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Published in Agron J 99:833-841 (2007)
DOI: 10.2134/agronj2006.0199
© 2007 American Society of Agronomy
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Corn

Relationship between P and N Concentrations in Corn

Noura Ziadia,*, Gilles Bélangera, Athyna N. Cambourisa, Nicolas Tremblayb, Michel C. Nolina and Annie Claessensa

a Agriculture and Agri-Food Canada (AAFC), Soils and Crops Research and Development Centre, 2560 Hochelaga Blvd., Quebec, QC, Canada G1V 2J3
b AAFC, 430 Gouin Blvd., St-Jean-sur-Richelieu, QC, Canada J3B 3E6

* Corresponding author (ziadin{at}agr.gc.ca)

Received for publication July 7, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 DATA ANALYSIS
 RESULTS AND DISCUSSION
 REFERENCES
 
Tools to diagnose P crop status are becoming increasingly important to minimize the risk of surface and groundwater contamination from excessive fertilization while still applying sufficient P to optimize crop yield. The objectives of this study were to establish the relationship between P and N concentrations of corn (Zea mays L.) during the growing season and, in particular, to determine the critical P concentration required to diagnose P deficiency. Shoot biomass and P and N concentrations were determined weekly in an experiment with four to six N rates conducted over 2 yr (2004 and 2005) at three sites with adequate soil P for growth. The P and N concentrations decreased with time and increasing shoot biomass at all sites. The P concentration in relation to N under nonlimiting N conditions is described by a linear relationship (P = 1.00 + 0.094N, R2 = 0.76, P < 0.001, n = 71) in which the concentrations are expressed in g kg–1 dry matter (DM). Under limiting N conditions, the relationship was different with greater P concentrations for a given N concentration. The present study establishes a predictive model for critical P concentration in corn shoots, as a function of the N concentration in the shoot biomass and the degree of N deficiency. This critical P concentration can then be used to quantify the degree of P deficiency during the current growing season.

Abbreviations: DM, dry matter • NNI, nitrogen nutrition index


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 DATA ANALYSIS
 RESULTS AND DISCUSSION
 REFERENCES
 
EFFECTIVE MANAGEMENT OF P fertilizer is critical not only to the economics of crop production but also to the environment. Accumulation of soil P beyond that removed by the crop increases the risk of P movement to surface and ground waters, which can be detrimental to aquatic ecosystems through eutrophication (Sims et al., 1998). The accuracy of P fertilizer recommendations, therefore, is essential for sustainable crop production. Several methods of soil analyses have been developed to estimate the amount of soil P available to growing crops. Phosphorus extracted by some of these methods has been significantly correlated to plant growth and P uptake under controlled conditions (Simard et al., 1991; Tran et al., 1992) and in field studies (Ziadi et al., 2001). Other reports indicate that soil analyses are poor predictors of crop P requirements under field conditions (Morel et al., 1992; Heckman et al., 2006) primarily because these soil test methods do not account for the slow release of sorbed P (Steffens, 1994) and the mineralization of soil organic P (Tiessen et al., 1994). Plant-based diagnostic methods can be used as an alternate, or to complement soil analyses. Multielement diagnostic systems, such as the diagnosis and recommendation integrated system (Walworth and Sumner, 1987) and the compositional nutrient diagnosis (Parent and Dafir, 1992), are based on empirical relationships that do not account for changes in plant nutrient concentrations during the growing season.

The importance of taking into account changes in N concentration during the growing season for diagnostic purposes was highlighted by Greenwood et al. (1990) for a large number of crop species. In corn, Plénet and Lemaire (2000) and Herrmann and Taube (2004) presented a model of critical N concentration based on this concept of N dilution. The concept of P dilution and its use for the development of a model of critical P concentration has only been studied in perennial grasses. A diagnostic test for P nutrition, based on the relationship between P and N concentrations during growth, was first proposed for perennial grasses (Salette and Huché, 1991; Duru et al., 1997). The positive relationship between P and N concentrations reflects the dilution of both elements with increasing shoot biomass and indicates the role of soil N availability and crop N status on P absorption by plants (Kamprath, 1987). For timothy (Phleum pratense L.) grown under nonlimiting P conditions, the strong relationship between P and N concentrations was used to establish the critical P concentration for shoot growth; it was defined as a function of the N concentration in the shoot biomass and the degree of N deficiency (Bélanger and Richards, 1999). In corn, the positive relationship between P and N concentrations in leaf tissues at silking indicates how N fertilization affects corn P concentration (Follett et al., 1974; Kamprath, 1987). We are not aware of any study of this relationship between P and N concentrations in the shoot biomass of corn grown under varying rates of N fertilization.

Our main objective was to establish the relationship between P and N concentrations in corn shoots during the growing season by using data obtained under a wide range of levels of N nutrition and pedoclimatic sites known to have adequate soil P for growth. More specifically, we wanted to determine the critical P concentration for shoot growth, which could be used to diagnose and quantify P deficiency during the growing season.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
DATA ANALYSIS
 RESULTS AND DISCUSSION
 REFERENCES
 
Site Description and Treatments
The relationship between P and N concentrations in corn shoots was studied using data from an experiment conducted over 2 yr (2004 and 2005) at three sites in Quebec, Canada: St-Louis (45°51' N; 73°00' W), St-Basile-de-Portneuf, referred to as St-Basile (46°48' N; 71°46' W), and L'Acadie (45°17' N; 73°20' W) in 2004; and St-Louis, Ste-Catherine-de-la-Jacques-Cartier, referred to as Ste-Catherine (46°49' N; 71°39' W), and L'Acadie in 2005. These sites were selected to represent three diverse soil textures and different previous crops within the area of corn cultivation in Quebec.

Soil properties are presented in Table 1. Organic matter content was determined by wet oxidation (Tiessen and Moir, 1993). Soil pH was measured in distilled water with a 1:2 soil:solution ratio (Hendershot et al., 1993). The quantity of Mehlich-3 extractable nutrients was determined according to the method of Tran and Simard (1993). The P and Al concentrations in the extracts were measured on plasma (ICP–OES, Perkins Elmer, Model 4200DV). Saturation in P was determined as the ratio of soil P to Al content as extracted by Mehlich-3. Particle size analysis was performed by the hydrometer method after oxidizing the organic matter (Sheldrick and Wang, 1993). Precipitation and temperature data were collected at the Environment Canada Fleury station (45°48' N; 73°00' W) for the St-Louis sites, at the Environment Canada Ste-Christine-de-Portneuf station (45°49' N; 71°55' W) for the St-Basile and Ste-Catherine sites, and at the Environment Canada L'Acadie station (45°17' N; 73°21' W) for the L'Acadie sites. Appropriate corn hybrids and planting and fertilization dates were adapted to each site (Table 1).


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  		Table 1. Site characteristics and cropping practices.

 
All plots received 20 kg N ha–1 as 11–52–0 and 27–0–0 at planting, except at L'Acadie in 2005 where 30 kg N ha–1 was applied. At either the V8 or V10 stage of development (Table 1), a second N application in the form of calcium ammonium nitrate (27–0–0) was banded 10 cm from the corn plant by hand to obtain the desired N rate for each plot. Treatments consisted of six N rates (20, 50, 100, 150, 200, and 250 kg N ha–1) except at L'Acadie, where four N rates were used (20, 73, 125, and 178 kg N ha–1 in 2004 and 30, 83, 135, and 188 kg N ha–1 in 2005). A randomized complete block design with four replicates was used at each experimental site. At planting, P and K fertilizers were applied according to soil analysis and local recommendations (Centre de Références en Agriculture et Agroalimentaire du Québec, 2003). Thus, 35 kg P ha–1 and 35 kg K ha–1 in 2004, and 35 kg P ha–1 and 50 kg K ha–1 in 2005, were mechanically applied at each site. Soil available P, following P application, was then considered adequate for high crop yields at all sites. The plot size was 9 by 10 m with 12 corn rows except at L'Acadie, where plot size was 6 by 10 m with eight corn rows; a 0.75-m interrow was used and plant density was {approx}93 300 plants ha–1.

Sample Collection and Analysis
Shoot biomass was sampled weekly for 8 wk in 2004 and 7 wk in 2005 using a 2-m section of a row in each plot. We excluded data from sampling dates for which the shoot biomass was <1.0 Mg DM ha–1 (Tables 2, 3, and 4). Whole plants were cut at ground level using pruning scissors. Shoot biomass was weighed fresh and subsamples were collected for analysis. At St-Louis, St-Basile, and Ste-Catherine, subsamples consisted of five plants randomly selected within a 2-m row section, whereas at L'Acadie, all plants from the 2-m row section were mechanically shredded and a subsample of {approx}500 g was collected. Subsamples were dried at 55°C in a forced-draft oven for 7 d, ground to pass through a 1-mm screen in a Wiley mill, and stored at room temperature before laboratory analyses. Samples of 0.1 g of dried and ground corn were mineralized using a mixture of sulfuric and selenious acids, as described by Isaac and Johnson (1976). The P and N concentrations in plant tissue were measured on a QuikChem 8000 Lachat autoanalyzer (Lachat Instruments) using the Lachat methods 13-107-06-2-E and 15-501-3, respectively (Lachat Instruments, 2005). Grain yield was determined in each plot by harvesting whole plants manually on a 10- by 0.75-m area. The harvested ears were wet shelled and a grain subsample was dried at 55°C until constant weight; grain yield was adjusted to 14% moisture.


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  		Table 2. Phosphorus and N concentrations of corn shoots on different sampling dates at St-Louis in 2004 and 2005.{dagger}

 

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  		Table 3. Phosphorus and N concentration of corn shoots on different sampling dates at St-Basile in 2004 and Ste-Catherine in 2005.{dagger}

 

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  		Table 4. Phosphorus and N concentration of corn shoots on different sampling dates at L'Acadie in 2004 and 2005.{dagger}

 

   DATA ANALYSIS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 DATA ANALYSIS
RESULTS AND DISCUSSION
 REFERENCES
 
Data for each sampling date at each site were subjected to ANOVAs using the PROC GLM (SAS Institute, 2001), and SEMs were calculated. The relationship between P and N concentrations under nonlimiting and limiting N conditions was described by linear regressions (SAS Institute, 2001). The nitrogen nutrition index (NNI) of the crop was used to distinguish nonlimiting and limiting N conditions. This NNI at each sampling date was determined by dividing the N concentration of the shoot biomass by the critical N concentration, an approach previously used on tall fescue (Festuca arundinacea Schreb.) (Bélanger et al., 1992), timothy (Bélanger and Richards, 1997), and potato (Solanum tuberosum L.) (Bélanger et al., 2001). The critical N concentration (Nc), the minimum N concentration required to achieved maximum shoot growth, was defined as a function of shoot biomass as proposed for corn by Plénet and Lemaire (2000; Nc = 34.0 x DM–0.37, where DM is the shoot biomass in Mg ha–1. The NNI was averaged across three to six sampling dates depending on the site.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 DATA ANALYSIS
 RESULTS AND DISCUSSION
REFERENCES
 
Effect of N Fertilizer on Corn Grain Yield and N and P Concentrations during the Growing Season
Corn grain yield increased with increasing N fertilization rates (Tables 2, 3, and 4). Across all sites and years, grain yield ranged from 2.3 to10.2 Mg ha–1 with the lowest N fertilization rate (20–30 kg N ha–1) and from 6.3 to 12.5 Mg ha–1 where 250 kg N ha–1 was applied. Those yields are within the normal range of corn yields for the area where the experiment was conducted (Simard et al., 2001). The increase of corn grain yield with fertilizer N application is consistent with others studies conducted in Eastern Canada (Isfan et al., 1995; Tran et al., 1997) and in the USA (Onken et al., 1985; Jokela and Randall., 1989; Halvorson et al., 2005).

Increasing N fertilization increased shoot N and P concentrations on most sampling dates (Tables 2, 3, and 4). This positive effect of N fertilization on N concentration has often been reported (Tran et al., 1997; Simard et al., 2001). The effect of N fertilization on P concentration, however, has not been studied as much and has been attributed to the increasing capacity of roots to exploit more volume of soil and therefore absorb more nutrients (Barber, 1995).

Decrease in P and N Concentrations with Time
Phosphorus and N concentrations in the shoot biomass generally decreased with time (Tables 2, 3, and 4). A decrease in N concentration with time or advancing maturity has been reported for wheat (Triticum aestivum L.) (Justes et al., 1994), potato (Duchenne et al., 1997), timothy (Bélanger and Richards, 1997; 1999), and corn (Plénet and Lemaire, 2000). Nitrogen concentrations varied from a maximum of 34.4 g kg–1 DM at Ste-Catherine on 11 July 2005 to a minimum of 6.1 g kg–1 DM observed at St-Basile on 25 Aug. 2004 (Tables 2, 3, and 4). A similar range of N concentration (7–34 g N kg–1 DM) was reported by Plénet and Lemaire (2000) for corn grown with different N rates in France.

A decrease in P concentration with time has also been reported in corn (Plénet et al., 2000a). Phosphorus concentrations varied from a maximum of 4.5 g kg–1 DM at St-Louis on 6 July 2004 to a minimum of 1.5 g kg–1 DM at St-Basile on 20 and 25 Aug. 2004 (Tables 2, 3, and 4). When P was not limiting shoot growth, Plénet et al. (2000a) report P concentrations in corn from {approx}5.0 to 2.5 g kg–1 DM during the growing season. Corn P concentrations were particularly low at St-Basile in 2004, where P concentrations varied from 2.7 to 1.5 g kg–1 DM during the growing season (Table 3). This may be partly explained by the amount of precipitation in May and June of 2004 (181 mm) that was less than the 30-yr average (239 mm) at that location. It is well known that water stress, especially early in a season, decreases root growth and consequently the capacity to absorb P (Barber, 1995).

Decrease in P Concentration with Shoot Biomass
Shoot P concentration decreased as shoot biomass increased at all six sites (Fig. 1). For a given shoot biomass, P concentration tended to decrease with decreasing N rates, particularly at St-Louis in 2005 (Fig. 1). Bélanger and Richards (1999) reported that P concentrations for a given shoot biomass markedly increased with increasing N fertilization rates; this indicates a direct positive effect of N fertilization on P concentration. On the last sampling dates, corn grown under all N rates had similar shoot biomass P concentrations (Tables 2, 3, and 4). High N fertilization rates increased shoot biomass and thus increased P dilution, resulting in a P concentration similar to that obtained under limiting N conditions. This effect has also been observed in timothy (Bélanger and Richards, 1999).


Figure 1
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  		Fig. 1. Phosphorus concentration as a function of shoot biomass of corn fertilized with various N rates in an experiment conducted over 2 yr (2004 and 2005) at three sites.

 
Relationship between P and N Concentrations
The shoot P concentration increased with increasing N concentration at all six sites (Fig. 2). In a field study, Follett et al. (1974) reported that increased leaf P concentration was associated with increasing leaf N concentration in corn production. For a given N concentration, the P concentration generally decreased with increasing N fertilization rates (Fig. 2). Similar results were reported by Osborne et al. (2002), who found an increase in P concentration with the reduced growth resulting from a N deficiency.


Figure 2
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  		Fig. 2. Phosphorus concentration as a function of N concentration of corn in an experiment conducted over 2 yr (2004 and 2005) at three sites.

 
The relationship between P and N concentrations is reportedly different for limiting and nonlimiting N conditions (Duru and Ducrocq, 1997; Bélanger and Richards, 1999). To determine these two contrasting N conditions, we used the NNI. Values of NNI greater or equal to 1.0 indicate that the crop is in a situation of nonlimiting supply of N, whereas values of NNI smaller than 1.0 indicate a N deficiency. Studies on other crops, however, have shown that maximum yield can be reached at a NNI of 0.90 (Bélanger et al., 1992, 2001; Bélanger and Richards, 1997). This result is confirmed in our study on corn, where grain yield did not generally increase significantly with NNI ≥ 0.90 (Tables 2, 3, and 4). Therefore, we considered that N was not limiting corn grain yield when NNI was ≥0.90. On the basis of this assumption, we selected N fertilization treatments with a NNI ≥ 0.90 to determine the relationship between P and N concentrations under nonlimiting N conditions. For limiting N conditions, we used treatments with a NNI < 0.80 (Tables 2, 3, and 4). Data from the site at St-Basile were not used in determining the relationship between P and N concentrations because of its unexplained low corn P concentration.

Nonlimiting N Conditions
The relationship between P and N concentrations under nonlimiting N conditions can be described by the following linear relationship:

Formula 1[1]
in which both concentrations are expressed in g kg–1 DM (Fig. 3).


Figure 3
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  		Fig. 3. Phosphorus shoot concentration as a function of N concentration of corn in an experiment conducted at two sites in 2004 and three sites in 2005 under nonlimiting N conditions (NNI ≥ 0.90). Solid line, linear regression [P = 1.00 + 0.094N, R2 = 0.76, P < 0.001, n = 71] for corn; dashed line, model of Bélanger and Richards (1999) for timothy.

 
This relationship approximates the critical P concentration under nonlimiting N conditions, that is, the minimum P concentration needed to achieve maximum shoot growth. It assumes that soil P availability did not limit shoot growth. Under nonlimiting N conditions, P concentrations ranged from 2.1 to 4.4 g kg–1 DM (Fig. 3). Under nonlimiting P conditions in France, Plénet et al. (2000a) report a range of corn P concentrations of 2.5 to 5.0 g kg–1 DM. In Canada, Bélanger and Richards (1999) report a linear relationship between P and N concentrations for timothy (P = 1.46 + 0.069N with P and N in g kg–1 DM). The intercept and the slope of the relationship between P and N concentrations observed in this study on corn are similar to those in Bélanger and Richards (1999) for timothy (Fig. 3), even though these two species are from different metabolic groups. The adjustment of P and N concentrations during biomass accumulation, both of which are related to nutrient dilution, appears to be of a general nature and may apply to several crop species. Further research is required to validate this relationship for a broader range of environments and species.

N Limiting Conditions
The relationship between P and N concentrations under limiting N conditions can be described by the following linear relationship:

Formula 2[2]
For a given N concentration, the P concentration was higher under limiting N conditions than under nonlimiting N conditions (Fig. 4). Proportionally, for a given sampling date, the decrease in shoot biomass N concentration was much more important than the decrease in shoot biomass P concentration (Fig. 5). Similar observations are given by Bélanger and Richards (1999) for timothy and Duru and Ducrocq (1997) for permanent pastures.


Figure 4
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  		Fig. 4. Phosphorus shoot concentration as a function of N concentration of corn in an experiment conducted at one site in 2004 and two sites in 2005 under limiting N conditions (NNI < 0.80). Solid line, linear regression (P = 1.00 + 0.094N) for nonlimiting N conditions as presented in Fig. 4; dashed line, linear regression [P = 1.25 + 0.104N, R2 = 0.95, P < 0.001, n = 13] for limiting N conditions.

 

Figure 5
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  		Fig. 5. Phosphorus shoot concentration as a function of N concentration of corn at St-Louis in 2004 and Ste-Catherine in 2005 under limiting (20 kg N ha–1) and nonlimiting N conditions (100 kg N ha–1). Solid line, linear regression (P = 1.00 + 0.094N) for nonlimiting N conditions as presented in Fig. 3; dashed line, linear regression (P = 1.25 + 0.104N) for limiting N conditions as presented in Fig. 4; dotted lines, data from one sampling date.

 
On a single corn leaf, Kamprath (1987) reports that leaf P concentration at silking was increased by N fertilization and was highly correlated with leaf N concentration. In our study for shoot biomass, P concentration was adjusted to N concentration during crop growth (Fig. 4). On individual sampling dates, however, the adjustment of P concentration to N concentration with increasing fertilization (Fig. 5) was much less than that observed at the leaf level by Kamprath (1987).

Implications for P Diagnostic in Corn
Our results show that P and N concentrations decrease during growth, and that critical P concentration is a function of the N concentration and the degree of N deficiency. If producers apply adequate amounts of N to optimize corn yield, we propose using the relationship for nonlimiting N conditions. However, when N is deficient (NNI < 0.80), a different relationship should be used. These relationships provide tools to estimate the critical P concentration and, in turn, assess the P status of corn during the growing season.

The P:N ratio has also been proposed for diagnostic purposes. Our results indicate that the P:N ratio differs between nonlimiting and limiting N conditions; it is higher when N is limited. Consequently, in using the P:N ratio as a diagnostic tool, the level of N nutrition should also be considered. Also, the relationship between P and N concentrations for both N conditions indicates that the P:N ratio changes during the growth cycle, although the concentration of both elements decrease. For example, the P:N ratio is 0.13 early in the season (N concentration of 30 g kg–1 DM) and 0.16 late in the season (N concentration of 15 g kg–1 DM). The optimal P:N ratio therefore depends on the stage of development and on the level of N nutrition.

The importance of adequate tissue P concentration for early corn growth (Miller, 2000; Plénet et al., 2000b) and crop yield (Grant et al., 2001) is well documented. The present study provides a relationship to estimate the critical P concentration that is essential for quantifying the degree of P deficiency. When it is combined with the index of N nutrition, developed by Plénet and Lemaire (2000), it provides a diagnostic tool for N and P deficiencies and imbalances in corn. Since early P deficiency in corn cannot be remedied with later applications (Barry and Miller, 1989), producers can use this tool to adjust P fertilization in the following growing seasons.


   ACKNOWLEDGMENTS
 
This study was funded by Synagri Inc. and Agriculture and Agri-Food Canada (AAFC) through a matching investment initiative program and the GAPS program of AAFC. The assistance of Alain Larouche, Mario Deschênes, Sylvie Michaud, Danielle Mongrain, Carl Bélec, Edith Fallon, and Marcel Tétreault is greatly appreciated.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 DATA ANALYSIS
 RESULTS AND DISCUSSION
 REFERENCES
 




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