Influence of conservation tillage and rotation length on potato productivity, tuber disease and soil quality parameters on a fine sandy loam in eastern Canada

Soil & Tillage Research 63 (2001) 1–13

Influence of conservation tillage and rotation length on potato productivity, tuber disease and soil quality parameters on a fine sandy loam in eastern Canada M.R. Carter*, J.B. Sanderson Agriculture and Agri-Food Canada, Crops and Livestock Research Centre, P.O. Box 1210, Charlottetown, PEI, Canada C1A 7M8 Received 13 October 2000; received in revised form 25 April 2001; accepted 9 June 2001

Abstract Potato (Solanum tuberosum L.) farming systems generally use excess tillage and produce low levels of crop residue in the potato year, both of which are detrimental to soil quality. In eastern Canada the major form of primary tillage is autumn (September–October) mouldboard ploughing (20 cm depth), which leaves the soil bare and unprotected over the winter period prior to planting potato in the following spring (May). A study (split-plot randomized block with six replicates) was initiated in 1994 on a fine sandy loam (Orthic Podzol) in Prince Edward Island to evaluate the use of conservation tillage in both 2-year [barley (Hordeum vulgare L.)–potato] and 3-year [barley–red clover (Trifolium pratense L.)–potato] potato rotations. The main conservation tillage strategy was to shift the primary tillage event from the autumn to spring, by replacing the autumn mouldboard ploughing with a herbicide treatment, and use of a relatively shallow (15 cm depth) one-pass reduced tillage (chisel plough) just prior to potato planting. Mulches were used on all plots after potato harvest to provide soil cover over the cool season. Potato yield and quality, incidence of tuber disease, surface residue levels after potato planting, soil organic carbon and carbon fractions, and soil structure were evaluated from 1994 to 1999, over three and two cycles of the 2- and 3year rotation, respectively. Results for the 2-year rotation indicated that neither conventional nor conservation tillage were sustainable, due to Rhizoctonia disease pressure and a decline in tuber quality, in comparison to the 3-year rotation. Potato yields and tuber quality were similar between tillage systems in the 3-year rotation. Conservation tillage significantly increased the concentration of organic carbon and macro-organic carbon at the soil 0–8 cm depth, compared to conventional tillage, and significantly improved soil structural stability. Use of conservation tillage in 3-year potato systems has the potential to maintain crop productivity, protect the soil resource, and improve soil quality. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Conservation tillage; Potato; Rotation; Rhizoctonia; Fine sandy loam; Podzol; Soil organic carbon; Soil structure; Eastern Canada

1. Introduction Potato farming systems generally use excess tillage and produce low levels of crop residue in the potato *

Corresponding author. Tel.: þ1-902-566-6869; fax: þ1-902-566-6821. E-mail address: [email protected] (M.R. Carter).

year, both of which are detrimental to soil quality. In Prince Edward Island, in eastern Canada, up to 40% of the agricultural land is committed to potato production on a rotation basis, and this provides 25% of Canada’s total potato production. For most of eastern Canada, and especially for Prince Edward Island, the major form of primary tillage is autumn mouldboard ploughing (Carter et al., 1990; Vyn et al., 1994), although the

0167-1987/01/$ – see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 1 9 8 7 ( 0 1 ) 0 0 2 2 4 - 0

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chisel plough has also been utilized in cereals as a form of conservation tillage (Carter, 1994). Concerns about excessive tillage in potato rotations have generated a need to assess alternative tillage systems, ranging from use of zone tillage (e.g., Pierce and Burpee, 1995) to full width tillage utilizing chisel in place of mouldboard ploughs for the primary tillage (e.g., Carter et al., 1998a). Studies conducted in Norway have indicated that the intensity of tillage in potato rotations can be reduced without any adverse effect on potato yield and quality, although some influence may be evident on potato maturation and optimum harvest date (Ekeberg and Riley, 1996; Riley and Ekeberg, 1998). In Prince Edward Island, use of spring rather than autumn primary tillage and replacement of the mouldboard with a chisel plough, within the potato phase of a 3-year rotation, had no adverse effect on potato yield and quality, and caused little change in soil physical quality (Carter et al., 1998a). Holmstrom et al. (1999) on a range of sites in Prince Edward Island, using short-term studies, showed that adoption of forms of chisel ploughing, as a conservation tillage technique, was feasible in both 2- and 3-year potato rotations. Chisel ploughing tends to concentrate crop residues near the soil surface with potential benefits for organic matter increase, improved soil hydraulic properties, and increased protection from the impact of rainfall and soil movement via erosion. A major concern in potato production is the sustainability of short-term rotations. Long-term studies have indicated that length of crop rotation and frequency of potato cropping has an important bearing on the incidence of plant disease (Hoekstra, 1989). In addition to rotation length, the type of tillage practice may also influence the incidence of potato disease. Gudmestad et al. (1978) and Leach et al. (1993) showed that deep chisel ploughing, in comparison to mouldboard ploughing, significantly reduced both soil populations and the incidence and severity of stem lesions on potato caused by Rhizoctonia solani. It was hypothesized that the retention of crop residue within the surface soil depth (0–10 cm), under chisel ploughing, and the resulting increase in microbial activity and competition among saprophytic organisms may result in a suppression of pathogen activity (Gudmestad et al., 1978; Sturz et al., 1997).

In contrast to cereal or grass rotations where type of tillage can be constant and not related to any specific crop, rotations that include root crops such as potato in combination with cereals or grasses present a varied tillage requirement over time (Keller, 1989; Carter, 1994). Thus, the type of conservation tillage will be crop specific in potato rotations to accommodate both grass and root crops. In addition, for potato rotations in humid temperate climates, conservation tillage needs to be combined with other technologies (e.g., mulching) to ensure adequate soil cover is provided during the cool season after the potato harvest (Carter, 1994). This implies that the evaluation of conservation tillage practices for potato need be conducted on a rotation basis to assess interactions among the mix of crops, types of tillage, and supplemental technologies over time. A study was initiated to assess the feasibility and long-term effects of using conservation tillage practices in combination with crop residue mulches (after the potato harvest) on potato production in 2- and 3-year potato rotations on a fine sandy loam in Prince Edward Island. Conservation tillage in the potato phase involved replacing the conventional autumn mouldboard ploughing with a herbicide treatment followed by reduced tillage in the spring. Specific objectives were to assess the influence of rotation length, tillage system, and soil management on potato productivity and indicators of soil quality. Productivity indicators were potato yield and quality, and incidence of tuber disease, specifically Rhizoctonia (R. solani) and Common Scab (Streptomyces scabies), while indicators of improved soil management were surface residue levels after potato planting, soil organic carbon and organic carbon fractions, and soil structural stability.

2. Materials and methods 2.1. Experimental site Agricultural soils in Prince Edward Island are mainly fine sandy loams developed on acidic glacial till parent material. Similar soils are also found in other regions of Atlantic Canada (e.g., northern Nova Scotia). The annual precipitation ranges from 800 to 1100 mm per annum (Fig. 1), while the soil moisture

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Fig. 1. Growing season precipitation at Harrington Research Farm, Prince Edward Island, over the 6-year period of the study compared to the long-term (110 years) average.

regime and soil temperature is humid to per-humid and cool boreal, respectively. The study was conducted at the Harrington Research Farm (latitudinal/longitudinal: 46.2/63.1) in central Prince Edward Island on a Charlottetown fine sandy loam, an Orthic Podzol (FAO) [clay (<2 mm), silt (2–50 mm), sand (50–2000 mm), and gravel (2–80 mm) content of 165, 280, 534, and 21 g kg1, respectively]. In 1994, a replicated experiment was initiated to assess conservation tillage in both 2- and 3-year potato rotations. The experimental design was a split-plot with five main plots and six replicates, which allowed each phase of a 2- and the 3-year rotation to be present each year. Main plot size was 23 m  5 m. The 2-year rotation consisted of spring barley and potato (cv. ‘Russet Burbank’), and the 3-year rotation was barley (under-sown with red clover), red clover and potato. In the potato year only, the main plots were split to provide conventional and the conservation tillage sub-plots ð23 m  2:5 mÞ treatments. Fig. 2 provides a conceptual overview of the rotations indicating time of crop growth and agronomic practices. Table 1 gives an outline of the management practices.

2.2. Tillage treatments 2.2.1. Potato phase of the rotations In the 3-year conservation tillage rotation, the red clover was sprayed with glyphosate [N-(phosphonomethyl)glycine] at 2 l (356 g l1 a.i.) ha1 in the autumn prior to the potato year and the residue left as a soil cover over the cool season (November–April). For the 2-year rotation, one pass with a chisel plough (15 cm deep) was conducted after the barley harvest in the autumn prior to the potato year. This allowed some regrowth from unharvested grain that provided some degree of live soil cover prior to winter kill of the volunteer barley in the cool season. For the potato phase in both rotations, the conventional tillage consisted of mouldboard ploughing (20 cm deep), in the autumn (October) prior to the potato year, followed by two or more passes (10 cm deep) with a disc and harrow in the spring (May). Conservation tillage consisted of one-pass (15 cm deep) of a chisel plough with wide (36 cm) sweeps in the spring for both rotations. During the potato growing season, both conventional and conservation tillage treatments were subject to the

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Fig. 2. Conceptual outline of tillage and other management practices for conventional (CT) and conservation (MT) tillage in the 2- and 3-year potato rotation.

same in-row cultivation for ridging (hilling), fertilizer and pesticide applications, and harvesting operations. The potato rows were ridged (2–3 times in growing season) using a 2-row ridger, which consisted of five S-shaped tines (to loosen the soil between the rows to 10–20 cm depth) placed ahead of a mouldboard implement that formed the ridges. All cultural practices, including pest control, were similar to those outlined in the Atlantic Canada Potato Guide (1993). Table 1 provides a description

of crop and soil management details for the potato phase. After the potato harvest, in both the 2- and 3-year rotation, a mulch was applied to the soil surface immediately after the potato harvest to provide winter cover. Wheat (Triticum aestivum L.) straw was applied in the 2-year rotation and red clover hay applied in the 3-year rotation. The specific rate (approximately 4 Mg ha1, <150 g kg1 water content) of mulch used corresponds to the traditional mulching rate, which is

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Table 1 Management practices for seed, fertilizer and pesticides in 2- and 3-year rotations, on a fine sandy loam Podzol in Prince Edward Island Management practice

Potato

Barley

Red clover

Seed variety Seed rate (kg ha1) Fertilizer rate (kg ha1)b Herbicide (rate)c

Russet burbank 1800–1900a 1110–1120 Metribuzin (138 g ha1) Diquat dibromide (304 g ha1) Chlorothalonil (405 ml ha1) Endosulphan (324 ml ha1)

Iona 120–126 330–340 MCPA (0.7 l ha1) Nil Nil

Marino 15–18 Nil Glyphosate (2 l ha1) Nil Nil

Fungicide (rate)d Insecticide (rate)e a

Potato planted at 38 cm spacing using Rhizoctonia free, whole seed. Fertilizer applied at planting in potato (N:P:K:Mg at 15:6.6:12.5:2, respectively); fertilizer applied at seeding in barley (N:P:K at 17:7.4:14.1, respectively). c Diquat dibromide applied prior to potato harvest in September to kill potato vines; glyphosate applied in October to kill clover. d Chlorothalonil applied several (8–14) times over the growing season. e Endosulphan applied several (2–4) times over the growing season. b

optimal for reduction of cool season soil erosion (Edwards et al., 1995). 2.2.2. Barley and red clover phase of the rotations In the barley phase of the rotations, the tillage treatment was the same for both rotations. Shallow (10 cm) tillage was employed in the spring after the potato harvest prior to seeding the barley using a hoeseeding drill. In the 3-year rotation, red clover was under-seeded in the barley year. During the clover phase, the crop was cut once (in June) with a flail mower and the residue retained on the plots. 2.3. Crop and residue measurements Potato yields were estimated by harvesting one row (23 m) per sub-plot. As a measure of yield quality, tubers were graded to obtain marketable yield. Barley and red clover yields (on a 150 g kg1 water content basis) were determined by harvesting a 40 m2 area on each plot using a plot combine harvester. Grain mean mass (1000 kernel weight) was determined each year. Red clover above ground biomass yield (generally second cut) was determined in September/October just prior to the glyphosate application. Surface crop residue cover measured after potato planting but before hilling each year was estimated using the tape-transect method (Hartwig and Laflen, 1978), by counting the pieces of plant residue under each mark (2.5 cm spacing) on a 30.5 m tape.

2.4. Potato disease measurements Disease measurements were conducted both in the field and in storage. Field measurements were conducted in early September just prior to die-back of the potato vines. In each plot, the stems, stolons, and roots of 10 plants were sampled and rated (from 1 to 8) for Rhizoctonia disease (R. solani) based on severity of lesions and degree of secondary invasion of the tissue. Disease measurements on potato tubers in storage were conducted both in early to mid-December and in early to mid-March. Thirty tubers from each plot were sampled and 15 rated for disease in the December and March assessments, respectively. Tubers were washed and the tuber surface area covered with Rhizoctonia sclerotia estimated (%), and then rated (from 1 to 4) for sclerotia prominence or thickness. At the same time, tubers were also assessed for common scab (S. scabies) based on surface area covered (%) and rated (from 1 to 3) for depth of scab infection. 2.5. Soil measurements In September 1999, three soil cores ð8 cm inside diameter  8 cm longÞ were obtained from the 0–8 cm soil depth of each plot in the potato phase (within the potato ridge) of each rotation. Soil from each core was removed, weighed and a portion retained at field moisture passed through a 4.75 mm sieve. The three samples from each plot were combined and a subsample obtained for measurement of water content

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(to estimate dry soil bulk density) and microbial biomass carbon. The remainder of the soil was air-dried for determination of structure and organic carbon.

with six replicates (Genstat 5 Committee, 1987). Each crop was analysed separately.

2.5.1. Soil structural stability Soil structural stability was determined using a wetsieving technique (Angers and Mehuys, 1993). A 30 g sub-sample of air-dry soil (<4.75 mm) was placed on a nest of flat sieves (2.00, 1.00, 0.50, and 0.25 mm), then the sieves (15 cm diameter) were placed in a wetsieving apparatus and the soil wet-sieved for 10 min in water. The water stable aggregates on each sieve were corrected for the presence of primary particles. The mean weight diameter (MWD), which is the sum of the proportion of soil on each sieve multiplied by the mean diameter of adjacent sieves, was calculated as an index of soil structural stability.

3. Results 3.1. Crop yield and quality 3.1.1. Total potato yield Moving the time of tillage from the fall to the spring, reducing the degree and intensity of tillage, and maintaining a significant increase of residue cover prior to hilling, did not adversely influence potato yield (Table 2). Total potato yield was similar among rotation and tillage over the 6-year period. Significant differences were periodic and not consistently related to type of management.

2.6. Statistical analysis

3.1.2. Marketable potato yield Comparisons of the marketable potato yield, except for the enhanced yield under the minimum tillage in the first year, showed no differences in the early years of the study (Fig. 3). However, after 3 years significant differences were noted among the various parameters. In the last 2 years a significant decline in marketable yield was evident under the 2-year, compared to the 3-year rotation. In 1998, yield under the 2-year conservation was significantly lower compared to the 2-year conventional tillage system.

Data was subjected to analysis of variance using a split-block model with length of rotation (2- and 3-year) as the main plot and tillage method as the sub-plot

3.1.3. Potato tuber number The 2-year, compared to the 3-year rotation, showed a significant increase in the number of tubers

2.5.2. Soil organic carbon and carbon fractions Soil organic carbon and macro-organic carbon, the latter obtained after vigorous soil shaking with glass beads and sieving (53 mm) (Carter et al., 1998b) were determined by dry combustion using an LECO CNS 1000 analyser. Microbial biomass carbon was determined on field moist soil samples using the chloroform fumigation–extraction method (Voroney et al., 1993).

Table 2 Comparison of crop rotation and tillage on total potato yield (Mg ha1) over 6 years, after three and two cycles in 2- and 3-year rotations, on a fine sandy loam Podzol in Prince Edward Islanda Rotation

Tillage

1994

1995

1996

1997

1998

1999

2-Year

Conventional Conservation

43.1 42.4

39.5 36.9

42.0 39.7

35.8 a 32.5 ab

32.4 27.8

27.8 31.1

3-Year

Conventional Conservation Standard error of the differenceb Significance level (R, rotation) Significance level (T, tillage) Significance level (RT)

44.4 45.4 1.45 0.14 0.86 0.29

36.2 37.3 1.76 0.38 0.46 0.10

40.1 39.8 2.77 0.73 0.20 0.33

35.5 a 29.9 b 1.93 0.38 0.01 0.34

28.5 31.0 2.35 0.85 0.46 0.03

31.3 32.6 3.41 0.22 0.27 0.63

a b

Values in the same column followed by the same letter are not significantly different at p  0:05. Number of observations: 6, error degree freedom: 10.

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Fig. 3. Comparison of crop rotation and tillage on marketable potato yield (Mg ha1) over 6 years, after three and two cycles in 2- and 3-year rotations, on a fine sandy loam Podzol in Prince Edward Island. Values for the same year with the same letter are not significantly different at p  0:05.

(Table 4). In four of the 6 years, barley produced lower grain yield under one or both of the 2-year, compared to the 3-year rotation treatments. Grain mass (1000 kernel weight) varied from 32 to 48 g over the 6-year period, and in three of the 6 years was significantly ðp  0:05Þ greater for the 3-year, compared to the 2-year rotation (data not shown). Tillage differences had no effect on grain mass. Barley straw biomass was also significantly ðp  0:05Þ greater under the 3-year, than under the 2-year rotation (data not shown).

per hectare after 3 years, indicating a decrease in tuber size (Table 3). This change in tuber distribution and decrease in tuber size decreased the processing quality of the potatoes from the 2-year rotation. Tillage differences had no significant effect on either yield or tuber number in either the 2- or 3-year rotation. 3.1.4. Barley and red clover yields Barley yields following the potato phase of the rotations were influenced mainly by rotation length

Table 3 Comparison of crop rotation and tillage on potato tuber number (103 ha1) over 6 years, after three and two cycles in 2- and 3-year rotations, on a fine sandy loam Podzol in Prince Edward Islanda Rotation

Tillage

1994

1995

1996

1997

1998

1999

2-Year

Conventional Conservation

359 331

302 312

313 a 313 a

301 a 278 a

283 a 251 ab

188 181

3-Year

Conventional Conservation Standard error of the differenceb Significance level (R, rotation) Significance level (T, tillage) Significance level (RT)

383 388 29.9 0.20 0.29 0.15

250 295 26.4 0.12 0.16 0.37

263 b 286 ab 17.6 0.03 0.34 0.33

279 a 237 b 18.2 0.07 0.02 0.44

195 b 229 ab 28.6 0.07 0.98 0.05

176 185 13.1 0.83 0.83 0.21

a b

Values in the same column followed by the same letter are not significantly different at p  0:05. Number of observations: 6, error degree freedom: 10.

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Table 4 Comparison of crop rotation and tillage on barley yield (Mg ha1) over 6 years, after three and two cycles in 2- and 3-year rotations, on a fine sandy loam Podzol in Prince Edward Islanda Rotation

Tillageb

1994

1995

1996

1997

1998

1999

2-Year

Conventional Conservation

3.0 2.9

1.8 b 2.0 ab

2.6 b 2.9 a

4.3 4.8

2.7 b 2.5 b

3.1 a 2.9 b

3-Year

Conventional Conservation Standard error of the differencec Significance level (R, rotation) Significance level (T, tillage) Significance level (RT)

3.1 3.0 0.17 0.47 0.50 0.59

2.1 ab 2.2 a 0.13 0.05 0.27 0.82

2.9 a 2.9 a 0.10 0.02 0.07 0.09

4.8 4.5 0.29 0.85 0.50 0.004

3.6 a 3.5 a 0.13 0.001 0.13 0.68

3.4 a 3.4 a 0.24 0.44 0.01 0.66

a

Values in the same column followed by the same letter are not significantly different at p  0:05. In the barley phase of the rotations, shallow tillage followed by seeding with a no-till drill was used for all treatments. Tillage differences (i.e., conventional vs. conservation) relate to the tillage used in the previous year (i.e., the potato phase) of the rotation, when the plots were split. Grain yield is expressed at 150 g kg1 water content. c Number of observations: 6, error degree freedom: 10. b

Red clover biomass yields (representing the second cut) ranged from 6.3 to 11.5 Mg ha1 and did not differ between tillage treatments (Table 5). 3.2. Potato tuber disease Data from the field measurements on Rhizoctonia infection of potato stems, stolons, and roots indicated periodic and slight differences between rotations and tillage practice in the early years of the experiment (data not shown). In the last 2 years (1998 and 1999), however,significantð p < 0:02ÞincreasesinRhizoctonia disease ratings were evident in the field for the 2-year (rating > 5), compared to the 3-year (rating < 5) rotation. Disease assessment of stored potato tubers indicated that common scab infection was relatively low

(severity < 7%; rating < 1:6) and not significantly different among treatments (data not shown). The severity (percent surface area) of Rhizoctonia on harvested tubers increased in the 2-year rotation over time, compared to the 3-year rotation, however, differences due to tillage were not evident until 1999 (Table 6). By contrast, the height of the Rhizoctonia sclerotia was initially similar among treatments. After 1996, however, the ratings for sclerotia height were significantly ðp < 0:02Þ greater under the 2-year (rating > 1:5) than the 3-year (rating > 1:5) rotation (data not shown). Tillage differences were also evident in some years with conservation tillage being at a disadvantage in the 2-year rotation in regard to tuber quality. Evidently, the 2-year rotation would be limited with regard to sustainability by disease pressure. By contrast, the 3-year

Table 5 Comparison of crop rotation and tillage on red clover yield (Mg ha1, dry matter basis) over 6 years, after two cycles in a 3-year rotation, on a fine sandy loam Podzol in Prince Edward Island Rotation

Tillagea

1994

1995

1996

1997

1998

1999

3-Year

Conventional Conservation Standard error of the differenceb Significance level

11.5 11.0 0.72 0.55

7.9 7.1 0.39 0.10

8.7 7.9 0.53 0.17

10.2 9.8 1.05 0.74

8.1 8.7 0.82 0.54

6.3 6.4 0.23 0.80

a In the barley phase of the 3-year rotation, red clover was established by under-seeding and obtained full growth the following year. Tillage differences (i.e., conventional vs. conservation) relate to the tillage used in the potato phase of the rotation, when the plots were split. Red clover yields were obtained in September/October just prior to glyphosate application. b Number of observations: 6, error degree freedom: 5.

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Table 6 Comparison of crop rotation and tillage on Rhizoctonia disease severity (% area of tuber affected by Rhizoctonia) of harvested potato tubers (after 3 months storage) over 6 years, after three and two cycles in 2- and 3-year rotations, on a fine sandy loam Podzol in Prince Edward Islanda Rotation

Tillage

1994

1995

1996

1997

1998

1999

2-Year

Conventional Conservation

4 4

9b 15 a

24 a 31 a

28 a 13 b

39 a 41 a

36 a 28 a

3-Year

Conventional Conservation Standard error of the meanb Significance level (R, rotation) Significance level (T, tillage) Significance level (RT)

4 4 0.5 0.95 0.95 0.81

3b 5b 2.6 0.04 0.19 0.48

6b 6b 5.0 0.003 0.54 0.56

5c 6c 3.1 0.007 0.53 0.51

18 b 4c 3.8 0.001 0.22 0.05

10 b 3b 2.9 0.002 0.03 0.99

a b

Values in the same column followed by the same letter are not significantly different at p  0:05. Number of observations: 6, error degree freedom: 10.

Table 7 Comparison of crop rotation and tillage on surface residue cover (%) over 5 years measured prior to potato planting in 2- and 3-year rotations, on a fine sandy loam Podzol in Prince Edward Island Rotation

Tillage

1994

1995

1996

1997

1998

1999

2-Year

Conservation

16.8

16.6

17.5

38.5

31.7

NDa

3-Year

Conservation Standard error of the meanb Significance level

26.0 1.37 0.005

29.3 0.83 0.001

21.5 0.69 0.009

37.1 4.48 0.84

27.0 2.67 0.27

ND

a b

Not determined. Number of observations: 6, error degree freedom: 5.

Fig. 4. Comparison of soil water stable aggregate (WSA) size distribution after 6 years, after three and two cycles in 2- and 3-year rotations, on a fine sandy loam Podzol in Prince Edward Island. Values for the same aggregate size with the same letter are not significantly different at p  0:05.

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rotation had relatively low and similar levels of disease between tillage systems, except for the last 2 years when conservation tillage showed a disease reduction compared to the conventional tillage. 3.3. Surface residue levels Residue levels at the soil surface in the spring after potato planting reflected the differences in both rotation and tillage system. Mouldboard plough systems subject to fall primary tillage and intensive spring tillage had no measurable residue on the soil surface (data not shown), however, relatively high residue was present in the conservation tillage systems (Table 7). In the early years percentage residue cover was higher in the 3-year conservation tillage (following a red clover phase), compared to the 2-year conservation tillage (following a barley phase). Overall, the mean and standard deviation for residue cover (%) in the 2- and 3-year conservation tillage was 24:2  10:2 and 28:2  5:7, respectively. 3.4. Soil structural stability After 6 years, even though a significant amount of soil disturbance occurred in the potato phase of the rotation, soil structural stability was improved under the conservation tillage systems, compared to the conventional (Figs. 4 and 5). Conservation tillage increased the soil stability to water of the >2.00 mm

Fig. 5. Comparison of soil structural stability MWD after 6 years, after three and two cycles in 2- and 3-year rotations, on a fine sandy loam Podzol in Prince Edward Island. Values for the same aggregate size with the same letter are not significantly different at p  0:05.

sized aggregates, compared to conventional tillage (Fig. 4). This was also expressed by the water stable aggregate MWD, which was significantly increased under conservation tillage, especially in the 3-year potato rotation (Fig. 5). Previous studies on the same soil type have shown that increases in soil organic matter and organic carbon fractions are the main factors influencing size distribution of water stable aggregates (Carter, 1992).

Table 8 Comparison of crop rotation and tillage on soil organic carbon, organic carbon fractions, and dry bulk density sampled just prior to potato harvest in the 6th year, after three and two cycles in 2- and 3-year rotations, on a fine sandy loam Podzol in Prince Edward Islanda Rotation

Tillage

Organic C (g kg1)

Macro-organic C (g kg1)

Microbial biomass C (mg g1)

Dry bulk density (Mg m3)

2-Year

Conventional Conservation

19.2 c 20.7 b

11.4 c 13.6 b

530 b 730 a

1.08 a 1.03 b

3-Year

Conventional Conservation Standard error of the meanb Significance level (R, rotation) Significance level (T, tillage) Significance level (RT)

20.3 bc 23.7 a 0.44 0.005 0.001 0.10

12.3 bc 17.2 a 0.69 0.01 0.004 0.19

570 b 722 a 45.7 0.59 0.007 0.66

1.03 b 0.97 c 0.013 0.01 0.006 0.77

a b

Values in the same column followed by the same letter are not significantly different at p  0:05. Number of observations: 6, error degree freedom: 10.

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3.5. Soil organic carbon and carbon fractions After 6 years, significant increases were evident in both soil organic carbon under the conservation tillage, compared to the conventional tillage systems. Conservation tillage, in both rotations, increased the concentrations of soil organic carbon and macroorganic carbon (Table 8). The latter, a measure of transitory organic matter between recent fresh residue and humified material, represented a mean of 63 and 66% of the soil organic carbon in the conventional and conservation tillage, respectively. The 3-year conservation tillage system showed greater increases in organic carbon and macro-organic carbon, compared to the 2-year conservation tillage system. Conservation tillage, in both rotations, increased the level of soil microbial biomass carbon. Soil bulk density, which was not limiting for crop growth in this soil type (Carter et al., 1998a), was significantly decreased by both the increase in rotation length and the use of conservation tillage.

4. Discussion Major attributes of conservation tillage in humid regions, subject to high precipitation and an extensive cool season, would be the continuum of soil cover over the relatively long winter period and the use of mulches and residue management (Carter, 1994). In this study, shifting the time of tillage from the fall to the spring and the use of mulches after the potato harvest presented a viable and practical form of conservation tillage for potato rotations. Except for the 6-month period after the potato harvest, when mulches were applied to provide soil cover, the soil was

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protected by live (i.e., crop canopy) or dead (i.e., crop residue) cover. In the case of the latter, where the red clover was treated with glyphosate in the fall, the surface cover approached or met the commonly recognized indicator or target (i.e., 30%) for conservation tillage even when measured after crop emergence just prior to hilling. This study clearly showed the benefit of 3-year over 2-year rotations for potato production. The latter rotation soon developed problems with disease severity and consequently tuber quality. Potato yield differences, between the 2- and 3-year rotation, however were only evident after 4 years (Fig. 3). Comparison of tuber quality parameters, such as the number of offtype tubers (i.e., misshapen tubers separated at tuber sizing) and Rhizoctonia severity, expressed as a ratio between rotations, provided a clear indication of the poor sustainability of the 2-year, compared to the 3-year rotation (Table 9). Rotation differences were also evident in the barley yields (Table 4), which were probably related to greater organic nitrogen (i.e., red clover residues) input under the 3-year, compared to the 2-year rotation. Generally, tillage differences did not influence yield or yield parameters. The 6 year average for marketable yield was 24:6  6:9 and 23:7  6:9 Mg ha1 , respectively for the 2-year conventional and conservation tillage, and 26:8  5:3 and 26:6  5:9 Mg ha1 , respectively for the 3-year conventional and conservation tillage. Thus, the 3-year conservation tillage produced similar yields as the conventional system, but had the added benefits of less tillage inputs and improved soil conservation potential. Improvements in the soil biological and physical condition, at the surface depth, under conservation tillage would be potentially beneficial for both protection of the soil resource from

Table 9 Comparison of ratio of off-type potato tubers and Rhizoctonia severity for the 2- and 3-year rotations, over time, on a fine sandy loam Podzol in Prince Edward Island Rotation

Parameter a

2-Year:3-year a

1994 b

Off-type tubers Rhizoctonia severity

1.2 1.0

1995 2.1 2.0

1996 2.1 4.6*

1997 1.0 4.1*

Ratio of 2-year rotation to 3-year rotation for both number of off-type potato tubers and Rhizoctonia severity. Misshapen tubers separated at tuber sizing. * Indicates significant increases ðp < 0:05Þ. b

1998 *

12.5 3.5*

1999 1.9* 5.0*

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erosive forces and enhancing soil water holding capacity. The latter is an important soil attribute for generally coarse to medium textured soils. Recent studies conducted in Prince Edward Island on fine sandy loams showed that potato yield was related to soil water retention, which was associated with changes in soil organic matter (DeHaan et al., 1999). Studies conducted in Prince Edward Island with conservation tillage for spring cereals have shown that elevated numbers of saprophytic trash micro-flora, concentrated in the residue rich surface depth of conservation tilled soils, may inhibit Rhizoctonia activity (Sturz and Carter, 1995). It is interesting to note that the lowest severity of Rhizoctonia in this study was generally found under the residue rich 3-year conservation tillage treatment. Sturz et al. (1997), in a review of the literature, showed that the ability to induce disease suppressiveness in soil is linked in creating aggressive competition among soil microbes in the root zone, a phenomenon found under cultural practices such as mulching and addition of organic amendments. The importance of tillage method is probably linked to the placement and distribution of residues. More research is needed to elucidate the relation between residue and potato health in conservation tillage systems. 5. Conclusions The 2-year, compared to the 3-year potato rotation, significantly changed the distribution in tuber size, increased the proportion of off-type tubers, and increased the severity of Rhizoctonia disease on harvested tubers. Overall, rotation length was the dominant factor influencing potato production. The longterm results indicate that conservation tillage in a 3-year rotation had no detrimental effect on potato growth and quality; has the potential to improve soil properties such as structure and organic matter at the soil surface; and appears to be a promising form of conservation tillage for intensive potato production systems in Atlantic Canada. Acknowledgements The authors are thankful for technical assistance provided by Agriculture and Agri-Food Canada technical

staff. This paper is Contribution No. 965 of the Charlottetown Research Centre.

References Angers, D.A., Mehuys, G.R., 1993. Aggregate stability to water. In: Carter, M.R. (Ed.), Soil Sampling and Methods of Analysis. Can. Soc. Soil Sci. Lewis Publishers/CRC Press, Boca Raton, FL, pp. 651–657. Atlantic Canada Potato Guide, 1993. Atlantic Provinces Agricultural Services Coordinating Committee, Pub. No. 1300/93. Carter, M.R., 1992. Influence of reduced tillage systems on organic matter, microbial biomass, macro-aggregate distribution and structural stability of the surface soil in a humid climate. Soil Till. Res. 23, 272–361. Carter, M.R., 1994. A review of conservation tillage strategies for humid temperate regions. Soil Till. Res. 31, 289–301. Carter, M.R., Kunelius, H.T., White, R.P., Campbell, A.J., 1990. Development of direct drilling systems for sandy loam soils in the cool humid climate of Atlantic Canada. Soil Till. Res. 16, 371–387. Carter, M.R., Sanderson, J.B., MacLeod, J.A., 1998a. Influence of time of tillage on soil physical attributes in potato rotations in Prince Edward Island. Soil Till. Res. 49, 127–137. Carter, M.R., Gregorich, E.G., Angers, D.A., Donald, R.G., Bolinder, M.A., 1998b. Organic C and N storage, and organic C fractions, in adjacent cultivated and forested soils of eastern Canada. Soil Till. Res. 47, 253–261. DeHaan, K.R., Vessey, G.T., Holmstrom, D.A., MacLeod, J.A., Sanderson, J.B., Carter, M.R., 1999. Relating potato yield to the level of soil degradation using a bulk yield monitor and differential global positioning systems. Comput. Electron. Agric. 23, 133–143. Edwards, L.M., Burney, J., DeHaan, R., 1995. Researching the effects of mulching on cool-period soil erosion control in Prince Edward Island, Canada. J. Soil Water Conserv. 50, 184–187. Ekeberg, E., Riley, H.C.F., 1996. Effects of mouldboard ploughing and direct planting on yield and nutrient uptake of potatoes in Norway. Soil Till. Res. 39, 131–142. Genstat 5 Committee, 1987. Genstat 5 Reference Manual. Oxford University Press, Oxford. Gudmestad, N.C., Huguelet, J.E., Zink, R.T., 1978. The effect of cultural practices and straw incorporation into the soil on Rhizoctonia disease of potato. Plant Dis. Rep. 62, 985–989. Hartwig, R.O., Laflen, J.M., 1978. A meterstick method for measuring crop residue cover. J. Soil Water Conserv. 32, 90–91. Hoekstra, O., 1989. Results of twenty-four years of crop rotation research at ‘De Schreef’ experimental site. In: Vos, J., Van Loon, C.D., Bollen, G.J. (Eds.), Effects of Crop Rotation on Potato Production in the Temperate Zones. Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 37–43. Holmstrom, D.A., DeHaan, R., Sanderson, J.B., MacLeod, J.A., 1999. Residue management for potato rotation in Prince Edward Island. J. Soil Water Conserv. 54, 445–448.

M.R. Carter, J.B. Sanderson / Soil & Tillage Research 63 (2001) 1–13 Keller, E.R., 1989. Crop rotation — an important aspect in integrated potato production. In: Vos, J., Van Loon, C.D., Bollen, G.J. (Eds.), Effects of Crop Rotation on Potato Production in the Temperate Zones. Kluwer Academic Publishers, Dordrecht, The Netherlands, pp. 291–301. Leach, S.S., Porter, G.A., Rourke, R.V., Clapham, W.M., 1993. Effects of moldboard plowing, chisel plowing and rotation crops on the Rhizoctonia disease of white potato. Am. Potato J. 70, 329–337. Pierce, F.J., Burpee, C.G., 1995. Zone tillage effects on soil properties and yield and quality of potatoes (Solanum tuberosum L.). Soil Till. Res. 35, 135–146. Riley, H., Ekeberg, E., 1998. Effects of depth and time of ploughing on yields of spring cereals and potatoes and on soil properties of a morainic loam soil. Acta Agric. Scand. B 48, 193–200.

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Sturz, A.V., Carter, M.R., 1995. Conservation tillage systems, fungal complexes and disease development in soybean and barley rhizospheres in Prince Edward Island. Soil Till. Res. 34, 225–238. Sturz, A.V., Carter, M.R., Johnston, H.W., 1997. A review of plant disease, pathogen interactions and microbial antagonism under conservation tillage in temperate humid agriculture. Soil Till. Res. 41, 169–189. Voroney, R.P., Winter, J.P., Beyaert, R.P., 1993. Soil microbial biomass C and N. In: Carter, M.R. (Ed.), Soil Sampling and Methods of Analysis. Can. Soc. Soil Sci. Lewis Publishers/ CRC Press, Boca Raton, FL, pp. 277–286. Vyn, T.J., Janovicek, K., Carter, M.R., 1994. Tillage requirements for annual crop production in eastern Canada. In: Carter, M.R. (Ed.), Conservation Tillage in Temperate Agroecosystems. Lewis Publishers/CRC Press, Boca Raton, FL, pp. 47–71.