Are some soil groups in Wisconsin more likely than others to have high phosphorus and potassium concentrations at the surface compared to the plow layer? Do minimum-tillage systems lead to more plant nutrient concentrated at the soil surface? Soil samplers are needed throughout Wisconsin this spring to help answer these questions. They are requested to take part in a project to measure the extent of nutrient stratification in common agricultural soils under different tillage systems. The primary objective of the study is to assess cropland P stratification by soil group and by tillage system for the Wisconsin P Index, but information on potassium stratification will also be collected.

The P Index is a tool to assess the risk of runoff P delivery from agricultural fields to surface water for phosphorus-based nutrient management planning. In Wisconsin’s P Index, surface (0-1 inch) soil test P is used to estimate runoff dissolved P and sediment P concentrations. Rather than require a separate “surface” soil sample, the P Index applies a stratification factor to the routine plow-layer soil test P to estimate surface soil test P. The stratification factors currently used in the P Index came from a soil sampling project conducted in 2002-2003 from 103 fields throughout the state. The majority of these samples had a higher concentration of soil test P at the surface than in the plow layer. One surprising finding of this study was a significantly higher stratification ratio for the red medium- and fine-textured soils found in eastern Wisconsin compared to medium- and fine-textured soils in other parts of the state. In addition, there was only a slight trend towards increased stratification in no-till and established alfalfa across all samples. However, there were not enough samples from different soil groups for valid statistical analysis of the effects of tillage on stratification. In this study, we hope to verify the stratification factors currently used in the P Index for soil groups, and to determine if additional factors should be used to account for the effects of tillage on stratification.

Participating soil samplers will be asked to sample a minimum of four fields of one soil type commonly used for growing crops in their areas. More samples will be welcome. We will need a large number of samples from medium- and fine-textured soils across the state to come to any conclusions about the effects of tillage on soil P stratification within each soil group. For each field, the sampler will take composite plow layer and surface samples to 0-6 inch and 0-1 inch depths around one point in the field. The samples will be analyzed for soil test P and K at the Marshfield or Madison UW-Soil and Plant Analysis Labs. Samplers will also be asked to report field location and management history in an on-line field data entry form.  You can review this form and a detailed protocol at http://www.soils.wisc.edu/extension/onfarmdemo/ .

 

We hope to get enough samples in this spring to be able to produce a summary report this summer on P and K stratification in Wisconsin’s medium and fine-textured agricultural soils. Contact Laura Good to participate (lwgood@wisc.edu, 608-262-9894).