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Published in Agron J 99:501-510 (2007)
DOI: 10.2134/agronj2006.0105
© 2007 American Society of Agronomy
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Soil Quality & Fertility

Potato Yield Stability under Contrasting Soil Management Strategies

Ellen B. Mallory* and Gregory A. Porter

Dep. of Plant, Soil and Environmental Sciences, Univ. of Maine, Orono, ME 04469

* Corresponding author (ellen.mallory{at}maine.edu)

Received for publication April 5, 2006.

   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Managing soil quality is recognized as a cornerstone of maintaining crop production potential. Here we show that soil management that improves soil quality characteristics can also reduce year-to-year variation in yields. Thirteen years of data from the Maine Potato Ecosystem Project were used to investigate the long-term effects of soil management, pest management, cultivar, and rotation in a factorial design on the yield and yield stability of potatoes grown in 2-yr rotations. Potato (Solanum tuberosum L.) yields in the amended soil system (manure, compost, green manure, and supplemental fertilizer) were up to 55% higher than yields in the contrasting nonamended soil system (synthetic fertilizer) in all but 1 yr. Yield stability was also enhanced in the amended system compared with the nonamended system, as demonstrated by lower CVs of total and U.S. no. 1 potato tuber yield. Stability analysis indicated that yields in the amended system were less influenced by adverse growing conditions, particularly low rainfall. Total and U.S. no. 1 treatment yields in the poorest-yielding year were 63 and 59% of maximum yields, respectively, in the amended system, compared with 45 and 46% in the nonamended system. Yields and yield stability were also influenced by pest management system and cultivar but not by rotation. These results indicate that management practices that improve soil quality can enhance potato yield stability by reducing the impact of adverse growing conditions.

Abbreviations: CEC, cation exchange capacity • IPM, integrated pest management • MPEP, Maine Potato Ecosystem Project • SOC, soil organic carbon


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
FARMERS emphasize yield potential when evaluating cropping systems but also consider the predictability, or stability, of those yields (Eghball et al., 1995; Varvel, 2000). Year-to-year variation in yields on a specific field is due primarily to weather-related environmental factors, pest and nutritional stresses, and management (Batchelor et al., 2002; Loomis and Conner, 1992; Smolik et al., 1995). A key question in designing cropping systems is whether management can buffer the effects of an unpredictable environment (Varvel, 2000).

Managing soil quality is fundamental to maintaining a soil's crop production potential (Christensen and Johnston, 1997). In fact, the term "soil quality" is commonly defined in terms of sustaining biological productivity as well as maintaining environmental quality and plant and animal health (Karlen et al., 1997). Evidence of the need to manage for soil quality comes from studies of degraded soils that exhibit reduced productivity even with high fertilizer inputs (Aref and Wander, 1998; Cassman, 1999; Parr et al., 1992) and from studies in which increased productivity is associated with enhanced soil organic carbon (SOC), achieved by amending with organic materials (Barzegar et al., 2002; Christensen and Johnston, 1997; Hornick and Parr, 1987), growing a sod crop (Díaz-Zorita et al., 2002; Johnston, 1991), or reducing tillage (Díaz-Zorita et al., 2002; Dick et al., 1997). Others have found predictive relationships between yield and soil quality characteristics, such as total soil carbon (C) (Alvarez et al., 2002), active soil C and macroaggregate stability (Stine and Weil, 2002), and total soil nitrogen (N) (Stenberg, 1998). Productivity increases in SOC-enhanced soils have been attributed to improvements in soil structure, water-holding capacity (Johnston, 1991), and nutrient supply (Cassman, 1999; Díaz-Zorita et al., 1999).

There is also a perception that high-quality soils may produce more stable yields by buffering environmental factors such as limited or excessive rainfall, pests, and diseases (Ellmer et al., 2000; Romig et al., 1995). Crops grown on soil that receives organic amendments and has enhanced soil quality characteristics have been shown to have access to greater soil moisture (Liebig and Doran, 1999; Lotter et al., 2003) and are more resistant to weed (Gallandt et al., 1998a) and insect (Alyokhin et al., 2005) pressures. However, evidence is scarce that these amended systems reduce yield variation. One recent study comparing two organic systems with a conventional system found similar maize (Zea mays L.) yields in years of adequate rainfall but higher yields in the organic systems in years of drought (Lotter et al., 2003). They attributed this difference to enhanced water-holding capacity of the organically managed soil. In contrast, analysis of two long-term experiments found no evidence that manure-amended systems, associated with improved soil quality, altered temporal yield variability (Aref and Wander, 1998; Eghball et al., 1995). Additionally, in some organic systems that included soil amendments, greater variation in yield has been reported (Clark et al., 1999; Spiertz, 1989). Work in this area is insufficient to make broad generalizations. Few studies are of adequate duration to assess temporal yield variability (Varvel, 2000), and those with longer-term histories often compare entire cropping systems in an experimental design that makes it difficult to isolate soil management effects from other effects, such as tillage frequency and rotation (Smolik et al., 1995; Stine and Weil, 2002).

Potato (Solanum tuberosum L.) may be particularly sensitive to weather-related variation in part because it has a shallower root system than other annual crops (Opena and Porter, 1999). Benoit and Grant (1980, 1985) noted that periods of water deficit or excess severely limited potato yields in northern Maine despite the fact that total rainfall amounts were generally sufficient. Compounding the sensitivity of potato to water stress is the fact that the water-supplying capacity of soil under potato production is often degraded. Potato production in Maine and northeastern Canada has resulted in lower SOC concentration and less structural stability (Saini and Grant, 1980) due to high levels of soil disturbance and low levels of crop residue returns (Angers and Carter, 1996; Grandy et al., 2002). Black and White (1973) noted an increase in potato yield with manure application, independent of applied fertilizer, which they attributed to increased organic matter, water-holding capacity, and cation exchange capacity (CEC). In Maine, the idea that degraded soils are limiting potato yields is supported by the observation that yields have remained relatively constant over the last 50 yr despite increasing inputs of pesticides and fertilizers (Economic Research Service, 2002; Westra and Boyle, 1991).

The Maine Potato Ecosystem Project (MPEP) was initiated in 1991 to investigate key factors limiting potato production. This cropping systems trial compared two contrasting soil management systems. The amended soil management system, designed to improve soil quality, received annual additions of organic amendments (manure and compost) supplemented by synthetic fertilizer. The nonamended soil management system used only synthetic fertilizers. These soil management systems were in factorial combination with the other experimental factors (pest management systems and cultivars from 1991 to 1998 and rotation treatments from 1999 to 2004), so the effects of soil management can be isolated from the effects of other factors. This aspect of the trial, and the fact that the soil treatments caused highly divergent soil characteristics, make the MPEP an ideal trial to investigate long-term effects of soil management. The objectives of this study were (i) to assess the impact of soil management on soil chemical and physical properties and (ii) to investigate the influence of soil management, pest management, cultivar, and rotation on yield and yield stability of potatoes grown in 2-yr rotations.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description
The experiment was conducted from 1991 to 2004 at the Maine Agricultural and Forest Experiment Station's Aroostook Research Farm in Presque Isle, Maine, on a gravely, well drained, Caribou loam soil (fine-loamy, mixed, frigid Typic Haplorthods). The 5.9 ha used for the study had a long history of commercial and research potato production. Details of the establishment of the experiment are given in Porter (1996).

Cultural Practices and Treatments
The experiment consisted of 96 plots (14.6 by 41.0 m) in four replicate blocks and can be divided into two phases. During Phase 1 (1991–1998), treatments were arranged in a randomized, complete-block, split-plot design. Main-plot factors were one of three pest management systems: conventional, reduced input, and bio-intensive. Subplots were a fully factorial combination of two soil management systems ("amended" vs. "nonamended"), two potato cultivars (‘Atlantic’ vs. ‘Superior’), and two rotation entry points (potato vs. rotation crop). Although pest treatments were randomly assigned to locations within blocks, and soil and cultivar treatments were randomly assigned to locations within main plots, entry points were assigned to alternating positions within the field in an effort to minimize the movement of insects.

The amended soil management system was designed to rapidly improve soil quality by adding organic amendments (raw beef manure and/or potato cull compost) and by rotating potato with a pea (Pisum sativum L. subsp. sativum)/oat (Avena sativa L.)/hairy vetch (Vicia villosa Roth) green manure crop. A secondary objective of this system was to reduce the need for fertilizer. Manure and compost were applied and incorporated before planting potatoes. These organic amendments were supplemented with fertilizer as needed to provide approximately the same nutrient levels as in the nonamended soil treatment. The nonamended soil management system followed industry standards, including rotating potato with barley (Hordeum vulgare L.) interseeded with red clover (Trifolium pratense L.) and using recommended rates of inorganic fertilizers. Table 1 provides average annual application rates of manure, compost, and fertilizer and the estimated average annual availability of N, phosphorus (P), and potassium (K) for the two soil management systems. No micronutrient fertilizers were applied to either soil treatment. Complete descriptions of the pest and cultivar treatments and of the cultural methods are provided elsewhere (Gallandt et al., 1998b; Porter and McBurnie, 1996).


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  		Table 1. Applications of amendments and fertilizer and estimated nitrogen (N), phosphorus (P), and potassium (K) availability from these sources during Phase 1 (1992–1998) and Phase 2 (1999–2004) of the Maine Potato Ecosystem Project.

 
The objectives of the project shifted in 1999 to include an investigation of crop diversity effects (Phase 2). The conventional and reduced input pest management plots were redistributed among three crop rotations: standard (potato–barley/red clover), intensive (potato–soybean [Glycine max (L.) Merr.]–potato–barley/red clover), and integrated (potato–soybean–barley/alfalfa [Medicago sativa L.]/timothy [Phleum pratense L.]–forage). These rotations were managed using conventional integrated pest management (IPM) and one potato cultivar (‘Atlantic’). The bio-intensive pest management plots were assigned to the integrated rotation only, with "biorational" IPM pest management (Alyokhin et al., 2005). The changes in the study for Phase 2 created two fully factorial experiments. Experiment 1 included crop rotation and soil management factors and was managed under conventional IPM. Experiment 2 compared pest management and soil management factors within the context of the integrated crop rotation. The two experiments shared the set of plots that were in the integrated rotation and managed with conventional IPM. For consistency with Phase 1, the only 1999–2004 results presented in this paper are those from the standard and intensive rotations of Experiment 1, in which potato was grown every other year.

Plot assignments for the soil management (and entry point) treatments remained constant from 1991 to 2004, giving the plots a continuous 14-yr history of amended or nonamended soil systems. During Phase 2, beef manure was applied before potato and barley crops (Table 1). No compost was applied, and amended system soybeans received no manure or fertilizer.

Soil Analyses
Cation exchange capacity, pH, and mineral nutrient content were determined from soil samples taken each fall after crop harvest. Ten soil cores were collected to a 15-cm depth from each plot, bulked, and mixed thoroughly. A subsample was dried, sieved through 2-mm screen, and submitted to the University of Maine Soil Testing Laboratory for pH and CEC analysis using standard methods (Hoskins, 1997; Northeast Coordinating Committee on Soil Testing, 1995). A modified Morgan procedure was used for phosphorus and cation extraction (Northeast Coordinating Committee on Soil Testing, 1995). Soil organic matter and water-stable aggregate content were determined from samples taken each spring before the application of organic amendments. Ten soil cores were collected from a 15-cm depth, air-dried, sieved through a 6.4-mm screen, bulked, and mixed thoroughly. Duplicate subsamples were analyzed for readily oxidizable SOC using the Walkley–Black method (Nelson and Sommers, 1996). A separate set of duplicate subsamples was analyzed for water-stable soil aggregate content according to the methods described by Porter and McBurnie (1996). Soil moisture content was estimated as part of the procedure to monitor soil mineral N content at biweekly intervals throughout the growing season. Ten soil cores were taken to a 20-cm depth and bulked. A 50-g subsample was sieved through a 2-mm screen, weighed wet, dried at 105°C for 24 h, and weighed again.

Yield and Tuber Quality
Potato crop yields were determined from the four center rows of each plot. Tubers were lifted with a two-row potato digger and collected by hand, and the yield of the entire four rows was weighed in the field. Any decaying tubers were weighed separately. Two 22.7-kg subsamples were collected from each plot and graded for tuber size and external defects. U.S. no.1 yields were calculated as the yield of tubers between 4.8 cm and 10.2 cm in diameter, excluding decayed, sunburned, misshapen, scabby, or growth cracked tubers.

Statistical Analyses
Significant treatment effects and interactions on the selected soil characteristics were identified using ANOVA. The normality of total and U.S. no. 1 tuber yield data was tested using Kolmogorov-Smirnov tests (p ≤ 0.01; SYSTAT v.10, 2000). Repeated-measures ANOVA was used to determine the significance of rotation cycle, treatment, rotation cycle by treatment, and treatment interaction effects on total and U.S. no. 1 tuber yields for each phase of the experiment. There were three rotation cycles for each entry point in both phases, corresponding to years 1993/1994, 1995/1996, and 1997/1998 for Phase 1 and 1999/2000, 2001/2002, and 2003/2004 for Phase 2. The first rotation cycle in Phase 1, 1991/1992, was not included because the treatment structure of the experiment was substantially altered after the 1991 season.

Yield stability was assessed by two methods. In the first, the CV of total and U.S. no. 1 yields over time was calculated for each plot within each phase of the experiment (Clark et al., 1999; Spiertz, 1989). The CVs were subjected to ANOVA to determine significant treatment effects and interactions. The second assessment of variability was stability analysis (Guertal et al., 1994; Raun et al., 1993), in which total and U.S. no. 1 yield for each soil management treatment was regressed on the annual mean yield of both treatments combined, designated the "environment mean yield." The environment mean yield reflects the overall growing conditions for each year, which includes temperature, rainfall, pest pressure, and effectiveness of pest and crop management. Regressing treatment yields on the environment mean yield allows one to evaluate the relative response of the treatments under the range of growing conditions that occurred, thereby providing a way to investigate significant year-by-treatment interactions that commonly appear in repeated-measures analyses of long-term trials (Raun et al., 1993). Data from the two phases of the experiment were combined for stability analysis because there were no significant interactions between the soil management treatment and other treatments (pest management, variety, and rotation) on total yield, U.S. no. 1 yield, and the CV of those yields during both phases. A subset of the plots were used that were consistent between Phases 1 and 2 in having potato grown every other year during both phases (this excluded the bio-intensive/biorational IPM plots) and in being planted to ‘Atlantic’ (this excluded the plots that were planted to ‘Superior’ in Phase 1). Data for the soil treatments were then averaged by replicate over the two remaining pest treatments in Phase 1 and the two rotation treatments in Phase 2.

The relationship between tuber yield and rainfall was explored to investigate the hypothesis that lower sensitivity to fluctuations in rainfall was a possible mechanism for enhancing yield stability. Using the subset of data used for stability analysis, tuber yield was regressed on rainfall amount (May–September, May–August, and June–August) and evenness (using Shannon diversity index [Bronikowski and Webb, 1996]). The residuals of the regression of yield on June–August rainfall (i.e., the variation in yield that was not explained by June–August rainfall) were regressed on the environment mean yield. The purpose of this second stability analysis, with the effect of rainfall removed, was to investigate the importance of rainfall as a driver of treatment differences in the original stability analysis. The regression lines for the soil treatments from stability analyses and from regressions of yield on rainfall were compared using the extra sums of squares procedure (Motulsky and Christopoulos, 2004). This procedure uses an F test to determine if there is a statistically significant difference between the error sum of squares for a model fitting treatments separately (sums of squares are added) and a model fitting all the data at once. We used a p value of 0.05 to indicate whether the model with separate fits provided a significant reduction in the error sums of squares, indicating that the treatment regression lines were significantly different.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Soil Characteristics
The amended soil management system substantially increased soil organic C, pH, CEC, and total water-stable aggregates (Table 2). Soil organic C in the amended soil was significantly greater than in the nonamended soil after only one season, and water-stable aggregates were significantly greater than in the nonamended soil after two seasons (Gallandt et al., 1998b). Soil organic C was 50% higher in the amended plots by the end of Phase 1 (1998) and 63% higher 5 yr into Phase 2 (2004). This is one of the largest increases in soil organic C among experiments investigating the effects of organic amendment on soil characteristics and crop performance (Clark et al., 1998; Fraser et al., 1988; Poudel et al., 2002; Wander et al., 1994). Consistent with these similar experiments, particulate organic matter carbon was disproportionately enhanced in the amended system, doubling by the end of Phase 1 (Griffin and Porter, 2004). Higher CEC and water-stable aggregates values in the amended system reflect the increase in soil organic C. The amended system substantially increased soil test P due to much greater rates of P inputs than in the nonamended system (Table 1). Modified Morgan P in the amended soil in 1998 and 2003 was above the level considered excessive according to Maine nutrient planning guidelines (45 kg ha–1), although it was below the action level for row crops grown on land that is not classified as highly erodable or located in a most at-risk watershed (USDA-NRCS, 2004). In practice, manure application rates should be adjusted to avoid excessive P accumulation in the soil and potential loss to the environment (Sharpley et al., 2001).


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  		Table 2. Selected chemical and physical properties of the amended (+) and nonamended (–) soil before initiation (1991), after 1 yr (1992), at the end of Phase 1 (1998), and 5 yr into Phase 2 (2003) of the Maine Potato Ecosystem Project.

 
Yields and Yield Variation
The amended soil management system produced total and U.S. no. 1 potato yields equal to or greater than those in the nonamended soil in all years but 1996 (Fig. 1 ). Repeated-measures ANOVA revealed significant rotation-cycle by soil system interactions for total and U.S. no. 1 yield (Table 3). Yield gains in the amended system, when statistically significant, were 4 to 54% for total yield and 8 to 36% for U.S. no. 1 yield. Foliar macronutrient concentrations in the amended system were lower or equal to those in the nonamended system in all years (Alyokhin et al., 2005; Porter and McBurnie, 1996) and were always in the sufficiency range for potato (Westermann, 1993). This suggests that yield enhancement in the amended system was not related to the supply of macronutrients. However, boron availability could have contributed to the difference in yields. Leaf tissue boron levels in the amended system (20–32 mg kg–1) were in the sufficiency range, whereas they were in the marginal range for the nonamended system (10–22 mg kg–1), with values of >20 mg kg–1 considered sufficient and 10 to 20 mg kg–1 considered marginal (Westermann, 1993). The yield results are consistent with many other trials that have demonstrated the ability of systems relying primarily on organic sources of fertility to produce crop yields comparable to those of synthetic fertilizer-based systems (Eghball et al., 1995; Johnston, 1991; Poudel et al., 2002; Stanhill, 1990). These results are also in agreement with others who have found that potato production can be limited by soil quality (Black and White, 1973; Porter et al., 1999).


Figure 1
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  		Fig. 1. Annual mean yields of (a) total and (b) U.S. no. 1 potatoes in the amended and nonamended soil treatments in Phase 1 (n = 24) and Phase 2 (n = 8) of the Maine Potato Ecosystem Project. Error bars correspond to 1 SE. Significant differences between the treatments in any given year are indicated with * p < 0.05 and ** p < 0.01.

 

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  		Table 3. Repeated-measures ANOVA of total and U.S. no. 1 potato tuber yield of the Maine Potato Ecosystem Project.

 
Temporal variation in annual potato yield was high (Fig. 1) and loosely followed annual rainfall patterns (Table 4). However, year-to-year variation was significantly reduced in the amended soil system for both yield classes and during both phases of the study (Tables 5 and 6). This finding contradicts results from two long-term trials in which repeated manure application enhanced crop yields compared with unmanured treatments but failed to reduce year-to-year variation in those yields (Aref and Wander, 1998; Eghball et al., 1995). It is possible that the discrepancies in soil physical and chemical characteristics between contrasting soil treatments were not as great in these studies as in the MPEP. Neither study reported soil characteristics for the treatments or time period reported. Additionally, the crops tested (maize, oats, and hay) may not be as sensitive to sources of temporal variation as potato.


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  		Table 4. Monthly rainfall amounts at Aroostook Research Farm, Presque Isle, Maine, 1992–2004.

 

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  		Table 5. Analysis of variance of year-to-year variation, expressed as the CV, in total and U.S. no. 1 potato tuber yield during Phase 1 (1992–1998) and Phase 2 (1999–2004) of the Maine Potato Ecosystem Project.

 

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  		Table 6. Effect of soil management, pest management, and cultivar on the CV of total and U.S. no. 1 potato tuber yield during Phase 1 (1992–1998) and Phase 2 (1999–2004) of the Maine Potato Ecosystem Project.

 
Pest and cultivar (Phase 1) affected the CV for yields, whereas rotation (Phase 2) did not (Tables 5 and 6). ‘Superior’ showed considerably more variation in yield than ‘Atlantic’ (CVs for ‘Superior’ were 33.5 and 34.7% compared with CVs for ‘Atlantic’ of 19.8 and 17.7% for total and U.S. no. 1 yield, respectively) presumably because ‘Superior’ matures earlier and is more susceptible to heat stress, water stress, and early dying than ‘Atlantic’. Mean CVs for total and U.S. no. 1 yield were higher in the bio-intensive pest management system (31.4 and 31.8%, respectively) than in the pesticide-based systems (23.7 and 21.7%, respectively, for conventional and 24.8 and 25.0%, respectively, for reduced input). Pest and cultivar effects are discussed in more detail elsewhere (Gallandt et al., 1998b).

Yield Stability
Regression of treatment yield on the environment mean yield was significant in all cases (Fig. 2 ), and extra sums of squares analysis distinguished between the responses of the two soil management treatments for total and U.S. no. 1 yield. The amended system produced more stable yields over the range of growing conditions occurring in this study. Total and U.S. no. 1 treatment yields in the poorest-yielding year were 63 and 59% of maximum yields, respectively, in the amended system compared with 45 and 46% in the nonamended system. Yields were most divergent between the soil management systems at the lowest environment mean (i.e., poorest growing conditions) and converged as growing conditions improved, suggesting that the amended system buffered one or more yield-limiting factors.


Figure 2
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  		Fig. 2. Linear regression of (a) total potato yield and (b) U.S. no. 1 potato yield on the environment mean yield for amended (filled triangle) and nonamended (open triangle) soil management treatments of the Maine Potato Ecosystem Project, 1992 to 2004. Individual data points are the mean of four replicates (n = 4). Probability values are from extra sums of squares (ESS) analysis comparing amended vs. nonamended soil regression lines (n = 52; 13 yr times four replicates).

 
Rainfall could be one such yield-limiting factor whose effects could be influenced by soil management. Variation in the amount and timing of rainfall is one of the primary causes of year-to-year variation in crop yields (Batchelor et al., 2002; Loomis and Conner, 1992; Runge and Hons, 1998). In this study, June–August rainfall produced the strongest linear relationship with total and U.S. no. 1 tuber yield (r2 between 0.07 and 0.15) compared with other rainfall periods and rainfall evenness measures. Although the relationships between yield and the rainfall evenness measures used were weak, the distribution of rainfall was also important. For instance, June–August rainfall in 1999 was in the middle of the range (283 mm), but much of that rainfall came late in the season. In contrast, in 2002 most of the 253 mm of rain occurred early in the season, with little rainfall from mid-July through August during the critical tuber-bulking period. The poor distribution of rainfall in these years is reflected in relatively low yields. Additionally, other influences, such as insect pests, diseases, nutrient availability, and growing degree days, are important influences on yield and should be included in a subsequent study of the major determinants of yield in this trial. Here, our intention was to identify a likely source of year-to-year variation whose effects were influenced by soil management.

The amended treatment was less sensitive to changes in rainfall than the nonamended treatment, as indicated by the lower slopes and r2 values (Fig. 3 ). Increased water-supply capacity is often associated with increased SOC (Barzegar et al., 2002; Liebig and Doran, 1999; Weil and Magdoff, 2004). Soil moisture, measured at biweekly intervals throughout the growing season, was almost always significantly higher in the amended versus the nonamended system. In the three representative years of soil moisture data presented in Fig. 4 , soil moisture was 4 to 29% greater in the amended than in the nonamended plots.


Figure 3
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  		Fig. 3. Linear regression of (a) total potato yield and (b) U.S. no. 1 potato yield on June through August rainfall for amended (filled triangle) and nonamended (open triangle) soil management treatments of the Maine Potato Ecosystem Project, 1992 to 2004. Individual data points are the mean of four replicates (n = 4). Probability values are from extra sums of squares (ESS) analysis comparing amended vs. nonamended soil regression lines (n = 52; 13 yr times four replicates).

 

Figure 4
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  		Fig. 4. Gravimetric soil moisture in the amended and nonamended soil treatments (a) in the first year soil moisture was measured (1995), (b) at the end of Phase 1 (1998), and (c) 6 yr into Phase 2 (2004) of the Maine Potato Ecosystem Project (n = 8). Error bars correspond to 1 SE. Significant differences between the treatments in any given year are indicated with * p < 0.05, ** p < 0.01, and *** p < 0.001.

 
The observation that the relationships between yields and rainfall paralleled those with the environment mean yield suggests that the different responses of the soil management systems to rainfall largely explain the difference in yield stability between the soil systems. Further evidence of this comes from the fact that when the residuals from regressing yield on rainfall were regressed against the environment mean, the soil management system effect was no longer significant (extra sums of squares p values were 0.181 and 0.504 for total and U.S. no.1 yields, respectively). In other words, removing the variation due to rainfall from the yields eliminated the difference in yield stability between the two soil management systems.

Lotter et al. (2003) recently reported increased drought tolerance of two organic cropping systems compared with a conventional system in Pennsylvania. In years of insufficient rainfall, the manure-based and legume-based organic systems, both of which had enhanced soil characteristics, produced higher yields of maize and soybean than a fertilizer-based system, but yields between the three systems were equal when rainfall was sufficient. Likewise, in the Argentine Pampa, wheat (Triticum aestivum L.) grain yields correlated with total soil C and soil water retention in years of rainfall deficit but correlated with total organic N and available P in higher rainfall years (Díaz-Zorita et al., 1999).

Buffering variable rainfall amounts may not be the only way in which an amended soil management system enhances crop yield stability. Results from the MPEP show that when weed biomass significantly affects yield, as occurred in the bio-intensive pest management treatments, both were reduced in the amended as compared with the nonamended system (Gallandt et al., 1998a). The authors proposed that the amended soil management system produced a more vigorous crop that was better able to compete with weeds than the nonamended soil system. Also in the MPEP, in situ densities of Colorado potato beetles [Leptinotarsa decemlineata (Say)] were lower in the amended system compared with the nonamended system (Alyokhin et al., 2005), as were reproduction and development of Colorado potato beetles caged on potato plants grown in the amended versus the nonamended soil (Alyokhin and Atlihan, 2005). Additionally, potato leaf mineral compositions in the two soil systems were highly discrepant and explained 40 to 57% of the variation in in situ Colorado potato beetle populations (Alyokhin et al., 2005). The authors proposed that, taken together, these results provide support for the mineral balance hypothesis (Phelan et al., 1996), which postulates that increased SOC and microbial activity associated with organically managed soils maintain a balanced nutrient profile in the plants that promotes good plant growth and resistance to herbivory. The relative importance of the amended soil management system's apparent ability to modulate the effects of weed pressure, insect pest pressure, and variable rainfall on potato yields cannot be separated in this study.

In addition to enhancing soil physical and chemical characteristics considered favorable for crop production, the amended soil treatment exhibited an increase in soil test P (Table 2), raising concerns of environmental export. From this standpoint, levels of manure such as these might not be desirable, nor are they likely possible given a limited supply (Christensen and Johnston, 1997; Magdoff and Weil, 2004). However, if the enhancement of organic matter and its associated soil quality characteristics are primarily responsible for increasing yield stability, then the relationship between organic matter and yield stability should be maintained for other management strategies that improve or maintain soil quality, such as reduced tillage (Díaz-Zorita et al., 2002; Dick et al., 1997), rotations with sod crops (Johnston, 1991), or reducing amendment applications to maintenance levels (Grandy et al., 2002).


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
A soil management system designed to improve soil quality through the addition of organic amendments provided the optimal combination of enhancing potato yields and reducing the year-to-year variability of those yields. Potato production in the contrasting nonamended soil system was more susceptible to adverse growing conditions, particularly low rainfall, and seemed to be limited by poor soil quality. These results demonstrate that managing for soil quality with an amended soil management system can be a viable strategy to buffer the effects of an unpredictable growing environment and stabilize yield.


   ACKNOWLEDGMENTS
 
The Maine Potato Ecosystem Project resulted from the inspiration, expertise, and persistence of many participants. We thank A. Randall Alford, Andrei Alyokhin, Francis A. Drummond, M. S. Erich, Eric R. Gallandt, Timothy S. Griffin, Eleanor Groden, David A. Lambert, Matt Liebman, Michele C. Marra, Jeffrey C. McBurnie, Bacilio Salas, and many others who provided technical support. This project has received support from USDA-CSRS Special Grants Program (91-3414-5904 and 94-34141-0040), USDA-SARE/ACE (LNE93-36/ANE93.18), USDA Northeast Region IPM Program Grant (93-34103-8413), USDA-ARS (58-5352-2-389; 58-5353-3-389), USDA-APHIS (94-8201-0128; 95-8201-0128), U.S. Army Corps of Engineers (013-01A-2005-012 and 014-01A-2005-012), USDA-CSREES-IFAFS (2001-52101-11308), and the Maine Potato Board.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Maine Agric. & Forest Exp. Stn. Publ. 2907.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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