In¯uence of water table management on
corn and soybean yields
M.N. Mejia, C.A. Madramootoo
*, R.S. Broughton
Department of Agricultural and Biosystems Engineering, McGill University, Macdonald Campus, 21 111 Lakeshore, Ste-Anne-de-Bellevue Que., Canada H9X 3V9
Accepted 16 November 1999
Abstract
A 2-year ®eld study was conducted in eastern Ontario to evaluate the effect of water table
management (WTM) on the yields of strip-cropped corn (Zea maysL.) and soybean (Glycine max
Merr.). The WTM treatments consisted of two subirrigation treatments with water table controls set
at 0.50 m (CWT0.5) or 0.75 m (CWT0.75) from the soil surface, and a free drainage (FD) treatment
(water table1.00 m below the soil surface. Both corn and soybean yields were higher with CWT
than with FD for both years. In 1995, corn yields were 13.8% and 2.8% greater and soybean yields
8.5% and 12.9% greater, respectively, in the CWT0.5and CWT0.75plots than in the FD plots.
Similarly, in 1996, corn yields were 6.6% and 6.9% greater and soybean yields 37.3% and 32.2%
greater, respectively, in the CWT0.5 and CWT0.75 plots than in the FD plots. Yield increases
obtained during the study were attributed to greater crop water uptake in the CWT plots as a result of the higher water tables. Comparison of 1995 and 1996 weather data with the long-term average of the region shows that the years of study had wetter-than-average conditions in the critical months of July and August, and that the yield increases due to WTM could be expected to be even greater
during drier years.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Controlled drainage; Glycine maxMerr.; Subirrigation; Water table; Management; yield increase;
Zea maysL.
1. Introduction
Corn (Zea mays L.) and soybean (Glycine max Merr.) are economically important crops in Ontario and Quebec, and, consequently, take up a large proportion of arable land. In total, 0.81 million ha (Mha) of soybean and 0.98 Mha of corn were grown in 1995 in
Agricultural Water Management 46 (2000) 73±89
*Corresponding author. Tel.:1-514-398-7778; fax:1-514-398-8387.
E-mail address: cam@agreng.lan.mcgill.ca (C.A. Madramootoo).
these two provinces. In Ontario, the area cropped to soybean has increased from 0.42 in 1984 to 0.73 Mha in 1995 (Statistics Canada, 1996). For the same period in Quebec, land cropped to corn has increased from 0.22 to 0.28 Mha, while the area cropped to soybean has increased from 239 ha in 1976 to 0.08 Mha in 1995 (Statistics Canada, 1996).
Along with this expansion in cropped area, water quality degradation from drainage and fertilizer use are likely to increase. Best management practices which reduce pollution from agriculture, use water resources efficiently, as well as increase productivity, are, therefore, needed for environmentally sustainable corn and soybean production in the region. Water table management (WTM) has been shown by various researchers to encourage the conservation of resources, increase productivity and reduce pollution. The environmental and economic benefits of WTM through reduced pollution and increased yields have been documented (Wright et al., 1992; Kalita and Kanwar, 1993; Tan et al., 1993; Drury et al., 1994; Broughton et al., 1995; Madramootoo et al., 1995).
In the humid St. Lawrence lowlands of Quebec and Ontario, drainage is required to remove excess water and is essential for field crop production. In summer, however, droughty conditions often occur in the field due to lack of rain and high evapotranspiration. The climatic factor most limiting to grain yields in fertilized corn is insufficient rainfall during the growing season, particularly in July, when crop yields in subsurface-drained fields are often reduced as a result of dry spells which can be exacerbated by excessive drainage (Drury et al., 1996).
With WTM, the proper amount of aeration and soil moisture can be provided to the crops in a more flexible manner. The water table can be lowered by drainage to facilitate field operations in the spring and fall, and raised by controlled drainage and/or subirrigation to provide plants with needed water during the growing season.
Thus, in addition to improving drainage water quality by reducing leaching of agrochemicals from the soil profile, WTM can increase the efficiency of corn and soybean production in two major ways: (i) by helping to retain more nitrate-N in the soil for plant use, fertilizer costs can be reduced, and (ii) from water being made available to plants during times of need, resulting in the improved crop yields. Under droughty conditions, WTM by controlled drainage alone cannot provide adequate water to plant roots. To supply plants with additional moisture in times of need, subirrigation can be employed to pump water back into subsurface drains. Water table management, through either controlled drainage or subirrigation minimizes the risk of crop losses from uncertain rainfall, thereby stabilizing yields from year to year.
irrigation systems, making it suitable for grain crops such as corn and soybean. Since WTM requires less energy, labour and water, it is more economical than a sprinkler system (Doty et al., 1983). WTM systems require flat topography, coarser textured soils, an impermeable soil layer at 1±2 m in depth and the presence, or installation of a pipe drainage system. Fortunately, these requirements are met in most of Ontario and Quebec. For instance, in two counties in Quebec, it is estimated that 15 000 ha are well suited to WTM (Papineau, 1988).
Lysimeter studies have been conducted to find optimum water table depths for corn and soybean production in eastern Canada (Tan et al., 1993; Broughton et al., 1995). This research supports the idea that there is a need for more field validation of potential crop production levels with optimum water table depths. The objective of this study is to evaluate the effects of three water table levels on corn and soybean grain yields under eastern Canadian field conditions.
2. Materials and methods
2.1. Field layout
The study was conducted on a 3.5 ha field in Bainsville, eastern Ontario (458110N, 748230W) in 1995 and 1996. The average field slope of the site was 0.06% and the soil was a stone free Bainsville silt loam (Dark Grey Glysolic soil group), underlain by a clay layer at a depth of 1.0 m.
A ridge-till system with corn±soybean strip-cropping was practised for both years of the study (1995, 1996), with rows strips perpendicular to drainage laterals running across all treatment plots. In 1995, two-thirds of the experimental field was cropped to soybeans and one-third to corn. The rotation was designed so that corn was cultivated on land that had been cropped to soybeans the two previous years. In 1996, the field was planted half to corn (C) and half to soybean (S) in alternating strips:
1994;1995 CSCSSSCSCSSSCSCSSSCSCSSSC 1996 SCSCSCSCSCSCSCSCSCSCSCSCS
Each strip had six 0.75 m wide rows. The seeding rates were 72 000 seeds/ha for corn and 432 000 seeds/ha for soybean. Glyphosate herbicide was applied prior to seeding and pre-emergent herbicides were used during planting. Both corn and soybean were mechanically cultivated twice to control weeds. The corn strips were fertilized with 28% urea/ammonium nitrate at a rate of 130 kg/ha in 1995 and 140 kg/ha in 1996. No fertilizer orRhizobiuminoculant was applied to the soybean strips in either year.
2.2. Drainage system and controlled water table treatments
The drainage system consisted of 15 subsurface lateral drains that discharge individually into a drainage ditch. Each lateral was 125 m long. The first 10 m section from the outlet was made of 75 mm diameter non-perforated polyethylene pipe to minimize water table drawdown by the ditch. The other 115 m section was made of
100 mm perforated polyethylene drainage pipe equipped with a filter sock. The average drain depth is 1 m and the laterals are sloped at 0.10%. The lateral drains were spaced 18.3 m apart and were centrally located beneath each plot. Therefore, each lateral drains an area 18.3 m wide by 115 m long, totalling about 0.21 ha.
The three treatments were: controlled water tables (CWT) at 0.5 m (CWT0.5) and
0.75 m (CWT0.75) below the soil surface, and conventional free drainage (FD) applied to
the 1.0 m deep laterals. Subirrigation maintained the water tables above the drains in the CWT treatments. Each treatment consisted of three laterals and was separated by buffer drains to isolate subsurface flows between the different treatment plots (Fig. 1). Laterals A through J had water table control structures at the outlets and riser pipes to enable subirrigation and maintain the water table treatments (Fig. 2), while drains K through O drained freely into the ditch.
Subirrigation was initiated during the early vegetative stage and maintained until near senescence. Water for subsurface irrigation was pumped from a ditch that was connected
to a nearby lake. A 0.75 kW Myers pump was used to convey lake water to the drain laterals. At the field, a flowmeter recorded the total volume of water supplied. The volume of drainflow was measured by tipping buckets at each of the monitored lateral drain outlets, and recorded by a datalogger. The subirrigation system had a peak delivery rate of approximately 0.95 I/s or 3.7 mm/day.
2.3. Field measurements
Water table depth from the ground surface was determined by lowering a graduated rod with a water sensor at the tip into observation wells placed in the field. The observation wells were 1.6 m long and made of 25.5 mm (1 in.) PVC pipes which were cut and plugged at the bottom with drainage tape (Broughton, 1972). The pipes had drilled holes of 6 mm in diameter along their lengths to let soil water in and were covered with geotextile to keep silt out. The wells, installed to a depth of 1.5 m at lateral drain mid-spacing, protruded at the soil surface, and were capped to prevent rain or particulate matter from entering. There were 43 wells in total, 12 observation wells per treatment, six additional wells placed to monitor water table shape across CWT plots, and one 2 m well used to monitor the water table level when it dropped below 1.5 m in the FD plot. The top of each observation well was surveyed relative to a benchmark, to the bottom of the drain outlets, and to the tops of risers of the water table control structures.
Fig. 2. Control structure outlet.
Soil samples were collected throughout the growing season to compare soil moisture contents across the treatment plots. The soil samples were taken at lateral drain mid-spacing, from three depths (0±20, 20±40 and 40±70 cm) and at 36 different locations in each treatment field. Soil moisture contents were measured gravimetrically by weighing the samples before and after oven-drying them at 1058C for 24 h (Gardner, 1965).
Rainfall and air and soil temperatures were recorded daily with an on-site datalogger to compare the weather during the study to the long-term average conditions of the region. Using the recorded daily temperatures, evapotranspiration (ET) and corn heat units (CHU) were calculated for both years. The Blaney±Criddle equation (Schwab et al., 1993) was used to calculate ET:
ETkp 0:46T8:13 (1)
where ET is the monthly evapotranspiration, mm;kis the crop coef®cient for corn (0.42 for May; 0.8 for June; 1.15 for July, 0.87 for August; 0.55 for September; FAO, 1977);p
is the monthly percent of total daylight hours (Environment Canada Climatic Normals, 1960±1990 at McGill University Weather Station, Montreal, Que.), (Environment Canada, 1993);Trepresents the average monthly temperature, 8C.
The corn heat units (CHU) were calculated using the following equation (Brown and Bootsma, 1993)
whereTmin,Tmaxare the minimum and maximum temperature, 8C
The CHU were accumulated from the date of seeding until the date of grain physiological maturity. The dates of seeding were 11 May and 21 May in 1995, for corn and soybeans, respectively, and 27 May for both crops in 1996. The grain maturity dates were assumed to be 60 days after tasselling (Hanway, 1963). This date was 17 September in 1995 and 28 September in 1996.
2.4. Yield parameters
The harvesting of corn and soybean was done by hand. Corn cobs were removed from plants 2.5 m on either side of the laterals. This was done six times per treatment plot, for a total of 30 m per treatment. Additionally, 60 corn plants were taken from each treatment to determine the harvest index. The soybean plants were cut and shelled manually in 1995, but shelled by a combine harvester in 1996. The cobs were shelled using a corn sheller and grain yields compared. The 100-seed weights for both corn and soybean were also compared. Finally, dry matter weight comparisons were made for corn, and number of pods per plant were compared for soybean.
2.5. Statistical analysis
in each of the monitored plots (Lalonde, 1993). This restriction, therefore, forced similar water table treatments to be situated adjacent to each other.
Conventional statistical tests for significance were, therefore, not applicable due to the lack of complete randomization. Therefore, a two-tailed Student's t-test was used to determine if treatments were significantly different from one another (Agriculture Canada, 1989; Lalonde, 1993). The treatments were compared in pairs: CWT0.5 versus
CWT0.75, CWT0.75versus FD, and CWT0.5versus FD. In cases where a certain outcome
was expected, an upper-tailed, or directionalt-test was applied to test the hypotheses. The compared samples were homoscedastic (from populations with the same variance), were independent and normally distributed. The t-test is considered an appropriate statistical method to test hypotheses under such conditions (Agriculture Canada, 1989; Howell, 1989; Devore and Peck, 1990).
3. Results and discussion
3.1. Water table levels
The practice of WTM through controlled drainage and subirrigation was effective in maintaining the water table levels in the CWT plots above the drains. Fluctuations of water table levels and rainfall distribution for both years are shown in Figs. 3 and 4. The water table fluctuations are reported as the mean value of all readings from each treatment plot. Although the water table control structures at the drain outlets were set at 0.5 and 0.75 m below the soil surface, the moisture losses due to ET and deep and lateral
Fig. 3. Water table ¯uctuations and rainfall distribution in 1995.
seepage prevented static equilibrium at these levels. In 1995, the average water table level for the CWT0.5, CWT0.75and FD plots were 0.91, 1.03 and 1.30 m, respectively. In 1996,
the average water table level for the CWT0.5, CWT0.75, and FD plots were 0.75, 0.85 and
1.21 m, respectively. However, the water table levels occasionally reached target levels and were significantly different (p<0.05) between treatments throughout most of the growing season, for both years.
3.2. Climatic and crop growth factors
Average climatic data are shown in Table 1. The average growing season temperatures in 1995 and 1996 were both 8.6% above the long term average. Accumulated CHU in 1995 and 1996 were very slightly above the long-term average, by 1.3% and 2.2%, respectively. Total ET during the 1995 growing season was 3.7% above the long-term average, while in 1996 it was 6.8% below average. For the entire growing season (May± September), total precipitation in 1995 was 13.8% below the long-term average, while it was 20.1% above average in 1996.
With respect to corn and soybean plants grown in the area, May±June is a period of germination and vegetation growth, while July±August is a period of flowering and grain filling. For both corn and soybean, moisture availability is most critical as flowering is initiated. The second most critical time is germination, followed by the vegetative and grain filling stages (FAO, 1979). Rainfall in September would have very little or no benefit to the crop, since maturity will have been reached. In eastern Canada, moisture reserves are generally abundant at the germination stages due to spring snowmelt.
For both 1995 and 1996, precipitation for the May±June period was below the 30-year average (by 15.1% and 14.3%, respectively), while precipitation for the July±August
Table 1
Climatic and growing conditions in southwestern Quebec and Eastern Ontario
Parameter Tempa(8C) Eta(mm) CHUa Precipitationb
May±September May±September May±September May±September May±June July±August
Long term avg. 16.7 744.2 2801 394.1 145.5 158.2
1995 average 18.1 771.8 2838 339.9 123.6 180.4
(% from avg.)c (8.6) (3.7) (1.3) (ÿ13.8) (ÿ15.1) (14.0)
1996 average 18.1 693.9 2862 473.2 124.7 193.4
(% from avg.)c (8.6) (ÿ6.8) (2.2) (20.1) (ÿ14.3) (22.3)
aEnvironment Canada Weather Station at CoÃteau-du-Lac, Quebec (1979±1995). bEnvironment Canada Weather Station at Lancaster, Ontario (1951±1980). cField site weather data recorded at Bainsville, Ontario (1995±1996).
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period was above the 30-year average (14.0% and 22.3%, respectively). In spite of this there was insufficient rainfall to meet the high ET and crop demands during the flowering and grain-filling stages of both corn and soybean (Table 2).
3.3. Soil moisture
The shallower water tables in the CWT plots substantially increased soil moisture levels in both years. The differences in soil moisture content among the treatments for 1995 and 1996 are presented in Figs. 5 and 6, respectively. During the flowering period in July, when soil moisture is most crucial, the soil moisture in the CWT plots were significantly higher than in the FD plots (p<0.05). The greater soil moisture in the subirrigated plots helped to partially overcome moisture deficits, which were very high during June, July and August for both years.
3.4. Water de®cit
In 1995, a total of 4743 m3 of water was applied to the CWT sections of the experimental field. This was equivalent to an irrigation depth of 223 mm. In 1996, 5315 m3 of water were applied for an irrigation depth of 248 mm. Table 2 shows the contributions of subirrigation and rainfall to meeting ET demand for both years. In 1995 and 1996, rainfall alone was not adequate to meet crop ET requirements in June, July and August for any of the treatment plots. The water deficits for these months were much higher in the non-irrigated FD plots than in the subirrigated CWT plots. Thus, WTM was extremely beneficial during these months, as it supplemented rainfall to meet crop ET requirements.
3.5. Crop yields
Substantial increases in corn and soybean yields were found with WTM for both years. The harvest results and treatment comparisons for statistical significance for corn and soybean are shown in Tables 3 and 4, respectively.
Table 2
Irrigation, precipitation, ET and water de®cit (mm) in 1995 and 1996
Month 1995 1996
Irrigation Rainfall ET Water deficit Irrigation Rainfall ET Water deficit
3.6. Corn yield
The highest corn yields were found in CWT plots for both years. In 1995, there were no significant differences between the yields of the CWT0.5 and CWT0.75 treatments, or
between the CWT0.75and FD plots. However, the difference between the CWT0.5and FD
plots was statistically significant (p0.05). This 13.8% increase in yield, as a result of the CWT0.5treatment, produced 1.53 t/ha more corn when compared to conventional FD. In
Fig. 5. Soil gravimetric water content Ð 1995.
1996, the highest yields were again found in the CWT plots. Compared to FD, the CWT0.5and CWT0.75plots gave 6.6% (0.45 t/ha) and 6.9% (0.47 t/ha) greater yields,
respectively. However, only the yield from the CWT0.75plot was significantly different
(p0.05) from that obtained with FD. Corn yield in 1996 did not show as dramatic an increase with WTM as in 1995. The wetter summer in 1996 probably reduced the relative beneficial effects of subirrigation. Overall, corn yield in 1996 was much lower than that of 1995, due most likely to a late spring and late planting date.
3.7. Grain size
The 100-kernel weights of subsamples from each treatment were taken to see if water table depth had an effect on grain size. Assuming that all grains had a uniform density, a higher 100-kernel weight would mean larger corn grains. In 1995, the CWT0.5 and
CWT0.75produced the largest grain sizes. Compared to FD, the CWT0.5 and CWT0.75
treatments produced grains that were larger by 14.0% and 3.1%, respectively. Although the size difference between CWT0.75and FD was not significant, the differences between
CWT0.5and CWT0.75, CWT0.5and FD treatments were significant. In 1996, the CWT plots
again produced the largest grains. Grain size was significantly different between CWT0.75
and FD (p0.05), marginally significant (p0.10) between CWT0.5and CWT0.75, but not
significant between CWT0.5 and FD. The CWT0.5 and CWT0.75 plots produced kernels
which were 4.1% and 10.1% larger, respectively, when compared to FD. Again, due to a wet year and late planting date, grain sizes in 1996 were smaller than in 1995.
3.8. Harvest index
Harvest index (HI) is the ratio of the above-ground plant biomass to total grain produced by the plant. It indicates how the plant allocates its resources (i.e., more leaf and stem production versus grain production). The CWT plants had higher HI compared to FD plants, indicating that the CWT plants produced more biomass than FD plants for both years (Table 3). However, none of the differences in HI between treatments was
Table 3
Effect of three water table depths on corn yield, grain yield and harvest index and on soybean grain yield, harvest index and number of pods per plant for 1995 and 1996
Year Crop Parameter Water table management
1995 Corn Yield (t/ha) 12.61 11.39 11.08 NS NS **
100-kernel wt. (g) 29.82 26.97 26.97 ** NS **
Harvest index 1.82 1.80 1.71 NS NS *
Soybean Yield (t/ha) 3.44 3.58 3.17 NS ** *
100-kernel wt (g) 19.02 19.49 18.04 NS NS NS
Pods/plant 23.89 26.30 24.84 NS NS NS
1996 Corn Yield (t/ha) 7.29 7.31 6.84 NS * NS
100-kernel wt. (g) 25.73 27.22 24.72 * ** NS
Harvest index 1.79 1.79 1.78 NS NS NS
Soybean Yield (t/ha) 3.24 3.12 2.36 NS ** **
100-kernel wt. (g) 20.72 21.48 19.77 * ** **
Pods/plant 18.46 25.46 13.94 NS ** **
aCWT
0.5, CWT0.75: water table controlled at 0.5 and 0.75 m from the soil surface, respectively. FD: free drainage, water table1.0 m from the soil surface.
significant atp0.05 in either year. Therefore, water table depth had no significant effect on the ratio of plant biomass to total grain produced in corn in both years.
3.9. Soybean yield
The highest soybean yields were found in the CWT plots for both years (Table 3). In 1995, although the grain yields in the CWT0.5 and CWT0.75 treatments did not differ
significantly, the comparison in yields between CWT0.75versus FD and CWT0.5 versus
FD were significantly different at the p0.05 and p0.10 levels, respectively. The CWT0.5and CWT0.75plots produced 8.5% (0.27 t/ha) and 12.9% (0.41 t/ha) higher
yields than FD, respectively. In 1996, the yields from the CWT0.5and CWT0.75treatments
did not differ significantly. However, the yields from the CWT0.5and CWT0.75treatments
were significantly greater than those from the FD treatment (p0.05). Compared to FD, the CWT0.5and CWT0.75plots gave 37.3% (0,88 t/ha) and 32.2% (0.76 t/ha) higher
yields, respectively. The yield increases in 1996 were greater than those in 1995, in spite of the wet year. The beneficial effect of subirrigation in June and August most likely boosted the yields in 1996. The late spring probably did not adversely affect the soybean crop as much as the corn since soybean is normally planted later than corn.
3.10. Grain size
The 100-seed weights were taken to see if WTM had an effect on soybean grain size (Table 3). Assuming that all the grains had a uniform mass and density, a greater 100-seed weight would mean larger soybean grains. In 1995, CWT0.5and CWT0.75plots
produced the largest grains. Compared to FD plots, the CWT0.5 and CWT0.75 plots
produced grains that were larger by 5.4% and 8.0%, respectively. However, these differences were not significant (p>0.05). In 1996, the CWT plots again produced the largest grains. Grain size differences were significant (p0.05) for all comparisons. The CWT0.5and CWT0.75plots produced soybean seeds that were 4.8% and 8.6% larger than
FD seeds, respectively.
3.11. Pods per plant
In 1995, the plants that produced the most pods were found in the CWT0.75plots, followed
by those in the FD and CWT0.5plots (Table 3). However, the differences in the number of
pods per plant were not significant. In 1996, the CWT0.5and CWT0.75plots produced the
most pods per plant. Although the difference in number of pods between the CWT0.5and
CWT0.75plots was not significant, both CWT's produced significantly more pods per plant
than FD plots (p0.05). In 1996, the CWT0.5and CWT0.75plots produced 32.0% and 82.2%
more pods per plant, respectively. Combined, the CWT plots produced 57% more pods per plant than FD plots, which was reflected in the 35% increase in 1996 grain yields.
Improved crop preference can be attributed to the higher water availability provided by WTM. More water reaches the crop roots by capillary rise from the shallower water tables. It has been shown that stomatal conductance and transpiration rates of corn grown with 0.3 and 0.6 m water table depths were greater than those grown with 0.8 m water table depth (Tan et al., 1993). Some studies have shown that soybean yields were greatest with a 0.6 m water table depth (Williamson and Kriz, 1970), while others found that a water table depth of 0.15±0.30 m maximized soybean yields (Nathanson et al., 1984).
In both years of this study, the best yields for both corn and soybean crops were achieved by setting a water table regime between 0.5 and 0.75 m. However, under our field conditions, these design water table depths are actually between 0.75 and 1.03 m since deep seepage and ET losses lowered the water tables. Research by Kalita and Kanwar (1993) and Madramootoo et al. (1995) showed that the highest corn and soybean yields were obtained with a water table depth of 0.6±0.9 m, and the lowest yields were obtained with a water table depth of 0.2±0.3 m.
Climatic conditions greatly affected the effects of WTM on crop production. Particularly in mid-July, 1995 and 1996, soil moisture content was much lower in the free drainage plots than in the subirrigated area. However, sufficient soil moisture was replenished in the free drainage area by timely rainfalls received in July to prevent wilting in the non-subirrigated section. The combination of high temperatures in June along with abundant rainfall in July resulted in very good crop growing conditions for both irrigated and non-subirrigated crops. Although there were substantial yield increases with WTM, the yield response to WTM was moderate and would most likely be higher in years when July and August are drier than normal.
4. Conclusions
Compared to free drainage, corn and soybean yields were increased by water table management in 1995 and 1996. In 1995, corn grain yield was increased by 13.8% and 2.8% by the CWT0.5 and CWT0.75 treatments, respectively, while soybean yield was
increased by 8.5% and 12.9% by the CWT0.5and CWT0.75treatments, respectively. In
1996, corn yields were higher in the CWT0.5and CWT0.75treatments than in FD by 6.6%
and 6.9%, respectively. Soybeans grown in the CWT0.5and CWT0.75treatments showed
yield increases of 37.3% and 32.2% over FD yields. The yield increases were also reflected in the larger grains and kernels of the crops grown with WTM.
When compared to the long-term growing conditions of the region, the yield increase responses during 1995 and 1996 are most likely moderate and could be higher in drier years. For similar soils and climatic conditions, a water table level of 0.75 m was recommended for corn and soybean production.
Acknowledgements
This research was supported by the Land Improvement Contractors of Ontario, the Natural Sciences and Engineering Research Council of Canada and Agriculture and
Agri-Food Canada. The assistance of the farm owner, Mr. R. McRae is appreciated. We are grateful to Mr. J. Perrone and Dr. G. Dodds, Research Assistants at McGill University for proof-reading and editing this manuscript.
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