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Production Papers

Alfalfa Yield and Soil Phosphorus Increased with Topdressed Granular Compared with Fluid Phosphorus Fertilizer

^a Plant Sciences Dep., Univ. of Arizona, Tucson, AZ 85721
^b Dep. of Soil, Water, and Environmental Sciences, Univ. of Arizona, Tucson, AZ 85721
^c Pioneer Hi-Bred International, Inc., P.O. Box 1150, Johnston, IA 50131

^* Corresponding author (mottman{at}ag.arizona.edu)

Received for publication September 5, 2005.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Fluid P fertilizers are commonly applied to established alfalfa (Medicago sativa L.) despite their higher cost compared to granular forms. The objectives of this experiment were to compare effects of fluid and granular P fertilizer on alfalfa yield, and availability and movement of P in the soil. The P fertilizers compared were fluid ammonium polyphosphate (APP, 10–34–0) applied in irrigation water and granular monoammonium phosphate (MAP, 11–52–0) topdressed and incorporated by flood irrigation on a calcareous soil at Maricopa, AZ. Fertilizer P was applied each December at rates of 1, 2, 3, and 5 g P m^–2. The soil was sampled about 1 mo later. In the first four cuttings of the first year, hay yields were 1358 g m^–2 for APP and 1501 g m^–2 for MAP. No differences in yield due to P source were measured in the remaining four cuttings of the first year, or in any cutting the second or third year. In the surface soil (0–7.6 cm), the bicarbonate-extractable soil P averaged 4.4 mg kg^–1 for APP and 7.1 for MAP. The depth of movement of the fertilizers was similar except in the first year where MAP moved deeper into the soil profile than APP. In this study, the higher cost of fluid APP compared with granular MAP was not recovered by increased yield. However, at low P rates, water-run APP may be more economical than topdressed MAP due to its low application cost.

Abbreviations: APP, ammonium polyphosphate • DAP, diammonium phosphate • MAP, monoammonium phosphate • PA, phosphoric acid • TSP, triple superphosphate

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
FOR MANY CROPS WORLDWIDE, a shift has occurred from granular to fluid fertilizers, due to their ease of handling and versatility in application (Havlin et al., 2004). This shift has been especially prominent in irrigated alfalfa, where mechanical incorporation of fertilizer is not practiced after establishment, due to the likelihood of damage to the crop. Without this mechanical incorporation, topdressed P fertilizer may not be available to the roots of an established crop. When mixed with irrigation water, fluid fertilizers may move through soil cracks and macropores (Jaynes et al., 1992) and thus reach greater soil depth than granular fertilizers do.

It is not clear whether fluid or granular P fertilizer is the most economical to use. The cost of fluid P fertilizer can be 44% greater than granular formulations (Meister, 2004). Applying the fluid fertilizer in the irrigation water, however, is less expensive than topdressing granular fertilizer. Further, some studies (Holloway et al., 2001, 2005; Venugopalan and Prasad, 1994) have shown that fluid forms of P are more effectively taken up by plants, and that the price differential between fluid and granular may be further compensated by lowering the application rate for fluids.

Direct comparisons between fluid and granular fertilizers are complicated by the fact that these fertilizers may differ not only in formulation (solid or liquid), but also in the chemical form of P (orthophosphate or polyphosphate). Many fertilizers contain P as orthophosphate such as fluid phosphoric acid (PA, 0–55–0) and the granular fertilizers MAP (11–52–0), diammonium phosphate (DAP, 18–46–0), and triple superphosphate (TSP, 0–45–0). In contrast, APP (10–34–0), a common fluid P fertilizer, contains about half of the P as polyphosphates (chains of orthophosphates) and the other half as orthophosphate (Rehm et al., 1998).

Therefore, both the formulation (liquid or solid) and chemical composition (orthophosphate or polyphosphate) of the fertilizer should be considered when comparing the two P fertilizer forms. When P fertilizers differing in both formulation and chemical composition have been compared, fluid APP was superior to granular fertilizers when applied in a subsurface band in alfalfa (Stein and Westerman, 1984) and triticale (X Triticosecale) (Holloway et al., 2005), whereas granular fertilizers were superior to fluid APP when broadcast in wheat (Triticum aestivum L.) (Anurag et al., 1992). When P fertilizers differing only in formulation were compared, similar yields were reported with APP whether applied as a liquid or solid (Venugopalan and Prasad, 1992), but higher yields were reported with fluid MAP than the granular formulation (Holloway et al., 2001). When P fertilizers differing only in chemical composition were compared, similar yields were obtained with fluid PA and APP (Locascio and Rhue, 1990).

Soil availability and movement of P from liquid and solid sources seem to vary depending on the method of application and soil properties. When fertilizer was applied in irrigation water, APP moved to only 60 to 70% of the depth of MAP and PA (Lauer, 1988a). When fertilizers were broadcast and sprinkler irrigation applied to incorporate them, movement of APP compared with MAP and TSP was deeper in a sand, similar on a calcareous silt loam, and less deep in a noncalcareous silt loam (Lauer, 1988b). Ammonium polyphosphate moved deeper and was more available than orthophosphate fertilizer in a sand (Chung et al., 1999), or Alfisol (Kudeyarova and Kvaratskheliya, 1984), or a Mollisol (Kravchuk, 1992). On a fine sandy loam (Typic Hapludult), the mobility of APP and DAP was similar, but more precipitation was noted with APP (Khasawneh et al., 1974). The APP precipitation was localized and irreversible while the DAP precipitation was dispersed and showed signs of reversibility (Khasawneh et al., 1979). Also, in waterlogged soil simulating lowland rice (Oryza sativa L.) conditions, MAP was more available than APP (Porananond and Searle, 1977). With MAP, diffusion and availability was greater with fluid than granular formulation on calcareous soils in southern Australia (Lombi et al., 2004).

In the southwestern USA, APP is commonly applied to established alfalfa in irrigation water. Many assume that fluid forms of P are more available to the crop due to their water solubility and potential for penetration into the soil, and that topdressed granular fertilizers need to be mechanically incorporated for them to be effective. However, no such comparison of APP applied in irrigation water to topdressed granular fertilizer has been reported in the literature. Therefore, the objective of this experiment was to evaluate effects of fluid APP applied in irrigation water and granular MAP topdressed as a granule on alfalfa hay yield, and movement and availability of P in soil.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Soil Characteristics
An experiment was conducted at the University of Arizona Maricopa Agricultural Center near Phoenix from 1996 to 1999. The soil type was a Casa Grande sandy clay loam (reclaimed fine-loamy, mixed, superactive, hyperthermic, Typic Natrargid). Bicarbonate-extractable P was 6.1 mg kg^–1 before planting. This soil is well drained and relatively deep with uniform texture except for occasional clay lenses from 0.6 to 1.5 m and gravel at 1.8 to 2.4 m. Chemical constituents of this soil have been characterized as follows: pH = 8.2, organic matter (OM) = 0.5%, ECe = 1.6 dS m^–1, cation exchange capacity (CEC) = 10.6 cmol kg^–1 (Post et al., 1988).

Alfalfa Establishment
Sudangrass (Sorghum sudanense L.) was grown on the experiment site in the summer of 1996, the forage harvested, and crop residue disked to prepare for planting of alfalfa in the fall. The alfalfa cv. CUF 101 was seeded with a grain drill on 8 Oct. 1996 at a rate of 28 kg seed ha^–1. Earthen dikes were erected around the perimeter and surface flood irrigation was applied on 11 Oct. 1996 to germinate the seed.

Phosphorus Treatments
Fertilizer treatments, initiated in December of 1996, consisted of various rates of P applied as granular MAP spread on the soil surface and fluid APP either applied in irrigation water or sprayed onto the soil surface (Table 1). The design of the experiment was a randomized complete block with 11 treatments and four replications. Earthen dikes were erected and fertilizer was applied 19 to 21 Dec. 1996, 16 to 18 Dec. 1997, and 9 to 10 Dec. 1998 on plots that measured 3.7 by 6.1 m. Depending on the specific treatment, the fluid APP fertilizer was either injected into irrigation water or sprayed onto the soil surface with a backpack sprayer followed by irrigation within 2 d. The granular MAP fertilizer was topdressed on the surface of the soil by hand and incorporated with irrigation. The amount of irrigation water applied to the plots was about 10 cm, except in December of 1996, when more water was applied than intended due to breaching of the earthen dikes with irrigation water. The treatments with fluid APP in the irrigation water were irrigated with 10 cm of water, followed by about 20 cm of water that overflowed from the treatments with surface-applied MAP and APP. Therefore, the treatments with fluid APP in the irrigation water received 30 cm of water and all other treatments received 20 cm of water in December of 1996.

Irrigation
Irrigation was scheduled to accommodate the forage harvest. Irrigation water was applied twice between cuttings, once after the bales were removed from the field and another time about halfway to the next cutting. During much of the year when the cutting cycle was about 28 d, irrigation water was applied every 14 d. The amount of water applied each irrigation was about 15 cm, except for the last cutting of the year when the annual fertilizer treatments were applied as noted above.

Soil and Plant Analysis
Soil was sampled for determining available P on 30 Jan. 1997, 20 Jan. 1998, 20 Jan. 1999, and 27 Jan. 2000. Twelve 5 cm-diam. cores were collected per plot. All treatments were sampled to a depth of 0 to 7.6 cm to characterize the surface soil since most soil P is located near the soil surface. Samples from lower soil depth were taken only from the unfertilized control and the three treatments receiving the high (5 g m^–2) rate of P. The depth increments of these samples were 7.6 to 15.2, 15.2 to 23.0, 23.0 to 30.5, and 30.5 to 61.0 cm. Sodium bicarbonate-extractable phosphate was determined in the soil samples by colorimetric method (Kuo, 1996).

Statistical Analysis
The data were statistically analyzed using the GLM procedure of Statistical Analysis System (SAS Institute, 1999).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Yield
Phosphorus fertilizer form and method of application generally had a small influence on alfalfa hay yield (Table 2, Fig. 1 ). A significant difference between the control and fertilized treatments was measured in earlier cuttings: the first six cuttings in 1997, the first four and last cuttings in 1998, and the first three cuttings in 1999, or in 14 of 24 cuttings. The optimum P rate for the conditions of this study was about 3 g P m^–2 (Fig. 1). In 1997, topdressed MAP resulted in higher yields than water-run APP for the first four cuttings in spring, but not for the following four cuttings that year. In 1998 and 1999, no differences in alfalfa yields were detected regardless of P fertilizer form and method of application. Summed over 3 yr, hay yield was similar for water-run APP and topdressed MAP, except for the March and April cuttings due to the differences measured in 1997. Total yield summed from all cuttings was slightly greater for topdressed MAP compared with water-run APP. Whether APP was water-run or sprayed on the soil surface did not affect yield, except for the last cutting of 1998 and the first cutting in 1999, where the water-run method resulted in higher yield than sprayed. Spraying APP on the soil surface is similar to topdressing a dry fertilizer since the APP dried before irrigation was applied. Therefore, the differences between APP and MAP were probably not due to the method of application.

View larger version (30K): [in this window] [in a new window]   Fig. 1. Annual alfalfa hay yield increases in a curvilinear fashion to rate of fluid and granular P fertilizer applied in December of the previous year. The P fertilizers used were fluid ammonium polyphosphate (APP, 10–34–0) applied in the flood irrigation water and granular monoammonium phosphate (MAP, 11–52–0) topdressed and incorporated with flood irrigation. Yields were higher with the granular MAP than fluid APP fertilizer in 1997, but were similar in 1998 and 1999 for both fertilizers. The symbols ns, +, *, ** indicate that the coefficient of determination (r2) of the fitted equation (y = b0 + b1/x) is not significant at P = 0.10, or is significant at P = 0.10, 0.05, or 0.01, respectively.

The results of this study suggest water-run APP is slightly less effective than granular MAP especially early in the first year where higher yields were obtained with MAP. The yield response to P fertilizer that we measured in earlier cuttings can be explained by reduced root interception and diffusion of P when soil temperatures are cool. The lack of response in later cuttings does not appear to be related to P supply running out or P fixation, since even the highest P rates did not increase yield.

One rationale for this research was to evaluate if the added cost of fluid P is warranted based on efficiency. Under the conditions of our study, fluid APP applied in irrigation water did not increase alfalfa yield compared with topdressed MAP. In fact, topdressed MAP resulted in higher yields for some cuttings. Reid et al. (2004) concluded that the higher cost of P in fluid PA was not justified by its agronomic performance compared with MAP and TSP with irrigated alfalfa. In our study, results may have been different if the APP had been knifed in a band below the soil surface as in the study of Stein and Westerman (1984) where knifed APP was superior to dry fertilizers applied in various ways. Our results suggest that APP is similar or inferior to granular fertilizer. This is in contrast to studies in India (Venugopalan and Prasad, 1994) and Australia (Holloway et al., 2001, 2005) where fluids were reported to be superior to dry P fertilizers. The difference between our study and some of those reported in the literature may be due to differences in methods of application or soil characteristics. Fluid P fertilizers may be superior to granular fertilizers when applied in subsurface bands (Holloway et al., 2005; Stein and Westerman, 1984). Also, granular P may not dissolve entirely or may form insoluble reaction products in highly calcareous soils as has been reported in Australia (Lombi et al., 2004).

Soil Phosphorus
Increased concentration of bicarbonate-extractable P was measured with topdressed MAP. At 0 to 7.6 cm, soil P averaged about 60% higher for MAP than APP applied in the irrigation water (Table 3). Soil P was also higher when APP was sprayed on the surface than applied in the irrigation water. Despite differences in soil P, no differences were measured in P concentration in plants sampled at the May harvest of each year (data not presented). The differences in soil P between fertilizer sources were greatest at higher P rates (Fig. 2 ). In addition, no differences in soil P between fertilizer sources were detected below 7.6 cm, except in 1997 (Fig. 3 ).

View larger version (30K): [in this window] [in a new window]   Fig. 2. Bicarbonate-extractable P in the surface soil (0–7.6 cm) increases with rate of fluid and granular P fertilizer applied. The P fertilizers used were fluid ammonium polyphosphate (APP, 10–34–0) applied in the flood irrigation water and granular monoammonium phosphate (MAP, 11–52–0) topdressed and incorporated with flood irrigation. Soil P was higher with granular MAP than fluid APP. The soil was sampled about 1 mo after the previous fertilizer application, except in the Year 2000, where the soil was sampled at the end of the experiment about 13 mo after the previous fertilizer application. Therefore, little or no increase in soil P was measured in 2000 in contrast to the previous years. The symbols ns, +, *, ** indicate that the coefficient of determination (r2) of the fitted equation (y = b0 + b1x) is not significant at P = 0.10, or is significant at P = 0.10, 0.05, or 0.01, respectively.

View larger version (23K): [in this window] [in a new window]   Fig. 3. Most of the bicarbonate-extractable P remains in the surface soil with fluid or granular fertilizer P application. The P fertilizers used were fluid ammonium polyphosphate (APP, 10–34–0) and granular monammonium phosphate (MAP, 11–52–0). The APP was applied in the irrigation water (water-run) or sprayed on the soil surface (sprayed) and MAP was topdressed. The data presented includes the unfertilized control and P fertilizer applied at 5 g P m–2. Where differences in soil P were detected, higher values were generally measured with granular MAP than fluid APP. Movement of P in the soil profile was limited and not affected by P fertilizer source, except in 1997 where an accumulation of soil P was measured between 15 and 23 cm for granular MAP perhaps due to the greater amount of irrigation water applied at the time of fertilizer application that year. Soil P was usually not affected by whether APP was water-run or sprayed. The error bars represent the least significant difference at P = 0.05 [LSD(0.05)], or if the LSD is not significant, an "ns" appears next to the data points rather than an error bar.

The most important consideration when evaluating crop performance with different P sources may be reaction of the fertilizer with the soil. Lauer (1988b) found that, compared with MAP and TSP, movement of APP was similar on a calcareous silt loam, more deep on sand, and less deep on a noncalcareous silt loam. On calcareous soils in southern Australia, P diffusion and availability was greater with fluid APP than granular fertilizers (Lombi et al., 2004). Fluid APP precipitated and formed insoluble compounds in a noncalcareous fine sandy loam (Khasawneh et al., 1974) and in a waterlogged soil (Porananond and Searle, 1977). In our experiment, the conversion of polyphosphates to orthophosphates, which normally occurs within a few days (Huffman, 1970) or a few weeks (Venugopalan and Prasad, 1989), may have been slowed by cold weather since the applications occurred in December, and thereby affected fertilizer efficiency (Englestad and Allen, 1971). Pyrophosphate, a polyphosphate containing two orthophosphate molecules, is available to plants but taken up at a slower rate than orthophosphate (Huffman, 1970). The disadvantage of granules in some instances is that they may not dissolve completely (Lombi et al., 2004), but surface flood irrigation may have sufficiently dissolved the granules in our study. Topdressing P in the soils of the present study may be better than mechanical incorporation because of limited fertilizer/soil contact. The surface soil remains moist for some time after irrigation, particularly after the second irrigation in the cutting cycle where the canopy shades the soil surface, and roots near the soil surface may be able to access the topdressed P.

Whether fluid or granular P fertilizer is the most economical to use depends on fertilizer price, P rate, application cost, and yield benefit from the respective fertilizers. If we assume (i) the fertilizers in question are equally effective, (ii) the material costs are $1.46 kg^–1 P for granular MAP and $2.09 kg^–1 P for liquid APP (Meister, 2004), and (iii) the application costs are $20 ha^–1 for applying granular fertilizer and no cost is associated with dripping liquid fertilizer into an irrigation ditch (Meister, 2004), then the rate at which the material and application cost for both fertilizers are equal is 32 kg P ha^–1. At higher rates, granular MAP is more economical and at lower rates, liquid APP is more economical using the assumptions above. The rate of P that is applied to alfalfa annually in the southwestern USA is usually about 50 kg P ha^–1 (Meister, 2004), so MAP would be more economical at this rate. However, when liquid P is applied in the irrigation water, typically the annual rate is split among several applications. Since application cost is negligible for applying liquid fertilizer in flood irrigation, costs are not increased by splitting the annual application. Also, multiple applications may have the additional advantage of providing the crop with more opportunity to take up P before it is inevitably fixed by the soil.

The assumption used in the economic analysis above that fluid APP and granular MAP are equally effective in increasing yields may not be true in all cases. In our research, APP applied in irrigation water, or sprayed on the soil surface, was not as effective as topdressed MAP since it resulted in less hay yield and lower soil P values. Even though we were not able to demonstrate increased productivity with fluid P in irrigated alfalfa production, fluid forms of P fertilizer still have the advantage of ease of handling, versatility of application, and low application cost in flood irrigation systems.

	ACKNOWLEDGMENTS

 
This research was supported by the Potash and Phosphate Institute. The technical assistance of Mark Rogers and Mary Comeau is greatly appreciated.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Ottman, M. J. Articles by Doerge, T. A. Search for Related Content PubMed Articles by Ottman, M. J. Articles by Doerge, T. A. Agricola Articles by Ottman, M. J. Articles by Doerge, T. A. Related Collections Forage Management Phosphorus Alfalfa Production Agriculture Soil Fertility and Productivity