Sustainability of crop production systems is dependent on many factors, from the cost/benefit of the many operations involved to the inputs and outputs obtained to the continued health of the soil and overall agroecosystem. Probably the most important single attribute to growers is crop productivity, usually measured by yield. Crop yield is the final result, but numerous aspects of crop development and growth may be involved in or responsible for the resultant yield observed, and may give indications of where production problems may be occurring. Soil health, defined as the continued capacity of soil to function as a vital living system to sustain biological productivity; maintain environmental quality; and promote plant, animal, and human health [1
] is a critical component of agricultural productivity, sustainability, and ecosystem function. Incorporation of soil and crop management practices that promote soil health, such as crop rotations, cover crops and green manures, organic amendments, and conservation tillage, into improved cropping systems may help maintain and/or improve soil health and enhance productivity, sustainability, economic vitality, and environmental quality [3
Potato (Solanum tuberosum
L.), an important crop in the US and throughout the world, can be particularly hard on soils due to the intensive tillage operations and cropping patterns used. Potato production in the northeast U.S., as well as in other potato growing regions, has been characterized by short (2 y) rotations, extensive tillage, minimal crop residue return, and minimal crop diversity, often taking a toll on soil health and crop productivity over time [5
]. Increasing rotation length from 2 years to 3 or more years between potato crops has been shown to improve productivity, as well as reduce soilborne diseases in multiple studies [6
]. Other practices, such as the addition of cover crops and green manures [11
], amendments of compost or animal manure [13
], and reduced tillage [6
], have all shown promise for having positive effects on tuber yield and quality, as well as other benefits to various soil properties and soil health in potato systems. However, most previous research has focused on the assessment of individual practices or rotations, and not necessarily on the combined effects of multiple different practices in integrated cropping systems for the total system effects on productivity and plant and soil properties.
In this research, which builds upon our previous work with improving potato cropping systems [17
], we assessed the effects of cropping systems incorporating multiple soil health management practices focused on specific soil and crop management goals, on various plant characteristics of the potato crop itself, including growth, productivity, and nutrient concentrations. In 2004, we established field trials for long-term evaluation of different potato cropping systems to better determine what factors were most limiting to potato production in Northeastern U.S., and how these limitations could be addressed through cropping systems [20
]. Three specific cropping systems were established to address the crop and soil management goals of soil conservation, soil improvement, and disease suppression, and these were compared to a standard rotation and a non-rotation control.
Previously, we characterized the effects of these cropping systems on various management concerns, such as soilborne potato diseases and soil microbiology [20
], soil health (represented by various soil physical, chemical, and biological properties) [24
], and soil nutrient-related enzyme activities and P status [26
]. In the present research, we examined cropping system effects on crop productivity using such crop characteristics as photosynthetic potential (measured as leaf area index and chlorophyll content), tuber yield and quality (total and various size class distributions, misshapenness, and specific gravity), biomass production (both above- and below-ground), and plant tissue nutrient concentrations.
In this research, multiple individual soil health management practices were combined into cropping systems with specific management goals of soil conservation, soil improvement, and disease suppression, and effects on crop growth, nutrition, and tuber yield and quality were assessed over up to five full cropping seasons (and after cropping systems in place for 3 to 7 years) under both non-irrigated (rainfed) and irrigated conditions. Combined data from all five seasons demonstrated that cropping system significantly affected virtually all of the crop and plant characteristics measured, ranging from crop growth (photosynthetic potential and biomass) and tissue nutrient concentration to potato tuber yield and quality, with the soil improving (SI) system, which included compost amendments, cover crops, and reduced tillage in a 3 y rotation, producing the greatest overall effects and improvements in these crop production parameters, particularly without irrigation. Irrigation effects were also significant for most parameters. This research demonstrated that improved cropping systems can substantially enhance characteristics associated with potato crop productivity.
Concurrent research on these same cropping systems over the same years documented the system effects on soil physical, chemical, and biological properties, and that effects tended to increase over time [24
]. In these studies, all rotations increased aggregate stability, water availability, microbial biomass C, and total C and N compared to no rotation (PP), and the 3 y systems (SI, SC, DS) increased aggregate stability relative to the 2 y system (SQ). Additionally, the 3 y systems with reduced tillage (SI and SC) increased water availability and reduced bulk density relative to the other systems. However, the SI system resulted in greater increases in total and particulate organic matter (POM) C and N; active C; microbial biomass C; water availability; CEC; concentrations of P, K, Ca, Mg, and S; and lower bulk density than all other cropping systems [24
]. SI was also shown to increase microbial activity and greatly affect soil microbial community characteristics, whereas PP showed the lowest microbial activity, with the others in between [20
]. These changes all constitute parameters associated with improved soil health.
In the present study, under non-irrigated conditions, all crop rotations increased total and marketable tuber yields over no rotation (PP), but the SI system resulted in the highest tuber yield of all systems (both total and marketable), averaging 30 to 40% higher than SQ and PP systems over all years. Yield differences were greatest in the drier years (2007 and 2010), when SI yields were 40–90% higher than SQ and PP. In addition, SI resulted in the highest percentage of large and extra-large size-class tubers, and fewer small or under-sized tubers. It is also noteworthy that with irrigation, all cropping systems, with the exception of SI, produced substantially higher yields than their non-irrigated counterpart, with total and marketable yields averaging 27 and 37% higher, respectively, demonstrating that only SI produced comparable (and high) yields under both irrigated and non-irrigated conditions. These yield effects indicate the importance of adequate soil water in potato production and, as has also been previously demonstrated, that in most years, supplemental irrigation is needed in Maine to increase productivity [29
]. However, the data also strongly suggest that the yield increases observed in SI are related to soil health improvements associated with increased water-holding capacity and plant-available water. Thus, the improvements in soil characteristics, and particularly the increased organic matter and ability to store and hold available water provided by the compost amendments, apparently enabled SI to produce higher yields when not irrigated than all other cropping systems. Essentially, under these conditions, the improvements resulting from the compost amendments -effectively substituted for water additions through irrigation in these studies. This aspect was noted and explored in previous research examining the economics of potentially using compost amendments as an alternative to irrigation [34
]. In other research, compost amendments have been shown to provide similar increases in organic matter, water availability, various soil quality parameters, and generally higher tuber yields [5
], although in some cases, tuber yields were not significantly increased with compost amendments even when there was substantial improvement in soil quality parameters [37
]. Organic matter amendments have been shown to improve soil structural stability primarily through increases in aggregate stability, as well as improvements in bulk density, aeration, porosity, and water movement [39
There are many aspects and changes in soil characteristics involved with the compost amendments and other factors within the SI system, and thus the specific cause of the yield increases observed cannot be conclusively determined. However, based on the results observed and the characteristics of these systems, it is apparent that improvements in properties associated with the ability to store and hold soil water were at least partially, if not primarily, responsible for the yield increases observed in the SI system, rather than such aspects as nutritional improvements. First, all systems were supplied with adequate (above optimal NPK) fertilization so as not to limit productivity based on numerous studies in this area [29
]. Additional NPK fertility above these levels provided by the compost amendments would not be expected to increase yield further, as studies have indicated depressed yields, not increased yields, with above optimal fertilizer additions [29
]. Most importantly, SI had higher yields than other systems when not irrigated, but no improvement with irrigation, even though all other systems showed increased yields when irrigated. If yield increases were primarily related to increased nutrition, then we would expect to observe an irrigation effect, as with all other systems, but in SI, comparable yields were produced under irrigated and non-irrigated conditions. Additionally, under irrigated conditions, SI would be expected to produce higher yields than the other systems, but again, this was not observed, thus further indicating that added nutrition was not the primary cause of increased yields with SI. Overall, differences in water availability appeared to explain a substantial part of the cropping system yield differences observed. Under irrigated conditions, DS produced the highest overall yields, while PP still resulted in lower yields than all other cropping systems. Interestingly, these irrigation effects on yield were observed even in 2009, a year in which no irrigation treatments were applied (as were not needed), yet effects were still observed, possibly a result of cumulative beneficial effects due to a history of previous irrigation.
The DS system also resulted in overall significant increases in total and marketable yield relative to SC, SQ, and PP under non-irrigated conditions. These increases were presumably due to the beneficial effects of the added green manure and cover crops in reducing potential pathogens and soilborne diseases, and maintaining various soil health parameters, as has been observed in other potato systems [12
]. As previously reported, DS resulted in lower incidence and severity of multiple soilborne diseases (including stem canker, black scurf, and common scab), as well as significant effects on soil microbial community characteristics, but more modest effects on soil chemical and biological parameters [20
]. The SC system, however, despite increased rotation length, use of cover crops, and reduced tillage, resulted in comparable tuber yield to the standard 2 y SQ rotation through most years of this study, although it did show indications of higher yields than SQ in the later years (following the second full rotation cycle). Other researchers have also noted that significant effects due to increased rotation length and cover crops alone may take several years to develop [6
], and this was also indicated in the overall comparable soil properties observed for SC and SQ through the early years of the study [24
Overall, average yield values for DS, SC, and SQ under non-irrigated conditions were comparable to average state-wide values for commercial production in Maine for this period (~32 Mg/ha, 2006–2010) [47
], whereas SI averaged higher, and PP lower than average, as the majority of commercial production in Maine is not irrigated. However, under irrigated conditions, all cropping systems resulted in yields above the state-wide averages (by 8 to 28%).
Specific gravity is an important quality characteristic for processing potatoes, as it represents the dry matter content of tubers. Higher specific gravity means higher dry matter content, which produces lighter color, absorbs less oil, and requires fewer tubers and less time to produce the same yield of finished product (thus less costly to produce) [48
]. Specific gravity varied somewhat among cropping systems, with PP resulting in the overall highest values, and SI resulting in lower values. However, acceptable specific gravity values for Russet Burbank of 1.082 or higher (representing total solids content of >21.5%) were observed for all cropping systems in all years except 2010, which was the warmest and driest summer, with higher than normal temperatures and lower than normal rainfall observed throughout July, August, and September. High temperatures and water stress are known to depress specific gravities, as well as excessive water and/or fertilization [49
]. The slightly lower specific gravity observed in SI is probably due to the higher organic matter content and lower bulk density of those soils, as organic amendments may also reduce specific gravity [36
]. Although there was no overall irrigation effect on specific gravity, there was a significant interaction between cropping system and irrigation, and it appears that for most of the systems, there was a slight increase in specific gravity associated with irrigation for most systems, but a slight decrease in SI, resulting in no overall effect.
Under non-irrigated conditions, SI also resulted in the greatest photosynthetic potential, as represented by the leaf area index, leaf area index over time (leaf area duration—LAD), and leaf chlorophyll content. Yields are closely associated with the ability of a plant to intercept solar radiation and its efficiency in accumulating dry matter. LAD has been shown to be more closely related to yield than LAI and other indicators of leaf area [51
]. SPAD readings are closely related to actual chlorophyll content and have been used as an indicator of leaf N content, but recent research indicates that the relationship between SPAD readings and Leaf N content can be greatly affected by environmental conditions and crop species [52
]. SI also resulted in greater overall root and shoot biomass than the other cropping systems, demonstrating the impact of the improved soil quality parameters for SI on all aspects of crop growth dynamics. Previous research has also demonstrated that large additions of organic matter can dramatically affect these growth parameters [36
]. Surprisingly, however, SI resulted in overall lower tuber biomass than DS, but it must be taken into account that the biomass measurements were made in early August, when tubers were first developing, and do not represent any potential effects on yield. Although DS resulted in overall greater LAD than the remaining systems, and greater chlorophyll content than SQ and PP, there were fewer differences among the other cropping systems for biomass measurements under non-irrigated conditions, although PP generally resulted in lower values for most parameters. Irrigation resulted in overall increases in LAD and chlorophyll content for all systems, but irrigation effects on biomass were inconsistent. Although averages over all three years of biomass data indicated overall increases due to irrigation, individual years varied. There were some differences among cropping systems overall, including greater root biomass in SC than PP, greater shoot biomass in SI than all systems, and greater tuber biomass in DS than SI and SQ, but again, effects were variable between years. These differences reflect the generally more favorable conditions for biomass growth under irrigated conditions.
SI also resulted in generally higher levels of N, P, and K in above-ground shoot and tuber tissues, but lower concentrations of Ca, Mg, and Mn in shoot tissue, as well as generally lower concentrations of Fe, Cu, and Zn, relative to most other cropping systems. This observation indicated that SI management did not always increase the levels of these micronutrients, even though soil levels of these nutrients were generally increased in SI [24
]. However, this observation was consistent with studies of other cropping systems amended by organic fertilizers. For example, application with poultry litter resulted in a greater concentration of extractable soil P, K, Ca, Mg, Cu, Zn, and Na. However, these increases did not always result in greater concentrations of these elements in cotton plant parts [54
]. SQ and PP tended to have higher Ca and Mg concentrations in shoot tissue, as well as higher Fe, Cu, and Zn, than most other systems. Overall, tissue concentrations for all major nutrient elements (N, P, K, Ca, Mg) for all cropping systems were within the normal (sufficient level) ranges previously observed and reported for potato leaf and tuber tissues [56
Although only one potato variety, Russet Burbank, which is the predominant processing variety grown commercially in the northeast, was used in this study, observed results should be generally applicable to other potato varieties as well. Previous studies in this region have indicated similar responses to rotations, amendments, and fertilization in multiple different potato varieties [29
], and improvements in soil health have been associated with increases in yield across not only different potato varieties but many different crops as well [4
Overall, this study demonstrated that incorporating soil health management practices into integrated cropping systems can greatly affect crop growth and productivity parameters, and can be used to improve crop growth and yield, in addition to benefits in soil health and other soil properties. The integration of practices such as extending crop rotations, use of cover crops and green manures, reduced tillage, and, particularly, organic amendments, into existing, modified, and enhanced potato cropping systems may provide the basis for greater sustainability and productivity in potato production systems. This study also demonstrated that development of improved cropping systems can substantially enhance productivity from the standard cropping system currently used throughout Northeastern US for potato production. The SI system, which incorporated large organic amendments along with a longer rotation period, use of cover crops, and reduced tillage, resulted in substantial effects and improvements in crop growth and yield, particularly under non-irrigated conditions. Organic matter affects and influences many different soil physical, chemical, and biological properties in various ways, and has often been cited as the single most important aspect of soil health [4
]. Characterization of the water-extractable organic matter samples within the cropping systems suggested that these management practices stimulated the decomposition of the humic fraction in the soil organic matter pool, implying healthier soil conditions with these practices than in continuous potato growth [59
]. The current study further revealed that large organic matter amendments had the most immediate and substantial effects of all the cropping systems. This research also emphasized the importance of soil water, and that under normal environmental conditions during cropping seasons in Maine, irrigation provides a definite yield benefit under most cropping systems, but also that one of the benefits of the large organic amendments in SI was that it could be an effective substitute for irrigation and produce high yields without irrigation, at least under the conditions occurring during this study. Although SI and the effects of organic amendments appeared to provide the most substantial effects, the DS system, which included disease-suppressive rotation crops, green manures, crop diversity, and increased tillage (for incorporation of cover crops and green manures), also resulted in high yields (highest under irrigation) throughout, demonstrating the impact of reduced disease levels and other benefits provided by green manure crops and crop diversity. However, for the relatively short duration of this study, a small increase in rotation length, cover crops, and reduced tillage, as provided in the SC system was not sufficient to produce an overall increase in yield and growth parameters relative to the standard 2 y rotation, although by the second rotation cycle, indications of higher SC yields were evident. Additional time, or more aggressive changes, appear to be necessary to achieve enhanced productivity in this system. However, all these approaches still may provide some benefits in contributing to the overall goals of maintaining and/or improving soil health. Research is continuing to integrate the principles of these systems into more productive and economically viable enhanced cropping systems for growers in the northeast and elsewhere.