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Published in Agron J 99:999-1008 (2007)
DOI: 10.2134/agronj2006.0237
© 2007 American Society of Agronomy
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Phosphorus Management

Phosphorus Extraction by Cotton Fertilized with Broiler Litter

H. Tewoldea,*, K. R. Sistanib, D. E. Rowec and A. Adelic

a AWMRU, USDA-ARS, 810 Hwy. 12 East, Mississippi State, MS 39762
b AWMRU, USDA-ARS, 230 Bennett Ln., Bowling Green, KY 42104
c WMFRU, USDA-ARS, 810 Hwy. 12 East, Mississippi State, MS 39762

* Corresponding author (htewolde{at}ars.usda.gov)

Received for publication August 22, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Knowledge of the magnitude of P extracted and removed by harvested crop is an important component of effectively managing poultry litter to minimize or prevent the buildup of P in soil. This knowledge does not exist or is not well documented in cotton fertilized with litter as the primary fertilizer. The objective of this research was to quantify the magnitude of P extracted by cotton when fertilized with broiler litter and to determine whether supplementing litter with inorganic N improves P extraction. The research was conducted from 2002 to 2004 on two commercial farms representing a conventional-till at Cruger and a no-till at Coffeeville, MS, USA. At each location, the treatments consisted of an unfertilized control; a farm standard (STD) fertilized with inorganic fertilizers; and broiler litter of 2.2, 4.5, and 6.7 Mg ha–1 in an incomplete factorial combination with 0, 34, or 67 kg ha–1 N as urea ammonium nitrate (UAN) solution. The unfertilized control extracted an average across years of 27.7 kg P ha–1 at Cruger and 26.0 kg P ha–1 at Coffeeville. Application of both litter and UAN-N decreased tissue P concentration but increased extracted amount of P because of increases in dry weight. The largest end-of-season P extraction in this research, which included 53.9 kg P ha–1 in 2004 at Cruger and 49.3 kg P ha–1 in 2002 at Coffeeville, was recorded for the treatment that received the largest litter rate of 6.7 Mg ha–1 supplemented with 34 or 67 kg ha–1 UAN-N. Applied P always exceeded extracted P in all 3 yr at both locations when the litter rate was 4.5 or 6.7 Mg ha–1. Extracted P equaled or exceeded applied P when 2.2 Mg ha–1 litter was applied. Increasing litter rate decreased phosphorus extraction efficiency (PEE) while supplemental UAN-N increased PEE. An average of 53% of the total P extracted was partitioned to seed with an additional 2.4% partitioned to lint for a total of 55% that would be removed with harvested crop. Nitrogen fertilization appeared to shift P partitioning from vegetative to reproductive parts. Supplementing litter with inorganic N may be an effective strategy not only in extracting additional P from soils but also in increasing the fraction partitioned to seed so that more P is removed from the field.

Abbreviations: DAP, days after planting • LnNn, litter (Mg ha–1) and nitrogen as urea ammonium nitrate (kg ha–1) • NTPA, applied total plant available nitrogen • PEE, phosphorus extraction efficiency • STD, farm standard fertilization with inorganic fertilizers • UAN, urea ammonium nitrate


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
POULTRY LITTER, which is often viewed as a waste, is land-applied as a way of its disposal but also as a source of plant nutrients or as a soil amendment. Because of the convenience and cost of transportation, it is typically applied on pastures and hayfields repeatedly for many years within short distances of poultry production houses, a practice that can lead to environmentally unacceptable overload of P in the soil. In the southeastern USA, cotton can ease the burden placed on pastures near poultry houses because research has shown that poultry litter is an effective cotton fertilizer (Mitchell and Tu, 2005; Tewolde et al., 2007) and cotton is one of the most dominant field crops in the same states that also produce the majority of USA poultry. On the basis of 13 yr of research under conventional-till and no-till in Alabama, USA, Mitchell and Tu (2005) reported the performance of litter equaled that of ammonium nitrate fertilization and stated that the "... relative N availability in broiler litter was about 92 to 113% of N in ammonium nitrate." In a neighboring state in Mississippi, USA, Tewolde et al. (2007) showed that 4.5 Mg ha–1 broiler litter supplemented with 67 kg ha–1 inorganic N supports lint yield equivalent to that of farm standard fertilization with conventional inorganic fertilizers under conventional-till and no-till systems.

However, mismanaged litter use and a resulting environmental damage is often a concern when using litter as a soil amendment or a fertilizer. Phosphorus is the single most important nutrient of environmental concern when litter is applied as a source of crop nutrients. This is because litter contains P in excess of the crop requirement when applied to meet the N need. Typically, the N:P ratio of broiler litter is about 2 (Collins et al., 1999). But, under optimum production, the tissue N:P ratio, which is a function of the N and P uptake, varies between 1.5 for cereal crops and 20 for oilseed and legume crops (Sadras, 2006). The N:P ratio of container-grown cotton before maturity can be as large as 6.0 (Tewolde et al., 2005). Cotton in certain parts of the southeastern region in the USA receives up to 140 kg N ha–1 but only 0 to 30 kg P ha–1, a 4.7 N:P ratio. Therefore, applying litter to meet the N need of cotton would result in excess P application and, over time, in the accumulation of P in the soil.

Effective management of litter to minimize or prevent the buildup of P, when litter is used as the primary cotton fertilizer in soils that historically received no litter, requires adequate knowledge of the amount of P extracted and removed by harvested crop. Limited amount of research has been conducted to generate this kind of knowledge in forage grasses (Evers, 2002; Brink et al., 2001). In a year-round forage production system, Evers (2002) determined a maximum P uptake of 36.6 kg P ha–1 by annual ryegrass (Lolium multiflorum L.) and 18.6 kg P ha–1 by ‘Coastal’ bermudagrass [Cynodon dactylon (L.) Pers.] for a total of 55.3 kg P ha–1 uptake from the same soil that received 9 Mg ha–1 litter applied once in October plus 168 kg N ha–1 as ammonium nitrate applied three times during the year. Brink et al. (2001) measured the P uptake by 12 temperate legume and four temperate grass species that received 9 Mg ha–1 poultry litter and reported a maximum 2 yr average uptake of 23.4 kg P ha–1 by annual ryegrass.

The fraction of litter P that can be extracted by cotton and the consequences of applying litter to meet the nutritional needs of cotton in part or in full on soil accumulation of P are not well investigated. There is only one recent research that reported the extraction efficiency of litter-derived P by container-grown cotton (Tewolde et al., 2005). This research which used an inert media in a greenhouse showed that cotton extracted a maximum of only 16% of applied litter P in the absence of any other source of P. To our knowledge, no other research has documented the efficiency of cotton in extracting litter P.

When supplied with conventional inorganic fertilizers, reported amount of P extracted by cotton is variable. According to Halevy (1976), irrigated cotton in Israel extracted 44 to 46 kg P ha–1 with {approx}44% of the total (19–21 kg P ha–1) harvested in the seed. Others reported substantially less P uptake. Bassett et al. (1970) reported an average P uptake of only 19 kg ha–1 by mature irrigated cotton in California USA with only 9 to 12 kg ha–1 removal of P with harvested seed. Mullins and Burmester (1990) reported total P uptake of 16.3 to 18.2 kg ha–1 in Alabama USA with only 9.1 kg P ha–1 removed from the field in harvested seed. More recently, Dorahy et al. (2004) reported an average extraction across 17 field experiments in Australia of 21 kg P ha–1 at cut-out (defined as ≤5 nodes above white flower), 2/3 of which (15 kg ha–1) accumulated in seed. Although the report by Halevy (1976) that cotton can extract up to 46 kg P ha–1 is encouraging, the other reports suggest that use of litter on cotton can lead to accumulation of P that may not be environmentally sustainable. This research was conducted to quantify the magnitude of P extracted by cotton when fertilized with broiler litter and to determine whether supplementing litter with inorganic N improves P extraction. The research was part of a larger program with an overall goal of developing best management practices for using poultry litter as a primary fertilizer for optimum cotton production in the southeastern USA.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The research was conducted on two commercial farms under conventional-till at Cruger (33°18' N, 90°15' W, 32.9 m altitude) and under no-till at Coffeeville (33°58' N, 89°41' W, 71.2 m altitiude), MS, in 2002, 2003, and 2004. The soil at Cruger was a Dubbs silt loam (fine-silty, mixed, active, thermic Typic Hapludalfs) and the soil at Coffeeville was an Ariel silt loam (coarse-silty, mixed, thermic Fluventic Dystrochrepts). Cotton was grown on the same soil continuously for >25 yr at Cruger and for 4 yr at Coffeeville before initiating this research. Under each system, total amount and efficiency of P extraction by cotton fertilized with fresh broiler litter rates of 2.2, 4.5, and 6.7 Mg ha–1 in an incomplete-factorial combination with 0, 34, or 67 kg ha–1 N side-dressed as UAN solution was determined. The combinations included L2.2N0, L2.2N34, L2.2N67, L4.5N0, L4.5N34, L4.5N67, L6.7N0, and L6.7N34, where L is litter, N is nitrogen as UAN, and the subscripts represent the litter (Mg ha–1) or N (kg ha–1) rates. Extraction from an unfertilized control (L0N0) and an STD were included to make a total of 10 treatments.

The 10 treatment combinations within each location were tested in a randomized complete block design with three or four replications. The plots consisted of four 119 m-long rows spaced 1.02 m apart at Cruger and eight 73 m-long rows spaced 0.97 m apart at Coffeeville. Each treatment was applied to the same plot in all 3 yr. Each year, UAN (32% N) was applied between square and first flower stage as a sidedress using a commercial liquid fertilizer applicator equipped with coulters which opened slits about 0.15 to 0.20 m away from the row center into which the UAN solution was injected to a depth of {approx}0.10 m. Inorganic N, P, and K fertilizers were applied to the STD at the same rate as adjacent fields as practiced by the respective farm. This treatment received 135 kg N ha–1 yr–1 as UAN under the conventional-till at Cruger. Under the no-till at Coffeeville, the treatment received 101 kg N ha–1 in 2002 and 118 kg N ha–1 in 2003 and 2004 as UAN. Phosphorus was applied as triple superphosphate (0–46–0 N–P2O5–K2O) to the STD at 0, 20, and 0 kg P ha–1 at Cruger and 29, 20, and 0 kg P ha–1 at Coffeeville in 2002, 2003, and 2004, respectively. Potassium was applied to the STD as KCl (0–0–60 N–P2O5–K2O) at 140, 98, and 93 kg K ha–1 at Cruger and 56, 75, and 112 kg K ha–1 at Coffeeville in 2002, 2003, and 2004, respectively. All P and K fertilizers were applied to the STD as a broadcast by hand before planting.

Litter was applied 3, 1, and 10 d before planting at Cruger in 2002, 2003, and 2004, respectively. At Coffeeville it was applied 22 d before planting in 2002, 25 days after planting (DAP) in 2003, and 9 DAP in 2004. Each year, the litter was broadcast-applied with a commercial fertilizer spreader equipped with ground speed-sensing radar, an electronic scale, and a rate-control computer system (Barrons & Brothers, Inc., Gainsville, GA). Litter was soil-incorporated in the same day of application under the conventional-till at Cruger but was not incorporated under the no-till at Coffeeville. Cotton was planted on 19 Apr. 2002, 16 Apr. 2003, and 19 Apr. 2004 at Cruger; and on 21 May 2002, 2 May 2003, and 28 Apr. 2004 at Coffeeville. Additional details on soil properties, crop management, and weather are provided elsewhere (Tewolde et al., 2007).

Extracted P was determined based on aboveground whole-plant samples collected from 0.5- to 0.6-m2 center rows of each plot. Plant samples were taken four times during the 2002 and 2003 growing seasons from Cruger and during the 2002 growing season from Coffeeville. The samples from Cruger were taken 68, 97, 112, and 130 DAP in 2002, and 78, 98, 121, and 135 DAP in 2003. The samples from Coffeeville were taken 50, 71, 93, and 112 DAP in 2002. In other years, plants were sampled only at the end of the season: 134 DAP in 2004 from Cruger, and 137 DAP in 2003 and 138 DAP in 2004 from Coffeeville. Each year, cotton started flowering in late June at Cruger and in early to mid-July at Coffeeville.

Plants from both locations were cut at soil level, separated into leaves (leaf blade + petioles), stems (branches + main stem), and reproductive parts (squares + flowers + bolls). Reproductive parts were further separated into burs, seed, and lint when bolls were mature enough to make the separation feasible. The lint was separated from seed using a 10-saw gin. All other separations were accomplished by hand. All plant parts were dried in a forced-air oven at 80°C to constant weight, weighed, and ground to pass a 1-mm sieve. Seed samples were delinted with H2SO4 before grinding as linters on seed made homogenization extremely difficult.

Phosphorus concentration in plant parts was determined using an inductively coupled dual axial Argon plasma spectrophotometer (ICP, Thermo Jarrell-Ash Model 1000, Franklin, MA) (Donohue and Aho, 1992). Approximately 0.2 g of the dried and ground sample was ashed in a muffle furnace at 500°C for 4 h. The ash was digested by adding 1.0 mL 6 M HCl for 1 h and 40 mL of a double-acid solution of 0.0125 M H2SO4 and 0.05 M HCl for an additional 1 h. The digested solution was then filtered using a 2V Whatman (Maidstone, UK) filter paper and analyzed for total P concentration by the ICP. Phosphorus concentration in the litter was determined by the same method used for the plant tissues.

Accumulation of P in each plant part was calculated as the product of P concentration and dry weight of each plant part. Total P extracted by aboveground plant parts was determined as the sum of P accumulation in leaves, stems, and reproductive parts. Phosphorus concentration was not analyzed in all lint samples as the P content of lint was expected to be low. Total P accumulation in lint was therefore determined by multiplying the lint dry weight of each sample by an average lint P concentration measured on all lint samples from 1 yr-location. Each lint sample used for analysis was thoroughly cleaned by hand to remove contaminating debris. The fraction of P partitioned to a plant part was calculated as a percentage of the total P extracted by all aboveground plant parts. The efficiency of cotton in extracting applied P (PEE), which is sometimes referred to as apparent P recovery, was computed as total extracted P as a percentage of the total applied P after accounting for P extracted by untreated plants as

Formula 1[1]
where Pt = phosphorus amount extracted by a treatment that received litter with or without UAN-N, Pu = phosphorus amount extracted by the treatment that received no litter or UAN-N (L0N0), and Pa = amount of total P applied in litter or inorganic fertilizer.

Litter and supplemental UAN-N effects on P concentration in each plant part and amount of extracted P by each plant part were tested by subjecting the data to ANOVA using MIXED model analysis on SAS (Littell et al., 2002). Preliminary ANOVA was performed for a randomized complete block design with a factorial treatment structure for litter and UAN-N factors. Additional analysis was performed using a trend to describe the litter and UAN-N treatment structure as a response surface model where the full model had three slope parameters that included litter linear (LL), UAN-N linear (NL), and their interactions (LL x NL). The significance of the linear effects of litter (LL) and UAN-N (NL) were tested by omitting the LL x NL interaction term from the full model when the interaction was not significant. Regression analysis was used to test the relationship between amount of extracted P and tissue P concentration with applied total plant available N (NTPA). The NTPA was estimated by summing applied litter N and UAN-N with the assumption that 50% of the total litter N (Tewolde et al., 2007) and 100% of the UAN-N become plant available in the cotton growing season. All differences mentioned in the discussion are significant at P ≤ 0.05 unless stated otherwise.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
The two locations were selected to represent a conventional- and a no-till cotton production system. In addition to differences in tillage, cultivars, and other management methods, these locations represent two contrasting litter application methods. While applied litter at Cruger was thoroughly incorporated with the soil within 1 d of application, the litter at Coffeeville was left on the soil surface without incorporation. The results are therefore presented separately by location. The data were also analyzed and are presented separately by year within a location because a plot received the same treatment each of the 3 yr within a location which may have resulted in a cumulative residual effect of the treatments.

Phosphorus Concentration in Plant Parts
Phosphorus concentration of bulk whole leaves (petioles + blades) on any of the sampling days in 2002 and 2003 at Cruger and in 2002 at Coffeeville ranged between 2.4 and 7.7 g kg–1 (Table 1). This range is well within or above the published sufficiency range of 1.5 to 6.0, which is based on the youngest fully mature leaf blades taken between early and late flowering (Mitchell and Baker, 2000). Among all plant parts excluding lint, seeds or reproductive parts had the largest P concentration and stems had the least at any stage in the season. At maturity, seed P concentration of all treatments at either location exceeded 6.4 g kg–1 and was as much as 10.6 g kg–1. Lint P concentration, measured on Cruger samples taken at the end of the season in 2002 only, did not exceed 0.5 g kg–1 which is even less than that of stem P concentration. The largest reported P concentration of cotton fertilized with conventional inorganic fertilizers is 7 g kg–1 in seed and 3 to 5 g kg–1 in leaves (Halevy, 1976). Others reported even lower leaf and seed P concentration (Bassett et al., 1970; Mullins and Burmester, 1990).


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  		Table 1. Phosphorus concentration in aboveground plant parts of cotton fertilized with broiler litter with or without supplemental N as urea ammonium nitrate solution (UAN-N) at Cruger (conventional-till) and Coffeeville (no-till) in Mississippi.{dagger}

 
When averaged across treatments, locations, and years, leaf P concentration slightly increased and stem and bur P concentration decreased during the season (Table 1). The trend of leaf P concentration across the season appeared to depend on the N nutrition which was best reflected in 2003. Leaf P concentration in 2003 at Cruger steadily increased during the season for the untreated control and other treatments that received insufficient N from litter or UAN. The trend was similar at Coffeeville in 2002. Leaf P concentration of treatments that received sufficient or moderate N fertilization remained unchanged or showed a tendency to decrease slightly during the season. The trend of P concentration across the season did not seem to depend on N nutrition in other plant parts. Phosphorus concentration in seed, unlike in the other plant parts, increased during the last two or three measurements. This increase most likely is because of remobilization of P from bur, stem, and leaves to the seed.

Treatments affected leaf P concentration, with litter application affecting leaf P concentration less consistently than UAN-N application despite applying different amounts of P in litter depending on the rate. The litter-linear (LL) effect on leaf P concentration was not significant in any of the four sampling days at Cruger or Coffeeville in 2002 and in the first two sampling days in 2003 at Cruger (Table 1). The UAN-N-linear (NL) effect, on the other hand, was significant on three of the four sampling days in 2002 at Cruger and in the last two sampling days in 2003 at Cruger and in 2002 at Coffeeville. Leaf P concentration decreased with increasing UAN-N rate. Usually the untreated control (L0N0), which received no UAN-N, litter, or any other fertilization, had the largest leaf P concentration. Leaf P concentration of this treatment at the end of the season was 4.1 g kg–1 in 2002 and 6.9 g kg–1 in 2003 at Cruger. End-of-season leaf P concentration of the L0N0 treatment at Coffeeville in 2002 was even greater at 7.7 g kg–1. Treatments that received adequate N fertilization usually had the least leaf P concentration on all days. The STD treatment, for example, which received adequate N and was among the top-yielding treatments (Tewolde et al., 2007), usually had the lowest end-of-season leaf P concentration with 3.0 g kg–1 in 2002 and 2.6 g kg–1 in 2003 at Cruger and 4.4 g kg–1 in 2002 at Coffeeville. The L4.5N67 treatment, which also received adequate N and yielded as good as or better than the STD (Tewolde et al., 2007), had leaf P concentration similar to that of the STD (3.5 g P kg–1 in 2002, 2.9 g P kg–1 in 2003 at Cruger, and 4.3 g P kg–1 in 2002 at Coffeeville).

The dependence of leaf P concentration on N nutrition became more apparent when leaf P concentration was regressed on NTPA, which showed that leaf P concentration measured later in the season decreased with increasing NTPA (Fig. 1 ). The response of P concentration to litter, UAN-N, or NTPA in the other plant parts was similar to the response of leaf P concentration at both locations. The cotton plant appears to continue extracting P even when applied N is inadequate regardless of the adequacy of soil P. Soil P extracted by Mehlich-3 solution (Mehlich, 1984) immediately before planting each season was 40.4 mg kg–1 in 2002 and 63.2 mg kg–1 in 2003 at Cruger and 8.1 mg kg–1 in 2002 at Coffeeville. Inhibited plant growth due to N deficiency while plants continued to absorb P probably leads to greater P concentration in leaves and the other plant parts. Evers (2002) reported an inverse relationship between P concentration in forage grasses and the amount of N applied each year and attributed this relationship to increased forage production due to increased applied N and therefore a dilution effect on the P concentration in the forage. When P is absorbed in excess of plant metabolic needs, it is probably stored in leaf cell vacuoles as an inorganic P and as a phytate P in seeds (Marschner, 1995). Up to 77% (6.4 g kg–1) of the total seed P in cotton is found in the form of phytate compared with 55% in wheat (Triticum aestivum L.), 73% in corn (Zea mays L.), and 65% in rice (Oryza sativa L.) grains (Godoy et al., 2005).


Figure 1
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  		Fig. 1. Relationship between applied total plant available N and bulk leaf P concentration on selected dates in cotton grown with broiler litter with or without supplemental N as urea ammonium nitrate solution at Cruger (conventional-till) and Coffeeville (no-till) in Mississippi. Each data point is an average of three or four replications.

 
Phosphorus Extraction
Trend of Extracted Phosphorus across the Season
Total extracted P by all aboveground plant parts when averaged across treatments, years, and locations increased from 10.8 kg P ha–1 at {approx}65 DAP to 36.5 kg P ha–1 at {approx}125 DAP (Fig. 2 ). The increase in total extracted P beyond {approx}90 DAP was solely due to rapid accumulation of P in seed. Phosphorus accumulation in leaves and stems peaked at {approx}90 DAP and gradually declined thereafter with the decline in stems being much smaller than in leaves. Amount of P accumulated in burs also declined from 11.0 kg P ha–1 at {approx}90 DAP to only 4.9 kg P ha–1 at {approx}125 DAP. The decreasing trend of P accumulation in leaves, burs, and stems and the increasing trend in seeds suggest that P is remobilized from burs, leaves, and stems to seed, although some P may have also been lost in self-defoliated senescent leaves.


Figure 2
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  		Fig. 2. Amount of extracted phosphorus in plant parts of cotton grown with broiler litter with or without supplemental N as urea ammonium nitrate solution at Cruger (conventional-till) and Coffeeville (no-till) in Mississippi. Each data point is an average of three or four replications, 10 treatments, two locations, and 3 yr (2002–2004).

 
Effect of Litter and UAN-Nitrogen on End-of-Season Phosphorus Extraction
Total P extracted by all treatments at the end of the season ranged between 24.9 and 53.9 kg P ha–1 at Cruger and between 23.0 and 49.3 kg P ha–1 at Coffeeville (Fig. 3 ). Each year, within each location, the L0N0 treatment extracted the least P with 27.7, 24.9, and 30.4 kg P ha–1 at Cruger and 29.8, 23.0, and 25.1 at Coffeeville in 2002, 2003, and 2004, respectively. Applying litter significantly increased P extraction at both locations in each of the 3 yr (Fig. 3, Table 2). The effect of increasing litter rate on P extraction was more consistent in the last year (2004) of the 3-yr research within each location. Plants that received 0, 2.2, 4.5, and 6.7 Mg ha–1 litter with no supplemental UAN-N in 2004, respectively, extracted 25.1, 28.3, 32.5, and 39.8 kg P ha–1 at Coffeeville and 30.4, 37.4, 41.5, and 45.0 kg P ha–1 at Cruger. Application of UAN-N also significantly increased P extraction in 2 of the 3 yr at each location, but the effect of UAN-N application was less consistent than the effect of litter application. These increases in P extraction with increasing litter or UAN-N rate were almost always due to increases in plant dry weight, because neither litter application nor UAN-N application increased P concentration in the different plant parts. To the contrary, increasing litter or UAN-application had a decreasing effect on tissue P concentration (Table 1).


Figure 3
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  		Fig. 3. Total P extracted by cotton grown with broiler litter with or without supplemental N as urea ammonium nitrate solution (UAN-N) at Cruger (conventional-till) and Coffeeville (no-till) in Mississippi in 2002 to 2004. Each bar segment is an average of three or four replications. {dagger} Applied UAN-N rates in 2004 under the conventional-till were 67 and 135 kg ha–1 instead of the 34 and 67 kg ha–1 applied in 2002 and 2003. STD = farm standard fertilization.

 

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  		Table 2. Statistical significance (P > F) of the linear effect of applied broiler litter (LL) or N as urea ammonium nitrate solution (NL) on the end-of-season P extracted by aboveground parts of cotton grown under conventional-till at Cruger and no-till at Coffeeville in Mississippi (data shown in Fig. 3). Only the linear effects are shown as the interaction between LL x NL were not significant at P < 0.05.

 
The largest P extraction in this research, which included 53.9 kg P ha–1 in 2004 at Cruger and 49.3 kg P ha–1 in 2002 at Coffeeville, was recorded for the treatment that received the largest litter rate of 6.7 Mg ha–1 supplemented with 34 or 67 kg ha–1 UAN-N (Fig. 3). This treatment was among the best-yielding treatments (Tewolde et al., 2007). The L4.5N67 treatment, which yielded consistently better than most treatments at both locations, extracted only slightly less P than the treatment (L6.7N34) that extracted the most P. The STD treatment often extracted the least amount of P among the best yielding treatments at Cruger.

To our knowledge, the largest reported P extraction by mature cotton is 44 to 46 kg P ha–1 by Halevy (1976), who grew cotton in Israel under irrigation after applying N–P–K fertilizers presumably from inorganic sources. Bassett et al. (1970), who used one of the two cultivars used by Halevy (1976) under irrigation and applied only N fertilizers, reported a maximum P extraction of only 19 kg P ha–1 with only 9 to 12 kg P ha–1 removal from the field with harvested seed. More recently, Dorahy et al. (2004) reported P uptake by cotton that received P fertilizers in eastern Australia to range between 13 and 31 kg P ha–1 at cutout. This is still substantially lower than the extraction we recorded in our research, where we fertilized with broiler litter supplemented with inorganic N. This may be an indication that the cotton plant may extract P in excess of its normal metabolic need when fertilized with poultry litter supplemented with inorganic N. It seems a large fraction of this excess P is stored in the seed.

The maximum P extracted by one cotton harvest in one season recorded in our research was equivalent to the maximum P extracted by year-round forage harvested six or seven times during the year (Evers, 2002). Evers (2002) grew ‘Coastal’ bermudagrass which was overseeded with annual ryegrass in the winter and measured a maximum P uptake of 18.6 kg ha–1 by bermudagrass and 36.6 kg ha–1 by ryegrass for a total of 55.3 kg P ha–1 uptake from the same soil that received 9 Mg ha–1 litter applied once in October plus 168 kg N ha–1 as ammonium nitrate applied three times during the year. When harvested only once, the maximum P uptake by temperate legume and grass species was less than half that of cotton in our research (Brink et al., 2001). This shows cotton extracts as much P as or more P than forage and pasture crops to which litter has traditionally been applied.

Extracted Total Phosphorus at Coffeeville versus Cruger
Relative comparison of extracted P by certain treatments shows that plants at Coffeeville extracted as much P as plants at Cruger despite producing less lint and aboveground dry weight. For example, the L6.7N0 treatment extracted 37.5, 32.4, and 39.8 kg P ha–1 at Coffeeville compared with 29.4, 36.7, and 42.5 kg P ha–1 at Cruger at the end of the season in 2002, 2003, and 2004, respectively (Fig. 3). The L6.7N0 treatment produced 1269, 1405, and 1252 kg ha–1 lint at Coffeeville and 1643, 1484, and 1424 kg ha–1 lint at Cruger (Tewolde et al., 2007). Mehlich 3 extractable soil P (Mehlich, 1984) before planting each season at Coffeeville (19.4 mg P kg–1 soil averaged across years) was also much less than at Cruger (60.7 mg P kg–1 soil averaged across years). The ability of the cotton at Coffeeville in extracting P comparable to that of the cotton at Cruger, despite producing less lint and aboveground biomass, may be because lower soil pH at Coffeeville than at Cruger may have contributed to greater plant availability and extraction of P from the soil. The greater P concentration in plant parts at Coffeeville than at Cruger (Table 1) may be an indication that P was more available and extractable at Coffeeville than at Cruger. Averaged across all treatments, preplant soil pH in the top 0.15-m soil depth was 5.5, 5.7, and 5.8 at Coffeeville and 6.0, 6.1, and 6.4 at Cruger in 2002, 2003, and 2004, respectively. Cultivars may also have contributed to a greater P extraction at Coffeeville than at Cruger, but some reports indicate that cultivars may not differ in P uptake (Mullins and Burmester, 1990).

End-of-Season Phosphorus Extraction Efficiency
When averaged across all treatments that received litter, cotton at the end of the season extracted less P than applied each of the 3 yr within a location. Plants extracted an average of 38.7, 12.1, and 13.9 kg ha–1 less P than applied at Cruger and an average of 40.7, 24.9, and 25.6 kg ha–1 less P than applied at Coffeeville in 2002, 2003, and 2004, respectively (Table 3). The greater discrepancy between extracted and applied P in 2002 than in 2003 or 2004 is most likely due to a greater litter P concentration in 2002 ({approx}18 g kg–1) than in 2003 and 2004 ({approx}12 g kg–1) and therefore due to applying more litter P in 2002 than in the other 2 yr. Applied litter remained the same each year. The litter used for both locations in 2002 was supplied by a commercial broiler chicken producer in southern Mississippi and the litter used in 2003 and 2004 was supplied by another commercial broiler chicken producer in central Mississippi. The difference in P concentration between the two litter sources is probably indicative of the importance of P management in poultry production practices to minimize excess P in the litter.


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  		Table 3. Applied, extracted (end-of-season), and extraction efficiency of phosphorus by cotton fertilized with broiler litter ± supplemental N as urea ammonium nitrate solution (UAN) at Cruger (conventional-till) and Coffeeville (no-till) in Mississippi.{dagger}

 
The discrepancy between applied and extracted P was dependent on the amount of applied litter. Plants extracted P equivalent to the total applied when only 2.2 Mg ha–1 litter was applied in 2003 and 2004 at both locations. At this litter rate, plants extracted 5.2 and 12.1 kg ha–1 more P than applied at Cruger and 2.3 kg ha–1 more P and 1.8 kg ha–1 less P (averaged across the UAN-N treatments) than applied at Coffeeville in 2003 and 2004, respectively (Table 3). Applied P exceeded extracted P by 10.4 kg ha–1 at Cruger and by 4.5 kg ha–1 at Coffeeville in 2002 when 2.2 Mg ha–1 litter with high P concentration was applied. Applied P always exceeded extracted P in all 3 yr at both locations when 4.5 or 6.7 Mg ha–1 litter was applied. At 4.5 Mg ha–1 litter, applied P exceeded extracted P averaged across supplemental UAN-N rates by 42.3, 12.7, and 19.2 kg P ha–1 at Cruger and by 47.4, 31.1, and 29.1 kg P ha–1 at Coffeeville in 2002, 2003, and 2004, respectively. At 6.7 Mg ha–1 litter, applied P exceeded extracted P by 75.7, 37.0, and 45.0 kg P ha–1 at Cruger and by 84.7, 56.5, and 55.9 kg P ha–1 at Coffeeville in 2002, 2003, and 2004, respectively.

Phosphorus extraction efficiency, which is the efficiency by which cotton extracted litter-derived P after accounting for P supplied by the soil reserve, was as large as 46.6% at Cruger and 40.9% at Coffeeville among all treatments that received litter (Table 3). Within each level of UAN-N, PEE usually was largest with 2.2 Mg ha–1 litter application and smallest with 6.7 Mg ha–1 litter application. When averaged across all years and the three UAN-N levels, PEE was 23.3, 16.3, and 12.8% at Cruger and 21.3, 15.7, and 13.9% at Coffeeville when applied litter was 2.2, 4.5, and 6.7 Mg ha–1, respectively. This decrease of PEE with increasing litter rate is because extracted amount of P did not proportionally increase with increasing applied litter rate and therefore with increasing litter-derived P rate. The low PEE found in this field research, to some extent, agrees with a finding by Tewolde et al. (2005) who reported a maximum of only 16% litter P extraction by immature cotton grown in an inert media in a greenhouse.

Supplementing litter with UAN-N improved PEE, although this was only occasionally significant. Phosphorus extraction efficiency averaged across all years and litter rates was 12.7, 19.8, and 26.6% at Cruger and 13.4, 15.8, and 26.6% at Coffeeville when 0, 34, and 67 kg ha–1 supplemental UAN-N was applied, respectively (Table 3). The increase in PEE with increasing supplemental UAN-N rate most likely is because of increases in seed cotton yield and plant growth in proportion to the UAN-N rate. This may be one reason applying litter to meet a portion of the N need and applying inorganic N to meet the other portion of the cotton N need may be the best strategy of managing litter-derived P when litter is used as the primary cotton fertilizer in high-P soils.

Overall, our results indicate that applying the higher rates of litter will lead to accumulation of P in the soil over time. Of the three litter rates tested in this research, soil P accumulation to detrimental levels is less likely to happen when the lower rate of 2.2 Mg ha–1 litter is used. However, applying 2.2 Mg ha–1 litter may not be an attractive practice for a grower because this rate supplies too little ({approx}25%) of the N need of cotton (Tewolde et al., 2007) for growers to invest and make litter use as part of their fertilization program. Litter rate of 6.7 Mg ha–1 supplied {approx}75% of the N need for cotton production in this research. But, our results show cotton extracts much less P than supplied by 6.7 Mg ha–1 litter, a rate which is likely to result in accumulation of P in the soil. The intermediate rate of 4.5 Mg ha–1 may be a more attractive rate because it supplies {approx}50% of the N need of cotton, and the buildup of soil P when this rate is used is less than that of the 6.7 Mg ha–1 litter rate. Further, the 4.5 Mg ha–1 litter application supplemented with 67 kg ha–1 UAN-N resulted in the best lint yield each of the 3 yr at both locations (Tewolde et al., 2007). If it is necessary to apply the higher rates of litter rate, another viable strategy may be to draw down accumulated soil P by stopping litter application for a few years and by fertilizing with inorganic N sources until an amount of P equivalent to that added by previous years' litter application has been removed with harvested crop after which litter fertilization may be resumed.

Partitioning of Phosphorus to Plant Parts
Phosphorus extracted by aboveground plant parts at the end of the season was recovered largely in reproductive parts including seed, lint, and bur. An average across treatments, years, and locations of 71% of the total extracted P was partitioned to reproductive parts. Seed alone contained 52.7% of the total extracted P and lint contained an additional 2.4% for a total of 55.1% that would be removed with harvested crop.

Litter and UAN-N applications significantly affected partitioning of P to plant parts mostly in 2003 at Cruger (Table 4). Increasing litter rate at this location in 2003 linearly decreased P partitioning to leaves but linearly increased P partitioning to seeds. For example, when no UAN-N was applied, partitioning of P to leaves at 0, 2.2, 4.5, and 6.7 Mg ha–1 litter was 22.7, 20.2, 17.2, and 12.3%, respectively, with a corresponding partitioning to seed of 50.6, 52.2, 54.2, and 60.5%. Increasing applied UAN-N also had similar effect on P partitioning. The P partitioning to leaves of the 2.2 Mg ha–1 litter treatment supplemented with 0, 34, or 67 kg ha–1 UAN-N was 20.2, 13.9, and 12.0%, respectively, with a corresponding partitioning to seed of 52.2, 60.6, and 61.6%.


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  		Table 4. End-of-season partitioning of extracted phosphorus to aboveground plant parts of cotton fertilized with broiler litter ± supplemental N as urea ammonium nitrate solution (UAN) at Cruger (conventional-till) and Coffeeville (no-till) in Mississippi.{dagger}

 
In the other years at either location, both litter and UAN-N significantly affected partitioning of P only occasionally (Table 4). In general, increasing litter or UAN-N application increased partitioning of P to reproductive parts and decreased partitioning to leaves and stems although these effects were stronger in certain years and locations than in others. These effects seemed to be related to the N application rate. Regressing P partitioning on NTPA showed that P partitioning to leaves and stems mostly is negatively related to NTPA while partitioning to seeds, burs, and lint is positively related to NTPA (Fig. 4 ). This suggests increasing N fertilization shifts the partitioning of P from vegetative to reproductive parts. So, supplementing litter fertilization with inorganic N may be an effective strategy not only in extracting additional P from soils but also in increasing the P fraction partitioned to seed so that more P is removed from the field with harvested cotton. Earlier, we reported that litter fertilization of 4.5 Mg ha–1 supplemented with 67 kg ha–1 UAN-N resulted in yield equal to or greater than the fertilization with farm standard fertilization with 100% inorganic fertilizers (Tewolde et al., 2007). To our knowledge, the effect of adequate N fertilization in shifting P partitioning from vegetative to reproductive plant parts has not been reported previously.


Figure 4
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  		Fig. 4. Relationship of P partitioning to plant parts with applied total plant available nitrogen (NTPA) of cotton grown with broiler litter with or without supplemental N as urea ammonium nitrate solution at Cruger (conventional-till) and Coffeeville (no-till) in Mississippi in 2003.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Knowledge of the magnitude of P extracted and removed by harvested crop is an important component of effectively managing litter to minimize or prevent the buildup of soil P. The results of this research show cotton can extract about the same amount of P supplied by 2.2 Mg ha–1 litter with ≤ 18 g kg–1 P concentration but removes only about 55% of it in harvested seed and lint. Applying litter ≥ 4.5 Mg ha–1 supplies P in excess of the amount that can be extracted by cotton and would result in the buildup of soil P over time. Supplementing litter with inorganic N fertilization increases P extraction but not to the extent that all of the litter-derived P is extracted. Supplemental inorganic N increases extracted litter P by increasing dry weight, not by increasing tissue P concentration. Increasing supplemental inorganic N or litter rate, to the contrary, decreases tissue P concentration. In addition to increasing total P extraction, supplementing litter with inorganic N benefits litter-P management because it seems to shift P partitioning from vegetative to reproductive parts and therefore increases the P amount removed in harvested crop. Overall, this research shows cotton extracts as much P as or more P than forage and pasture crops to which litter has traditionally been applied although cotton may not extract all of the P supplied by ≥ 4.5 Mg ha–1 litter.


   ACKNOWLEDGMENTS
 
We extend our appreciation to cooperating farmers Mr. Robert W. Farmer and Mr. Coley L. Bailey, Jr. for supporting this research by providing plot space. We are also grateful to Richard Switzer, USDA-ARS, for providing technical assistance and Dr. Tim Fairbrother and Quinnia Yates, USDA-ARS, for analyzing P concentration of all samples.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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