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

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

Summary

Between September 2008 and December 2009, a variety of methods were used in an attempt to create small amounts (10-40 kg) of soil with particular calcium:magnesium (Ca:Mg) ratios. Of the different methods used, leaching with large volumes of water containing concentrated calcium and magnesium chloride salts resulted in a set of soils with Ca:Mg ratios most different from those naturally found in associated field plots. The treated soils were then used in a set of greenhouse experiments conducted from December 2009 to April 2010. These experiments assessed growth of soybean plants in the different soils, as well as levels of calcium oxalate crystals produced in the leaves of the plants, population growth of soybean aphids feeding on the plants, and weight gain of beet armyworms feeding on the plants. A protocol used in for calcium oxalate determination in urinalysis was modified to allow for quantification of calcium oxalate in soybean leaf tissue samples, and all samples were successfully analyzed. Soybean leaf tissue must still be analyzed for possible differences in mineral content. No statistical analyses have been conducted to date, but graphical exploration of the data suggests no large effect of either high or low Ca:Mg ratios on aphid reproduction, beet armyworm weight gain or development, or soybean plant growth.

Objectives/Performance Targets

Objective 1: Cation ratio modification — To determine what method(s) are most effective for changing the calcium:magnesium ratio of volumes of soil sufficient for greenhouse pot experiments.

Objective 2: Greenhouse experiments using modified soil — To determine in controlled greenhouse conditions whether calcium:magnesium ratio of soils affects either growth and tissue quality of soybean plants or growth and development of two common soybean pests with different feeding habits.

Accomplishments/Milestones

Objective 1: Cation ratio modification – As of December 2009, we determined that we had achieved Ca:Mg ratios in the lab that were as different from those naturally occurring in our field soils as we were likely to get on a feasible timescale. This involved multiple modifications of a procedure used by other researchers. While other researchers have had success letting soil sit for long periods saturated by solutions containing particular ratios of calcium and magnesium chloride salts (followed by rinsing with dilute sulfate salts to remove the chloride), this method did not measurable alter the cation ratios of the silt loam we are working with. Instead, we ended up leaching soils from above with similar solutions of chloride salts, followed by rinsing with dilute sulfate salts. Perhaps because the leaching solution carried away displaced cations and forced their replacement in a particular ratio with cations from the solution, this method was relatively effective.

Despite multiple attempts, achieving particular target Ca:Mg ratios proved difficult even under lab conditions. Soil taken from an unamended field plot in 2008 has a naturally occurring Ca:Mg ratio of 2.1:1. Though we had hoped to attain ratios as low as 1:1 and as high as 4:1, all ratios obtained were greater than or equal to the naturally occurring ratio, with a high of 4.65:1. This meets our objective insofar as we were able to obtain at least some soils with ratios different from those in the field.

Objective 2: Greenhouse experiments using modified soil — In order to move ahead with greenhouse experiments, a set of four soils was selected for greenhouse work from the soils described under objective 1: a control, consisting of screened but otherwise unmodified soil from the field, a “treated control,” consisting of screened field soil rinsed with dilute sulfate salts but not chloride salts, and two soils with different Ca:Mg ratios (2.95 and 4.65). Soil of each of these four types was weighed into uniform amounts for potting, amended with carefully weighed N-P-K and S fertilizers and soybean inoculant, to be used in a set of three experiments:

Experiment 1: Soybean growth, calcium oxalate content, and tissue mineral analysis – This experiment will involve growing plants in soils of different cation ratios (without any insect feeding involved) and analyzing the resulting tissue for macro- and micronutrients and content of calcium oxalate. We hypothesize that plants grown in soils of higher Ca:Mg ratios will have higher levels of some nutrients and/or calcium oxalate than soils grown in lower Ca:Mg ratio soils.

Experiment 2: Soybean aphid reproductive rate and total lifetime reproduction – This experiment will involve growing plants in soils of different cation ratios, placing individual aphids on those plants, and comparing the performance (lifespan, measures of reproduction) of aphids on plants exposed to different catio ratios. We hypothesize that aphids feeding on plants grown in soils of higher Ca:Mg ratios might do less well (have lower lifetime reproduction and/or worse survival outcomes) than plants grown in lower Ca:Mg ratio soils.

Experiment 3: Beet armyworm growth and development – Individual beet armyworm larvae will be placed on plants raised in soils of varied cation ratios. Their survival, weight gain, and days to pupation will be measured. Our main hypothesis with respect to the larvae is the same as for the aphids just mentioned.

Most of the soil modification and greenhouse experiments just described are now completed, though some tissue analysis and all of the statistical and writing work remain to be done.

Impacts and Contributions/Outcomes

Collaborators: