F.H. Andrade. 2005. Los nutrientes del suelo y la determinaci&n del
rendimiento de los cultivos de granos. p. 43-65. In Echeverr&a, H.E.,
y F.O. Garc&a (eds.) Fertilidad de suelos y fertilizaci&n de
cultivos. Ediciones INTA, Buenos Aires, Argentina.
&&&&&&&&[  ]
Andri&n, M.L., R.J. Melchiori, O.P. Caviglia, P.A. Barbagelata, y O.F.
Paparotti. 2001. Evaluaci&n de la nutrici&n nitrogenada en trigo
por medio de un &ndice de verdor en hojas. p. 52-61. In Actas V Congreso
Nacional de Trigo – III Simposio Nacional de Cereales de Siembra Oto&o
- Invernal, Carlos Paz, C&rdoba, Argentina. Septiembre 2001. Federaci&n
de Centros de Acopiadores de Cereales, Carlos Paz, C&rdoba, Argentina.
&&&&&&&&[  ]
Arregui, L.M., M. Merina, y A.M. Mingo-Castel. 2000. Aplicaci&n del
medidor port&til de clorofila en los programas de fertilizaci&n
nitrogenada en patata de siembra. p. 157-170. In Pascualena, J., y E. Ritter
(eds.) Libro de Actas del Congreso Iberoamericano de Investigaci&n
y Desarrollo en Patata, Vitoria-Gast&is, Espa&a. 3-6 Julio de
&&&&&&&&[  ]
Booij, R., J.L. Valenzuela, and C. Aguilera. 2000. Determination of crop nitrogen
status using non- invasive methods. p. 72-81. In Haverkort, A.J., and D.K.L.
MacKerron (eds.) Management of nitrogen and water in potato production. Wageningen
Pers., Wageningen, The Netherlands.
&&&&&&&&[  ]
Caldiz, D.O. 2006. Producci&n, cosecha y almacenamiento de papa en
la Argentina. 226 p. BASF Argentina-McCain, Buenos Aires, Argentina.
&&&&&&&&[  ]
Denuit, J.P., M. Olivier, M.J. Goffaux, J.L. Herman, J.P. Goffart, J.P. Destain,
and M. Frankinet. 2002. Management of nitrogen fertilization of winter wheat
and potato crops using the chlorophyll meter for crop nitrogen status assessment.
Agronomie (Paris) 22:847-853.
&&&&&&&&[  ]
Di Salvo, M. 2001. Estimaci&n del estado de nutrici&n nitrogenada
de pasturas de agropiro y festuca mediante un &ndice de verdor. 66
p. Tesis M.Sc. Universidad Nacional de Mar del Plata, Facultad de Ciencias
Agrarias, Unidad Integrada Balcarce, Balcarce, Argentina.
&&&&&&&&[  ]
Echeverr&a, H.E. 2005. Papa. p. 365-378. In Echeverr&a, H.E.,
y F.O. Garc&a (eds.) Fertilidad de suelos y fertilizaci&n de
cultivos. Instituto Nacional de Tecnolog&a Agropecuaria, Buenos Aires,
Argentina.
&&&&&&&&[  ]
Echeverr&a, H.E., y R. Bergonzi. 1995. Estimaci&n de la mineralizaci&n
de nitr&geno en suelos del sudeste bonaerense. Informe T&cnico
N& 135. 15 p. Estaci&n Experimental Regional Agropecuaria Balcarce,
Balcarce, Argentina.
&&&&&&&&[  ]
Echeverr&a, H.E., R.A. Strada, y G.A. Studdert. 2000. M&todos
r&pidos de an&lisis de plantas para evaluar la nutrici&n
nitrogenada del cultivo de trigo. Ciencia del Suelo 18:105-114.
&&&&&&&&[  ]
Esfahani, M., H.R. Ali Abbasi, B. Rabiei, and M. Kavousi. 2008. Improvement
of nitrogen management in rice paddy fields using chlorophyll meter (SPAD).
Paddy Water Environ. 6:181-188.
&&&&&&&&[  ]
Fal&tico, J., G.A. Studdert, y H.E. Echeverr&a. 1999. Nutrici&n
nitrogenada del trigo bajo siembra directa y labranza convencional. Ciencia
del Suelo 17:9-20.
&&&&&&&&[  ]
Gallardo, C. 2000. Perfiles de luz y nitr&geno foliar en canopeos de
papa y sus efectos sobre la eficiencia en el uso de la radiaci&n. 72
p. Tesis M.Sc. Universidad Nacional de Mar del Plata, Facultad de Ciencias
Agrarias, Unidad Integrada Balcarce, Balcarce, Argentina.
&&&&&&&&[  ]
Gianquinto, G., J.E. Goffart, M. Olivier, G. Guarda, M. Colauzzp, L. Dalla
costa, et al. 2004. The use of hand-held chlorophyll meters as a tool to assess
the nitrogen status and to guide nitrogen fertilization of potato crop. Potato
Res. 47:35-80
&&&&&&&&[  ]
Giletto, C.M. 2002. Comparaci&n de m&todos para evaluar la nutrici&n
nitrogenada en papa. 74 p. Tesis M.Sc. Universidad Nacional de Mar del Plata,
Facultad de Ciencias Agrarias, Unidad Integrada Balcarce, Balcarce, Argentina.
&&&&&&&&[  ]
Giletto, C.M., J. Rattin, H.E. Echeverr&a, y D.O. Caldiz. 2006. Evaluaci&n
de la nutrici&n nitrogenada en nuevas variedades de papa aptas para
el procesamiento industrial. Ciencia del Suelo 24:65-75.
&&&&&&&&[  ]
Haverkort, A.J., and D.K.L. MacKerron. 2000. Management of nitrogen and water
in potato production. 353 p. Wageningen Pers, Wageningen, The Netherlands.
&&&&&&&&[  ]
Love, S.L., W.H. Bohl, D.L. Corsini, J.C. Stark, and N. Olsen. 2005. Cultural
management of Gem Russet potatoes. University of Idaho, College of Agricultural
and Life Sciences. CIS 1093. Available at
(accessed 13 August 2005).
&&&&&&&&[  ]
Minotti, P.L., P.E. Halseth, and J.B. Sieczka. 1994. Field chlorophyll measurements
to assess the nitrogen status of potato varieties. HortScience 29:.
&&&&&&&&[  ]
Nelson, D.W., and L. Sommers. 1973. Determination of total nitrogen in plant
material. Agron. J. 65:109-112.
&&&&&&&&[  ]
Olivier, M., J.P. Goffart, and J.F. Ledent. 2006. Threshold values for the
chlorophyll meter used as a decision tool for supplemental nitrogen application
in the potato crop. Agron. J. 98:496-506.
&&&&&&&&[  ]
Piekielek, W.P., R.H. Fox, J.D. Toth, and K.E. Macneal. 1995. Use of chlorophyll
meter at the early dent stage of corn to evaluate nitrogen sufficiency. Agron
J. 87:403-408.
&&&&&&&&[  ]
Sainz Rozas, H., y H.E. Echeverr&a. 1998. Uso del medidor de clorofila
para el monitoreo de la nutrici&n nitrogenada del cultivo de ma&z.
Revista Facultad Agronom&a, Argentina 103:37-44.
&&&&&&&&[  ]
Samborski, S.M., N. Tremblay, and E. Fallon. 2009. Strategies to make use
of plant sensors-based diagnostic information for nitrogen recommendations.
Agron. J. 101:800-816.
&&&&&&&&[  ]
SAS Institute. 1988. SAS/STAT User's guide. Statistics version 6.0. 3rd ed.
SAS Institute, Cary, North Carolina, USA.
&&&&&&&&[  ]
Schepers, J.S., D.D. Francis, M. Vigil, and F.E. Below. 1992. Comparison of
corn leaf nitrogen concentration and chlorophyll meter readings. Commun. Soil
Sci. Plant Anal. 23:.
&&&&&&&&[  ]
Snapp, S., D. Smucker, and M. Vitosh. 2002. Nitrogen management for Michigan
potatoes. Ext. Bull. E-2779. 4 p. Michigan State University, East Lansing,
Michigan, USA.
&&&&&&&&[  ]
Soil Survey Staff. 2008. Official soil series descriptions [Online]. Soil
Survey Staff, Natural Resources Conservation Service, United States Department
of Agriculture. USDA-NRCS, Lincoln, Nebraska, USA. Available at
(accessed 10 February 2008).
&&&&&&&&[  ]
Varvel, G.E., J.S. Schepers, and D.D. Francis. 1997. Ability for in-season
correction of nitrogen deficiency in corn using chlorophyll meters. Soil Sci.
Soc. Am. J. 61:.
&&&&&&&&[  ]
Vidal, I., L. Longeri, and J.M. H&tier. 1999. Nitrogen uptake and chlorophyll
meter measurements in spring wheat. Nutr. Cycl. Agroecosyst. 55:1-6.
&&&&&&&&[  ]
Westermann, D.T. 1993. Fertility management. p. 77-86. In R.C. Rowe (ed.)
Potato. Health management. The American Phytopathological Society Press, St.
Paul, Minnesota, USA.
&&&&&&&&[  ]
Westermann, D.T. 2005. Nutritional requirements of potatoes. Amer. J. Potato
Res. 82:301-307.
&&&&&&&&[  ]
Wood, C.W., P.W. Tracey, D.W. Reeves, and K.L. Edmisten. 1992. Determination
of cotton nitrogen status with a hand-held chlorophyll meter. J. Plant Nutr.
&&&&&&&&[  ]
Zebarth, B.J., M. Younie, J.W. Paul, and S. Bittman. 2002. Evaluation of leaf
chlorophyll index for making fertilizer nitrogen recommendations for silage
corn in a high fertility environment. Commun. Soil Sci. Plant Anal. 33:665-684.
&&&&&&&&[  ]
Ziadi, N., M. Brassard, G. B&langer, A. Claessens, N. Tremblay, A.N.
Cambouris, et al. 2008. Chlorophyll measurements and nitrogen nutrition index
for the evaluation of corn nitrogen status. Agron. J. 100:.
&&&&&&&&[  ]
Instituto de Investigaciones Agropecuarias, INIAAvda. Vicente Méndez 515, Casilla 426Chillán, ChileTel.: (56-42) 2206800 - 2206780}