We conducted a series of experiments assessing the effects of soil cation ratios on both plant health and plant-feeding insects. Solutions containing varied ratios of dissolved calcium and magnesium salts were used to force changes in the ratios of exchangeable cations present in a Plano silt loam from Wisconsin. These soils were then used in a series of experiments to assess the impact of soil calcium:magnesium (Ca:Mg) ratios on soybean tissue nutrient content and production of calcium oxalate (known to be a storage form of calcium and potentially a form of defense against insects). Additional soybean plants raised in soils of varied cation ratios were used as hosts for soybean aphids and beet armyworm larvae to determine if soil cation ratios affect either survival and fecundity (aphids) or survival and weight gain (beet armyworms) of plant-feeding insects. Plants growing in soils of elevated Ca:Mg ratios had higher levels of some macro and micronutrients than unmodified control soils. Ca:Mg ratios also appeared to be positively correlated with the calcium oxalate content of soybean tissue, which was lowest in a control soil with a Ca:Mg ratio of 2.05:1 and highest in a modified soil with a Ca:Mg ratio of 4.65:1. Despite these plant tissue differences, performance of the two insect species varied only minimally between cation ratio treatments.
Because organic farmers have relatively few control options when insect pest populations reach problem levels, a preventive approach to pest management is essential in organic systems. However, given the limited research base regarding relationships between soil fertility, plant health, and insect growth and reproduction, it’s unclear in many situations exactly what this should mean to farmers in terms of inputs and practices.
One specific fertility management approach that has been advocated to help plants repel or tolerate feeding by insects is the “basic cation saturation ratio” (BCSR) concept (sometimes referred to as the “soil balance” approach). The BCSR concept, use of which is not limited to organic agriculture, proposes that chemical, physical, and biological soil conditions are optimal for plant growth when the negatively charged exchange sites on soil clay and humus are filled with particular proportions of the cations Ca, Mg, and K (Exner 2007). For Bear et al. (1945, 1948, 1951), on whose work the idea primarily rests, these proportions were 65% Ca, 10% Mg, and 5% K, with protons filling the remaining exchange sites. Graham (1959) and Albrecht (1975), important proponents of the concept, later gave ranges from 65-85% Ca, 6-20% Mg, and 2-5% K that they felt were acceptable. As a practical matter, since many soils of the Upper Midwest have Ca saturation levels lower than these target ratios, growers interested in the BCSR approach usually try to add Ca ions to their soil (and, consequently, displace Mg and K ions) by fertilizing with either calcitic limestone [calcium carbonate, CaCO3], or gypsum [calcium sulfate, CaSO4•(H2O)2].
As a guide to fertilizer application, the BCSR concept is often contrasted in the literature and in recommendations made by soil testing labs with the “sufficient levels of available nutrients” (SLAN) concept (McLean 1977, Eckert 1987, Exner 2007). Under the sufficiency level concept, there are “Definable levels of individual nutrients in the soil below which crops will respond to added fertilizers with some probability and above which they likely will not respond” (Eckert 1987).
Reviews of early work by Bear, Albrecht, and others on the BCSR concept reveal significant methodological flaws. In particular, the method by which given Ca saturations were obtained resulted in changes in pH such that what was actually being measured was plants’ response to pH and not Ca or ratios of cations (Kopittke and Menzies 2007). Dozens of studies reviewed by Kelling and Peters (2004) and Kopittke and Menzies (2007), including a series of field and laboratory experiments by McLean, one of Albrecht’s students, failed to find significant benefits in yield or tissue composition by using the BCSR concept as a guide to fertilization rather the SLAN concept. Work by Olson et al. (1982), Exner (2007) and others has also demonstrated higher costs to the BCSR approach relative to the SLAN concept.
Despite the lack of definitive research support for the BCSR concept, McLean (1977) and Kopittke and Menzies (2007) document that it remains popular around the world among both conventional and organic growers, as well as consultants and some private soil testing labs. With the exception of Schonbeck (2000), working in vegetable systems in the southeastern U.S., no one has explored the impact of BCSR-based fertilization in certified organic production or with respect to insect pest problems. As a result, with the encouragement and participation of seven certified organic growers throughout southern Wisconsin (three of whom use the BCSR approach as a guide to fertility management), we are conducting a long-term field study, separate from this NCR SARE graduate student project, with the primary objective of evaluating the role of the BCSR concept in crop plant nutrient uptake and insect pest and natural enemy response in an organic field and forage crop rotation.
At the same time, to control for variations in weather, weed pressure, and other factors in the field, we have conducted a set of greenhouse experiments exploring the impact of BCSR-like soil fertility treatments on soybeans and two insects that feed on soybeans, the soybean aphid (Aphis glycines) and beet armyworm (Spodoptera frugiperda). In these experiments, we sought to determine a) whether increasing Ca:Mg ratios in soil would increase the Ca:Mg ratio of plant tissue, and b) if higher Ca:Mg ratios in soils would translate into some changes in the survival, feeding, or reproduction of either a piercing-sucking insect (soybean aphid) or a chewing insect (beet armyworm), possibly as a result of changes in levels of calcium oxalate. Calcium oxalate is a mineral that most vascular plants accumulate in crystalline form in their tissue. In some studies by other researchers (e.g., Korth et al. 2006), calcium oxalate appears to help plants defend themselves against chewing insects. As a result, possible increases in calcium oxalate concentration as a result of BCSR fertility management could provide a mechanism to explain insect pest effects observed in the field.
The initial proposal for this project listed a variety of intended outcomes. These are listed below together with notes on our actual achievements in each area.
1. Summary of field operations, yields, and weed pressure involved in transitioning a four-year grain/forage system to organic certification
All data needed for this summary has been collected and is being compiled into a paper intended for publication in a peer-reviewed scientific journal.
2. Demonstration of ability to modify cation ratios of soils for greenhouse experiments
Though the process proved hard to control precisely, modification of methods developed by other researchers resulted in the creation of soils with cation ratios significantly different from those of the baseline soil.
3. Comparison of effects of BCSR and SLAN fertility management systems on insect pests in the greenhouse
Greenhouse trials were conducted as described with modified soils, soybean plants, and two different insect pests.
4. Comparison of effects of BCSR and SLAN fertility management systems on insect pests, plants, and soils in large experimental plots
Four field seasons of data have now been collected and most analyses completed; results are being compiled into a paper intended for publication in a peer-reviewed scientific journal.
5. Scientific knowledge required to support informed choice of organic fertility management program
Results from greenhouse and field trials together may prompt growers to change their methods.
6. Baseline information collected to design effective related experiments on working farms
Some such experiments (e.g., Exner 2007, Schonbeck 2000) have been conducted with great care on the part of both researchers and participating growers, and those studies have not found a benefit to using the BSCR approach. That may because the time scale involved in changing soil cation ratios (not to mention the amounts of calcium inputs and logistics of collecting and analyzing soil, tissue, grain, and insect pest data) is such that work on real farms that might detect cation ratio effects on insects just isn’t feasible. The logistical difficulties involved in the greenhouse and research plot trials reported here certainly confirm the authors’ feeling that a farm-scale trial of sufficient statistical power would be too difficult.
7. Summary of WI grower attitudes and practices with respect to use of BCSR and SLAN approaches.
Creation of modified soils and the subsequent greenhouse trials with soybean plants and insect pests consumed more time than initially expected that conducting and analyzing a proper survey was not feasible. Informal surveys conducted during conference and webinar presentations found that at least 20 percent of attendees (a mixture of growers, researchers, certified crop advisors, and the general public) either used the BCSR approach themselves and/or are familiar with it.
Working in a Plano silt loam at the Arlington Agricultural Research Station near Madison, Wisconsin, we have been comparing two different organic soil fertility systems for their impact on a variety of insect pests since 2007. The first of these two systems, the standard organic fertility (SOF) system, relies on livestock manure, alfalfa hay in the rotation, and cover crop green manures for nitrogen and most other crop nutrients. The second (BCSR) system involves all of the management practices just described but also entails addition of either high-calcium aglime (a single application averaging 3,000 lbs per acre in spring 2007) or gypsum (average of 2,600 lbs per acre in spring 2008, and average of 3,000 lbs per acre in late fall 2009) regardless of soil pH levels. There are 32 0.77-acre plots, 16 of each organic fertility treatment, and each plot is being moved through a four-year rotation (alfalfa/forage grass plus small grain, alfalfa/forage grass, corn, soybeans) typical of many farms in the region. The plots received formal organic certification in fall 2009.
Plant tissue mineral analyses and standard tests of soil physical and chemical properties (including organic matter, pH, and ratios of exchangeable cations) have been conducted at appropriate times (for example, after exposure to a full season’s rainfall, in the case of calcium inputs) either annually or as needed to choose levels of inputs. Weed management has involved delayed planting dates typical of organic field crop production, pre-plant flushing of weeds with shallow cultivation (average of three separate operations), and post-plant cultivation with rotary hoes, tine weeders, hilling cultivators, and other tillage implements common to both organic and conventional production (average of 5-6 total separate operations). Data on weed populations and weed species composition (not presented here) have now been collected for four growing seasons.
Beginning in 2008, we also added a set of conventionally managed plots (rotated between corn and soybeans) in an adjacent field of the same soil type. These plots receive urea and other synthetic fertilizers as dictated by soil tests and crop removal. They also receive preplant application of appropriate herbicides but no insecticides. Rather than comparing organic and conventional systems per se, the purpose of these plots is to help determine whether any plant or insect effects seen in the organic plots are a function of organic management in general as opposed to the BCSR method in particular.
In all three systems, we have focused our data collection on three crop-pest associations: soybean aphid on soybeans, potato leafhopper on alfalfa, and a set of Lepidopteran larvae (European corn borer, corn earworm, and western bean cutworm) on corn. Phelan et al. (1995, 1996) found that the history of soil fertility management (standard organic versus conventional, in their studies) affected egglaying preferences by European corn borers. Together with differences in population densities and timing of population establishment, this represents the kind of changes we might expect to find.
Because it’s possible that one or another of our fertility management approaches might help plants regulate pests to levels that naturally occurring predators are better able to suppress, we are measuring both pest populations and the interactions between pests and natural enemies. Work with natural enemies thus far has focused largely on predators of the soybean aphid.
To complement the long-term field experiments, we have also conducted a set of greenhouse studies. These studies involved rearing two different insects (soybean aphids and beet armyworm larvae) on soybean plants grown in soil modified in the laboratory to a set of different cation ratios. Cation ratios were modified by leaching aquarium-sized blocks of soil with large volumes of solutions containing the desired target ratio of calcium to magnesium (present as highly concentrated dissolved salts of calcium and magnesium chloride). Small amounts of soils of four different ratios (two controls with Ca:Mg ratios of approximately 2.0:1 and two modified soils of 2.95:1 and 4.65:1) were placed into pots and used to grow soybean plants under controlled greenhouse conditions. In one study, the resulting soybean tissue was simply analyzed for macro and micronutrients and calcium oxalate. In two other studies, soybean plants growing in treated soils were used as hosts for either newly born aphids or newly hatched beet armyworm larvae. The aphids were monitored until they died to determine how long they lived and how many offspring they produced. The beet armyworm larvae were monitored for survival and weight gain to the point of pupation.
Selected characteristics of soil from the standard organic and BCSR organic treatments are presented in Tables 1 and 2. Values from before the start of the experiment (fall 2006) are contrasted with values obtained in late summer 2010. Values from conventional plots from 2010 are shown for comparison purposes.
Statistical analysis across years within each treatment will likely prove informative (for example, both soil organic matter and total cation exchange capacity may have increased in both organic treatments), but these analyses have not been completed. Comparisons within years and across treatments are also preliminary and will need to be adjusted for block effects and other factors. However, at present it seems that the BCSR organic plots have significantly higher levels of exchangeable soil calcium than standard organic plots (an average of 2321 ppm Ca for BCSR in 2010 compared to 1991 ppm Ca for standard). At the same time, BCSR plots also have significantly lower levels of exchangeable Mg and K than standard plots (492 ppm Mg for BCSR in 2010 compared to 600 ppm Mg for standard, and 173 ppm K for BCSR compared to 199 ppm K for standard). These results are consistent with the replacement of Mg and K by Ca on surfaces of soil clay and organic matter. In all treatments, however, quantities of Ca, Mg, and K are above deficiency ranges defined for Wisconsin according to the SLAN concept (Laboski et al. 2006).
The ratio of percent saturation with Ca to percent saturation with Mg appears to be significantly higher for the BCSR organic soil than for the standard organic soil (2.84 compared to 2.00 in 2010). The BCSR plots have significantly higher percent saturation with Ca than the standard plots (70.7% compared to 61.0% for the standard plots in 2010), and this percentage brings the soil into the range suggested by advocates of the BCSR approach.
Despite approaching the target cation ratios in the field BCSR organic plots, few measures of yield or insect pest response have differed significantly by treatment thus far. Yields of corn in 2010, for example, averaged 192 bushels per acre for all treatments, while soybean yields ranged insignificantly between 63 and 65 bushels per acre for the three treatments (soil balance, standard organic, conventional). Data on grain quality and forage quality have been collected and statistical analyses of these data are underway.
One representative set of insect data is the graph of soybean aphid population growth shown in Figure 1. As with a similar set of data from 2008 (not shown), aphid populations appeared to diverge between the three treatments near the end of the season, with populations lowest in the BCSR organic plots, next lowest in the standard organic plots, and highest in the conventional plots. However, due to wide variation in aphid counts between plants, populations did not differ significantly by treatment except on the final sampling date, when plants in conventional plots harbored significantly more aphids than plants in either of the two organic treatments. We have not yet analyzed the analogous 2010 data, but preliminary summaries suggest that the pattern of data is the same as in the previous two years. We are now analyzing a corresponding set of plant tissue data for each pest (mineral content of soybean leaves at R1-R2, in the case of soybean aphids) that may allow us to establish soil-plant-insect interaction effects.
Research results from the greenhouse largely mirror those obtained in the field. Using methods modified from Favoretto et al. (2006), we did successfully create soils with ratios of exchangeable Ca:Mg as high as 4.65:1. There was a significant difference between soil treatments in the length of time it took for beet armyworm larvae to reach pupation, with the larvae on plants grown in 4.65:1 soil pupating slightly faster than larvae in other treatments. However, weight gain and final pupal weight do not appear to have been significantly affected by feeding on soybeans grown in soils of different cation ratios. Soybean aphids feeding on similar soybean plants similarly did not differ in lifespan or total lifetime reproduction.
Though there was no apparent response by insects to the cation ratios in which their soybean host plants were grown, leaf tissue from the plants themselves did vary in content of calcium oxalate (see Figure 2). Plants grown in soil modified to Ca:Mg ratios of either 2.95:1 or 4.65:1 contained significantly more calcium oxalate than plants grown in unmodified control soil with a Ca:Mg ratio of 2.0:1. Plants grown in soil of the 2.95:1 ratio also had more calcium oxalate than plants from a second control group grown in soil rinsed with water in a way that mimicked the cation ratio modification process, though this was not true of plants grown in soil with a 4.65:1 ratio.
The results of the field experiments discussed here suggest that changing cation ratios on a field scale appears possible, but the amounts of inputs required are large. Though aphid data from the field hints at a possible difference in insect response between the two organic treatments, the BCSR approach has overall not shown significant effects on either crop yield or insect pest populations over four years. However, the high Ca:Mg ratios advocated by BCSR proponents have only been achieved recently and may not yet have had time to affect plants or insects. By comparison with adjacent conventional plots, the same field experiment has provided suggestive evidence that organic fertility management (with or without BCSR-related inputs) might result in lower pest populations than a conventional fertility system without insecticides.
In greenhouse experiments, use of the BCSR approach does not appear to have had significant effects on insect pests. However, it does appear to have some measurable, statistically significant effects on mineral content of soybean tissue.
Our goal is to develop research-based information on how different organic fertility management practices affect crop resistance to insect pests, and what (if any) particular soil fertility management practices are most helpful. With that goal in mind, it is not obvious that the higher material and management costs of the BCSR approach provide a return on investment in terms of either yield or pest control. However, the practices described here represent only one possible fertility management variant with one set of crops and pests on a particular soil type and should not be interpreted as conclusive for any other production system.
Educational & Outreach Activities
Updates on project progress have been presented to the grower advisory board and to attendees of the UW-Madison CALS Organic and/or Agronomy Field Days (total of more than 100 participants/year) in 2007, 2008, and 2010. Results from the project were also presented in talks by Robin Mittenthal at the 2011 Upper Midwest Organic Farming Conference, LaCrosse, WI, the 2011 Wisconsin Crop Management Conference in Madison, WI, and in a 2011 e-Organic webinar. A publication intended for a peer-reviewed journal is in preparation, and the results will also be applied toward Robin Mittenthal’s PhD thesis in Entomology at UW-Madison.
-Talks given by Robin Mittenthal and/or Eileen Cullen (see details under Publications/Outreach) have provided more than 400 attendees at conferences, field days, and a webinar with knowledge about the effects of soil fertility on pest management.
-Successful modification of soil Ca:Mg ratios in the laboratory and resulting changes in levels of calcium oxalate in soybean tissue are outcomes that could support further research into the importance of the BCSR approach and/or the role of calcium oxalate in plant defense against insect pests.
A publication intended for a peer-reviewed journal is in preparation. This publication will describe the inputs (gypsum and aglime) used to achieve particular cation ratios in our field experiment. The paper will include a very brief, informal cost-benefit analysis of the use of these inputs for pest management (in a nutshell, results to date do not justify the application of large amounts of Ca-rich minerals for this purpose).
As noted previously, the time required to modify soils and test them in the greenhouse precluded more thorough or extensive survey effort with respect to farmer attitudes and adoption.
Areas needing additional study
-The methods reported here resulted in Ca:Mg ratios higher than those of the baseline soil, but the authors had hoped to create and test soils with ratios lower than those that naturally occur in this particular soil type. It is possible that use of different methods and/or a different soil type would allow for the creation of a wider range of cation ratios (as has been the case in other studies not involving insects).
-While the changes in calcium oxalate levels observed in this study do not appear to have affected the insect species used, changes in calcium oxalate were larger than expected, and it is possible that other methods (for example, the creation of knockout mutants that express either more or less calcium oxalate than wild type plants) could create soybean plants or other crops with enough calcium oxalate to deter insect feeding. Calcium oxalate-related knockout mutants of several crop species exist, and there is the potential for more work in this area.
-A survey of use of BCSR and SLAN methods in WI and/or the upper Midwest as a whole (planned but not conducted as part of this study) would still be useful in guiding future research in this area. If the survey instrument was accompanied by publications of the results of this study, survey questions could ask respondents who use the BCSR method if they would consider stopping use of BCSR given the lack of obvious effects on yield or insect pests.