THE IMPORTANCE OF SAMPLING DEPTH WHEN TESTING SOILS FOR THEIR POTENTIAL TO SUPPLY PHOSPHORUS TO SURFACE RUNOFF

Authors: Vadas, Peter; MALLARINO, ANTONIO - IOWA STATE UNIV; MCFARLAND, ANNE - TARLETONE STATE UNIV

Submitted to: Extension Fact Sheets
Publication Type: Other
Publication Acceptance Date: 12/1/2005
Publication Date: 1/1/2006
Citation: Vadas, P.A., Mallarino, A.P., Mcfarland, A. 2006. THE IMPORTANCE OF SAMPLING DEPTH WHEN TESTING SOILS FOR THEIR POTENTIAL TO SUPPLY PHOSPHORUS TO SURFACE RUNOFF. Extension Fact Sheets. p. 1.

Interpretive Summary: Soil-test P results are an inherent part of agricultural nutrient management. Soil testing has historically been conducted for agronomic production, but is now used environmentally to assess the risk of P transport in runoff. However, a clear statement on the importance of sample depth in environmental soil P testing has not been put forward. Agronomic soil samples are usually taken from 0-15 to 0-20 cm deep. Surface runoff interacts with only the top few cm of soil. Therefore, sampling depth is important when using soil-test P results for environmental purposes. In consistently well-tilled soils with little P stratification, an agronomic soil sample taken from 0-15 or 0-20 cm, or to the depth of tillage, should accurately assess soil P available to runoff. In no-till, pasture, or lightly tilled soils where distinct P stratification is likely, soil testing that accounts for P stratification will likely improve assessments of P available to runoff. Two options to account for soil P stratification are collecting samples from depths shallower than 0-15 to 0-20 cm or using a traditional 0-15 to 0-20 cm sampling depth and accounting for P stratification with different interpretation systems.

Technical Abstract: Soil-test P results are an inherent part of agricultural nutrient management, both agronomically and environmentally. Historically, soil testing has been conducted for agronomic production, but is now being used as an environmental parameter to assess the risk of P transport in runoff. While the relationship between soil P and runoff P has been well researched and discussed, a clear statement on the importance of sample depth in environmental soil testing for P has not been put forward. Soil samples for estimating nutrient sufficiency for crops usually are taken from 0-15 to 0-20 cm deep. Surface runoff in most cases interacts with only the top few cm of soil. Therefore, the issue of sampling depth is important when using soil-test P results for environmental purposes. This paper presents the current scientific understanding of the effect of sampling depth for environmental soil testing. In soils that are consistently well-tilled and have little P stratification, a traditional agronomic soil sample taken from 0-15 or 0-20 cm, or to the depth of tillage, should accurately assess soil P that is available to runoff. In no-till, pasture, or lightly tilled soils where distinct P stratification is likely, soil testing that accounts for soil P stratification will likely improve assessments of soil P available to runoff. Two possible options to account for soil P stratification are collecting soil samples from depths shallower than the traditional 0-15 to 0-20 cm agronomic depth or continuing to use the traditional 0-15 to 0-20 cm agronomic sampling depth and accounting for soil P stratification by developing different interpretation systems for P stratified soils