Phosphorus losses from agricultural land can promote eutrophication of lakes and streams through stimulation of algae and aquatic plant growth (Carpenter et al., 1998; Daniel et al., 1998). When the algae and plants die and decompose, they deplete the dissolved oxygen in the water which, in turn, affects the fish and other aquatic organisms. If plant growth becomes excessive, fishing, and other recreation, aesthetics, drinking water and even industrial uses may be limited (Correll, 1998). In most areas where much of the land is used for agriculture this is the major nonpoint source of P to surface waters (Sharpley et al., 1993). For this reason a national strategy for implementing P-based approach for nutrient management planning has been proposed by the NRCS.
 

As summarized by Bundy (1998) implementation of a P strategy may include several alternatives: 1) limiting buildup of soil test P values; 2) application of threshold values for typical major soils (benchmark soils), and 3) use of a P loss index to identify sites with high risk of losing P in runoff on a site-specific basis. From a practical standpoint, this shift in approach raises questions about the management of fertilizer phosphorus and manure and the soil test levels at which crop responses will or will not occur.
 

Soil test calibration curves are developed to target soil test levels that are optimum for crop production. They provide a critical link between soil testing, expected yields, and probabilities of response. Soil test calibration curves, like the one in Figure 1, show the relationship between relative yield (%) and soil test levels These curves are created by collecting yield response data from P rate studies conducted for a site at varying initial soil test levels. Relative yields for a given location and year are calculated by comparing the yield of a plot to the yield of the plots receiving fertilizer rates that were not yield-limiting. Figure 1 demonstrates the principle common to most calibration curves: low yields at lower soil test levels and highest yields near the critical level and above. In some situations, yields may decline at excessive levels if the nutrient becomes toxic or significantly disrupts the nutrient balance. An important feature of soil test calibration is the critical level. The critical level is the soil test below which yield reductions are more likely. Above the critical level, soil test levels are not expected to limit yield.

Some estimates of yield response can be made using the soil test calibration curve. The underlying assumption of such estimates is that following university ___________________________

^1/Professor and Extension Soil Scientist, Department of Soil Science, University of Wisconsin-Madison.
 

recommendations will boost yields to 98 to 100% relative yield. The Potash and Phosphate Institute assembled examples of calibration data from several states and crops. The assembled data appear in PKMAN (PPI, 1996) software documentation and are shown in Table 1. These data represent the general conditions in the upper Midwest, but to the extent possible, local data should be used to more accurately make individual decisions. In general, these show that for both corn and soybean almost no chance for further response is seen if soil test P (Bray P₁) exceeds 20 ppm. For wheat this level may be slightly higher. There have been many studies that have contributed to this conclusion (Webb et al., 1992; Mallarino et al., 1991; Olsen et al., 1962; deMooy et al., 1981; Cope 1981; Rehm et al., 1981; Randall et al., 1997). In some cases, the critical level may differ between soils (Randall et al., 1997; McCallister et al., 1987), however, the fluctuation tends to be relatively small [e.g. 13 to 19 ppm soil test P in the Randall et al. (1997) study].
 

Wisconsin studies have similarly shown that both soil and crop impact the P critical level as reviewed by Kelling et al. (1990). For example, when alfalfa studies were conducted for multiple years at several sites, P responses were not observed at soil test P levels above 25 ppm except on sandy soils where the Bray P₁ extractant is much more efficient (Attoe and Truog, 1950; Kelling, 1984; Smith and Powell, 1979). However, Schulte's review of corn and soybean data showed no responses above 18 and 14 ppm, respectively, except to periodic responses to starter or row placed fertilizer (E.E. Schulte. 1989. Review of corn and soybean P and K response data. UW-Dept. of Soil Science internal document). More recently, Bundy and Andraski (1998) conducted a series of on-farm starter fertilizer corn trials and observed that when soil test P levels were in the very high range or above

(> 30 ppm) responses to starter were seen about 40% of the time, but the responsiveness was most closely related to soil test K and a planting date/crop maturity factor, not soil test P. These data do not support the raising of the corn P critical level.
 

Other crops, such as potatoes and high value vegetable crops, have shown responses to higher P levels (Kelling et al., 1992; Kelling and Speth, 1997). For example, these studies showed that responses continue to be seen with potatoes on the acid silt loam soils in the Antigo area to soil test P levels of 160 ppm or more, but rarely on sandy soils above 100 ppm. Others have observed similar responses (Liegel et al., 1981; Nelson and Watson, 1947; Dyson and Watson, 1981). A variety of reasons have been suggested as to why responses occur at the levels including poor root systems, plant effects, interfering ions and poor crop transport of P, but no single clear reason is obvious. In general, however, with a few site specific exceptions, even with these very P-inefficient crops few responses are seen above soil tests of 80 ppm Bray P₁.
 

Because of the variation in P responsiveness observed between different crops and different soils, the Wisconsin soil test recommendation system attempted to recognize these distinctions (Kelling et al., 1998). A system was established that included both crop demand level and subsoil nutrient supplying power (Table 2). This then allowed for more specific recommendations for different crops on different soils. In this system, the critical level is defined as the break point between the optimum and high soil test P categories. It is expected that the probability of a yield response to additional P is less than 30% if the Bray P₁ soil test is above the optimum range. Furthermore, it approaches zero and no additional fertilizer P is recommended if soil test P is more than about 50% higher than the top end of the optimum range.
 

Other states have adopted a somewhat similar approach in their recommendation systems. As shown in Table 3, most of the states in the North Central Region are within 10 ppm of Bray P₁ of that used by Wisconsin where no additional phosphate fertilizer is recommended. Note that for these crops all of the states except Ohio suggest no additional phosphate fertilizer be applied at soil test levels generally below 40 ppm Bray P₁.
 

This review of the crop response data and soil test recommendations from the NC states, therefore, confirms that even the most stringent phosphorus-based nutrient management planning guidelines will not be a hindrance to crop production and may actually enhance farmer profitability. However, as pointed out by Bundy (1998) a phosphorus-based nutrient management strategies using only P soil test levels and/or on experimentally determined threshold soil P levels would pose major implementation obstacles, particularly on farms where substantial amounts of manure are land-applied. Use of the modified P index may be a workable solution to nutrient management, since excess P would be allowed to continue to accumulate in soils at sites with low P loss risk. The ultimate solution to avoiding nutrient losses to the environment is to bring nutrient inputs from all sources into balance with crop nutrient removals. The P index approach provides a method for minimizing P losses to the environment until this balance is achieved.
 

Attoe, O.J., and Truog., E. 1950. Correlation of yield and quality of alfalfa and clover

hay with levels of available phosphorus and potassium. Soil Sci. Soc. Amer. Proc. 14:249-253.
 

Bundy, L.G. 1998. Implementing a phosphorus strategy for nutrient management

plans. New Horizons in Soil Science, 8-98. Dept. of Soil Science, Univ. of Wisconsin-Madison, 6 p.
 

Bundy, L.G., and T.W. Andraski. 1998. Site-specific factors affecting corn response

to starter fertilizer: Results from 100 on-farm trials. New Horizons in Soil Science, 1-98. Dept. of Soil Science, Univ. of Wisconsin-Madison. 13 p.
 

Carpenter, S.R., N.F. Caraco, D.L. Correll, R.W. Howarth, A.N. Sharpley, and V.H.

Smith. 1998. Nonpoint pollution of surface waters with phosphorus and nitrogen. Ecological Applications 8:559-568.
 

Combs, S.M., K.A. Kelling, and E.E. Schulte. 1991. How Wisconsin's soil test

recommendations compare to those of surrounding states. Proc. Wisconsin Forage Council Symposium 15:79-85.
 

Cope, J.T., Jr. 1981. Effects of 50 years of fertilization with phosphorus and

potassium on soil test levels and yields at six locations. Soil Sci. Soc. Am. J. 45:342-347.
 

Correll, D.L. 1998. The role of phosphorus in the eutrophication of receiving waters:

A review. J. Environ. Qual. 27:261-266.
 

deMooy, C.J., J.L. Young, and J.D. Kaap. 1973. Comparative response of soybeans

and corn to phosphorus and potassium. Agron. J. 65:851-855.
 

Daniel, T.C., A.N. Sharpley, and J.L. Lemunyon. 1998. Agricultural phosphorus and

eutrophication: A symposium overview. J. Environ. Qual. 27:251-257.
 

Dyson, P.W., and D.J. Watson. 1971. An analysis of the effects of nutrient supply

on the growth of potato crops. Ann. Appl. Biol. 69:47-63.
 

Kelling, K.A. 1984. The case for fertilizing alfalfa. Dept. of Soil Science, Univ. of

Wisconsin-Madison. mimeo 9 p.
 

Kelling, K.A., L.G. Bundy, S.M. Combs, and J. B. Peters. 1998. Soil test recom-

mendations for field, vegetable and fruit crops. UWEX Bull. A2809. 54 p.
 

Kelling, K.A., L.G. Bundy, E.E. Schulte, and S.M. Combs. 1990. Agronomics,

economics and the environment: The research basis for the Wisconsin recommendations. Dept. of Soil Sci., Univ. of Wisconsin-Madison. mimeo

15 p.
 

Kelling, K.A., and P.E. Speth. 1997. Influence of phosphorus rate and timing on

Wisconsin potatoes. Proc. Annual Wisconsin Potato Meetings 10:68-79.
 

Kelling, K.A., R.P. Wolkowski, J.G. Iyer, R.B. Corey, and W.R. Stevenson. 1992.

Potato responses to phosphorus application and using petiole analysis in determining P status. Proc. Annual Wisconsin Potato Meetings 5:39-50.
 

Liegel, E.A., C.R. Simson, P.E. Fixen, R.E. Rand, and G.G. Weis. 1981. Potato

responses to phosphorus and potassium and recommendations for P and K fertilization. UWEX Potato Prod. Manual. 16 p.
 

Mallarino, A.P., J.R. Webb, and A.M. Blackmer. 1991. Corn and soybean yields

during 11 years of phosphorus and potassium fertilization on a high-testing soil. J. Prod. Agric. 4:312-317.
 

McCallister, D.L., C.A. Shapiro, W.R. Raun, F.N. Anderson, G.W. Rehm, O.P.

Engelstad, M.P. Russelle, and R.A. Olson. 1987. Rate of phosphorus and potassium buildup/decline with fertilization of corn and wheat on Nebraska Mollisols. Soil Sci. Soc. Am. J. 51:1646-1652.
 

Nelson, W.L., and A. Hawkins. 1947. Response of Irish potatoes to phosphorus and

potassium on soils having different levels of these nutrients in Maine and North Carolina. Agron. J. 39:1053-1067.
 

Olson, R.A., A.F. Dreier, C.A. Hoover, and H.F. Rhoads. 1962. Factors responsible

for poor response of corn and grain sorghum to phosphorus fertilization: I. Soil phosphorus level and climatic factors. Soil Sci. Soc. Am. Proc. 26:571-574.
 

Potash & Phosphate Institute. 1996. PKMAN: A tool for personalizing P and K

management. Version 1.0. Potash & Phosphate Institute, Norcross, GA.
 

Randall, G.W., T.K. Iragavarapu, and S.D. Evans. 1997. Long-term P and K

applications: I. Effect on soil test incline and decline rates and critical soil test levels. J. Prod. Agric. 10:565-571.
 

Rehm, G.W., R.C. Sorensen, and R.A. Wiese. 1981. Application of phosphorus,

potassium, and zinc to corn grown for grain or silage: Early growth and yield. Soil Sci. Soc. Am. J. 45:523-528.
 

Sharpley, A. N., T. C. Daniel, and D. R. Edwards. 1993. Phosphorus movement in

the landscape. J. Prod. Agric. 6:492-500.
 

Smith, Dale, and R.D. Powell. 1979. Yield of alfalfa as influenced by levels of P and

K fertilization. Commun. In Soil Sci. and Plant Anal. 10(3):531-543.
 

Webb, J.R., A.P. Mallarino, and A.M. Blackmer. 1992. Effects of residual and

annually applied phosphorus on soil test values and yields of corn and soybean. J. Prod. Agric. 5:148-152.
 
 
 
 

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Table 1. Phosphorus calibration examples (Bray P₁, except where noted).
 

                                                                          Relative Yield (%)                                                               
 

                                                                                                                                  Winter             Winter

Soil test                                                                         Spring       Spring Wheat        wheat               wheat

level         Corn         Corn         Corn      Soybean          wheat           N. Great           Kansas            Kansas

(ppm)       Iowa      Missouri      Illinois       Illinois        S. Dakota           Plains^*            (Bray)            (Olsen)⁺
 

2.5            66.5         31.0          42.0         42.0             75.2                 61.2                 35.0                 41.0

5.0            77.3         40.0          54.8         54.8             79.5                 78.0                 56.4                 68.0

7.5            86.7         48.5          69.3         69.3             83.1                 85.9                 73.6                 82.1

10.0          91.3         58.0          81.3         81.3             86.0                 90.4                 82.1                 89.9

12.5          94.1         66.5          90.2         90.2             88.6                 93.3                 87.9                 93.9

15.0          95.9         75.3          94.7         94.7             91.0                 95.4                 92.3                 97.0

17.5          97.1         84.5          97.3         97.3            93.1                 97.0                 95.0                 98.5

20.0          98.0         90.0          98.0         98.0            94.8                 98.2                 97.1                 99.9

22.5          98.7         93.5          98.6         98.6            96.4                 99.1                 98.2               100.0

25.0          99.3         96.0          99.1         99.1            97.8                 99.9                 99.3

27.5          99.6         98.0          99.5         99.5            98.8               100.0               100.0

30.0          99.8         99.3          99.8         99.8            99.6

32.5          99.9         99.9        100.0       100.0            99.9

35.0        100.0       100.0                                           100.0

^*Olsen P, ⁺Calculated from Bray P₁ assuming Olsen P = 0.75 Bray P₁.

Data: Potash & Phosphate Institute.
 
 
 
 

Table 2. Soil test interpretation ranges for phosphorus optimum category
 

                                                                                    Crop demand level                                                                  

                                1                     2                         3                         4                             5                         6

                                                (soybean                 (alfalfa             (red clover

Soil                     (corn)             low field)             high field)             med. field)             (vegetable)             (potatoes)

                         ------------------------------Bray P₁ soil test (ppm)--------------------------------
 

South forested    11-15             6-10                     16-23                 16-20                     31-45                 161-200

South prairie       16-20             6-10                     18-23                 16-20                     31-45                 161-200

Red soils             16-20             8-13                     18-25                 18-23                     31-45                 161-200

North silt loams   13-18             6-10                     16-23                 13-18                     31-45                 161-200

Sands                  23-32           10-15                     26-37                 23-30                     36-50                   91-125

Organic               23-32           10-15                     26-37                 23-30                     36-50                   91-125

High pH                9-15             6-10                     11-15                 11-15                     26-40                    61-75

Adapted from Kelling et al., 1998.
 
 
 
 
 
 

Table 3. Soil test level above which no phosphate (P₂O₅) is recommended for 140 bu corn and 5T alfalfa on medium-textured soils.
 

State                             Corn                 Alfalfa

                            --------------ppm-------------
 

Ohio                             45                 > 30-45*

Illinois                         32.5                     32.5

Michigan                     40                         40

Wisconsin                     30                         30

Iowa                         30.5                         30.5

Minnesota                     21                         21

Nebraska                      16                         25

*receives 65 lb P₂O₅/a.

Adapted from Combs et al. 1991.
 
 
 
 

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