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Published online 8 January 2009
Published in Agron J 101:82-90 (2009)
DOI: 10.2134/agronj2007.0031
© 2009 American Society of Agronomy
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Potato Yield and Quality Response to Subsoil Tillage and Compaction

Michael E. Copasa, Alvin J. Bussana,*, Michael J. Driliasa and Richard P. Wolkowskib

a Dep. of Horticulture, University of Wisconsin-Madison, Madison, WI 53706
b Dep. of Soil Sci., Univ. of Wisconsin-Madison, Madison, WI 53706

* Corresponding author (ajbussan{at}wisc.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Compacted soils have been found in intensively cultivated vegetable crop regions of Central Wisconsin, resulting in the wide scale use of subsoil tillage by growers. The goal of this project was to assess potato (Solanum tuberosum L.) yield and quality response to soil compaction and subsoil tillage. Potato quality factors evaluated were marketable yield, tuber size distribution, internal quality, and sugar concentration. A controlled small plot experiment and several field scale experiments located in collaborating grower fields were conducted to assess potato and soil responses to subsoil tillage. Cone index profiles showed the potential for limited root growth below the compacted soil layer with values >2.0 MPa. Subsoil tillage reduced cone index values to <1.0 MPa below 33 cm in 2 of 3 yr. Total and U.S. no. 1 yields were not influenced by subsoil tillage. Likewise, no consistent differences were seen in the size distributions of U.S. no. 1 tubers across treatments, but subsoil tillage tended to decrease proportion of tubers 113 to 170 g. Subsoil tillage did not affect tuber glucose or sucrose concentrations at harvest or following storage for 120 d. Internal tuber defects were not affected by either compaction or subsoil tillage. The lack of consistent effects of subsoil tillage on potato yield raises questions regarding the validity of this practice. The recommendation that potato growers use subsoil tillage may be linked to increased tuber size distribution or factors other than yield such as water, nutrient, or disease management.

Received for publication January 18, 2007.
   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
SOIL COMPACTION in irrigated vegetable cropping systems has increased in frequency of occurrence and intensity in sandy soils of Central Wisconsin. Compacted layers were commonly found in sandy soils through measurements of bulk density and cone index (Hilfiker and Lowery, 1988; Keisling et al., 1995; Laboski et al., 1998; Tanner et al., 1982). The compacted soil layer occurred at 22 to 23 cm below the surface and was approximately 5 to 20 cm thick. The compacted layer was partially caused by shear forces from plowing at the same depth on coarse soils (Tanner et al., 1982). Wheel traffic also led to increased bulk density and reductions in infiltration rates causing compacted surface and subsoil layers in sand soils (Meek et al., 1992, Hilfiker and Lowery, 1988). Wheel traffic has been a major concern on sand soils of Wisconsin due to intensive management and short crop rotations with processing vegetables that rely on large equipment in mechanized harvest operations. Potato and corn (Zea mays L.) rooting were limited on irrigated sand soils below 30 cm suggesting that the compacted layer restricted access to subsoil moisture (Hilfiker and Lowery, 1988; Laboski et al., 1998; Tanner et al., 1982). Plants responded to compacted soil layers by shortening root lengths, altering root architecture, and increasing root density in less compact areas of the soil profile.

Cone penetration resistance or cone index has become the primary means of gauging the level of compaction in soils. The constant rate cone penetrometer developed by Lowery (1986) successfully quantified the cone index of compacted soils. Increasing cone index was related to decreasing plant rooting and yield across several crops (Busscher et al., 2000; Hilfiker and Lowery, 1988; Keisling et al., 1995; Lowery, 1968; Laboski et al., 1998; Lowery and Schuler, 1994). Corn roots could not penetrate soils when resistance exceeded 1.8 MPa (Barley et al., 1965).

The presence of a highly compacted zone just below the plow layer reduced yield and quality in potato and other commercial vegetable crops under different soil types and environments (Flocker et al., 1960; Timm and Flocker, 1966; van Loon and Bouma, 1978; van Loon et al., 1985; Wolfe et al., 1995). van Loon and Bouma (1978) found soil compaction limited water availability during periods of high evapotranspiration. As a result, secondary tuber growth was initiated when water stress was elevated leading to higher cull rates, smaller tubers, and lower marketable yield. Soil compaction can limit potato growth and development at different tuber development stages resulting in reduced yield and quality (Lesczynski and Tanner, 1976; Stahlman and Allen, 2001). Factors contributing to effects of soil compaction include: reduced soil porosity leading to lower water holding capacity, lower soil 02 concentration, lower hydraulic conductivity that limited water movement to roots during high evapotranspiration periods, reduced diffusion of nutrients, and mechanical resistance to root growth and elongation leading to a limited area for both water and nutrient uptake (Hilfiker and Lowery, 1988; Laboski et al., 1998; Lowery and Schuler, 1994; Wolkowski, 1990).

Slow plant emergence under compacted soils has been documented in corn (Laboski et al., 1998; Lowery and Schuler, 1991) and potato (Blake et al., 1960; Timm and Flocker, 1966). Final plant stands were not affected in either crop, but compaction delayed emergence 4 to 8 d. Compacted soils reduced total aboveground biomass in potato (Timm and Flocker, 1966; van Loon and Bouma, 1978), cabbage (Brassica oleracea L.), snap bean (Phaseolus vulgaris L.), cucumber (Cucumis sativus L.), and sweet corn (Wolfe et al., 1995), and plant height of field corn (Lowery and Schuler, 1991).

The use of subsoil tillage has been recommended by potato agronomists to alleviate the effects of compacted layers on potato productivity across the United States (Stark and Love, 2003). Subsoil tillage has not consistently improved yields of potato in compacted soils. Holmstrom and Carter (2000) found no evidence of improved potato yield or quality, and only marginal improvements in soil bulk density when subsoil tillage was used to loosen compacted subsoils. Potato yield was only marginally affected by subsoil tillage because proper irrigation and water management alleviated any potential benefits (Alva et al., 2002; Halderson et al., 1993; Ibrahim and Miller, 1989; Miller and Martin, 1990; Ross, 1986; Tanner et al., 1982).

The goal of this project was to assess potato yield and quality response to soil compaction and subsoil tillage. Specific objectives were to (i) determine the impact of subsoil tillage on cone penetration resistance of a compacted soil, (ii) quantify potato yield and quality response to compaction and subsoil tillage, and (iii) verify results on the influence of subsoil tillage on potato yield and quality in field scale experiments. Potato quality factors evaluated were marketable yield, tuber size distribution, internal quality, and tuber sugar concentration.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Multiple experiments were conducted to evaluate potato response to subsoil tillage and compaction, root distribution, storability, and development of tuber diseases. This paper focuses on small plot and field scale trials designed to quantify crop responses to subsoil tillage and compaction.

Small Plot Experiment
Small plot experiments were conducted from 2003 to 2005 at the University of Wisconsin-Madison, Hancock Agricultural Research Station (44°8'23'' N, 89°31'23'' W, elevation: 328 m). The soil type was Plainfield loamy sand (sandy, mixed, mesic, Typic Udipsamments). Field corn was the previous crop each year. The experimental site was initiated on a new site within the same field each year of the project to minimize changes in soil characteristics. The experimental design was a randomized complete block with a split-plot factorial treatment arrangement and eight replications.

Whole plot treatments were the presence or absence of supplemental compaction. Subplot treatments consisted of either conventional tillage or conventional tillage supplemented with subsoil tillage. Both compaction and subsoil tillage treatments were performed in the spring before planting the experiment. Compaction treatments were implemented within 1 to 2 d of precipitation events to ensure soils were near field capacity. Supplemental compaction was achieved by repeatedly driving over the specified plots with a front end loader, wheel track to wheel track, until the entire plot was covered in 2003. The bucket of the front end loader was filled with gravel to increase weight on the forward axle. The total weight of the front end loader and gravel was 14.26 Mg. In 2004 and 2005, a dump truck loaded with limestone gravel was driven in a similar pattern over the plots to increase the level of compaction. The weight of the truck and limestone totaled 29.84 and 26.16 Mg in 2004 and 2005, respectively, but no measurement of pressure per unit area was determined. The wheel track compaction was designed to mimic high traffic levels by heavy trucks and other equipment used in annual vegetable and potato rotations.

Subsoil tillage was completed with a Sunflower 4000 series disc ripper (Sunflower Manufacturing, Beloit, KS). Depth of tillage was between 38 and 46 cm. All plots were prepared for planting via moldboard plow and culti-packer following subsoil tillage. Potato (var. Russet Burbank) was planted each year of the study. Seed tubers were machine cut into 65 to 75 g pieces and allowed to suberize for 1 wk before planting. Plots were machine planted at a depth of 12 to 15 cm, at an in-row seed piece spacing of 35.5 cm, and rows spaced 91.4 cm apart. In 2003, plots consisted of six rows, 6.1 m in length. In 2004 and 2005, plots consisted of eight rows, 6.1 m in length to accommodate more extensive destructive in-season sampling. Starter fertilizer consisted of 6–24–24 impregnated with imidacloprid {1-[(6-Chloro-3-pyridinyl)methyl1]-N-nitro-2-imidazolidinimine}, at a rate of 616 kg ha–1. The starter fertilizer was applied in bands on either side of the potato seed piece. Nitrogen (21–0–0) and calcium sulfate were broadcast at emergence at rates of 392 and 560 kg ha–1, respectively. Hilling of potatoes was conducted 2 wk following emergence with a supplemental fertilization of 34–0–0 at 420 kg ha–1. Two additional N applications (56 kg ha–1 each) were applied via irrigation in June of 2004 based on petiole nitrate tests collected after side-dress fertilizer applications were made. May and June precipitation in 2004 was 150% of normal causing nitrate leaching (Table 1 ). Supplemental irrigation with a traveling gun started in early June and continued through early September following vine desiccation.


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  		Table 1. Climatic data as collected from Hancock Agricultural Research Station, Hancock, WI. Irrigation values apply to the small plot experiment only. Values are given by year and mean of previous 20 yr is included.

 
Linuron {3-[3,4-dichlorophenyl]-1-methoxy-1-methylurea} was applied in early May for weed control. Sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} was used for late season grass control, as needed, at a rate of 2.34 L ha–1. Weekly spraying of fungicides began once predetermined thresholds were reached for early blight and late blight and continued until vine desiccation. Fungicides were a combination of chlorothalonil (tetrachloroisophthalonitrile) and azoxystrobin {methyl (E)-2-{2-[6-(2-cyanophenoxy) pyrimidin-4-yloxy]phenyl}-3-methoxyacrylate}. Vine desiccation was accomplished with split applications of diquat dibromide [6,7-dihydrodipyrido (1,2-a:2',1'-c) pyrazinediium dibromide] at 4 and 2 wk before harvest.

A portable constant-rate cone penetrometer, as described by Lowery (1986), was used to quantify penetration resistance. The penetrometer was constructed with specifications according to ASAE standards (30° circular cone, base area of 129 mm2, 9.5 mm diam. shaft, and a constant penetration rate of 30.5 mm s–1) (ASAE, 1983). Weights were used to calibrate the load cell before use in each experiment. All sampling procedures were consistent with those described by Lowery (1986).

Penetrometer readings were taken 1 to 2 wk after plant emergence. Penetration resistance was measured from the center of the hill at four random locations within each plot. Measurements were recorded every centimeter of depth from the surface to a maximum of 40 cm. Measurements were not taken below 40 cm due to the physical limitations of the penetrometer. The penetrometer was relocated and measurements retaken when the cone was impeded by stones. Readings were collected within 6 to 12 h of precipitation or irrigation events each year.

Potato tubers were harvested the second week of September from a single plot row using a research model harvester. Tubers were washed and graded according to industry size categories of: <113, 113 to 170, 170 to 283, 283 to 368, 368 to 453, and >453 g. B-sized potatoes (tubers <4.75 cm in diameter) and cull potatoes (including rotted, off-shaped, growth cracked, sunburned, and green tubers) were removed and weighed separately before size grading.

Specific gravity of potatoes was determined by obtaining approximately 2 to 4 kg of potato and weighing them in air in a suspended basket. The basket was then lowered into a water tank and weighed again. Specific gravity was calculated using the following formula:

Formula
A subsample of 10 tubers was taken from the mid-sized (170–283 g) tubers to estimate proportion of internal defects. Tubers were cut in half longitudinally and examined for physiological or pathological quality limiting internal disorders brown center, hollow heart, internal brown spot, and vascular discoloration.

Two subsamples of six tubers were collected for postharvest sugar analysis. The first subsample was analyzed immediately following harvest. The second subsample was placed in a production scale storage facility and managed according to recommended practices. Potato were preconditioned for 2 wk after harvest at plenum set point of 12.8°C, fan speed necessary to maintain pile temperature difference of 0.7°C, relative humidity of 95%, and enough outside air to maintain carbon dioxide levels in the storage atmosphere below 2000 µL/L. Potatoes were cooled to plenum set point of 8.4 by lowering plenum temperature 0.05°C every 12 h while all other parameters were maintained as stated above. Outside air was used for cooling when wet bulb temperatures were low enough to cool the pile. Refrigeration was used for cooling when outside air temperatures were too warm. The subsample was removed from storage and sugars were analyzed after 120 d. Glucose and sucrose concentrations were measured using procedures based on Sowokinos et al. (2000). The terminal 2.5 to 5 cm on the bud and stem ends of six potatoes from each plot (200 g) were juicerized separately with a model 6001 Acme Supreme Juicerator in 50 mmol/L sodium phosphate buffer, pH 7.2. The supernatant solution was brought to a final volume of 275 mL. Glucose and sucrose concentrations of the supernatant were measured with a YSI 2700 select analyzer using grade VII invertase from yeast (Sigma-Aldrich, St. Louis, MO) as per manufacturers recommendations.

Field-Scale Experiment
Field-scale experiments were conducted from 2002 to 2004 to quantify subsoil tillage effects on yield and quality. Potato growers in Central Wisconsin interested in the outcomes of the project agreed to help conduct large-scale experiments in their potato fields. Production fields were chosen based on their proximity to the research station and with a history of tuber disease issues and storability problems. Each factor could be influenced by subsoil compaction. All fields were located within a 25 km radius of the University of Wisconsin Agricultural Research Station at Hancock. Plainfield sand was the primary soil type within each cooperator's field. Slight variations from field to field were due to nature of glacial outwash and erosion of topography causing slight variations in organic matter and fine soil particulates. The design of each field-scale experiment was a randomized complete block with three replications. Six different fields were studied over 3 yr (1 in 2003, 3 in 2004, and 2 in 2005). Russet Burbank was the variety used in all cases except for Paramount Farms in 2004, which planted Frito Lay 1879. Fertilizer and irrigation management strategies used at each site were based on recommendations developed by researchers at the University of Wisconsin (Boerboom et al., 2006; Kelling et al., 1998).

Conventional tillage treatment varied by grower, but typically included primary tillage with a chisel plow followed by a soil finishing implement. Conventional tillage was typically no deeper than 30 cm. The subsoil tillage treatment followed the same practices as the conventional treatment, but included the addition of subsoil tillage during the autumn before planting. Subsoil tillage was done with several different tillage tools, but depth was maintained across fields between 40 and 50 cm. Field sizes ranged between 16 and 32 ha. Tillage strips were between 400 and 800 m in length by 11 m in width.

Subplots were established within each plot following emergence. Each subplot included five plants and the distance between the first and fifth plant was recorded. Subplots were spaced evenly across the length of the tillage strip. Sprayer track lanes and center pivot irrigation wheel tracks were avoided; around which highly variable microclimates and severe compaction could occur. Yield was sampled from 12 subplots within each field scale trial except for two of the three experimental sites in 2004 where only four subplots were sampled. Only four subplots could be sampled in 2004 due to time limitations imposed by grower's commercial harvest.

A constant rate cone penetrometer was used as described previously to develop cone index profiles in each field. Soil samples were collected at the time cone index data was collected and gravimetric moisture content determined in each field. Cone index values were only collected in two subplots per plot due to difficulty in transporting the apparatus beyond the end of the fields. Final harvest was coordinated with the cooperating growers and all subplots were hand harvested before field digging operations. The harvested subplots were graded at the Hancock Research Station for yield, quality, size distribution, specific gravity, and internal defects, as described in the small plot experiment.

Sugar analysis was conducted via the same methods as described in the small plot experiment with the Russet Burbank fields. For the chipping variety from the Paramount Farms site in 2004, orientation of bud and stem end was disregarded, and a whole tuber sample was taken from fresh weight equal to 200 g. Sampling for sugars was only conducted once following a storage period of 120 d.

Data Analyses
Data from each penetration measurement was compiled into a single array at 3 cm intervals to a maximum depth of 45 cm. Penetration resistance values were natural log transformed and subjected to ANOVA at each depth interval. Log transformation was completed to account for potential variation in data between years due to differences in soil moisture content (Hilfiker and Lowery, 1988; Laboski et al., 1998; Lowery and Morrison, 2002). Data were considered statistically significant when P values were 0.05 or lower. Main effect differences tested were the presence or absence of supplemental compaction and conventional tillage vs. subsoil tillage (conventional tillage with the addition of subsoil tillage).

In the small plot study, data were subjected to an orthogonal contrast to limit Type I and Type II errors. Main effect differences in the contrast compared subsoil tillage, soil compaction, and a linear contrast. The tillage contrast compared conventional and subsoil tillage treatments. The soil contrast compared the presence or absence of supplemental compaction. The linear contrast evaluated crop response to increasing levels of penetration resistance seen in the treatments (Fig. 1 ). Potato yield and quality data were pooled across years due to limited effects across repeated small plot experiments.


Figure 1
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  		Fig. 1. Cone penetration resistance (cone index) profiles under potato as influence by supplemental compaction and subsoil tillage in small plot trials on a Plainfield Loamy Sand near Hancock, WI, in 2003, 2004, and 2005.

 
Data in the field-scale experiment were subjected to pairwise comparison of means of variance to test for the main effect of subsoil tillage. Analysis of cone index was similar to methods described for small plot experiment. Potato yield and quality data were pooled across years and sites due to limited effects across field-scale trials.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Small Plot Experiment
Penetrometer Results
Soil penetrometer results showed compaction increased cone index values 18 cm below the surface of the hill in 2003 and 2004 (Fig. 1). Cone index increased at 18 cm from the top of the potato hill and extended through the full depth of the measured profile of 45 cm. Log transformation allowed for comparisons of cone index profiles. Gravimetric soil moisture data was lost requiring the transformation for comparison of treatments. The natural log of 1 MPa (1000 kPa) was 6.9 and 3 MPa was 8.0. In 2003, cone index increased (P > F = 0.05) in compacted plots vs. noncompacted treatments (Fig. 1). The cone index increased in compacted soil with depth to a maximum of more than 3.0 MPa and was greater than noncompacted treatments (P > F < 0.01). Subsoil tillage disrupted the compacted soil from 18 to 21 cm and decreased cone index to 1.0 MPa in 2003. As depth increased so did differences in cone index between subsoil and conventional tillage treatments to a maximum reduction of 0.58 MPa at 39 cm. Resistance values >1.0 MPa were seen at shallower depths under conventional compared to subsoil tillage. Interactions were limited and primarily evident in the soil surface where maximum penetration resistance was <1.0 MPa, and not likely to influence potato growth.

Compacted treatments had higher cone index values than noncompacted treatments at multiple depths >15 cm in 2004 (Fig. 1). Subsoil tillage reduced cone index, but only at depths >33 cm. Cone index values exceeding 1.0 MPa were evident below 25 cm in depth. A strong increase in cone index from 25 to 40 cm followed by a sharp decrease indicated the likely position of a plow pan from tillage during previous production years.

Cone index profiles during 2005 were similar to patterns seen in 2003 and 2004, but values were lower in the noncompacted soils (Fig. 1). Compaction appeared to cause similar effects on penetration resistance from 18 to 33 cm in 2005 as 2003 and 2004, but differences were minimal. Subsoil tillage reduced resistance to penetration at 33 and 36 cm. Cone index did not exceed 1.0 MPa until depths of 36 cm or greater in compacted treatments without subsoil tillage. Some interaction effects of compaction and subsoil tillage were seen at 27 to 33 cm due to low cone index values in noncompacted treatments. Cone index was <1.0 MPa, suggesting little effect of the interaction of compaction and subsoil tillage on potato growth.

Potato Yield and Quality
No differences were observed in total yield across treatments over the 3 yr the study was conducted (Table 2 ). Compaction effects on the proportion of U.S. no. 1 potato yield during these years were not evident, possibly due to variations in the amount of culls and B-sized tubers from year to year. The cull rate was not affected by compaction or subsoil tillage. Specific gravity was not consistently influenced by either compaction or subsoil tillage treatments. There was a difference in 2003 where subsoil tillage reduced specific gravity, but this did not manifest itself in the following seasons and was not evident in the combined analysis (data not shown).


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  		Table 2. Potato yield, yield proportions, and specific gravity as influenced by compaction and subsoil tillage in small plot study averaged across 2003–2005.

 
No differences in the proportional size distribution of U.S. no. 1 potato yield were seen (Table 3 ). Subsoil tillage decreased the proportion of 113 to 170 g tubers across all 3 yr (P = 0.0740). Compaction or subsoil tillage had no affect on proportion of tubers within all other size categories. In most cases the proportions of U.S. no. 1 tubers were highly variable, across years and treatments.


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  		Table 3. Size distribution of U.S. no. 1 tubers as influenced by compaction and subsoil tillage in small plot study at Hancock, WI, averaged from 2003 to 2005.

 
Analysis of the sugar concentrations at harvest and after 120 d in storage showed no response in tuber sucrose or glucose levels across all years (Table 4 ). Soil compaction did not affect tuber sugar levels. Subsoil tillage did not affect tuber sugar concentrations either. The most prevalent internal tuber defect was the combined response of brown center and hollow heart (Table 5 ). Brown center and hollow heart were analyzed together because brown center is generally accepted to be the precursor of hollow heart in Russet Burbank (Rex and Mazza, 1989). Brown center and hollow heart were found to be higher following subsoil tillage.


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  		Table 4. Sucrose and glucose concentrations in the bud and stem end of tubers at harvest and following 4 mo in storage as influenced by compaction and subsoil tillage in small plot study at Hancock, WI averaged from 2003 to 2005.

 

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  		Table 5. Proportion of internal disorders within marketable U.S. no. 1 tubers as influenced by compaction and subsoil tillage in small plot study averaged from 2003 to 2005.{dagger}

 
Field-Scale Experiment
Penotrometer Results
Cone index values increased with soil depth in field scale trials (Fig. 2 ). Cone index values were only presented for two field sites as they represented patterns observed across all six site years. Gravimetric soil moisture content and soil bulk density values indicated that loamy sand soils within these experiments were at field capacity when cone index values were collected (data not shown). Cone index was below 1.0 MPa (6.9 ln kPa) through the top 25 cm of the soil profile. In 2004, cone index values exceeded 3.0 MPa (8.0 ln kPa) at 36 and 39 cm when soils were at or near field capacity. Subsoil tillage effects on cone index did not differ at depth of tillage or 35 to 45 cm (P ~0.15). Spatial variability across the field landscape inhibited quantification of the effect. In addition, field-scale trials were subsoil tilled in the fall the year before planting. The effect of subsoil tillage on cone penetration resistance may have decreased before measurement during the subsequent spring. Cone index values >1.0 MPa (6.9 ln kPa) suggests soils could have limited potato root growth.


Figure 2
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  		Fig. 2. Cone penetration resistance (cone index) profiles under conventional and subsoil tillage in potato in commercial field trials on a Plainfield Loamy Sand near Coloma, WI in 2003 and 2004.

 
Yield results from the field-scale study showed no evidence to support the use of subsoil tillage for improving productivity of potato (Table 6 ). Tuber yield was highly variable both within each field and across treatments making determination of treatment effects difficult. Some of this variation was due to landscape and topographical positioning among reps within sites. No general trend was observed where subsoil tillage or conventional tillage increased total tuber yield, U.S. no. 1 potato yield, or the B-sized tuber yield. Subsoil tillage increased the cull yield by 0.18 Mg ha–1 compared to conventional tillage treatments. The increase in cull tubers was influenced by the amount of green and misshapen tubers. Cull tubers affected by rot or other diseases accounted for <1% of all culls across all sites and treatments (data not shown). The proportion of U.S. no. 1 tubers, and tuber specific gravity was unaffected by either treatment.


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  		Table 6. Potato yield, yield proportion, and specific gravity as influenced by subsoil tillage in commercial field trials.

 
The size distribution of U.S. no. 1 tubers showed some differences between the conventional and subsoil tillage treatments (Table 7 ). The proportion of 113 to 170 g tubers was decreased by subsoil tillage, while the proportion of 368 to 453 g tubers was increased. Differences between tillage treatments in other size categories were not seen.


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  		Table 7. Size distribution by proportion of U.S. no. 1 tubers as influenced by subsoil tillage averaged across six commercial field trials.

 
Glucose and sucrose levels were not affected by tillage treatments (Table 8 ). Sugar analysis for the Paramount Farms site was displayed separately since values were determined from whole tuber values used for evaluation of chipping potatoes as opposed to separating bud and stem end values used for sugar analysis of processing russet potatoes. The low sugar levels relative to other sites seen at the Paramount Farms site in 2004 was due to variety. Frito Lay 1879 was developed to be a low sugar chipping variety.


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  		Table 8. Sucrose and glucose concentration of tubers as influenced by subsoil tillage in commercial field trials.

 
The occurrence of internal disorders was not affected by tillage treatments in the field-scale study (Table 9 ). Analysis across internal disorders combined or specific disorders individually yielded similar results of no observable differences. High rates of brown center and hollow heart were seen in Russet Burbank in 2004 across much of the region including the experimental sites, but differences were not seen between treatments.


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  		Table 9. Proportion of internal disorders within U.S. no. 1 tubers{dagger} as influenced by subsoil tillage in commercial field trials.

 

   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Soil compaction was identified with penetrometer measurements beginning at 27 cm in depth and continuing through measured profile (Fig. 1 and 2). The cone index values exceeded 1.0 MPa at 25 to 30 cm and 3.0 MPa at 30 to 40 cm depending on treatment and year in small plot and field-scale trials. These cone index levels at or near field capacity have been shown to limit crop growth and plant rooting on coarse textured and other soils (Barley et al., 1965; Busscher et al., 2000, Keisling et al., 1995; Laboski et al., 1998; Lesczynski and Tanner, 1976; Lowery and Schuler, 1991; Tanner et al., 1982). Soil moisture data was lost from small plot trials, but data collection was done within 6 to 12 h of irrigation or precipitation events. Hart and Lowery (1996) showed Plainfield loamy sand reached field capacity within 15 to 30 min after precipitation and remained constant in the absence of crop growth. Crop water use could affect soil moisture status, but crop canopy was only 15 to 20% of soil surface area 2 wk after emergence suggesting minimal transpirational loss occurred between the time of soil wetting and measurement of cone index. Log transformation was used to help alleviate differences in cone index profiles due to variation in soil moisture content (Hilfiker and Lowery, 1988; Lowery and Morrison, 2002).

Artificial compaction and subsoil tillage changed the depth in the soil profile where limiting cone index values were realized. Results of the small plot trial in 2004 indicated the presence of residual tillage pans denoted by abrupt changes in cone index between 15 and 20 cm in depth (Lowery and Schuler, 1994)(Fig. 1). Tillage practices on the research station include moldboard plowing as the primary means of spring soil preparation which may have contributed to increased cone index values at 15 to 20 cm in 2004. Wheel traffic compaction treatments led to increased cone index values through the soil profile as well (Hilfiker and Lowery, 1988). The relative size and mass of harvest machinery and other equipment which traverse vegetable production fields has increased leading to higher potential for compaction due to wheel traffic. Increased wheel traffic was thought to be one of the major contributors to the development of compacted layers in commercial fields (Fig. 2). Consultation with other researchers and industry agronomists concur that cone index measurements from grower fields usually reveal one or two compacted layers with a cone index commonly exceeding 1.5 to 2 MPa and often times 3.0 MPa. Cone index within field-scale experiments was highly variable due to the nature of spatial variability in soil parameters. No subsoil tillage effect was evident on examination of cone index data collected from field scale trials.

Cone index was decreased by subsoil tillage at several depths during the 3 yr of the small plot study (Fig. 1). The greatest influence of subsoil tillage was seen at depths > 33 cm, where cone penetration resistance was reduced to 1.0 MPa in 2003 and 2005. Decreasing cone penetration resistance and disruptions in the compaction layer by tillage shanks traveling at 40 to 50 cm would be conducive to improved potato root growth and potentially plant growth and yield. Potato produces dense roots within the top 30 cm of soil, but has been shown to penetrate up to 60 cm deep into the soil if compaction was not a factor (Lesczynski and Tanner, 1976). The ability of subsoil tillage to disrupt compacted soil layers may be critical for growers with fields having long histories of compaction problems or where moldboard plowing has been used.

Subsoil tillage had minimal effects on total or marketable yield of potatoes grown in the presence of a compacted plow layer (Table 2 and 6). Compaction had no affect on potato yield or quality in the small plot study either even though cone index was increased. Previous research has documented yield reductions in potato due to soil compaction (Flocker et al., 1960; Timm and Flocker, 1966; van Loon and Bouma, 1978). The effect of subsoil tillage on yield has been one of debate with different results across soil types, climates, and management strategies (primarily irrigation). The lack of potato yield response to compaction or subsoil tillage has been documented on sand and irrigated soils in previous research (Alva et al., 2002; Halderson et al., 1993; Miller and Martin, 1990; Sojka et al., 1993; Tanner et al., 1982). No benefit in marketable yield was observed in this research, but tuber size was increased with subsoil tillage. We did not observe increased U.S. no. 1 potato yield as noted in several of those reports. Field-scale trials confirmed observations of increased tuber size that tended to occur in the small plot trials (Table 3 and 7). The effect of subsoil tillage on increased tuber size may be enough to warrant adoption of subsoil tillage by growers, as an increased yield of larger tubers raises potato price in many market classes. Other management factors within field sites may have had larger influences on potato yield and masked yield responses to subsoil tillage, especially irrigation (Alva et al., 2002; Tanner et al., 1982). Deeper rooting of potato promoted by subsoil tillage conceivably would decrease potential effects of heat and drought stress. However, proper irrigation management would alleviate the potential benefits of subsoil tillage by preventing heat and drought stress events in the first place (Alva et al., 2002; Halderson et al., 1993; Miller and Martin, 1990; Sojka et al., 1993; Tanner et al., 1982).

Subsoil tillage had no affect on tuber glucose values (Table 4 and 8). The accumulation of reducing sugars (glucose and fructose) has been shown to cause darkening of potato tissue during the frying process (Shallenberger et al., 1959). Sucrose was lowered by subsoil tillage at harvest and out of storage in one of 3 yr, but this trend was not consistent throughout the entire duration of the study. While sucrose does not directly influence fry color, sucrose has been shown to convert to reducing sugars during prolonged storage periods, and increased levels of sucrose ultimately translate into increased levels of reducing sugars (Sowokinos, 1978). Sucrose levels were indicative of tuber age and physiological stress experienced during the growing season. The reduced sucrose level of subsoil tilled treatments would suggest those plants did not experience the amount of stress seen under conventional tilled conditions. Changes in sugar levels have been directly related to periods of water stress and high temperatures during tuber development (Eldredge et al., 1996). This may show that despite the presence of moisture stress periods during the course of the study, rooting of the potato plants in subsoil tilled treatments was sufficient to improve continuity of water uptake through the stress period compared to conventional tillage. Sugar response to subsoil tillage may have been more evident if stored tubers were managed differently in storage (i.e., longer preconditioning).

Compaction and subsoil tillage had limited effects on internal disorders over the course of the study (Table 5 and 9). Hollow heart and brown center has been traced to cool soil temperatures followed by uneven soil moisture management and subsequent fast tuber growth (Rex and Mazza, 1998). In 2004, the growing season was cooler than average with intermittent periods of warm temperatures and heightened evapotranspiration rates (Table 1). The cool temperatures throughout the growing season were accompanied by increased rainfall and irrigation resulting in 180 mm of excess water in August. The increased incidence of hollow heart observed in 2004 (data not shown) was likely linked to the cooler conditions.

The ability of sandy soils to form a dense compacted layer at approximately 30 cm has been previously documented (Hilfiker and Lowery, 1988; Laboski et al., 1998; Lesczynski and Tanner, 1976 Parker et al., 1989; Tanner et al., 1982; van Loon et al., 1985). While the compacted layer has been shown to limit vertical water flow and drainage, inhibit root growth, and increase pathogen numbers, limited evidence suggests subsoil tillage increased yield and quality in potato. This project showed that under intensive vegetable cropping systems in Central Wisconsin, subsoil tillage had limited effects on potato yield. Potato internal defects were low in most cases suggesting the observed crops were not subjected to any undue stresses during the three growing seasons under which the project was conducted. Potato agronomists and processors continue to recommend, and in some cases require subsoil tillage. Subsoil compaction can be a multi-faceted issue causing tuber diseases and influencing storability. Water management may be a more important factor for optimizing potato productivity and compensate for yield and quality effects due to compaction and subsoil tillage (Alva et al., 2002; Stahlman and Allen, 2001; Tanner et al., 1982).

This research, coupled with other reports, suggests no reason to continue subsoil tillage as an agronomic means of improving potato yield. Subsoil tillage as a means to manage compaction tended to improve tuber size distribution across the small plot and field-scale trials. The value of increased size of U.S. no. 1 tubers and decreased tuber sucrose concentrations resulting from subsoil tillage must be evaluated for economic benefits relative to energy, equipment, and labor costs.


   ACKNOWLEDGMENTS
 
The authors would like to thank the Wisconsin Potato and Vegetable Growers Association, Frito Lay, McCain Foods, and the University of Wisconsin-Madison Graduate School for their financial support of this project. We appreciate the willingness of Coloma Farms, H&J Williams Farms, Robert Johnson Farms, and Paramount Farms for participating in this project. Finally, thanks to the staff at the University of Wisconsin-Madison Hancock Agricultural Research Station staff, University of Wisconsin-Madison Arlington Horticulture Research Station staff, Andrea Klahn, Lindsey Breunig, Melissa Miller, Aurelien Bisson, and Lauren Schwersinske for their tireless work in completing these field experiments.


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   REFERENCES
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 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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