inorganic fertilizer on these eroded soils at rates that would restore grain yields. Key words: ..... Plot data were used directly in the yield estimation. Additional ...
Economics of N and P fertilization to restore wheat yields on three artificially eroded sites in southern Alberta E. G. Smith1, Y. Peng2, M. Lerohl2 and F. J. Larney1 1Agriculture and Agri-Food T1J 4B1; 2Department of
Canada, Lethbridge Research Centre, Box 3000, Lethbridge, Alberta, Canada Rural Economy, University of Alberta, Edmonton, Alberta, Canada T6G 2H1. Lethbridge Research Centre contribution no. 3879850, received 1 March 1999, accepted 2 July 1999.
Smith, E. G., Peng, Y., Lerohl, M. and Larney, F. J. 2000. Economics of N and P fertilization to restore wheat yields on three artificially eroded sites in southern Alberta. Can. J. Soil Sci. 80: 165–169. An important economic concept in the evaluation of soil conservation is the ease with which production inputs can be substituted for one another. This concept and economic optimum application rates of N and P fertilizer for wheat grown after fallow were applied to three artificially eroded soils in southern Alberta. Each site had topsoil removed to depths of 0, 10 and 20 cm. Four rates of N (0, 50, 75 and 100 kg ha–1) and three rates of P (0, 11 and 22 kg ha–1) were applied to each eroded depth. There were four replications. Fertilizer N and P did not easily substitute for the loss of topsoil. The economic optimum level of N and P was nearly constant across depths of eroded topsoil. Increases in fertilizer N and P applied to eroded soils were primarily to replace lost soil nutrients. It was not economical to apply inorganic fertilizer on these eroded soils at rates that would restore grain yields. Key words: Eroded soil, economics, inorganic fertilizer, wheat Smith, E. G., Peng, Y., Lerohl, M. et Larney, F. J. 2000. Considérations économiques touchant la fumure N et P dans la remise en état des terres à blé à trois emplacements artificiellement érodés dans le sud de l’Alberta. Can. J. Soil Sci. 80: 165–169. Un concept économique important dans l’évaluation des mesures de conservation du sol est la facilité de substitution d’un intrant de production à l’autre. Ce concept ainsi que les doses d’épandage économiquement optimales de fumure azotée (N) et phosphorée (P) utilisées pour la sole de blé après jachère a été appliqué à trois sols artificiellement érodés du sud de l’Alberta. À chaque emplacement, le sol était décapé de la couche arable jusqu’à la profondeur de 10 et de 20 cm. Quatre doses de fumure N (0, 50, 75 et 100 kg ha–1) et trois de P (0, 11 et 22 kg ha–1) étaient employées pour chaque profondeur d’érosion. Il y avait quatre répétitions. La fumure N et la fumure P ne palliaient pas facilement la perte de terre de surface. Le niveau économiquement optimal de N et de P était quasiment constant quelle que soit la profondeur d’érosion. L’augmentation des apports de N et de P dans les sols érodés avait surtout pour résultat de remplacer les éléments nutritifs perdus. L’apport d’engrais minéraux nécessaires pour restituer la productivité céréalière des sols érodés s’est révélé non rentable. Mots clés: Sol érodé, économie, engrais minéral, blé
The soils in southern Alberta have undergone differing degrees of erosion over the past 90 yr of cultivation. While erosion prevention is important in maintaining soil productivity, once erosion has occurred producers need to know how best to restore productivity. One management strategy that may be employed on eroded soil is to apply additional inorganic fertilizers (Dormaar et al. 1986). Nitrogen and P are removed with the eroded soil and fertilizing with N and P can replace these lost nutrients. Erosion may also cause a change in the chemical and physical composition of the soil, which can affect nutrient availability. The Brown and Dark Brown Chernozemic soils of southeastern Alberta have a C horizon that is high in CaCO3, and loss of topsoil will effectively increase the concentration of CaCO3 in the surface layer, which tends to immobilize fertilizer P (Larney et al. 1995). Studies estimating the impact of topsoil erosion have utilized artificially eroded sites to exert control over variables that impact production and productivity (Ives and Shaykewich 1987; Tanaka and Aase 1989; Mahli et al. 1994; Larney et al. 1995). Ives and Shaykewich (1987) found that for Black Chernozemic soils, the addition of fer-
tilizer N and P compensated for 0 to 77% of the yield loss from soil erosion, but the impact was dependent on soil grouping and year. Tanaka and Aase (1989) determined that P was the most limiting nutrient for an eroded soil at Sidney, Montana and that yield loss from erosion could nearly be recovered with the addition of N and P fertilizer. Malhi et al. (1994) determined that barley yields in a controlled-environment greenhouse experiment for two eroded Black Chernozemic soils responded to applied N rates up to 200 mg kg–1 soil, but there was little response to P. Larney et al. (1995) found grain yield response to applied N and P fertilizer on eroded soils depended on the soil. Grain yield on a Brown Chernozemic soil did not respond to applied N and P with increased eroded depth, an irrigated Dark Brown Chernozemic did not respond to applied P or to applied N and P with increased eroded depth, and a dryland Dark Brown Chernozemic did not respond to applied N with increased eroded depth. Soil mineral N and P explained some of the differences by soil. The economic benefits of applying N and P fertilizer to restore productivity on eroded soils are not well known. Smith and Shaykewich (1990) determined that it was prof165
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itable to apply additional fertilizer to eroded soils in the Black soil zone, which recovered some of the yield loss due to soil erosion. However, these results cannot be extrapolated to other soils because the soil, weather, crops and tillage systems differ. The objectives of this study were to present concepts central to an economic evaluation of restoring productivity of an eroded soil, and to illustrate these concepts with an application to three artificially eroded soils in southern Alberta. MATERIALS AND METHODS Grain yield is a function of the inputs available to the growing crop (e.g. water, soil quality, soil nutrients, temperature, crop variety and management practices). A given level of yield can generally be obtained with a multitude of input combinations. Inputs can be substituted for one another, not chemically, but rather in terms of the quantity of output produced. This does not conflict with the “law of the minimum”, where yield is assumed to be determined by the most limiting factor. Each input combination that produces the same output has a different input, or combination of inputs, that are limiting production. If an input is severely limiting, it will not be easy to substitute that input for other inputs. That is, a large amount of other inputs would be required to replace a small amount of this latter input. There are many functional forms used for relating grain yield to these controlled and environmental inputs. The form of the equation is important because it will impact directly on the estimated optimum level of inputs, input substitution, and other properties. One functional form that reflects biological responses is a modified Mitscherlich-Spillman (MS) function (Van Kooten et al. 1989). The M-S equation, in which the inputs considered are soil mineral plus applied N (TN, kg ha–1), soil mineral plus applied P (TP, kg ha–1) and soil organic carbon (OC, g kg–1), is as follows: Y = α0 + β0(1 – β1TN)(1 – β2TP)(1 – β3OC) + ε
(1)
where, Y is grain yield (kg ha–1), α0 is the estimated intercept, the βi are estimated parameters, and ε is the random error. A measure of the actual depth of eroded soil was not included in the model because eroded depth and soil OC were highly correlated in this study, eroded topsoil depth is unknown unless all previous erosion is known, and the analysis was to link the economic concepts to soil attributes. Soil erosion can be estimated from the change in OC. The nonlinear regression was estimated using the nonlinear procedure in SHAZAM (SHAZAM 1997). Initial starting values were required for the procedure. Different initial values were specified to determine whether the system would converge on the same solution. If a different solution resulted, the log-likelihood function value was used to determine the better of the two solutions. The contribution of additional input levels can now be determined from the estimated yield equation. When the additional contribution to yield is small, it is an indication that the input level was adequate given the level of other inputs. A large contribution to yield likely indicates that the input level is severely limiting yield. These general results,
Table 1. Estimated grain yield (kg ha–1) equation coefficients for wheat grown on artificially eroded soils with applied N and P fertilizers at three locations Location Variable Intercept Constant TNy TP OC Corrected R2
Lethbridge Dryland
Lethbridge Irrigated
Taber
–2122.6 (–1.88)z 3207.9** (3.04) 0.8842** (41.9) 0.6304** (10.6) 0.8153** (18.7) 0.64
–2801.8** (–8.8) 11499.** (13.7) 0.9273** (119.0) 0.6856** (17.4) 0.9678** (295.5) 0.70
–2354.5** (–23.4) 6985.6** (12.4) 0.8982** (97.7) 0.4804** (7.7) 0.9448** (241.1) 0.72
**Significant at P = 0.05 and P = 0.01, respectively. zValues in parenthesis are t-values. yTN = mineral plus applied N (kg ha–1), TP = mineral plus applied P (kg ha–1), OC = soil organic carbon (g kg–1).
while not as rigid as the law of the minimum, indicate that under certain conditions altering some of the production inputs will not influence output from production. The optimum rate of an input to apply, for a given soil and environmental condition, is derived from the yield equation by determining the rate that maximizes gross revenues less input costs. The economic optimum application rate occurs where the additional grain yield from applying another unit of input equals the ratio of the unit cost of the input to the unit price of the product (Eq. 2). Px ∂f = i ∂xi PW
(2)
where, f is the yield function, Px is the unit cost of input xi, i and PW is the unit price of the product being produced. A measure of how easily inputs substitute for each other in production is given by the marginal rate of technical substitution (MRTS). The MRTS is also derived from the yield function as defined in Eq. 3: MRTSi, j =
∂f / ∂x j ∂xi =− ∂x j ∂f / ∂xi
(3)
where, MRTS measures the change in input xi in response to a one-unit change in input xj that is required to maintain a constant level of output or yield. These values are only relevant at the margin, that is, for relatively small changes in the input variables. A small MRTS absolute value indicates relatively easy substitution of xj with xi, that is, a small amount of additional input xi is required for a one-unit decrease in input xj in order to maintain yield. A large MRTS absolute value indicates input substitution is more difficult, that is, a large amount of additional xi is required for a one-unit decrease in xj. The MRTS does not mean that fertilizer N and P, for example, can be interchanged, but it
SMITH ET AL. — ECONOMICS OF RESTORING YIELDS ON ERODED SOILS
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Table 2. Optimum soil mineral plus applied and applied N and P fertilizer, wheat yield and net returns over fertilizer costs at the base prices and costsz Applied TNx
TP
N (kg ha–1)
P
Wheat yield
Net returns over N and P ($ ha–1)
Lethbridge Dryland 0 cm (15.76) 10 cm (10.44) 20 cm (8.06)
39 39 39
11 11 11
5 11 10
0 5 0
926.0 662.9 427.9
150.44 97.22 65.91
Lethbridge Irrigated 0 cm (17.56) 10 cm (13.03) 20 cm (11.53)
65 62 60
15 15 13
25 9 17
0 0 6
2187.1 1139.0 748.9
349.13 183.71 107.18
Taber Dryland 0 cm (13.98) 10 cm (10.08) 20 cm (9.43)
46 44 44
8 8 7
16 17 10
0 0 2
1445.1 650.0 501.1
230.93 99.26 75.16
Eroded topsoil (soil OC g kg–1)y
zWheat price = $0.165 kg–1, N cost = $0.47 kg–1, P cost = $1.40 yThe OC concentration is the average for four replications. xSoil mineral plus applied N (TN) and P (TP).
kg–1.
means there are different combinations of N and P that will produce the same yield. Application Two sites, one dryland and one irrigated were established in the spring of 1990 on a Dark Brown Chernozemic sandy clay loam at the Agriculture and Agri-Food Canada Research Centre at Lethbridge, Alberta. A third site was established at Taber, Alberta on a Brown Chernozemic clay loam (Larney et al. 1995). The two dryland sites were fallowed in 1989. Three levels of soil erosion were simulated by removing 0, 10 and 20 cm of topsoil with an excavator. The depth of the Ap horizon was about 10 to 15 cm and the Bt horizon 2 to 7 cm, so the C horizon was exposed with 20 cm of topsoil removed. Soil samples to 60 cm (six depths of: 0 to 7.5 cm, 7.5 to 15 cm, 15 to 22.5 cm, 22.5 to 30 cm, 30 to 45 cm and 45 to 60 cm) were taken from the main experimental plots after removal of the topsoil to determine soil mineral N and P, and soil OC. Soil mineral N in the analysis was from 0 to 60 cm, soil P was from 0 to 15 cm and soil OC was from 0 to 15 cm. Four N rates (as ammonium nitrate) of 0, 50, 75 and 100 kg N ha–1 were applied as random strips on each of the erosion treatments, and three P rates (as triple superphosphate) of 0, 11 and 22 kg P ha–1 were applied in random strips perpendicular to the N treatments. Fertilizer rates on the irrigated site were twice those of the dryland sites reported above. Zero tillage was used on the plots to minimize mixing of soils between plots. There were four replications of the 36 treatments for a total of 144 plots. Plot data were used directly in the yield estimation. Additional details of this experiment are reported by Larney et al. (1995). Data after 1990 were not used in this analysis because soil mineral N and P were not measured in 1991 and yield could not be estimated without this information, and the treatments were changed in 1992. The average farm gate wheat grain price (
