1. Lab 8: Soil Testing and Organic Matter

2. 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

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

9. 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

10. 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 ( ) 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)

11. 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)

12. 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

14. 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.