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Lab 8: Soil Testing and Organic Matter

Lab 8: Soil Testing and Organic Matter. Today. 3 Tests: 1) Phosphorus test: recommend P fertilizer 2) pH/lime test: recommend Lime 3) Humus content: fertility assessment. Soil Test: Phosphorus. Purpose – estimate P-supplying capacity of a soil, need for P fertilizer

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Lab 8: Soil Testing and Organic Matter

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  1. Lab 8: Soil Testing and Organic Matter

  2. Today 3 Tests: 1) Phosphorus test: recommend P fertilizer 2) pH/lime test: recommend Lime 3) Humus content: fertility assessment

  3. Soil Test: Phosphorus Purpose – estimate P-supplying capacity of a soil, need for P fertilizer • Deficiency : stunted, thin-stemmed plants with bluish-green dark foliage • Sufficient: photosynthesis, nitrogen fixation, flowering, fruiting, seed production, maturation, and root growth.

  4. Soil Test Phosphorus • Rinse all glass ware in deionized water • Weigh 5.0 g soil , Add to 40 mL plastic tube • Add extract- 30 mL dilute double acid (HCl and H2SO4), cap tightly and shake 5 min. • Filter immediately using FAST (No. 1) filter paper

  5. Soil Test Phosphorus • Transfer 2 mL solution into 50 mL volumetric flask • Add 20 mL of deionzed water and mix • Pump in 8mL of molybdate reagent • Add deionized water to 50 mL, seal with parafilm, and invert

  6. Soil Test Phosphorus • Wait 10-15 min. for color to react • Tansfer to cuvette (used in the colorimeter) • Discard contents down the drain and rinse glassware

  7. Soil Test Phosphorus • Molybdate (MoO4)- forms a bright blue-colored chemical complex with PO4 • Intensity can be measured by colorimeter • Use standard curve to find concentration of PO4 • Standard curve made by plotting absorbance reading on colorimeter vs. know concentrations of P

  8. Calculations • Copy absorbance readings and concentrations (plot on attached graph to obtain standard curve • Draw a straight line thru zero that best fits the points to get ppm (mg P/L) in solution • mg P/L x 0.03L/0.005 kg = mg P / kg ( ppm in soil) • mg/kg = ppm x 2 = lbs/a • Use calibration table in lab to rate the soil and make a fertilizer recommendation for P

  9. Lime Requirement Soils in the Southeast are acidic due to leaching over geologic time and N fert. Lime (CaCo3) is added to soils to bring the pH back to optimum levels (5.5-7.0) for crop growth Buffering capacity- soils ability to resist pH change determined by exchanageable acids (CEC) and pH dependant (CEC/AEC on oxides and humus)

  10. Lime Requirement Soil pH – 20 g of soil (in paper cup) + 30 mL of DI water - Allow to sit for 15 min. Measure pH with glass electrode (swirl cup gently) If pH is < 6.5, need lime to raise pH Add 2 mL Ca(OH)2 solution , wait 15 min., reread pH (limed pH)

  11. Lime Requirement • On graph, plot water pH and limed pH vs. ppm CaCO3 added (mg CaCO3/kg soil) • Draw a line through the two points, this is the buffer curve for your soil; read off ppm CaCO3 needed to reach target pH • Multiply this by 4 to get lbs ag lime to add per acre

  12. Organic Matter (Humus) • Special test (not routine) • Solubilize humus using sodium pyrophosphate (high pH, extracts humus from clay-organic complexes) • 1.00 g soil in centr. tube; pump in 30 mL Na4P2O7 • Shake about 5 min; centrifuge. • Filter thru FINE (No. 42) filter paper in a 100 mL cylinder till you collect 10 mL. • Dilute to 100 mL with water; read absorbance on spectrometer. Use calibration curve to get % organic C.

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