Relationship between organic fertility management, plant nutrition, and insect response

View the project final report

• 2010 annual report

Commodities

• Agronomic: corn, oats, soybeans, wheat, grass (misc. perennial), hay

Practices

• Animal Production: feed/forage, pasture fertility
• Crop Production: crop rotation, organic fertilizers, tissue analysis
• Education and Training: demonstration, extension
• Pest Management: biological control, economic threshold, integrated pest management, physical control, prevention
• Production Systems: agroecosystems, holistic management, organic agriculture, transitioning to organic
• Soil Management: organic matter, soil analysis, soil chemistry, soil quality/health

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.

Introduction:

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.