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
 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
Lime Buffer Curve
500
450
400
ppm CaCO3 added
350
300
250
200
150
100
50
0
4
4.5
5
5.5
6
soil pH
6.5
7
7.5
8
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.