Current Limitations of Soil Testing for Environmental Protection

Southern Region Committee on Nutrient Analysis of Soils,

Plants, Water and Waste Materials (SERA-IEG-6).

Soil testing is a reliable, scientifically based method for assessing a soil's nutrient level relative to crop production. Past emphasis has led to a database for determining critical levels of nutrients for agronomic crop production, and for determining rates of additional nutrients required to optimize yields. This body of scientific information has not incorporated environmental criteria in the calibration and correlation process.

There is a keen widespread interest for using soil testing to predict phosphorus levels in water leaving the land and going into lakes, streams and rivers. Although there may be a correlation between soil test phosphorus and potential water-soluble phosphorus in surface runoff from some soil-landscape combinations, the use of soil testing alone for the purpose of assessing the effects of a soil's nutrient level on water quality, is an inappropriate use. An extensive re-evaluation of criteria used in developing the correlation database would be required. Using soil test phosphorus for environmental purposes as currently proposed by several agencies and groups would require speculation beyond the range of crop response. This use without proper collection and validation for the new calibration criteria is unjustified and therefore scientifically indefensible. Soil test phosphorus may be used to identify soils that would require additional evaluation by other testing methods and factors with corresponding databases. The movement and changing bioavailability of phosphorus can be affected by such factors as soil type, soil pH, clay percentage, soil roughness, soil residue cover, tillage intensity, land slope, vegetative cover, distance to a sensitive water body, etc. These factors can be used in conjunction with soil testing to alert producers to consider the need for managing phosphorus in surface runoff with best management practices that increase nutrient trapping and reduce total runoff.

A more detailed soil-landscape combination evaluation will require the use of more specific testing methods for assessing environmental potentials of the phosphorus loss. At this point, databases for these methods are in the early stages of development.