Wednesday, November 7, 2007 - 1:15 PM

Use of Remote Sensing to Study the Effects of Soil pH and Carbonate Content on Soybean Cyst Nematode Population Densities and Soybean Yield.

Natalia Rogovska, Alfred M. Blackmer, and Gregory L. Tylka. Iowa State University, Ames, IA 50011

Iron deficiency chlorosis (IDC) and soybean cyst nematode (SCN; Heterodera glycines) infestation are recognized as major factors that reduce soybean (Glycine max Merr.) yield in the Midwestern US. IDC is often associated with soybean grown on high pH, calcareous soils. It has been documented that SCN population densities are greater in high pH soils. The objective of this research was to assess the percentage of variability in soybean yield and SCN population densities that could be explained by soil pH, calcium carbonate equivalent (CCE), and a previously defined alkalinity stress index (ASI), which combines soil pH and CCE in one value. The study was conducted within 21 fields expected to have marked variability in these soil properties and showing marked variability in soybean growth. Remote sensing of soybean canopy was used to help identify appropriate fields and sampling areas within fields. Ten to 16 plant and soil samples were collected from each field and analyzed for pH, CCE, and SCN densities. The plant samples were used to calculate relative yields. Regression analyses within and across sites were used to study relationships between relative yields and measured soil variables. Results showed that soybean yield decreased with an increase in pH, CCE, and SCN densities. The yield reduction due to high pH, CCE, and SCN was greater for SCN-susceptible than resistant varieties. The SCN population densities were influenced to a greater extent by soil pH than CCE, with greatest numbers of SCN tended to occur in soils having high pH and greater carbonate content. These relationships would be difficult to describe without the use of the new alkalinity index.