Introduction

Developing fertilization recommendations that are based on soil tests involves three important processes. The first process is correlation, which determines the relationship between plant nutrient uptake and the plant available nutrient in the soil using a particular soil test extractant. The next process is calibration, which determines the crop nutrient requirement at different soil test values. The final process is interpretations where nutrient application recommendations are developed based on soil test concentration, and other soil, crop, economic, environmental and climatic information. This process of correlation begins with identifying an appropriate soil test/chemical extractant. A soil test/chemical extractant mimics the extraction of nutrients by the plant root so the extraction estimates the soil’s nutrient-supplying capacity for a particular. There are several extractants used by soil testing laboratories in the US. Some of these extractants were developed for a specific nutrient i.e. Bray I, or multiple nutrients i.e. Mehlich 1 and 3, DTPA, Lancaster. Mehlich 1 and 3 are two widely used extractants by soil testing laboratories within the Southeast US. Mehlich 1 is the first universal extractant developed and is suitable for acidic and soils with low organic matter. The University of Tennessee began using the Mehlich 1 extraction method in December of 1981. Mehlich 3 was developed for a wide range of soil pH, cation exchange capacity, and base cation saturation.

To identify the appropriate extractant, laboratory or field experiments are carried out by testing the several extractants to identify which extractant has the best relationship between nutrient uptake and extracted nutrient. After a soil test extractant has been selected, the next step in the correlation process is to identify the critical soil test value (CSTV). The CSTV is the maximum soil test value below which crops are expected to show a yield increase with fertilizer application. 

Currently fertilizer recommendations by the Land Grant University in Tennessee is based on Mehlich 1. However, since the adoption of this method, improved soil organic matter and therefore buffering capacity associated with no-till management practices have been documented. Mehlich is suitable for use in well buffered soils, thus it may be a better alternative. To address this knowledge gap several field trial were initialed to compare the relationship between nutrient uptake and Mehlich 1 and 3 soil extractable phosphorus and potassium.