Soil Testing Methods: How they affect fertilizer and lime recommendations F.J. Sikora, Ph.D. University of Kentucky 20 th annual Kentuckiana Crop Production Seminar Dec. 9-10, 2003 Executive Inn – Owensboro, Kentucky

Nutrient recommendations Soil acidity Available plant nutrients Interpretation of lab results

Nutrient recommendations Soil acidity Available plant nutrients Interpretation of lab results

Nutrient recommendations Available plant nutrients Interpretation of lab results

Soil acidity Available plant nutrients Interpretation of lab results First stool leg

Most agronomic crops 6.4 Tobacco 6.6 Alfalfa 6.8 Target pH

Water pH Buffer pHSoil pH Active acidityResidual acidity They measure……

Mg 2+ K+K+ H+H+ Ca 2+ Mg 2+ K+K+ Ca 2+ Na + K+K+ H+H+ Ca 2+ K+K+ Mg 2+ K+K+ H+H+ H+H+ H+H+ Ca 2+ Mg 2+ K+K+ H+H+ Soil pH measures Active acidity 10 g soil + 10 ml water pH electrode H+H+ H+H+ H+H+ H+H+

H + H + H + H + H + H + H + H + H + soil particle B - B - B - B - B - B - B - B - H +

H + H + H +

pH of buffer alone 7.5 Soil Buffer pH 6.8 Soil Buffer pH 6.4 Active and Residual acidity x y Acidity (meq/100g) = 6.2 x (7.5 – Buffer pH) SMP Buffer

SMP buffer Mehlich buffer Adam and Evans buffer Initial pH Woodruff buffer 7.0 UK standard

Soil acidity Available plant nutrients Interpretation of lab results Second stool leg

P PotassiumPhosphorus Concentration of P and K in soil that is related to plant-available P and K They measure…… K

Common soil extractants Bray I Mehlich I Ammonium Acetate (for K) Bray II (for P) Mehlich III UK standard

Mehlich I = Mixture of 2 acids whole pie is total P SOIL

Mehlich III whole pie is total P SOIL Mixture of 1 acid + fluoride + salt + chelate + pH buffer

ExtractantP (lbs/acre) Bray I112 Bray II212 Mehlich I46 Mehlich III90 Comparison of 6 extractants for soil test P NAPT sample : Rozetta silt loam from Bureau Co., IL. Soil pH = 6.5, CEC = 8.3

ExtractantK (lbs/acre) Bray I272 Mehlich I250 Mehlich III332 Am. Acet.330 Comparison of 6 extractants for soil test K NAPT sample : Rozetta silt loam from Bureau Co., IL. Soil pH = 6.5, CEC = 8.3

Variation in how 45 Laboratories in North America report Mehlich III Soil Test P and K Source - NAPT, 2nd quarter 2000  1 lab reports as lbs/acre-6 inches  2 labs report as lbs/ppm  2 labs report as ppm/lbs/acre  1 lab repors an index  3 labs reports as kg/ha  1 lab reports as mg/L  20 labs report as mg/kg  15 labs report as lbs/acre

Mehlich III in lbs/acre UK standard for P and K mg/kg x 2 = lbs/acre

Soil acidity Available plant nutrients Interpretation of lab results Third stool leg

High Med Low V.Low Soil Test Level 100% % 50-70% <50% Sufficiency Level Nutrients available from soil External nutrients required soil fertilizer fert. Relative Sources of Nutrients

01 Soil Test P (lbs/acre) Yield (% of maximum) Soil Test P (lbs/acre) Low Med High V.low Agronomic Threshold

3 philosophies with a high soil test Time (years) Soil test P or K Agronomic Threshold maintenance sufficiency buildup

Fertilizer costs for Corn Production * Data from H.J. Savoy, Univ. of Tenn.

Corn Yields * Data from H.J. Savoy, Univ. of Tenn.