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Published in Agron. J. 96:1299-1305 (2004).
© American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA

Phosphorus Management

Effects of Phosphorus and Potassium on Forage Nutritive Value and Quantity

Environmental Implications

H. K. Panta,*, P. Mislevya and J. E. Rechciglb

a Univ. of Florida, IFAS, RCREC, 3401 Experiment Station, Ona, FL 33865
b Univ. of Florida, IFAS, GCREC, 5007 60th Street East, Bradenton, FL 34203

* Corresponding author (hari{at}mail.ifas.ufl.edu)

Received for publication May 3, 2003.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Management practices minimizing P application in agricultural catchments ultimately reduce P export to waters. To determine stargrass (Cynodon nlemfuensis Vanderyst var. nlemfuensis) response to P and K, eight rates of P and K were applied on experimental units located on Pomona fine sand (sandy, siliceous, hyperthermia Ultic Alaquods) Spodosols, and arranged in a randomized complete block design with four replicates. The forage yield was less from 39:0 (P/K; kg ha–1 yr–1) treatment than the experimental units supplied with 93 kg K ha–1 yr–1 and low P (10 and 20 kg ha–1 yr–1) in all years with exception in 1998 (i.e., the year of grass establishment), indicating efficient P utilization due to K applications. No significant differences were obtained in in-vitro organic matter digestibility (IVOMD) from the applications of 10 kg P ha–1 yr–1 and 93 kg K ha–1 yr–1. The applications of 10 and 93 kg ha–1 yr–1 of P and K, respectively, provided efficient P utilization. Phosphorus mass balance showed that stargrass receiving 10 and 93 kg ha–1 yr–1 of P and K, respectively, removed maximum P (161% of the applied P) by uptake from soils. This may indicate the capability of stargrass to mine P from subsoils if sufficient K is supplied, and also suggests that stargrass may be useful for crop phytoremediation on P-impacted sites. In general, this study indicates that applications of 10 kg P ha–1 yr–1 in combination with 93 kg K ha–1 yr–1 will maintain forage nutritive value and quantity, and maximize P removals by stargrass. Moreover, the supply of sufficient K appears to be crucial for efficient P utilization by forages, reducing potential adverse effects of P over-fertilization on water quality.

Abbreviations: CP, crude protein • DM, dry matter • IVOMD, in-vitro organic matter digestibility • NPK, nitrogen, phosphorus, and potassium fertilizers


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
STARGRASS IS A MAJOR forage species being propagated in the southern half of peninsular Florida and throughout the tropics worldwide. Stargrass will grow on nearly all soil types ranging from sands (Mislevy and Blue, 1981) to clays (Mislevy et al., 1990), and is found across climatic zones. In Florida and throughout parts of the tropics, the soils on which stargrass is grown are sandy, low in cation exchange capacity (CEC), pH, and nutrients (Rechcigl and Mislevy, 1997). Soils of the tropics make up 4.9 x 109 ha worldwide (Sanchez, 1976). The increase in stargrass usage in recent years has been due to high yields, persistence, good nutritive value, ease of establishment, drought tolerance, tolerance to most pests and diseases, and extended forage production during short photoperiod (Vicente-Chandler et al., 1974; Sinclair et al., 2001; Mislevy, 2002).

Stargrass, like other highly productive forages, have high nutrient requirements, especially when grass is removed through a cut and carry system (Vicente-Chandler et al., 1974; Mislevy, 2002). Despite the importance and large acreages of stargrass in production, few field studies have evaluated P and K requirements. Vicente-Chandler et al. (1974) indicated that stargrass yielded 28 Mg ha–1 annually, which removed 390, 65, 470, 150, and 55 kg ha–1 yr–1 of N, P, K, Ca, and Mg, respectively. Nitrogen fertilization greatly increases removal of fertilizer K by grasses through increased yields. An average of 140 kg K ha–1 was removed annually by tropical grass when no N was applied, compared with 500 kg K ha–1 when 450 kg N ha–1 yr–1 was applied (Vicente-Chandler and Pearson, 1960).

In a study evaluating rates of K fertilization on the yield and grass composition of napiergrass (Pennisetum purpureum Schum.), guineagrass (Panicum maximum Jacq.), pangolagrass (Digitaria eriantha steud.), and caribgrass (Eriochloa polystachya H.B.K), Vicente-Chandler et al. (1962) applied annual rates of K ranging from 0 to 900 kg K ha–1 to soil (pH 5.5) containing 100 and 670 kg ha–1 P and N, respectively. Each fertilizer element was split in six equal applications. When no K was applied, the grasses exhibited severe K deficiency symptoms and slowly died. Potassium concentration of forage increased with application rate up to 900 kg K ha–1 yr–1; however, DM yield maximized at about 450 kg K ha–1 yr–1. This would indicate that more K was taken up than needed for maximum growth. Luxury consumption of K is not desirable because cattle (Bos taurus) only require small quantities of K (National Research Council, 1984). Luxury consumption of K can be limited by applying only enough K to optimize yields, and by several split applications yearly (Vicente-Chandler, 1966).

In addition to being acidic, many tropical soils are naturally low in P and can react with fertilizer P to form fixed P rendering it relatively unavailable to plants. However, this fixed P slowly becomes available, and may be used over time to meet P requirements of grasses. The effects of P fertilization on the DM yield of grasses growing for 2 to 4 yr on Ultisols with all other nutrients provided in abundance were determined by Figarella et al. (1964). Phosphorus was applied at 0, 75, and 145 kg P ha–1 in a single annual application. Napiergrass responded strongly to 75 kg P ha–1 yr–1 on both Catalina (very-fine, ferruginous, isohyperthermic Typic Hapludox) and Mucara (fine-loamy, mixed, superactive, isohyperthermic Dystric Eutrudepts) soils by yielding 32.1 and 38.5 Mg ha–1 yr–1 DM, respectively.

When P was applied to a Fajardo clay soil (fine, mixed, active, isohyperthermic Chromic Epiaquerts) over a 4-yr period with a cropping history of sugarcane (Saccharum officinarum L.), napiergrass, guineagrass, and pangolagrass, DM yield and concentrations of P did not generally increase. This soil released 48 kg P ha–1 yr–1 compared with 25 kg P ha–1 yr–1 for Catalina and Mucara soils, which showed a high response to P. Pangolagrass responded to P fertilization on only 2 of 12 soils (Figarella et al., 1964). Their data indicated that grasses harvested by cutting responded highly to applications of about 75 kg P ha–1 yr–1 on soils with little or no previous P fertilization.

Recent environmental regulations have set strict rules on the application of P because it degrades water reservoirs. Phosphorus loss to ground or surface waters depends on various factors including physicochemical characteristics of soils, and the amount and forms of P stored in soils (Pant and Reddy, 2003). In flooded soil conditions, the dissolution of Ca-bound P, hydrolysis of Fe/Al-bound P, and mineralization of organic P to soluble P could export significant amounts of P to nearby water bodies. Phosphorus along with other nutrients loading from adjacent agricultural catchments threatens the water quality of many lakes, rivers, estuaries and other water bodies in many parts of the world, including Florida (Pant et al., 2003, 2004). Management practices that reduce the application of P fertilizer in agricultural parcels, such as pastures, would be particularly crucial for sustainable reduction in P export to water bodies. Little information is available regarding P and K requirements of tropical grasses, especially stargrass. Thus, the purpose of this study was to determine the response of stargrass to rates of P and K, and to determine if low P fertilization would compromise nutritive value and quantity of forage biomass. The specific objectives of the study were to: (i) determine the effects of various combinations of P and K fertilizer applications on DM yield, crude protein (CP), and IVOMD of stargrass; and (ii) estimate the potential loss of P to runoff.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Grass Establishment
Stargrass was established in 1997 and the experiments were conducted over a 3-yr period (1998–2000) on a Pomona fine sand soil at the University of Florida Range Cattle Research and Education Center, Ona, FL (82°55'W and 27°26'N). Soil tests, before grass establishment, indicated that soil had mean pH = 4.4; Ca = 336 mg kg–1; Mg = 44 mg kg–1; K = 28 mg kg–1; P = 5.8 mg kg–1; Zn = 1.08 mg kg–1; and Cu = 0.82 mg kg–1. Dolomitic limestone was applied at 4.0 Mg ha–1 and disked into the soil to a depth of 10 to 15 cm on 7 July 1997. ‘Florona’ stargrass was planted using vegetative stem cuttings at a rate of 1700 kg ha–1 on 14 July 1997. Seven days after planting, the area was treated with 0.28 kg ha–1 dicamba (3, 6-dichloro-2-methoxybenzoic acid), and 0.81 kg ha–1 2,4-D amine (2,4-dichlorophenoxyacetic acid; Weedmaster), and fertilized with 35–12–45 kg ha–1 N–P–K. The sources of N–P–K fertilizer amendments were ammonium nitrate, triple super phosphate, and potassium chloride, respectively. In addition, Zn, Cu, Mn, and Fe (in sulfate form) were applied at a rate of 1.4 kg ha–1, and 5.6 kg S ha–1 and 0.14 kg B ha–1 were also applied. Thirty-five days after planting, the grass was again fertilized with 56 kg N ha–1, applied as ammonium nitrate (34–0–0).

The experimental unit layout was a randomized complete block design with four replications. The treatments consisted of a control (no P or K), and split applications of eight combination rates of P (0, 10, 20, and 39 kg P ha–1 yr–1) and K (0, 23, 46, and 93 kg K ha–1 yr–1) to cover the range of rates commonly used by commercial producers. Applications of P and K fertilizers were made after each harvest for a total of six times a year. Each experimental unit measured 3.1 by 6.1 m. Nitrogen (as ammonium nitrate) was applied 30 d before initial harvest, and immediately following each successive harvest at a rate of 56 kg ha–1 for an annual rate of 336 kg N ha–1 yr–1. A micronutrient package TEM 300 containing B2O3 (96.5 g kg–1), CuO (37.5 g kg–1), Fe2O3 (257.3 g kg–1 of which 72.0 g kg–1 was water soluble), MnO (96.8 g kg–1), ZnO (87.1 g kg–1), and MoO3 (3.0 g kg–1) was applied annually at a blanket rate of 11 kg ha–1.

Forage Sampling and Chemical Analysis
The center of each experimental unit (1.5 m2) was harvested at a 5-wk interval using a Sensation "MOW-BLO" Mower (model 11F4-0), a conventional rotary blade plot harvester set at a 7.5-cm stubble height, to determine DM yield. Experimental units were cut six times per year from April to October. Forage samples were clipped with hand shears separately at each harvest to determine nutritive value and tissue analyses. Harvested forage samples were dried at 60°C in an oven, and ground to pass through a 1-mm stainless-steel screen. Samples were analyzed for IVOMD and total N as described by Moore and Mott (1974) and Gallaher et al. (1976), respectively. Crude protein was calculated by multiplying total N by 6.25.

A ground tissue subsample of 0.4 g was ashed at 450°C for 5 h, dissolved in 40 mL of 0.30 M HCl, and filtered as described by Rechcigl et al. (1995). Concentrations of P, K, Ca, Mg, Cu, Mn, Fe, and Zn were determined using an inductively coupled argon plasma spectrometry (Model 61E; Thermo Jarrel Ash Co., Franklin, MA). Forage yield and elemental composition are reported on a DM basis.

Soil Sampling and Chemical Analysis
Soil samples were taken from the A horizon (0–15 cm) of each experimental unit before establishment of the grass, and in the spring of each year before application of fertilizer treatments. Soil samples were air-dried and sieved through a 2-mm sieve in preparation for analysis. Soil pH was determined in the suspensions after equilibrating soil and deionized distilled water, respectively, at 1:2.5 (wt/vol) ratio for 30 min. Extractable K, P, Ca, Mg, Mn, Fe, Cu, and Zn were determined by the Mehlich I Method (0.05 M HCl and 0.01 M H2SO4) as described by Rhue and Kidder (1983).

Groundcover Estimation
We customarily evaluate groundcover of grass at the end of every study period to estimate persistence. Thus, the groundcover of stargrass was also assessed by visual estimation of persistence following a severe freeze at the end of the study period (Table 1) to determine whether there was any influence of fertilizer treatments. Groundcover was evaluated by a single evaluator before and 90 d after the severe freeze so that any effects on groundcover of the grass stands were clear.


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  		Table 1. Mean monthly rainfalls, and average and extreme low ground temperature at the experimental location during the study period from 1998 to 2000 and partial data for 2001.

 
Phosphorus Removal
For each treatment, P removal was calculated from average total amount of P recovered in forage DM each year. The P removal percentage was calculated, based on the total amount of P applied in each experimental unit per year. Control (i.e., no application of P or K) was not subtracted because it is known that P utilization efficiency of plants and microorganisms decreases as the availability of P, especially soluble reactive P, increases (e.g., reduction in phosphatase activities) (Pant and Warman, 2000). Values greater than 100% recoveries of applied P indicate uptake of native soil P by the grass. It should be noted that P uptake by grass may have come from applied fertilizer and native soil P sources disproportionately; thus, the P removal values need to be considered as general estimations. Note that the least significant difference (LSD) was performed only on average annual percentage removal of applied P during the study period to estimate effects of the fertilizer treatments on the P utilization.

Statistical Analysis
Unless otherwise stated, all analyses were performed in three replicates. Dry matter yield and concentrations of CP and IVOMD in forage tissue and selected chemical characteristics of soils were compared within years. The P mass balances, however, were compared for treatment effects based on pooled values for the whole study period to have general estimation of the P removal/movement phenomena in stargrass pasture. Statistical analysis was performed using SAS Program (SAS Inst., 1996), and significance was tested using Waller-Duncan K-ratio with K set at 100 (p ≤ 0.05). The regression equation associated with the study was fitted to the following basic model:

After fitting the full model to each response, the equations were examined for nonsignificant terms. A p ≤ 0.05 level of confidence was used to determine significance. The nonsignificant terms were deleted and the reduced model fitted. This procedure was continued until the only terms remaining in the model contributed significantly to the regression relationship or were the linear P or K terms when the interaction or the corresponding quadratic terms were significant. The unfertilized (0:0) fertilizer treatment was the most influential data point in these regressions, but its influence was never considered to be a leverage point (Belsley et al., 1980). Thus, regardless of which model was fitted to the data, the unfertilized treatment did not have undue influence on the fitted regression equations. Moreover, years are considered as a random model factor because the treatments effects could vary depending on the age of the grass stands, especially between the year of the stands establishment and thereafter. Hence, year x treatment interaction was not considered to avoid undue masking effect of the years on qualitative and quantitative data of the forage while analyzing treatment effects. Thus, the analysis of forage data is presented on year-by-year basis.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Forage Nutritive Value and Quantity
Dry Matter Yield
Differences among treatments were observed for DM yield in all harvests, and on a total seasonal basis between 1998 through 2000. The experimental units that received applications of 10, 20, or 39 kg P ha–1 yr–1 had higher forage yield than control that received no P and K in 1999 and 2000 (Fig. 1). However, the forage yield from 39:0 (P/K; kg ha–1 yr–1) treatment was not different from the control with the exception in 1998. The forage yield from 39:0 (P/K) treatment had less forage yield than the experimental units supplied with 93 kg K ha–1 yr–1 and low P (10 and 20 kg ha–1 yr–1) in all years with the exception in 1998, thereby indicating a more efficient utilization of P at higher K applications.



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  		Fig. 1. The effects of various fertilizer treatments on total dry matter (DM) yield of stargrass in 1998, 1999, and 2000. Total DM yields in a specific year with letters in common are not significantly different (P > 0.05). Minimum significant differences (MSD) are 0.415, 0.466, and 0.639 for years 1998, 1999, and 2000, respectively.

 
The coefficient for P was greater than that for K, suggesting the effects of K fertilization on DM yield during the first year was not as great as the effect of P fertilization, as indicated by the fitted model (DM yield = 3.16 + 0.027P + 0.008K – 0.0002P2; R2 = 0.82; P = 0.0001). However, the effect of K was greater during the second (DM yield = 1.45 + 0.033P + 0.050K – 0.0003P2 + 0.0002PK – 0.0004K2; R2 = 0.93; P = 0.0001) and third (DM yield = 1.17 + 0.0001P + 0.053K + 0.0002PK – 0.0004K2; R2 = 0.83; P = 0.0001) years as indicated by their respective coefficients. A significant interaction between P and K fertilizations in the second and third years suggested that the effect of one (P or K) on DM yield varied with the supply rate of the other. Moreover, as indicated by R2, these models explained 82, 93, and 83% of variability in DM yields in first, second, and third year of study, respectively.

Crude Protein
Grass from the treatments receiving ≤23 kg K ha–1 yr–1 generally had higher CP than grass receiving >23 kg K ha–1 yr–1 (Table 2). Faster growth and greater production of forage DM may have diluted concentrations of CP (Mislevy and Blue, 1981), thereby suggesting that greater herbage production may not necessarily guarantee better nutritive value. Fitted models indicated inverse relationships between CP concentration and P and K fertilization in the first year (CP = 15.51 – 0.024P – 0.034K; R2 = 0.71; P = 0.0001), while only the K effect was significant in the second (CP = 17.08 – 0.16K; R2 = 0.84; P = 0.0001) and third year (CP = 19.39 – 0.14K + 0.001K2; R2 = 0.62; P = 0.0001). These models explained 71, 84, and 62% of the variability in CP concentration in the first, second, and third year of the study, respectively. As the grass grew older, perhaps, the effect of P treatment diminished due to growth of roots into the P-rich Bh horizon.


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  		Table 2. Effects of different combinations of P and K fertilizer on crude protein (CP) concentration in stargrass obtained from various harvests from 1998 to 2000.

 
In-Vitro Organic Matter Digestibility
Although no significant difference in IVOMD of forage obtained from four of six harvests during the first year, overall digestibility generally was higher for forage that received no P or ≤20 kg P ha–1 yr–1 and 93 kg K ha–1 yr–1 compared with other treatments during the second and third year of the study (Table 3). The application of K, however, seemed to have greater influence than P.


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  		Table 3. Effects of different combinations of P and K fertilizer on in-vitro organic matter digestibility (IVOMD) of stargrass obtained from various harvests from 1998 to 2000.

 
Typical fitted models showed variable effects of P and K fertilizations on IVOMD. As indicated by the models, P supply inversely affected IVOMD in the first year (IVOMD = 49.19 – 0.027P + 0.024K; R2 = 0.60; P = 0.0001) while the effect was not significant in the second year (IVOMD = 41.39 + 0.088K; R2 = 0.58; P = 0.0001), but it was positive in the third year (IVOMD = 43.5 + 0.038P + 0.10K – 0.0008PK; R2 = 0.58; P = 0.0005). As indicated by R2, these models explained from 58 to 60% of the variability in IVOMD during the study period. The supply of K, however, had a positive effect on concentrations of IVOMD in all 3 yr. A significant interaction between P and K fertilizations in the third year of the study implied that the effect of one (P or K) on IVOMD varied with the supply rate of the other.

Groundcover
Groundcover was higher for forage receiving 93 kg K ha–1 yr–1, whereas forage receiving 46 kg K ha–1 yr–1 or less had less groundcover (Fig. 2). Although high rates of K application are often used to increase stress tolerance of hybrid bermudagrass [Cynodon dactylon (L.) Pers. x Cynodon transvaalensis Burtt Davy] turfs (Miller, 1999), and have a beneficial effect on alfalfa (Medicago sativa L.) vigor and persistence (Simons et al., 1995), such effects have not been reported for stargrass. After severe episodic freezes in January and February 2001 (Table 1), the groundcover of stargrass fertilized with 0 and 93 kg K ha–1 yr–1 ranged from 3 to 93%, respectively. No correlation of any type was obtained between groundcover and tissue P concentration. However, a highly logarithmic relation between groundcover and tissue K concentration obtained in this study (Fig. 3) indicates that K supply is critical for cold tolerance/persistence of stargrass during winter in the subtropics.



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  		Fig. 2. The groundcover of stargrass as affected by various combinations of P and K application rates to episodic freezing during the winter of 2001. The groundcover of the grass with letters in common are not significantly different (P > 0.05).

 


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  		Fig. 3. The relationship between tissue K concentrations and groundcover of stargrass (% green grass cover of the total) to occasional freeze spell at the end of the study (i.e., during the winter of 2001).

 
Phosphorus Removal
No significant differences were obtained for Ca, Mg, Zn, and Mn concentrations in soils collected from unfertilized experimental units and those fertilized with various combinations of P and K on any sampling date throughout the 3-yr study. However, soil P and K concentrations varied significantly (P < 0.05) among treatments in the second and third years of study (Table 4). Soil pH was not significantly affected by the treatments in 1998 and 1999. However, the pH was positively influenced by K application in the rest of the study period. Although the changes in pH were significant, the difference in the pH range (4.8–5.4) was not high enough to have a significant effect on forage production. The effects of the treatments on soil P and K were not significant in the first year of the study; however, the effects were variable from the second year depending on the ratios and quantities of P and K applied. Phosphorus removals by forage were substantially influenced by K application (Table 5). Maximum removals of P (161% of the applied fertilizer P) were achieved from experimental units that received combinations of 10 and 93 kg ha–1 P and K, respectively, followed by the experimental units receiving combinations of 20 kg P ha–1 and 93 kg K ha–1 (102% of the applied fertilizer P). Efficiency of P removal by the grass was increased with K applications, but increased P applications above 10 kg P ha–1 yr–1 reduced this efficiency. It was apparent that stargrass was able to take up (mine) native P from soils apart from the applied fertilizer P as indicated by >100% of the applied P removal. Although the amount of P removed was relatively higher when the higher rate (39 kg P ha–1 yr–1) of P was applied, the percentage removal of applied P was highest from experimental units receiving 10 kg P ha–1 yr–1. This indicates efficient utilization of P when grass was supplied with high rates of K and low rates of P. Phosphorus removal was greater each successive year during the 3-yr study period. As the grass stands got older, their roots may have penetrated deeper into the P-rich Spodic horizon, and translocated P into aboveground biomass. These results indicate that stargrass is capable of removing (mining) substantial amounts of P from soils when appropriate rates of P and K fertilizers are applied. This finding suggests that stargrass could possibly be effective for phytoremediation of P-impacted sites.


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  		Table 4. Effects of various combinations of P and K on selected chemical characteristics of soils from 1998 to 2000, and after the final harvest of 2000.

 

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  		Table 5. Removal of P by stargrass receiving different combinations of fertilizers from 1998 to 2000.

 
In general, increasing P and K fertility increased their concentrations in forage tissue (data not shown). However, differences in DM yield, CP, IVOMD, and persistence of the grass were relatively small at >10 kg P ha–1 yr–1, provided 93 kg K ha–1 yr–1 was applied. Because of potential detrimental effects on ground or surface waters, environmental regulations target reductions or avoidance of P build-up in soils. Hence, efficient utilization of P should dictate the choice of fertilizer application rates and ratios. As indicated previously, the residual P in soils could be minimized by proper supply of K to stargrass pastures. It is evident from this study that the application rates of 10 kg P ha–1 yr–1 and 93 kg K ha–1 yr–1 may help to maintain quantitative production of stargrass with good nutritive value, while minimizing the potential of P loss to ground waters, surface waters, or both through efficient P utilization.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study indicated that application of 10 kg P ha–1 yr–1 and 93 kg K ha–1 yr–1 would result in efficient utilization of P, while providing adequate fertility to maintain yield potential and persistence to stargrass during winters on Pomona fine sand Spodosols. Tissue CP concentration and IVOMD remained adequate for diets of beef cattle (National Research Council, 1984) when forages received the fertilizer combination of 10 kg P ha–1 yr–1 and 93 kg K ha–1 yr–1. This indicates that using low P fertilization rates, while supplying high or adequate K would enhance stargrass herbage production, and minimize P accumulation in soils. Reduction in soil P would also reduce its loss to ground and surface waters, and have a positive effect on water quality. It is apparent that the supply of K is not only critical for greater forage production, but is also essential to forage to withstand freeze spells during winter in the subtropics. It can be concluded from these observations that applications of 10 kg P ha–1 yr–1 in combination with 93 kg K ha–1 yr–1 will neither deteriorate forage nutritive value as indicated by concentrations of CP and IVOMD, nor substantially reduced DM yield of stargrass on Pomona fine sand Spodosols. In addition, this fertilizer combination may reduce potential short-term and long-term detrimental effects of P fertilization on water quality.


   ACKNOWLEDGMENTS
 
The authors acknowledge the assistance from C. Holley and A. Dunlap, IFAS Range Cattle Research and Education Center, University of Florida. This research was supported by the Florida Agricultural Experiment Station and approved for publication as Journal Series no. R-09521.


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





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