Lab 8: Soil Testing and Organic Matter

Today 3 Tests: 2) pH/lime test: recommend Lime 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 Deficiency : stunted, thin-stemmed plants with bluish-green dark foliage Sufficient: photosynthesis, nitrogen fixation, flowering, fruiting, seed production, maturation, and root growth.

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

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

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

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

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

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)

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)

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

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.