Effect of Foliar Application of Phosphorus on Winter Wheat Grain Yield, Phosphorus Uptake and Use Efficiency

Department of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK 74078

Contribution from the Oklahoma Agricultural Experiment Station

*Correspondence: William R. Raun, 044 North Ag Hall, Department of Plant and Soil Sciences, Oklahoma State University, OK 74078; Fax: (405) 774 5269; E-mail: wrr@mail.pss.okstate.edu.

ABSTRACT

To date, the best phosphorus (P) fertilizer use efficiency is around 16% when knifed or applied with the seed in winter wheat. Intuitively, one would expect foliar applied P to have higher use efficiencies than when applied to the soil, but limited information is available concerning this. Small amounts of P required to correct deficiencies could theoretically be introduced to the plant by a foliar P application. Nine trials were conducted in 2002, 2003 and 2004 at Lahoma, Lake Carl Blackwell and Perkins, OK to determine whether foliar applications of P can result in increased winter wheat (Triticum aestivum L.) grain yields, P uptake and use efficiency. A randomized complete block design with three replications was used to evaluate 12 treatments containing varying foliar P rates (0, 1, 2 and 4 kg ha-1 in 2002 and 2003 and additional 8, 12, 16 and 20 kg ha-1 in 2004 ) with and without pre-plant rates of 30 kg ha-1. Foliar application of P at Feekes physiological growth stage 7 (two nodes detectable) generally increased grain yields and P uptake versus no foliar P. Use efficiency was higher when P was applied at Feekes 10.54 (flowering completed). Results from this study suggested that low rates of foliar applied P might correct mid-season P deficiency in winter wheat, and that might result in higher P use efficiencies when compared to soil applications. Foliar P appeared to be more beneficial when yield levels were lower, likely due to moisture stress.

INTRODUCTION

In many agricultural production systems, P has been identified as the most deficient essential nutrient after nitrogen (N). Nutrient inputs into production systems have increased as a result of the need for high yielding crops to sustain the growing population around the world. In Oklahoma, phosphate inputs in winter wheat production ranged from 37.91 x 106 kg/ 2.18 x 106 ha in 1997 to 29.88 x 106 kg /1.42 x 106 ha in 2002 (1, 2). Even though the average is 21 kg ha-1, these inputs may become excessive where there were already high levels of soil Pleading to many environmental concerns, especially pollution issues. The most essential function of P is storage and transfer of energy in the form of ATP (adenosine triphosphate), ADP (adenosine diphosphate) and the important structural component of nucleic acids, coenzymes, phospholipids, and nucleotides.

Phosphorus originates from the weathering of soil minerals and other stable soil geologic materials and exists in both inorganic and organic forms of which the inorganic fraction is dominant. The inorganic forms are dominated by hydrous sesquioxides, amorphous crystalline aluminum and iron phosphates in acidic soils and as calcium phosphates in alkaline soils. The amount of available soluble P depends on pH, extent of contact between the precipitated Pand the soil solution, the rate of dissolution and diffusion of solid phase phosphorus, time of reaction, organic matter content, temperature and type of clay present. When the available P is less than the crop requirement, P is applied to the soil in the form of both inorganic and organic fertilizer.

Although inorganic fertilizers are readily available, they are slowly converted to unavailable forms due to precipitation. During early growth stages, plants may utilize the readily available form, while they compete for the slowly available forms in the later stages of growth.

P fertilizer use efficiency (PUE) averaged 8% when P was broadcast and incorporated and 16% when P was either knifed with anhydrous ammonia or applied with the seed in winter wheat (3, 4). Eghball and Sander (5) reported that 13.8 to 26.4 kg P ha-1 was taken up in corn grain at yield levels between 4.24 and 8.83 Mg ha-1, and a concentration of 0.31% P. Similarly, total P taken up in corn grain ranged from 21.4 to 47.4 kg P ha-1 at yield levels from 8.10 to 14.47 Mg ha-1, or 0.30% P (6). The diffusion coefficient of P in soil is very low, hence the root zone P is depleted and plants cannot get it when it is needed (7). Therefore, the utilization of P as a foliar application becomes increasingly important. The mechanistic processes by which foliar applied nutrients are taken up are through leaf stomata (8) and hydrophilic pores within the leaf cuticle (9).

In general, P deficient soils require pre-plant broadcast-incorporated rates of 11 to 22 kg P ha-1 to correct the deficiency in either wheat or corn. At a PUE of 16%, this addition results in only 1.7 to 3.5 kg of fertilizer P taken up in the grain. Although the literature does not provide information on relative efficiencies (soil applied versus foliar applied P), intuitively, one would expect the foliar applied P to be much higher. Thus, small amounts required to correct deficiencies can be easily introduced to the plant by a foliar P application. This approach has been overlooked for decades because it was assumed that the amounts of fertilizer P required by the crop were too large to be satisfied by a single foliar application. That assumption was easily accepted when P fertilizers were first used because soil deficiencies tended to be greater than today and solution fertilizers were uncommon.

Leach and Hameleers (10) reported that there was a significant increase in the starch content and cob index but no effect on dry matter production in maize due to foliar application of P and Zinc. Foliar applications of KH2PO4 were also found to delay leaf senescence and increase winter wheat grain yields during hot and dry summers (11, 12). Batten (13) later reported that net CO2 assimilation, N concentration and chlorophyll content decreased when wheat leaf P concentration falls below a critical level. Increased yields in barley were obtained using dilute solutions of foliar P (14). Barel and Black (15) reported findings in corn that 66% of foliar applied P to youngest mature leaf in a pot culture experiment as ammonium triple-phosphate was absorbed within 10 days and 87% of that absorbed was translocated within that time. However, Harder et al. (16) presented contradicting results showing that the foliar application of P applied 2 weeks after silking, significantly reduced grain yields.

Foliar fertilization with nitrogen, phosphorus, and potassium (NPK) can be supplemented with soil applied fertilizers but cannot replace soil fertilization in the case of maize (17), because demand for P is 1/10 that of N hence, foliar application might be beneficial. Therefore, correcting the plant's deficiency by foliar application seems plausible. Very little research has been conducted on the use of P as foliar spray at early stages of wheat and corn. However, recent work by Benbella and Paulsen (18) showed that foliar applications after anthesis of 5 to 10 kg KH2PO4 ha-1 (1.1 to 2.2 kg P ha-1) increased wheat grain yields by up to 1 Mg ha-1. Wheat grain yields are hindered due to senescence of wheat during grain fill. Therefore, to effectively prolong senescence, P has to be applied during later stages of growth, which is why foliar application seems particularly promising (18). Elliott et al. (19) reported a critical P concentration of 0.19 to 0.23% (at 90% maximum grain yield) in wheat grain. Earlier, Bolland and Paynter (20) reported that critical P concentration in wheat decreased from 0.91% to 0.23% (in shoot) with the growing season and 0.27% in grain.

Haloi (21) reported that when initial P deficiency symptoms appeared 25 days after sowing in wheat, higher doses of ammonium phosphate as a foliar spray gave the greatest reduction in P deficiency and highest yields. The efficiency of basal and/or foliar application of P was found to be similar (22).

The objectives of this study were to determine whether foliar applications of P can result in increased wheat grain yields and P uptake, and improve use efficiency.

MATERIALS AND METHODS

Three experimental sites were established in the fall of 2001 at Lahoma (Grant silt loam-fine-silty, mixed thermic Udic Argiustoll), Lake Carl Blackwell (Port silt loam-fine-silty, mixed, thermic Cumulic Haplustolls), and Perkins (Teller sandy loam-fine-loamy, mixed, thermic Udic Argiustoll), Oklahoma for evaluating the response of foliar application of P in winter wheat for three consecutive years. Initial soil test data is reported in Table 1.

A completely randomized block design with three replications was used to evaluate 12 treatments (Description of treatments is given in Table 2). Plots were 2.43 m by 3.04 m in size. At all sites, a fixed pre-plant N rate of 80 kg ha-1 was applied using ammonium nitrate (NH4NO3). Varying foliar P rates of 0, 1, 2 and 4 kg ha-1 were evaluated with and without pre-plant rates of 30 kg P ha-1 at different growth stages at all three sites in 2002 and 2003. In 2004, the treatment structure was modified to contain additional foliar P rates of 8, 12, 16 and 20 kg ha-1. Pre-plant P was broadcast and incorporated using triple superphosphate (Ca(H2PO4)2.H2O) for all trials. Foliar P was applied at Feekes growth stage 7 (second node of stem formed), Feekes 10.1 (heads emerging) and Feekes 10.54 (flowering completed) (23) in 2002 and 2003, while it was applied at Feekes 7 in 2004 using KH2PO4 solution with a pulse modulated handheld sprayer.

The winter wheat varieties used were ‘Jagger’ at Lahoma in all years and at Perkins in 2003 and 2004, ‘Custer’ at Lake Carl Blackwell (2003) and Perkins (2002), and ‘2174’ at Lake Carl Blackwell in 2003 and 2004. Wheat was planted in October in all years. Wheat was harvested with a Massey Ferguson 8XP experimental combine in June, removing an area of 2.0 x 3.04 m from the center of each plot. It was then weighed and sub sampled for P analysis. Grain samples were dried in a forced-air oven at 66oC, ground to pass a 140 mesh sieve (100 μm), and analyzed for total P. The concentration of P in the wheat grain was determined with a wet acid digestion procedure (24), and analyzed using a high-resolution inductively coupled plasma spectrophotometer (Thermo-Jarrell Ash IRIS ICP). Soft winter wheat flour standard reference material (SRM) (National Institute of Standards and Technology) was used to evaluate the wet acid digestion procedure of the grain tissue and resulted in 94% recovery of P in the grain.

Phosphorus use efficiency (PUE) in the wheat grain was calculated based on the following relationship:

All data were subjected to statistical analysis using SAS/STAT analytical tools (SAS, 2001). Single degree of freedom contrasts were performed for evaluating the differences in grain yield, grain P concentration and grain P uptake. The description and number of significant trials for each contrast are given in Table 3. The PUE data was transformed before analysis using arc sin variance stabilization method as follows.

PUEtrans = 2*(arc sin √PUE) [2]

Where PUEtrans refers to the transformed PUE data. Means were detransformed to the original scale for reporting.

RESULTS

Grain Yield

Grain yield significantly varied among treatments at Lahoma for all trials and at Perkins in 2002. Some single degree of freedom contrasts were also significant for all trials except at Lake Carl Blackwell in 2002 (number of total significant contrasts across six trials are given in (Table 3). Neither overall treatment effects nor single degree of freedom contrasts were found to be significant at Lake Carl Blackwell in 2002. Even though this site had high grain yields and the initial soil test results showed a low extractable P level, no actual P deficiencies were noted for this specific trial (mean grain yields across 12 treatments are presented in Table 4). Grain yield from plots fertilized with only pre-plant P significantly exceeded those plots which received only foliar P (Treatment 5 vs. 2,3,4, 9 and 11) in both years at the Perkins site (556 and 746 kg ha-1 increases in yield in 2002 and 2003, respectively). A comparison made between a combination of pre-plant + foliar P fertilization vs. only 30 kg ha –1 pre-plant rate showed a 630 kg ha –1 increase at Lahoma in 2002 (treatments 6, 7, 8, 10 and 12 vs. 5).

Mean grain yields were higher at Lahoma (2002) by 644 kg ha-1 and Perkins (2003) by 567 kg ha-1 (2003) when 2 kg ha-1 P was foliar applied at Feekes 7 compared to Feekes 10.54 (treatment 3 vs. 11). At Lahoma in 2003, the opposite was observed, where 2 kg ha-1 foliar P applied at Feekes 7 resulted in lower yields than the same rates applied at Feekes 10.54 (a decrease of 834 kg ha-1).

At Lake Carl Blackwell with no pre-plant P, 2 kg P ha-1 applied at Feekes 10.54 significantly increased yields when compared to the check and other 0 pre-plant P treatments that received P at Feekes 7 in 2003. This increase was not noted at all sites. At Lake Carl Blackwell and Lahoma in 2003, it was apparently advantageous to delay applying foliar P until Feekes 10.54 when compared to Feekes 7. At Lahoma in 2002, foliar P applied at Feekes 10.1 increased mean grain yield by 513 kg ha-1 compared with that at Feekes 10.54 (treatment 9 vs. 11), while at Lahoma in 2003 and Lake Carl Blackwell in 2003, mean grain yield was superior by 1172 and 335 kg ha-1, respectively at Feekes 10.54 compared with Feekes 10.1. At Lahoma in 2002, 2 kg ha-1 foliar applied P at Feekes 7 vs. that applied at Feekes 10.1 + 10.54 (Treatment 3 vs. 9and 11) resulted in a grain yield increase of 387 kg ha-1.

Trend analysis of mean grain yields for foliar P applied at Feekes 7 with no pre-plant P revealed a significant quadratic relationship between foliar P rates and grain yield at Lahoma in 2002 (Figure 1) . On the other hand, at a pre-plant rate of 30 kg P ha-1, foliar P at Feekes 7 showed a linear trend at Lahoma in 2002 (Figure 2). In 2004 at this site a quadratic response was observed for foliar rates up to 8 kg ha-1 where maximum yields were achieved at 4 kg ha-1( Figure 3).

Grain P Concentration

Much kike grain yield, grain P was high (>0.31%) at Lake Carl Blackwell in both years and low (0.18%) at Perkins in 2003, while it ranged between 0.20 and 0.26% for the remaining trials (Table 5). Some contrasts were also significant for this variable (Table 3). At Lahoma in 2002, grain P was higher by 0.017% for P applied pre-plant compared to that applied as foliar (Treatments 5, 6, 7 and 8 vs. 2, 3, 4, 9 and 11). On the other hand, the pre-plant + foliar treated plots showed a 0.022 and 0.039% lower grain P at Lahoma in 2002 and 2003, respectively, compared with only pre-plant treated plots. Similarly, at Lake Carl Blackwell in 2003, 0.039% more P was measured in pre-plant + foliar treated plots. At Lake Carl Blackwell in 2002, foliar P applied at Feekes 7 showed lower grain P concentration than rates applied at both Feekes 10.1 (Treatment 3 vs. 9) and at Feekes 10.1 + 10.54 (Treatment 3 vs. 9 and 11) by 0.033%, and at Feekes 10.54 (Treatment 3 vs. 11) by 0.031%.

Grain P concentration had a linear relationship with foliar P rates at Lahoma in 2002, and a quadratic relationship at Lake Carl Blackwell and Perkins in 2003 at 0 kg ha-1 pre-plant rate (Figure 4). At 30 kg P ha-1 pre-plant rate, two linear trends, one at Lake Carl Blackwell 2002 and another at Perkins 2003 (Figure 5) were significant while at Lahoma and Perkins (Figure 6) in 2002, a quadratic trend was revealed.

Grain P Uptake

Grain P uptake was significant in three of six trials for 2002 and 2003. The highest was at Lake Carl Blackwell (>13.50 kg ha-1) and the lowest (<4.32 kg ha-1) was at Perkins in 2002, while it ranged between 5.29 and 9.80 kg ha-1 for other sites (Table 6). For all trials, one or more contrasts were significant (Table 3). A trend for increased grain P uptake was observed when foliar P was applied with pre-plant P (Treatments 5-8) but this was not consistent over sites. At Lahoma in 2002, 1.17 and 1.68 kg ha-1 more P was taken up when foliar P was applied at Feekes 7 compared with that applied at Feekes 10.1 and 10.54, respectively. On the other hand, at Lake Carl Blackwell in 2002, grain P uptake was lower by 2.01 and 2.59 kg ha-1 at Feekes 7 than Feekes 10.1 and 10.54, respectively. At Lahoma in 2003, grain P uptake was increased by 2.84 and 3.06 kg ha-1 when P was applied at Feekes 10.54 than at Feekes 7 and 10.1, respectively.

Phosphorus Use Efficiency

Over all sites and years, PUE was higher when P was foliar applied at 2 kg P ha-1 (Table 7). PUE was as high as 86, 16, and 159% at Lake Carl Blackwell (2002), Lahoma (2002) and Lahoma (2003), respectively when 2 kg P ha-1 was foliar applied at Feekes 10.54 (Table 7). On average, PUE was higher when P was applied at Feekes 7 (39%) and Feekes 10.54 (47%).

DISCUSSION

Conventional P-soil test correlation utilizes knowledge that soil deficiencies may be represented as a percentage of the maximum yield when there is no P deficiency (26). This is appropriate for soil-applied P as rates do not need to be adjusted for yield level. However, rates of foliar P need to address uptake deficiencies of the plant, which are influenced both by potential yield (biomass) and available soil-P.

Grain yield and P concentration were not highly correlated. The poor correlation between P concentration and grain yield is not surprising since the role of foliar P on growth of wheat is more on delaying maturity. P concentrations in plants can be affected by limited P uptake due to variations in soil moisture status (27), root temperature (28) and various other environmental factors (29).

Regardless of the method of P application, response to P fertilization should have been observed across all trials. This is because initial soil test P levels were all below 100% sufficiency. Despite this, only 50% of the trials showed significant response to applied P. The significant grain yield response to P at Lahoma can be explained by the fact that the soil has a relatively low level of initial soil P compared to the other two sites. At Lahoma, the number of significant single degree of freedom comparisons obtained was more than the other two sites (with the exception of Perkins 2003) owing to the low initial soil P level.

Pre-plant P application consistently increased grain yield compared with top-dress P. Application of P pre-plant with supplemental foliar P also resulted in a better grain yield than pre-plant application in most instances where significance was observed. This suggests that wheat grain yield can be improved by supplementing P in foliar form when the plant is in need. Luxurious vegetative growth due to excess supply of N might induce hidden P hunger and the foliar correction of this hunger would likely improve yield. Similar explanation was given in high yielding environments with low soil P supply where foliar application of P helped to correct deficiencies and maintain higher yield (30). Green and Racz (31) reported a 300 kg ha-1 grain yield increment of wheat due to foliar P applied to a P deficient wheat crop.

In plots treated with only foliar rates at Feekes 7 and flowering, there was an apparent response which indicates that foliar P in wheat is still a potential option to manage P deficiency in wheat. Chambers and Devos (32) reported that depending on soil P status, foliar feeding of small amounts of P after heading increased yields over no P up to 672 kg ha-1 and added up to 538 kg ha-1 to the pre-plant P plots. However, the results were from trials conducted on a soil testing low in P and one would not expect to see these large yield increases on higher P fertility soils by foliar fertilization. Benbella and Paulsen (18) also showed that foliar applications after anthesis of 5 to 10 kg KH2PO4 ha-1 (1.1 to 2.2 kg P ha-1) increased wheat grain yields by up to 1 Mg ha-1.

The foliar rates considered in this study also showed apparent grain yield, and PUE increases. The 2004 grain yield data revealed that an addition of foliar P in excess of 8 kg ha-1 did not improve grain yield. The results from single degree of freedom comparisons generally lacked consistency.

Foliar application of P at Feekes 7 was generally better than applied at Feekes 10.1 or 10.54 in terms of grain yield and P uptake. The PUE data suggests that 10% more can be achieved if foliar fertilization is delayed until Feekes 10.54. However, it is preferable to apply at Feekes 7 since at this stage producers can simultaneously apply both N and P using the same equipment. In a preliminary foliar rate study made in Virginia, yield obtained from foliar rates applied at vegetative wheat stages surpassed that of the foliar rate applied at reproductive stages (33,34 ). Haloi (21) suggested that the delayed P applications resulted in a “stay green” effect whereby photosynthesis continued to take place during grain fill and that without the foliar P, more rapid senescence would be present. In order to realize any “stay green” benefit, environmental conditions must have been ideal (no moisture stress) from post flowering to maturity. Whenever plants are under moisture stress P uptake is reduced (27, 35).

When looking at Tables 4 and 7, data suggests that increases in grain yield from foliar P generally took place when yield levels were lower, likely due to increased moisture stress. This would make sense since P uptake due to contact exchange would be less under moisture stress, thus enhancing the benefits of foliar P in these years.

CONCLUSIONS

Results presented here confirm the beneficial use of foliar P fertilization in wheat, although the conditions in which this method would be used should be sought carefully. The Feekes 7 growth stage identified as the optimum growth stage for foliar P application is also the stage in which N is applied to avoid yield loss. Consequently, this allows reduction of cost associated with separate application.

Research on improving uptake of P by wheat leaves needs to be further studied. This might include study of formulations that might improve retention and penetration into the wheat leaves. Also, research has to be directed to see if foliar P applications during early stages of plant produce significant results. However, increased P use efficiency from low rates of foliar application was encouraging and will be pursued further.

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Table 4. Mean wheat grain yields (kg ha^‑1) for treatments at Lahoma, Lake Carl Blackwell, and Perkins, OK during 2002 and 2003 crop years.

^† Refer to table 2 for treatment description.

Table 5. Mean wheat P concentration (%) at Lahoma, Lake Carl Blackwell and Perkins, OK during 2002 and 2003 crop years.

^† Refer to table 2 for treatment description.

Table 6. Mean grain P uptake (kg ha^-1) for treatments at Lahoma, Lake Carl Blackwell, and Perkins, OK during 2002 and 2003 crop years.

^† Refer to table 2 for treatment description.

Table 7. Detransformed mean PUE (%) for treatments at Lahoma, Lake Carl Blackwell (LCB), and Perkins, OK during 2002 and 2003 crop years.

^† Refer to table 2 for treatment description.

Figure 1. Relationship between grain yield and foliar P rates applied at Feekes

7 without pre-plant P at Lahoma, 2002.

Figure 2. Relationship between grain yield and foliar P rates applied at Feekes

7 with pre-plant rate of 30 kg ha^-1 at Perkins, 2002.

Figure 3. Relationship between grain yield  and foliar P rates applied

at Feekes 7 without pre-plant rate at Lahoma in 2004.

Figure 4. Relationship between grain P concentration and foliar P rates applied

at Feekes 7 without pre-plant rate at Perkins, 2003.

Figure 5. Relationship between grain P concentration and foliar P rates applied at Feekes 7 with pre-plant P rate of 30 kg ha^-1 at Perkins, 2003.

Figure 6. Relationship between grain P concentration and foliar P rates applied at Feekes 7 with pre-plant P rate of 30 kg ha^-1 at Perkins, 2002.