1/20/2004The Texas Cooperative Extension Soil, Water and Forage Testing Laboratory has changed its primary soil nutrient extractant from the TAMU method (acidified ammonium acetate with EDTA) to the Mehlich III method. The former extractant, orignially derived from the Morgan method underwent multiple changes during its 57 use by the laboratory. Recent research by the laboratory and supporting Extension faculty and staff, Texas Agricultural Experiment Station researchers, agricultural industry, and cooperative agricultural producers indicated the former extractant over-estimated plant available phosphorus in several isolated agriculturally productive areas of Texas.
A review of historical geological surveys and preliminary research by the Soil, Water and Forage Testing Laboratory suggested the over-estimation of phosphorus in these soils was due to the presence of rock phosphate. Exclusively, the soils of concern are dominated by high a concentration of free-calcium carbonates (pH>7.6) developed in the Upper Austin Chalk geological deposits. These areas included a narrow band of the from North Waco to Oklahoma and along the Rio Grande River.
An exhaustive laboratory research program evaluated each of the major soil nutrient methods used in the south-central United States, along with several others used in North-America and one developed by the laboratory. Using field research samples, the following criteria was used in selecting a new test (or maintaining TAMU extract):
1. accuracy in predicting signficant economic yield increases to phosphorus fertilization
2. capacity to extract and analyze multiple nutrients
3. ease of use by laboratory
4. use of extractant by neighboring Land-Grant universities
5. relationship between new extractant and TAMU extractant (facilitates use of historic datasets)
6. acceptance by state and Federal environmental and agricultural agencies
7. potential acceptance by private laboratories
The only method evaluated that adequately predicted economic yield response to phosphorus fertilization across the numerous research study sites was the Mehlich III extractant. Fortunately, this method currently is being used by Univ. of Arkansas, Oklahoma State Univ., Kansas State Univ., and many private soil testing laboratories throughout the United States. While published research suggests the Mehlich III extractant also be used for micro-nutrient analysis (iron, zinc, copper and manganese), the Soil, Water and Forage Testing Laboratory did not find a strong enough relationship between exisiting methodology or plant uptake data to support this claim. The method does however work well for predicting available potassium, calcium, magnesium, sodium and sulfur. Sulfur extracted by the Mehlich III, while strongly correlated with the TAMU method, was significantly lower and thus existing sulfur recommendations were altered to reflect the difference in sulfur extractability by the methods. The other four elements' recommendations have not been altered at this time.
Impact of Phosphorus Change
Nine hundred eleven soil samples submitted by Texas clientele were analyzed by the laboratory using multiple metholodogies, and data was comparied to the TAMU method. The only criteria used for selecting these samples were:
1) non-urban samples
2) non-manured samples
3) orginated from Texas
4) Non-research samples
5) TAMU phosphorus levels below 200 ppm
6) no apparent issue with salinity
7) soil pH levels between 4.5 and 9.0
8) adequate soil to complete all tests.
Mehlich III data can be used with the following equation to predict EDTA phosphorus levels:
EDTA phosphorus=-40.63 + 4.313*pH*pH + 0.104*M3Mg-0.0000824*M3Mg*M3Mg + 0.718*M3P + 0.00128*M3P*M3P
r=0.896 P=<0.001
where M3Mg = Mehlich 3 magnesium and M3P = Mehlich 3 phosphorus
All data, except pH, are in ppm or mg/kg. Divide data expressed in lbs/acre by 2 prior to using this equation.
Effectively, soils low in pH (4.5-5.9) will have higher Mehlich III phosphorus levels than the same soils analyzed with the TAMU method. Additionally, soils high in pH will have lower phosphorus levels when analyzed with the Mehlich III than the TAMU method. These differences have been taken into account by the re-working of the laboratory's phosphorus fertilzer recommendations.
Additional information will be posted in the near future.