Agronomy Journal 94:48-51 (2002)
© 2002 American Society of Agronomy
SOIL FERTILITY
Point-Injected Phosphorus Effects on Established Cool-Season Grass Yield and Phosphorus Content
Ron H. Gelderman*,a,
James R. Gerwinga and
Edward Twidwellb
a Plant Sci. Dep., Box 2207A, South Dakota State Univ., Brookings, SD 57007
b Louisiana State Univ., P.O. Box 25100, Baton Rouge, LA 70894-5100
* Corresponding author (Ronald_Gelderman{at}sdstate.edu)
Received for publication June 5, 2000.
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ABSTRACT
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Phosphorus applied to established grass has been shown to increase forage yield. However, broadcast P applications could result in more P runoff to surface waters compared with subsurface P application. A field study was conducted to evaluate if subsurface point-injected P is more effective than surface-applied P at increasing forage yield and tissue P concentration for established grass. Nine site-years with very low P soil tests and long-term (>10 yr) established cool-season grass stands were utilized. Fertilizer treatments were no fertilizer P and 29 kg P ha-1 [66 kg ha-1 phosphoric oxide (P2O5)] applied on the surface or point-injected beneath the surface. Fertilizer P significantly increased dry matter yields at four site-years. Placement of P did not influence forage yields at the P responsive sites, except for one site-year. Forage P concentration was increased by added P at seven of nine site-years. Forage P concentrations from point-injected P treatments were lower compared with broadcast P at five of the seven P responsive site-years. Results of this study suggest that point-injected P offers no advantage over surface-applied P for increasing forage yield or P concentration of established cool-season grass stands.
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INTRODUCTION
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PHOSPHORUS FERTILIZATION of established forage grasses on low P–testing soils can result in significant yield responses (Gelderman et al., 1988; Lamond and Moyer, 1983; Moldenhauer et al., 1965; Rehm, 1990). Response to P as influenced by grass specie is not well documented, and a number of state extension recommendations for fertilizer P applications to established grass are not separated by specie (Anderson and Shapiro, 1990; Franzen, 1996; Gerwing and Gelderman, 1998; Rehm et al., 1994).
Traditionally, P fertilizer has been surface-broadcast on grasslands. Even though significant response to P applications is noted, the probability and magnitude of responses to such applications are often less than expected (Gelderman et al., 1988; Smika et al., 1961). Reasons for the less-than-expected response to applied P on grass sods could be positional unavailability of broadcast P, a lower P requirement in forage grasses compared with other crops, or the beneficial association of mycorrhizae with roots of permanent grass sods. Phosphorus moves very little with soil water, especially at low soil test levels, and as a result, grass roots need to contact applied P relatively close to where it is applied. Therefore, subsurface placement of P may increase P uptake by grasses over broadcast placement, particularly during periods when the surface soil dries.
Subsurface P fertilization of grass stands has been accomplished by knives (Lamond and Moyer, 1983), shoes (Smika et al., 1961), or double disk-drill openers (Black, 1968). Smika et al. (1961) found that the cultivating action of the shoe opener produced higher yields on native grass and crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schultes] regardless of fertilizer placement. Previous work with subsurface placement of P on grass produced yields that were lower (Karlovsky, 1957; Moore et al., 1968; Neller and Hutton, 1957; Smika et al., 1961) or, in one case, comparable to broadcast P (Lamond and Moyer, 1983). Some of these attempts at subsurface P placement caused stand injury due to physical disturbance (Karlovsky, 1957; Smika et al., 1961). Root shearing was implied as a possible explanation for lower tall fescue (Festuca arundinacea Schreb.) yields when fertilizer was applied by knife rather than broadcast (Sweeney et al., 1996).
Most point-injection fertilizer applicators utilize a rolling wheel with spokes to place fertilizer in a discontinuous band beneath the soil surface. This equipment was developed for no-till systems in which surface residue can plug conventional subsurface fertilizer applicators. In his review of point-injected fertilization research, Fixen (1990) indicates that most were N studies, and almost all indicated the point-injection system to be equal to or better than other placement methods. Less NH3 volatilization and better positional availability from subsurface N placement were the reasons given for the success of this system. Nevertheless, this method of fertilizer placement has not been widely adopted, and work with P on established grass has not been documented.
The point-injection system should cause less stand damage compared with knives, shoes, or disks for subsurface P fertilizer placement on sod while theoretically providing a more position-available P source for grass, especially when surface soils are dry. However, placement of P in discontinuous bands could be a disadvantage for root uptake of P (Eghball and Sander, 1987; Fixen, 1990). The objective of our work was to compare forage yield and P concentration under subsurface-applied, point-injected P with surface-broadcast P in established grass stands.
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MATERIALS AND METHODS
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A field study utilizing nine site-years with cool-season grasses was used to evaluate P placement. The characteristics of the sites are given in Table 1. All sites had grass stands that had been established for at least 10 yr. Phosphorus Bray P-1 soil tests were considered very low and indicated a response to P fertilizer would be likely (Gerwing and Gelderman, 1998). Available plant N was considered low while other soil tests were considered adequate for plant growth at all sites (Gerwing and Gelderman, 1998).
A randomized complete block design in a split-plot treatment arrangement with four replications was used. Whole-plot treatments were no P fertilizer and 29 kg P ha-1 (66 kg ha-1 P2O5). Subplots consisted of broadcast and point-injected P. The P rate used was 1.5 times the recommended P rate for established grass at a very low P soil test (Gerwing and Gelderman, 1998).
Phosphorus was applied as ammonium polyphosphate solution (10–15–0 N–P–K). The broadcast treatment was sprayed on the sod with standard spray-boom equipment. The point-injection placement utilized a Cady system of rolling spoked wheels to place the P solution at a 7.5-cm depth and 30-cm row spacing. Rate of P solution for both placement methods was delivered with a power takeoff pump and adjusted for ground speed with electronically controlled spray components. The ammonium polyphosphate was diluted (1:3 fertilizer/water) to facilitate flow through the spokes. No P subplots with a subsurface placement were treated with the applicator without fertilizer P. All treatments were equalized to 112 kg N ha-1 by broadcasting ammonium nitrate [NH4NO3] (34–0–0 N–P–K). We assumed this N rate would meet the N needs of a 9000 kg ha-1 yield goal (Gerwing and Gelderman, 1998). Treatments were applied in September of each year for the Brookings site and in early spring for all other sites. Site-year Brk94, Brk95, Hmr95, and Brn95 had treatments applied for 2, 3, 2, and 2 consecutive years, respectively. All other site-years received one application of P.
Yields were estimated by harvesting a 0.9- by 6.1-m area with a sickle-bar mower for sites Hyd93, Hgs93, Brk93, Brk94, Brk95, Hmr95, and Brn95 or a 0.4- by 6.1-m area with a rotary mower for sites Hmr94 and Brn94 from the middle of a total plot area of 1.5- by 7.6-m. Harvest dates and areas for each site-year are given in Table 1. Yields were determined on a dry matter basis. Forages were weighed wet, and a sample was taken for moisture determination. The sample was dried with forced air at 60°C for 7 d, weighed, and ground to pass a 1-mm screen. The ground forage sample was digested with nitric (HNO3) and perchloric (HClO4) acids and colorimetrically analyzed for P (Gelderman et al., 1990).
Analysis of variance was used to test for statistical significance (P < 0.10) of treatment effects and interactions. Mean separation for main-effect interactions was accomplished with LSD (P = 0.05).
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RESULTS AND DISCUSSION
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Forage Yield
Grass dry matter yields ranged from 1543 to 6820 kg ha-1 over the nine site-years (Table 2). The lowest site-year mean yields were from Hgs93, Hmr94, and Brn94 and corresponded to the greatest negative departure from normal precipitation for the March through June period (Table 3). Departure from normal precipitation from March through June at the site-years ranged from -122 to +252 mm (Table 3).
Phosphorus Effects
Application of P significantly increased forage yields with either placement at Brk93, Brk94, Brk95, and Brn95 (Table 2). Main-effect interactions occurred at Brk93 and Brn94 (Table 2). At Brk93, P significantly increased forage yield over check for the broadcast placement only (LSD = 999 kg ha-1). At Brn94, P did not significantly increase yields over check with either placement (LSD = 348 kg ha-1). The response to P over the four P responsive site-years ranged from about 600 to 2800 kg ha-1 dry matter (Table 2). Despite very low P soil tests (Gerwing and Gelderman, 1998), the other five site-years were nonresponsive to added P. The relative lack of response of forage grasses to added P is consistent with other studies (Gelderman et al., 1988; Sweeney et al., 1996).
Placement Effects
Placement of P did not significantly influence grass forage yield at three of the four P responsive site-years (Table 2). This indicates that broadcasted or point-injected P placement gave similar results. At Brk93, placement had no significant influence on yield without P, but broadcasting P significantly increased forage yield over point-injected P placement (LSD = 2666 kg ha-1). At Brn94, there was no significant difference in placement when P was applied, but a significant yield increase resulted from the injector applicator compared with broadcasting under the no-P treatment (LSD = 817 kg ha-1). Using the spoke applicator on the no-P treatments produced a mean increase of 179 kg ha-1 (over all site-years) compared with the no-disturbance (broadcast) treatments. Smika et al. (1961) found an increase in grass yields due to the mechanical disturbance of the applicator. Broadcast placement of P produced a mean increase of 400 kg ha-1 (over all site-years) compared with point-injected P.
Forage Phosphorus Concentration
Grass forage P concentration ranged from 0.80 to 2.11 g kg-1 dry matter (Table 4). Forage from fertilized plots (29 kg P ha-1) averaged 0.161 g P kg-1 dry matter compared with 0.115 g P kg-1 dry matter from the unfertilized plots. A P concentration of 0.12 and 0.22% of diet dry matter is suggested for a beef cow (Bos taurus) in midgestation and early lactation, respectively (Natl. Res. Counc., 1996). Phosphorus application significantly increased forage P levels at all site-years except Hgs93 and Brk95. Added P significantly increased forage P concentration at Hyd93, Hmr94, Brn94, and Hmr95 even though no significant differences in yields were obtained at these sites.
Placement of P significantly influenced forage P concentration at the Hyd93, Brk94, Hmr94, Brn94, and Hmr95 sites. A significant interaction of P and placement effects also occurred at these sites. The increase in forage P concentration due to added fertilizer P was significantly lower where P was point-injected compared with surface-applied at these five sites (Table 4). The lower forage P content from subsurface-applied P in this study is consistent with other research (Moore et al., 1968; Neller and Hutton, 1957). At two sites, Brk93 and Brn95, both application methods increased forage P concentrations similarly.
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SUMMARY
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The forage P concentration and yield results suggest that grass under the point-injected treatments utilized applied P to a lesser extent than plants under the broadcast treatments. Despite lower forage P concentrations from the point-injected treatments, plants had sufficient available P to produce similar yields compared with broadcast-treated plants, except at Brk93. This may not have been the case, however, if a lower rate of applied P had been used in the study. Perhaps the poorer utilization of P by the grass under the point-injected treatments may be due to the limited root contact of the discontinuous P bands, as suggested by Eghball and Sander (1987). However, other studies (Moore et al., 1968; Neller and Hutton, 1957) found less efficient P use by grass when subsurface P was applied in a continuous band.
On the basis of these data, and because of the more expensive equipment needs of subsurface P placement, point injection of P fertilizer is not recommended for established cool-season grass forages. It should be recognized, however, that on sloping soils, surface application of P fertilizer might result in more P loss in runoff to surface waters.
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ACKNOWLEDGMENTS
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We thank Anthony Bly, Dwayne Winther, Carolyn Hoffman, and Keri Skroch for their technical support.
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NOTES
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South Dakota Exp. Stn. Journal no. 3188.
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REFERENCES
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- Anderson, B., and C.A. Shapiro. 1990. Fertilizing grass pastures and haylands. NebGuide G78-406-A (revised 1990). Coop. Ext., Univ. of Nebraska, Lincoln.
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- Fixen, P.E. 1990. A review of point injection. p. 68–77. In J.L. Havlin and J.S. Jacobsen (ed.) Proc. Great Plains Soil Fertil. Conf., Denver. 6–7 Mar. 1990. Kansas State Univ., Manhattan.
- Franzen, D.W. 1996. North Dakota fertilizer handbook. EB-65. North Dakota State Univ. Ext. Serv., Fargo.
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ABSTRACT
INTRODUCTION
