Effect of minimum tillage and crop sequence on physical
properties of irrigated soil in southern Alberta
Xiying Hao
a,*, Chi Chang
a, Francis J. Larney
a,
Jennifer Nitschelm
b, Peter Regitnig
baAgriculture and Agri-Food Canada, Lethbridge Research Centre, PO Box 3000, Lethbridge, Alta., Canada T1J 4B1 bRogers Sugar Ltd., 5406, 64th Street, Taber, Alta., Canada T1G 2C4
Received 30 November 1999; received in revised form 30 March 2000; accepted 13 July 2000
Abstract
Adoption of conservation tillage practices has been much slower on irrigated land than on dryland in southern Alberta. This study investigates the effect of conventional tillage (CT) and minimum tillage (MT) on soil physical properties for two crop sequences on an irrigated Dark Brown Chernozemic clay loam from 1994 to 1998. For soft wheat and annual legumes, CT consisted of chisel plowing and double discing in the fall and light duty cultivation and harrow packing in spring. The MT treatment consisted of only light cultivation and harrow packing in spring. For sugar beets, CT consisted of moldboard plowing, double discing, light cultivation, harrow packing and ridging in fall and de-ridging in spring, while MT consisted of chisel plowing, harrow packing and ridging in the fall and de-ridging in the spring. Crop sequence 1 was spring wheat (Triticum aestivumL.)±sugar beet (Beta vulgarisL.)±spring wheat±annual legume, while sequence 2 was spring wheat±spring wheat±annual legume±sugar beet. Soil physical properties measured included bulk density (BD) and cone index (CI) after 1 and 5 years of treatment and soil aggregation and residue cover after 4 years of treatment. There were no signi®cant differences between MT and CT for BD and CI. Use of MT resulted in a larger geometric mean diameter (GMD) of aggregates (6.52 mm) and a lower erodible fraction (EF) of 23.4% compared to a GMD of 3.81 mm and an EF of 31.5% (EF) for CT. Use of MT also resulted in better residue cover than CT, reducing susceptibility to wind erosion. Crop sequence is crucial to the successful implementation of MT systems. Since the two crop sequences tested resulted in similar soil physical conditions after 5 years, each could be successfully used with MT for irrigated cropping in southern Alberta.# 2000 Published by Elsevier Science B.V.
Keywords:Physical properties; Tillage; Crop sequence; Residue cover
1. Introduction
In recent years, the area devoted to row crops (primarily peas, beans, potatoes and sugar beets) has increased in southern Alberta (Alberta
Agricul-ture, Food and Rural Development, 1998). For these crops to remain pro®table and environmentally sus-tainable, they have to be managed in systems that reduce erosion risk and preserve soil quality. In south-ern Alberta, frequent chinook winds (warm westerly winds that may gust to 120 km hÿ1
) can melt snow cover and cause wind and water erosion, especially under conventional tillage (CT), where soils are left bare of crop residue. CT, for irrigated land on the *Corresponding author. Tel.:1-403-317-2279;
fax:1-403-317-2187.
E-mail address: haoxy@em.agr.ca (X. Hao).
Canadian Prairies, usually consists of chisel plowing and double discing in the fall in addition to light cultivation and harrow packing in the spring. More intensive tillage (e.g., moldboard plowing) is used for row crop production.
Since productivity losses due to erosion are some-what masked by water inputs under irrigation, farmers may not perceive the need for soil conservation prac-tices on irrigated land. Thus, adaptation of minimum tillage (MT) and no-till (NT) systems has been slow on irrigated land in southern Alberta. These systems minimize soil disturbance and hence susceptibility to erosion. However, irrigated crops often experience reduced emergence rates under a no-till system (Hay-hoe et al., 1993) because of high amounts of crop residue when row crops follow cereals.
Sustainable agricultural production systems like MT and NT must also maintain the soil's physical quality. Tillage-induced changes in physical proper-ties, such as bulk density (BD), penetration resistance and aggregate stability are complicated and depend on soil texture, water content at the time of tillage and the time lags between tillage and physical property measurements. NT and MT can increase aggregate stability through the increase in organic matter (Larney et al., 1994b; Kitur et al., 1993), improve soil physical properties and soil water storage (Larney et al., 1994b) and increase in®ltration rates (Chang and Lindwall, 1989). However, an increase in soil pene-tration resistance, as measured by the cone index (CI), has been reported in NT systems due to traf®c asso-ciated with seeding and harvesting and a lack of tillage in seedbed preparation (Carter, 1991; Larney and Kladivko, 1989). On the other hand, Cassel et al. (1995) reported that CI was not affected by the tillage method.
The objective of this experiment was to study the effect of tillage systems (CT vs. MT) on soil physical properties and crop residue conservation for two crop sequences. The goal was to de®ne a management system that would not impact negatively on soil physical properties and reduce erosion risk.
2. Materials and methods
This study was initiated in fall 1993 at the Agri-culture and Agri-Food Canada Research Centre in
Lethbridge, Alta. The soil was a Dark Brown Cher-nozemic Lethbridge clay loam with an organic carbon content of 14.6 g kgÿ1
in the surface 0±15 cm. The site had been seeded to silage barley (Hordeum vulgare L.) for at least the previous 8 years.
The experiment was designed as a split±split-plot, with crop sequence as the main treatment, the crop type in the sequence as the sub-treatment, and tillage as the sub±sub-treatment. The plot size for the sub± sub-treatment was 6:1 m12:2 m. There were two crop sequences, each with four crops: sequence 1 (spring wheat±sugar beet±spring wheat±annual legume) and sequence 2 (spring wheat±spring wheat±annual legume±sugar beet). Sequence 1 allowed a cereal break, producing high amounts of residue, between two row crops producing low amounts of residue, and may be considered a more sustainable option. Sequence 2 placed the two low residue row crops back-to-back, which is a more conventional system in the area. Beans (Phaseolus vulgarisL.) were used as the annual legume from 1994 to 1996 and peas (Pisum sativum L.) from 1997 to 1998.
For the tillage treatments, CT represents the most widely used practices locally while MT uses fewer operations to enhance soil conservation. Pre-treat-ments were used to manage crop residues after har-vest. Annual legume residue was spread over the entire plot area, then chopped with a ¯ail mower for both CT and MT treatments. Wheat straw was spread with an oscillating harrow and ¯ail mowed in the fall if annual legumes or sugar beets were to be seeded the next year for the MT treatment.
For annual legumes and spring wheat under CT, soil was chisel-plowed (15±20 cm depth) and double disced (5±10 cm depth) in fall. No fall tillage opera-tions were conducted for the corresponding MT treat-ment. However, both tillage systems received a light-duty cultivation and a harrow packing operation in spring.
Gravimetric soil water content and soil BD were determined on September 28, 1994, and October 5±7, 1998. BD was measured using cores (Blake and Hartge, 1986) in 5-cm increments to 50-cm depth. CI was measured on September 27, 1994, and October 21, 1998 after harvest but before fall tillage. A mod-i®ed hand-held penetrometer (Anderson et al., 1980), with a 308cone, a base area of 1.29 cm2and a drive shaft diameter of 9.53 mm, was used to obtain CI in 3.5-cm increments to 52.5-cm depth. The penetrom-eter was modi®ed by adding an electric motor (400 W) and an actuator to obtain a steady penetration speed of 10 cm sÿ1
. Three penetrometer readings were obtained for each plot in 1994 and 10 in 1998.
In spring 1998, dry aggregate size distribution (ASD) and water stable aggregate (WSA) analyses were conducted. Soil samples from the 0 to 3 cm layer were collected from each plot before tillage treatments were applied. ASD samples were air-dried and passed through an improved rotary sieve (Chepil, 1952). Geometric mean diameter (GMD) was computed by regressing the cumulative mass of aggregates against the log of sieve size (Gardner, 1956). The wind erodible fraction (EF) is the mass of soil aggregates <0.84 mm diameter expressed as a percent of the total sample mass. Since the rotary sieve employed did not have a 0.84 mm sieve, the mass passing through 0.84 mm was computed using the regression equations from GMD calculations (Larney et al., 1994a).
WSAs were determined by measuring the propor-tion of 1.2±2.0 mm aggregates that were >250mm diameter, when subjected to the disruptive forces of water (Angers and Mehuys, 1993). The total carbon (C) in WSAs was determined using a C±N±S analyzer (Carlo Erba, Milan, Italy).
All data were analyzed using the GLM procedure (SAS Institute, 1990). When differences between treatment means were signi®cant at P<0:05, the multi-range Tukey test was performed.
3. Results and discussion
3.1. Fall soil water
There were no signi®cant tillage effects on fall soil water content in 1994 (Fig. 1a) or 1998 (Fig. 2a). Plots with sugar beets contained signi®cantly less water
Fig. 1. Effect of (a) tillage and (b) crop on soil water content in fall 1994.
than those with annual legumes at 5±20-cm depth in fall 1994 (Fig. 1b), but not in fall 1998 (Fig. 2b). The drier conditions under sugar beets in 1994 were prob-ably due to a later harvest date (September 20 com-pared to September 7, 1994, for annual legumes and spring wheat), which allowed greater soil water loss due to evapotranspiration by the sugar beet crop.
3.2. Bulk density
After the ®rst year of the study (fall 1994), there were no signi®cant effects of tillage, sequence or crop type on soil BD (Fig. 3a and b). After 5 years of cropping, BD was signi®cantly lower for CT (1.36 Mg mÿ3
) than for MT (1.48 Mg mÿ3 ) at the 10±15-cm depth, and at the 15±20-cm depth (CT: 1.44 Mg mÿ3
; MT: 1.51 Mg mÿ3
) (Fig. 4a). The lower BD value for CT was probably related to moldboard plowing for sugar beets each fall.
In fall 1998, the crop grown had a signi®cant effect on BD at the 5±10-cm depth, with spring wheat (1.42 Mg mÿ3
) resulting in a higher BD than sugar beets (1.33 Mg mÿ3
) (Fig. 4b). The lower BD of the sugar beet plots is likely due to soil loosening caused
by harvesting equipment (which pulls beets directly from the soil).
3.3. Soil penetrability
Increased CI with NT has been previously reported (Vyn and Raimbault, 1993; Vyn et al., 1998). How-ever, no signi®cant differences between the MT and CT treatment were found in this study. CI was not affected by tillage after either 1 (Fig. 5a) or 5 years (Fig. 6a) cropping. The lack of signi®cant tillage effects after 1 year may be due the short-term nature of the treatment effects. In addition, penetrometer measurements were conducted in late September, 5±12 months after the last tillage operation. In the interim, the soil may have settled, masking any short-term tillage effects on soil physical properties (Chang and Lindwall, 1989). The crop grown in 1994 affected CI (Fig. 5b) with signi®cantly higher values on spring wheat plots at depths of 3.5 and 7.0 cm and on sugar beet plots at depths of 10.5±51.0 cm (Fig. 5b). The higher CI readings at the 3.5 and 7-cm depths for spring wheat were probably related to combine har-vesting of spring wheat under moist soil conditions.
Fig. 3. Effect of (a) tillage and (b) crop on soil BD measured in fall 1994.
There was 15.2 mm of precipitation in the week pre-ceding wheat harvest. Annual legumes were hand-harvested with minimal impact on soil compaction. The higher CI values at lower depths (10.5±51.0 cm) for sugar beets were probably caused by the drier soil conditions associated with this crop (Fig. 1a).
After 5 years (fall 1998), a signi®cant tillage effect on BD at 10±15 and 15±20-cm depths (Fig. 4a) was not translated into an effect on CI (Fig. 6a). Similar to 1-year results, soil had a signi®cantly lower CI value at 7.0-cm depth after sugar beets than after peas and spring wheat (Fig. 6b). This is consistent with the BD data (Fig. 4b), which showed a lower BD for sugar beets at the 5±10-cm depth and is again related to soil loosening associated with sugar beet harvest. While BD values were lower for sugar beets than for peas and spring wheat at 7±20-cm, the CI showed no such trend. This is because the analysis and data interpretation of CI are complicated by the effects of both soil water and BD (Cristensen et al., 1989).
3.4. Aggregate size distribution and stability
The GMD of soil aggregates was signi®cantly larger, and hence EF was signi®cantly lower, for
MT than for CT in spring 1998 (Table 1). The results con®rm those reported by Unger et al. (1998), Hadas and Meirson (1978) and Chagas et al. (1995). The increase in aggregation (high GMD and low EF) associated with MT was probably partly related to an increase in organic matter (a binding agent for aggregate formation) (Gupta et al., 1979). Organic matter content was signi®cantly higher in the MT treatment due to less soil disturbance and slower crop residue decomposition (Hao et al., 1999). Since EF values for both tillage practices were
Fig. 5. Effect of (a) tillage and (b) crop on CI measured in fall 1994.
Fig. 6. Effect of (a) tillage and (b) crop on CI measured in fall 1998.
Table 1
GMD, EF and the WSA after 4 years of treatment (spring 1998) Treatments Dry aggregate WSA
GMD (mm) EF (%) Proportion (%)
Total C (g kgÿ1)
Tillage
CT 3.81 b 31.5 a 15.9 a 28.6 b MT 6.52 a 23.4 b 16.5 a 30.7 a Sequence
<60%, the critical value for wind erosion (Anderson and Wenhardt, 1966), the risk of wind erosion was relatively low for all treatments in the study.
There were no signi®cant effects of crop sequence or tillage on WSA (Table 1), although the total C content of WSA was signi®cantly higher under MT. Angers (1998) pointed out that additional organic
matter accumulation in the surface soil due to mini-mum or no-till would lead to an increase in WSA. The lack of a signi®cant tillage effect on WSA was prob-ably due to the relatively short experimental time. It is also possible that the rapid turnover of OM under irrigation and warm/hot summer conditions prevents the accumulation of active OM under both tillage systems.
3.5. Surface residue cover
Two of the main mitigating factors for wind erosion are adequate crop residue cover and large non-erodible aggregates (Larney et al., 1997). Visual assessment of photographic images taken in spring 1998, before the spring tillage treatment, indicated that the use of MT (Fig. 7e±h) retained more surface residue cover than CT (Fig. 7a±d), irrespective of the crops grown. This was related to lower tillage intensity with the MT treatments. As shown in Table 2, the MT treatment also resulted in larger soil aggregates. Thus, wind erosion losses are less likely to occur with the MT system than with the CT system.
As expected, peas and sugar beets produced very little crop residue and decomposed quickly since they contain more water and less ®ber than cereals. The visible residue in the MT plots with peas (Fig. 7h) and sugar beets (Fig. 7g) grown in 1997 was that left from spring wheat grown in 1996 and 1995. The lower straw cover for the plot in Fig. 7b than for the plot in Fig. 7a was probably due to localized salinity problems. Salinity was also a problem for the plot in Fig. 7c.
Fig. 7. Crop residue cover under CT (a±d) and MT (e±h) prior to seeding in spring 1998 from sequence 2.a: W, soft wheat; L, annual
legumes; S, sugar beets; the ®ve-letter sequence refers to the crops planted from 1994 to 1998.
Table 2
Aggregate size distribution for the selected ®eld in Fig. 7 Plot GMD (mm) EF (%)
CT
a 2.74 35.1
b 4.54 25.1
c 3.26 34.5
d 4.39 27.0
MT
e 3.99 26.9
f 7.16 19.6
g 8.10 20.3
4. Summary
In an irrigated cropping study in southern Alberta, there were no adverse effects on soil physical proper-ties (BD, CI) from using MT. On the contrary, use of MT resulted in higher aggregation (larger GMD and lower EF value) and better residue conservation than CT. This rendered MT plots less susceptible to wind erosion. For irrigated cropping, there are often reduced emergence rates under a no-till system because of high amounts of crop residue when row crops follow cereals. With the MT system proposed in this study, the soil has good residue cover over the winter and spring months and seeding problems were eliminated.
The effect of crop sequence is crucial to the suc-cessful implementation of MT systems. Although sequence 1 should provide better crop residue cover, the two sequences tested here resulted in similar soil physical conditions after 5 years. Thus, both crop sequences could be successfully used in an MT system for irrigated farms in southern Alberta.
Acknowledgements
Funding for this study was provided by the Canada-Alberta Environmentally Sustainable Agriculture (CAESA) Agreement and Alberta Agricultural Research Institute. The authors express their appre-ciation to Greg Travis, Elaine Nakonechny, Brett Hill and Tony Curtis for their technical assistance in carrying out the ®eld and laboratory work. The authors sincerely thank Rogers Sugar Limited for their con-tributions to the project and to Toby Entz for his advice on statistical analysis. This paper is Lethbridge Research Centre contribution No. 387-9984.
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