Published online 9 January 2007
Published in J Environ Qual 36:31-43 (2007)
DOI: 10.2134/jeq2005.0430
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
TECHNICAL REPORTS
Ecological Risk Assessment
Risk Assessment of Unsuitable Winter Conditions for Manure and Nutrient Application across Ontario
D. J. Fallow,
D. M. Brown,
J. D. Lauzon and
G. W. Parkin*
Dep. of Land Resource Science, Univ. of Guelph, Guelph, ON, Canada, N1G 2W1
* Corresponding author (gparkin{at}uoguelph.ca)
Received for publication November 16, 2005.
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ABSTRACT
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Practical guidelines addressing the timing of manure and nutrient application must consider the concerns of the farm operators while ensuring the protection of the environment. An approach was developed and analyzed through case studies to determine the first recommended day in the spring, and the last in the fall, for manure and nutrient application based on probability analysis. Since most manure and nutrient application guidelines recommend avoiding adverse conditions, the three criteria established to perform a risk assessment were: (i) a frost depth greater than 0.05 m; (ii) a snow accumulation of greater than 0.05 m; and (iii) a soil volumetric water content greater than or equal to that of the plastic limit for the soil. Climatic data and typical soil information for seven locations in Ontario were used to model volumetric soil water contents, frost depths, and snow accumulation from the simultaneous heat and water (SHAW) model for a 48-yr period (1954–2001). Applying the three criteria to the modeled output, the average range between the least limiting probability (0.1, or one in ten year occurrence) and the greatest limiting probability (0.001, or one in one thousand year occurrence) analyzed among the locations was 16 d in the spring as compared to 29 d in the fall. Although geographical location affected the predicted spring start and fall end recommended manure and nutrient application dates, local climate and soil hydraulic properties also played an important part in the determination of these days. Overall the prediction method developed performed reasonably well and provided insight into the environmental factors influencing manure and nutrient application timing.
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INTRODUCTION
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CRITERIA REGARDING BOTH THE QUANTITY AND THE TIMING of manure and nutrient application to agricultural soils include predicted crop nutrient requirements, management of manure storage facilities, and managing the potential for both surface and groundwater contamination (either chemically or bacteriologically). Determining the timing of application is complex and remains a contentious issue with both governments and agriculturalists. Practical application guidelines that address both the concerns of the farm operators with regard to the business of farming and the protection of the environment are necessary. The government of Ontario has begun to address these issues through legislation such as the Nutrient Management Act (Ontario Ministry of Agriculture, Food, and Rural Affairs, 2002), the Source Water Protection Plan (Ontario Ministry of the Environment, 2006), and the Clean Water Act (Ontario Ministry of the Environment, 2005).
There are many agronomic and environmental aspects that need to be considered when determining appropriate manure management strategies; however, this study will be limited to concerns about overland runoff to surface waters and tillage problems associated with winter conditions. Although studies have indicated that the environmental risks attributed to the winter spreading of manure may not be as hazardous as once considered (Young and Mutchler, 1976; Young and Holt, 1977), the authors still express caution in applying this practice. With an increased risk of overland flow due to frozen or saturated soils, soil freezing and thawing is an important factor in determining the appropriate timing of manure application in Ontario.
The objective of this study was to determine variation of risk associated with the application of manure and nutrients during winter conditions for seven regions in Ontario, Canada. The first recommended day of manure and nutrient application in spring, and the last allowable day for application in the fall based on probability analysis were determined using output from a hydrologic model. The limiting factors when considering manure and nutrient application were determined to be as follows: (i) frost depth greater than 0.05 m (frost criterion), (ii) snow cover on the ground greater than 0.05 m (snow criterion), or (iii) a volumetric water content equal to or exceeding the plastic limit of the soil (soil plastic limit criterion). The criteria were established using practical considerations on the ability to complete manure incorporation (tillage) and the Best Management Practices: Livestock and Poultry Waste Management Guide (Agriculture and Agri-Food Canada and the Ontario Ministry of Agriculture and Food, 1992).
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MATERIALS AND METHODS
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SHAW Model
The simultaneous heat and water model (SHAW) is an implicit finite-difference, vertical, one-dimensional model originally developed to simulate freezing and thawing in soils (Flerchinger and Saxton, 1988). The SHAW model has evolved through various revisions and now simulates water and heat flows through a user-defined profile that extends from a plant canopy through a snow layer (when present), plant residue layer, and finally through soil layers (Flerchinger and Saxton, 1989a, 1989b; Flerchinger et al., 1996; Flerchinger and Pierson, 1997; Flerchinger, 2000a, 2000b; USDA, 2005).
The SHAW model uses appropriate energy and mass-balance equations for each node within each defined layer to iterate to time-step solution (Flerchinger, 2000a). Energy and mass balances determined for each node are assumed to change linearly between nodes. It is these gradients that determine the magnitude and the direction of the energy and mass movements through the profile. An iterative Newton-Raphson technique is used to solve for the time-step changes for each flux solved. Initially the heat flux equations and temperature estimates for all nodes are solved. Then using this information the water flux (solving Richards' Equation), vapor flux, water content potentials (for unfrozen ground), and ice contents (for frozen ground) for all nodes are each solved in turn through iteration. The process is repeated for the time step until the user-defined tolerance level is reached for the flux determinations (Flerchinger, 2000a). The model version SHAW2.3 was used for this study.
Hayhoe (1994) found good agreement between SHAW-estimated and observed winter soil temperatures (at specific depths), liquid and total water contents for both snow-covered and snow-cleared sites, as well as estimates of snow depth, and timing and rate of snow melt, prompting the selection of the SHAW model for this study.
The SHAW model was used to determine the earliest acceptable manure application day in the spring and the last application day in the fall over a 48-yr period (January 1954 through December 2001) for seven Ontario locations: Emo, Guelph, Harrow, Kapuskasing, Mount Forest, Ottawa, and Smithfield (Fig. 1). Each location was modeled using a typical soil for the region. The location, climatic region, typical soil type, and general information for each location are given in Table 1. The annual and seasonal precipitation means for the 48-yr period from 1954 to 2001 are presented in Table 2 (Fallow et al., 2003).
Properties for the typical soil type for each location from the surface to a depth of 1.25 m were included as model inputs. The required soil properties included soil pore index, air entry value, saturated hydraulic conductivity, bulk density, porosity, texture, and organic matter. Soil textural information as well as the horizon thickness were estimated based on Table 2 of Webber and Tel (1966), Selirio et al. (1978), and soil survey reports [Emo: Ontario Institute of Pedology and Land Resource Research Institute (1984); Guelph: Hoffman et al. (1963); Harrow: Richards et al. (1949); Kapuskasing: Crane (1933); Mount Forest: Hoffman et al. (1963); Ottawa: Schut and Wilson (1987); Smithfield: Hoffman and Acton (1974)] (Table 3).
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Table 3. Horizon depths, hydraulic properties, and percentage of each texture class for each horizon of the typical soil type in each area of the seven study sites.
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The SHAW model inputs for the soil pore index and the air entry are based on the moisture release curve equation developed by Brooks and Corey (1966). The soil moisture release curves were approximated based on measurements reported by Crane (1933) and Webber and Tel (1966), and by utilizing the soils data set included with the HYDRUS 2D (Simunek et al., 1996) software package. The saturated hydraulic conductivity values (Ksat) were derived based on the typical soil type using Webber and Tel (1966) and the HYDRUS 2D soil texture and Ksat data set. The bulk density at each soil position throughout the profile was calculated based on the saturated volumetric water content determined for each moisture release curve and assuming a particle density for a mineral soil of 2650 kg m–3 (Fallow et al., 2003) (Table 3).
For this study, the SHAW model was run on a daily time step. The input file included daily maximum and minimum temperatures, dew point temperature, daily wind run, daily precipitation, and daily solar radiation. The climate data for the study locations were acquired from Meteorological Service of Canada, a section of Environment Canada. The climate input files were assembled to include daily values for 1 Jan. 1954 through 31 Dec. 2001, for a total of 48 yr.
The model was run with a representative corn crop on each site for each year of the duration of the simulation. The SHAW model requires that the height, a characteristic dimension (leaf width), dry biomass, leaf area index, and the rooting depth of each plant species be defined for various days throughout the year. To define these daily values during the year, a secondary program was created to generate values for the required parameters, including the planting, emergence, and harvesting dates based on the local accumulated crop heat units for each year.
Model Output
The model-generated snow depth, frost depth, and volumetric water content were extracted from the output files for the 48-yr duration of the study. The volumetric water contents for the 0.05-, 0.10-, and 0.15-m depths were averaged to estimate the volumetric water content in the upper 0.15 m of the typical profile for each day to provide the data necessary for the determination of the plastic limit criterion. The plastic limit for each location was set as the plastic limit of the soil type in the top 0.15 m of the profile as calculated from Dexter and Bird (2000) and McBride et al. (1997).
A rate of occurrence was determined by dividing the number of years a certain date met a criterion by the total number of years modeled. A data set for rate of occurrence for all days of the year was produced for each location and for each criterion. For any given day, a rate of occurrence of one indicates that the given criterion has been met in every modeled year and there is no opportunity to apply manure and nutrients on that day, whereas a rate of occurrence of zero denotes that in all of the modeled years (48) there were no instances of the criterion being met and application would be allowed based on that specific criterion. To remove some of the daily variability, smoothing was performed on the probability data set. Since the set probabilities should remain continuous through the end of the year back to the beginning, the data set was augmented by adding the last 28 d of the data set to the beginning of the data set and the first 28 d was added on to the last date of the set to perform the running average smoothing of the data. Various kernel sizes were evaluated to determine the smallest kernel size that minimized the difference in the rate of occurrence values between the last day of the probabilities (31 December) and the first day (1 January). The calculated kernel size was then used in the running average smoothing of the raw data set. The smoothing of the data also allows for calculated probabilities that occur with less frequency than 1 in 48 yr.
A set of seven probabilities ranging from 0.1 (a one in ten year probability) to 0.001 (a one in one thousand year probability) was used to determine the spring and fall recommended application dates for each location. A range of probabilities was selected to examine the spring and fall application date change associated with the change in risk. For the frost depth and the snow depth criteria, the smoothed rate of occurrence curve would return to zero at some point during the spring before ascending again in the fall. Determinations of a given probability start and end date for these criteria were straightforward. The first day of the year with a rate of occurrence less than the determined probability was flagged as the spring start date. Similarly, the last day of the fall where the smoothed rate of occurrence was less than the set probability before climbing to a greater rate of occurrence was the fall end date.
Determination of the recommended manure and nutrient spring start and fall end days using the final criterion, the soil plastic limit, was not as straightforward as the other two criteria, especially in clay or clay loam soils. In these soils the rate of occurrence of a volumetric water content greater than the set probability could happen at any time throughout the year since all that would be required would be a significant period of rainfall. It then became necessary to isolate the increased rate of occurrence of higher volumetric water contents due to snow and colder temperatures from the rates of occurrences observed throughout the remainder of the year. The effect of winter on the volumetric water contents was determined through the analyses of the slopes of the smoothed rate of occurrence functions. The slope steepness associated with the winter was used to isolate the duration of the winter's effect on the volumetric soil moisture. In determining the spring start day, the position where the slope became zero following the steep winter decline of the smoothed rate of occurrence line was taken to be the zero probability day of the year. Similarly, the last position where the rate of occurrence smoothed function was equal to zero before starting the steep rise in the late fall was set as the zero probability fall end day.
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RESULTS AND DISCUSSION
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The first spring and the last fall days of manure application for all seven modeled sites, for seven probabilities ranging from 0.1 to 0.001 established on the frost depth criterion, are presented in Table 4. Information regarding the snow depth criterion is presented in Table 5 and for the soil plastic limit criterion in Table 6. A range of probabilities are shown since the establishment of a spring manure application start day or a fall manure application cessation day will inevitably be a decision made based on acceptable risk. A range of probabilities will allow for the discussion of the SHAW model approach to include points based on the time frame in minimizing the risks associated with manure application. The range of the probability days will provide insight on how closely in time all criteria are met throughout the collection of probabilities examined.
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Table 4. First day of the year of spring manure application and last day of the year of fall manure application based on frost criterion.
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Table 5. First day of the year of spring manure application and last day of the year of fall manure application based on snow criterion.
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Table 6. First day of the year of spring manure application and last day of the year of fall manure application based on soil moisture criterion.
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Plots of the smoothed probability of exceeding a given criterion are presented for all three criteria and for the seven individual locations in Fig. 2
through 8.
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Fig. 2. Probability of exceeding the three criteria [(i) a frost depth greater than 0.05 m; (ii) a snow accumulation of greater than 0.05 m; and (iii) a soil volumetric water content greater or equal to that of the plastic limit for the soil] in Emo. One in 500 yr (0.002) probability spring start and fall end dates are indicated.
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Fig. 3. Probability of exceeding the three criteria [(i) a frost depth greater than 0.05 m; (ii) a snow accumulation of greater than 0.05 m; and (iii) a soil volumetric water content greater or equal to that of the plastic limit for the soil] in Guelph. One in 500 yr (0.002) probability spring start and fall end dates are indicated.
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Fig. 4. Probability of exceeding the three criteria [(i) a frost depth greater than 0.05 m; (ii) a snow accumulation of greater than 0.05 m; and (iii) a soil volumetric water content greater or equal to that of the plastic limit for the soil] in Harrow. One in 500 yr (0.002) probability spring start and fall end dates are indicated.
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Fig. 5. Probability of exceeding the three criteria [(i) a frost depth greater than 0.05 m; (ii) a snow accumulation of greater than 0.05 m; and (iii) a soil volumetric water content greater or equal to that of the plastic limit for the soil] in Kapuskasing. One in 500 yr (0.002) probability spring start and fall end dates are indicated.
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Fig. 6. Probability of exceeding the three criteria [(i) a frost depth greater than 0.05 m; (ii) a snow accumulation of greater than 0.05 m; and (iii) a soil volumetric water content greater or equal to that of the plastic limit for the soil] in Mount Forest. One in 500 yr (0.002) probability spring start and fall end dates are indicated.
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Fig. 7. Probability of exceeding the three criteria [(i) a frost depth greater than 0.05 m; (ii) a snow accumulation of greater than 0.05 m; and (iii) a soil volumetric water content greater or equal to that of the plastic limit for the soil] in Ottawa. One in 500 yr (0.002) probability spring start and fall end dates are indicated.
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Fig. 8. Probability of exceeding the three criteria [(i) a frost depth greater than 0.05 m; (ii) a snow accumulation of greater than 0.05 m; and (iii) a soil volumetric water content greater or equal to that of the plastic limit for the soil] in Smithfield. One in 500 yr (0.002) probability spring start and fall end dates are indicated.
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Earliest Spring Application Start Days
The first day of the year when all three criteria were not met for a given probability was considered the first spring day of manure application. The last criterion not to be met is considered to be the limiting factor for a particular probability in the determination of the spring application days. The predicted limiting factors for the spring application start dates based on the SHAW model output seemed to correspond reasonably with both the physical and climatic factors for each location.
For the Emo location the limiting factor for all given probabilities in the spring was the frost depth criterion. Of the four sites modeled with fine-textured soils (Emo, Harrow, Kapuskasing, and Ottawa), it was the only location that was not governed by the soil plastic limit criterion for the first application date in the spring. Although Emo did not follow the trend displayed by these other sites, this finding still seemed reasonable given that Emo was the location with the least annual precipitation (735 mm), with less fall and winter precipitation as compared to the other locations (Table 2) (Fallow et al., 2003). The lower fall and winter precipitation would logically lead to lower soil volumetric water contents that would minimize the risk of the soil plastic limit being exceeded and since there would be less snow cover, there would be an enhanced frost penetration.
At Emo there was a range of 23 d from the earliest application date when all three criteria are met, Day 120 (30 April for a 0.1 probability), limited by the frost depth criterion, to a first application date of Day 143 (23 May for a 0.001 probability), again controlled by the frost depth criterion (Table 7). The range of 23 d for the first spring application day on which all three criteria were met over all probabilities was the largest range for any of the sites, with the two most northern sites (Emo and Kapuskasing) both sharing this range of days. Although in the case of Kapuskasing, it was the soil plastic limit criterion that was the limiting factor. This 23-d span of the limiting factor for manure application in Kapuskasing ranged from Day 134 (14 May for a 0.1 probability) to Day 157 (6 June for a 0.001 probability; Table 7). The ranges for all criteria were very similar for both of these northern sites (Table 7); however, the difference between the earliest recommended day of spring manure application for the limiting factor at the 0.1 probability was 2 wk earlier in Emo.
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Table 7. Days between the 0.1 and 0.001 probabilities for manure application based on the frost depth criterion, snow depth criterion, and plastic limit volumetric water content criterion.
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As stated previously, the remaining fine-textured sites were all limited by the soil plastic limit criterion when determining the earliest manure application start date in the spring. For the two locations with fine-textured typical soils in southern Ontario, Harrow and Ottawa, the ranges of the earliest starting days as well as the starting days themselves were very similar. In Harrow, the range in the spring for the first application day is 9 d starting at Day 116 (26 April for 0.1 probability) to Day 125 (5 May for 0.001 probability; Table 6). The Ottawa location earliest manure application start days ranged from Day 113 (23 April for 0.1 probability) to Day 126 (6 May for 0.001 probability; Table 6), a range spanning 13 d. The fact that these two sites had such similar restrictions was due solely to the soil plastic limit criterion in Harrow. Harrow actually had an 8 d earlier start application day based solely on the frost depth criterion, whereas the snow depth criterion allowed for the earliest start day in Harrow, 13 d earlier than in Ottawa. The fact that the modeled soil profile in Harrow took a relatively longer time to drain below the plastic limit in the spring delays the start application day to late April or early May, which coincided with the Ottawa location. In fact, if one considers the maximum span between the earliest start application days between each criterion for all given probabilities as an indication of how close in time all three criteria are satisfied, then Harrow consistently displayed the largest series of gaps from 31 d at 0.1 probability, to 20 d for the 0.001 probability, whereas Ottawa had a smaller range of gaps between criteria of 14 d for 0.1 probability to 10 d for the 0.001 probability (Tables 4, 5, and 6).
For the locations with the coarser-textured soil profiles the earliest spring application days were limited by the frost depth criterion and the snow depth criterion. In fact, the soil plastic limit criterion was not even a factor at the Smithfield location as would be expected in this well-drained sand profile. The range of the earliest spring application days for which all three criteria were met is 16 d at Smithfield with the 0.1 probability day being Day 102 (12 April) and Day 118 (28 April) as the 0.001 probability day (Table 4). The limiting criterion for all probabilities was a snow depth greater than 0.05 m. This range was very similar to the other two coarser-textured profile locations. In Guelph, the limiting factor in determining the earliest spring application date was the snow depth criterion for the four higher probabilities (0.1, 0.04, 0.02, and 0.01 probability) and the frost depth criterion for the remaining probabilities. The range was 19 d from application Day 103 (13 April for a 0.1 probability) to Day 122 (2 May for a 0.001 probability) for the limiting factors (Tables 4 and 5). The soil plastic limit criterion was always satisfied first, generally 2 wk earlier than the other criteria. At Mount Forest, the limiting factor in determining the earliest spring application date was the snow depth criterion for the 0.1 probability and the frost depth criterion for the remaining probabilities. The range at Mount Forest was 21 d spanning from Day 110 (20 April for a 0.1 probability) to Day 131 (11 May for a 0.001 probability; Tables 4 and 5). All three coarser-textured southern Ontario locations had similar spring start days and ranges with the differences logically attributed to the differences in elevations and the proximities to the Great Lakes of the three sites (Fig. 1).
Last Fall Application Days
The last day in the fall when manure application would be recommended was determined in a very similar fashion to that of the first spring day of application. The last day in the fall when the likelihood of meeting all three criteria was lower than a determined probability was considered to be the last recommended day that would be acceptable for manure application. The last recommended day of manure application in the fall was the day before the date that any one criterion met or exceeded this set probability. This criterion was then considered to be the limiting factor.
The range from the 0.1 to the 0.001 probability spring start day for the limiting factor was on average about 16 d, with the two northern sites having the greatest span of 23 d. When considering the cessation of manure application in the fall, the average range in days from the limiting factor at the 0.1 probability to the limiting factor at the 0.001 probability day was almost 29 d. The two northern sites exhibited both the shortest span of 13 d, at Kapuskasing (limited by the soil moisture criterion at 0.1 probability and by the snow criterion at the 0.001 probability), and the longest span of 40 d at Emo (limited by the snow criterion at both probabilities) (Tables 4, 5, and 6). The increased range between the 0.1 and the 0.001 probabilities for the frost depth and the snow depth criteria (Table 7) in the fall as predicted by this approach seemed consistent when considering that the soil temperatures are generally warmer in the fall than in the spring. The warm soil temperatures in the fall would mean that cold air temperatures would first have to cool the soils before freezing to a depth greater than 0.05 m could take place. During the fall, a surface snow accumulation of 0.05 m may be likely, which would further delay the soil freezing process and may tend to cause a greater spread in the dates between the 0.1 and the 0.001 probabilities than observed in the spring dates (Table 7). In contrast to the fall, in the spring warm soil temperatures at depth combined with warmer air temperatures tend to thaw the soils in less time, decreasing the spread in dates between the 0.1 and the 0.001 probabilities.
The date of last allowable manure and nutrient application in the fall appeared to be more related to latitude and elevation than to soil type. For the northern sites, Kapuskasing, Emo, and Ottawa, as well as the location at the highest elevation, Mount Forest, the snow depth criterion (a snow depth greater than 0.05 m) was the limiting factor for most of the considered probabilities. At Kapuskasing the soil plastic limit criterion was the limiting factor for five of the seven considered probabilities; however, Kapuskasing was also the location with the greatest probability of exceeding this criterion throughout the entire year (Fig. 5). The Ryland clay profile used to model this location has a low hydraulic conductivity, and therefore drains slowly, resulting in the likelihood that a substantial rain anytime during the year will result in the soil plastic limit criterion being met. The probability for the soil plastic limit criterion began to rise sharply in September at the same time as the probability curve for the snow depth criterion began its climb. For the more restrictive lower probabilities (0.001 and 0.002), the snow depth criterion was the limiting factor; however, as the soil plastic limit criterion rises more steeply, it becomes the limiting factor in determining the last fall application day for the remainder of the considered probabilities.
It seems reasonable that Kapuskasing, the location farthest north, also had the earliest last application cessation date of the fall for the most restrictive probability (0.001). As mentioned previously, Kapuskasing also had a range of 13 d beginning at Day 263 (20 September for the 0.001 probability) limited by the criterion of a snow depth greater than 0.05 m to Day 276 (3 October for a probability of 0.1) limited by the soil plastic limit criterion. The next two northernmost sites, Emo and Ottawa, were the next earliest last application day of the fall. Both of these locations had the fall cessation of manure application limited entirely by the snow depth criterion. Emo is at a latitude approximately 315 km further north than Ottawa, and about 75 km south as compared to Kapuskasing (Table 1), which could explain why the 0.001 probability day in Emo (Day 268 or September 25) is only 5 d later than that of Kapuskasing, and 23 d earlier than the 0.001 probablility day in Ottawa (Day 291 or October 18). Both Emo and Ottawa have similar ranges between the 0.001 and 0.1 probability days, at 40 and 35 d, respectively. The 0.1 probability end day in Emo was Day 308 (November 4) whereas in Ottawa it was Day 326 (November 22). Although Mount Forest is about 130 km south of Ottawa, it is at a higher elevation (410 m at Mount Forest vs. 100 m at Ottawa), which may explain why the earliest fall application day is estimated to be the same as in Ottawa. The range from the 0.001 and 0.1 probability days is 27 d, and like the previous sites, it is the snow depth criterion that is the limiting factor for all probabilities.
Moving southward, the next two sites were Guelph and Smithfield, and although they are very close to Mount Forest in terms of latitude (within 45 km in a north–south direction) (Table 1), they are at a lower elevation. The elevation at Mount Forest is 410 m, whereas the elevations in Guelph and Smithfield are 340 m and 119 m, respectively. Even though Guelph is relatively close to Mount Forest, it was not influenced by lake-effect precipitation as seen by the annual precipitation for each location. In Mount Forest the average annual precipitation was approximately 135 mm greater than that of Guelph (Table 2) (Fallow et al., 2003). In fact, both the fall and winter precipitation recorded at Mount Forest exceed that at Guelph by about 60 mm (Table 2). Taking into consideration that Guelph received less precipitation than Mount Forest, it seems reasonable that the limiting factor in determining the last day of application in the fall was determined by the frost depth criterion for all probabilities instead of a snow depth >0.05 m. The range in Guelph was 28 d over all probabilities with the earliest end application date being Day 295 (October 22 for a 0.001 probability, limited by the frost criterion) to Day 323 (November 19 for a 0.1 probability, limited by the frost criterion). Smithfield had a range over all considered probabilities of 30 d, which was similar to Guelph. The earliest day for the fall manure application in Smithfield is Day 295 (October 22 for the 0.001 probability, limited by the snow criterion), and the latest day for the fall application of manure is Day 325 (November 21 for the 0.1 probability, limited by the frost criterion). Smithfield has no restrictions due to the soil moisture criterion, as the modeled site soil water content never exceeded that of the soil plastic limit. Of interest is that the separation between the limiting day as defined by the frost depth criterion and the limiting day defined by the snow depth criterion is a maximum of 5 d throughout the entire range of probabilities considered. This indicates that although a frost depth >0.05 m is the limiting factor for all of the probabilities, except at the 0.001 probability (which is limited by the snow depth criterion), the two factors are almost identical in determining the last day of acceptable manure application in the fall. This seems logical given the proximity of Smithfield to Lake Ontario, which would have a strong regulating influence on the local climate.
The southernmost location included as part of this study was Harrow. The limiting factor in Harrow for all probabilities in both the spring and the fall was the soil plastic limit criterion. In the fall, the range for the first end date is 20 d spanning from Day 304 (31 October for 0.001 probability) to Day 324 (20 November for a 0.1 probability).
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CONCLUSIONS
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The method used in the determination of the spring start and fall end days for manure application developed for this study appears to be consistent with the climatic and soil properties of the seven Ontario locations. Differences between the first application days in the spring and the last application days in the fall occur logically considering the physical and climatic conditions of each location.
The average range of spring start application days between the 0.1 and the 0.001 probabilities among all locations was approximately 16 d; therefore, there is a significant change in risk over a relatively short time span. Warmer soil temperatures in the fall help create a wider range of days between the 0.1 and the 0.001 probabilities with an average of 29 d suggesting that the selection of the appropriate risk level may be more critical in the fall than in the spring.
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ACKNOWLEDGMENTS
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The authors would like to acknowledge the Ontario Ministry of Agriculture Food and Rural Affairs, Ontario, Canada and the Canadian Water Network for the partial funding of this project. The authors thank Keith Reid of the Ontario Ministry of Agriculture, Food, and Rural Affairs for his input. Also the authors would like to thank Bryan Smith of the Meteorological Service of Canada, Environment Canada, for assistance in amassing the climate data for this study.
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REFERENCES
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- Crane, J.L. 1933. Preliminary study of the typical soil profile in the clay soil at the Dominion Experimental Station, Kapuskasing. M.S. thesis. Ontario Agricultural College, Dep. of Chemistry, Univ. of Guelph, Guelph, ON, Canada.
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ABSTRACT
INTRODUCTION
