Soil Core Sample

Note: if sample point is paved or not safe to sample soil, move to the nearest possible position;be especially considerate of underground irrigation lines! record the sample position on the field sheet.

Samples for chemical analyses (Nitrogen, Carbon, moisture content, soil organic matter content). Sample soil 10 meter from the plot center at North, East, South, and West four directions using 1 inch diameter, 12 inch deep slam hammer soil cores. Retrieve the plastic liner from metal core and cap both ends. Use Blue cap for Bottom end and red for the upper end.

Samples for pH, bulk density and texture determination. One meter north from the center, use 2 inch diameter, 6 inch long slam hammer soil core to take one intact soil core for bulk density analysis. [Note: you must obtain an intact core so you can get both the soil weight and soil volume. DO NOT push/compact the sample in any way when removing from corer.] RED cap on TOP; BLUE cap on BOTTOM

Soil Bulk Density Lab

Two inch cores (diameter: 5.08 cm) were used. The depth is 6 inch (15 cm) except the first plot (X18, 30 cm). The depth of the core is directly measured in cm (you frequently get cores less than 15 cm). That multiply with the surface area (20.259 cm2) give you volume.

The soil is sieved in the lab after air-drying into greater than 2mm portion (stone) and less than 2 mm portion (soil). Only the soil portion is used to calculate N, C budget. We were careful to "crush" the greater than 2mm portion to make sure soils were not just "stuck" together. That is an important issue for Ag soil since they tend to "harden" after air drying.

A subsample of soil is oven dried at 60C and further at 105C. We then calculated soil oven dry weight. We added soil weight (oven dry) and stone weight to calculate bulk density. We also calculated "soil density - soil/V" and used that to do our N and C calculation.

I purposely include stone in bulk density calculation. This is in line with most published studies, in which bulk density is measured by total weight (not sieved) divided by the volume.

Soil Texture

Particle Size Analysis

DRAFT

The method here will separate the sand, silt, and clay fractions. Additional hydrometer readings can be taken for greater discrimination within the size fractions is desired (see Gee and Bauder 1986)

I. Equipment and Reagents

1. Hydrometer, ASTM no. 152 H with Bouyoucos scale in g L^-1.

2. Large Reciprocal Shaker

3. Metal dispersing cups (for milk shake machines)

4. 500 ml Nalgene bottles

5. Plunger for sedimentation cylinders. (A long rod with a perforated disk attached to the bottom)

6. Sedimentation cylinders with 1 L mark.

7. Amyl alcohol (only if there is excess foam after shaking)

8. Sodium metaphosphate solution, 50 g/ L^-1.

9. 2 mm (no. 10) mesh sieve

10. Rolling pin and large rubber stopper (no. 10 to 14 works well)

11. Drying oven, weighing pans

12. Scale accurate to 0.1 g

13. Distilled water

14. Thermometer

II. Procedure

1. Sample preparation

A. Place samples out on a bench or table to air dry for one week. It is best to break up structures in samples that are high in clay as these can become very hard upon drying.

B. When the sample is dry it may be necessary to gently crush the structures with a rubber stopper or mortal and pestal.

C. Pass the sample through a 2 mm sieve. A rubber stopper makes a good device to break up and push through soil structures. Be sure that only gentle pressure is applied, too much pressure will distort the sieve mesh.

2. Analysis

Soil Dispersion

A. Weigh a sample for analysis into a 500 mL Nalgene bottle. If the soil is fine textured, silty or clayey use about 50 g. If the soil is coarse textured use about 75 g.

B. Weigh about 10 g into a small weighing pan to determine a soil moisture correction factor. Place in oven and dry at 105oC for 24 h. (This step has been completed for the entire 200-point survey.)

C. Add 250 mL of distilled water and 100 mL of Sodium MetaPhosphate solution to the sample in the Nalgene bottle. Place bottles in the reciprocal shaker and turn on ‘low’ setting for 24 hours.

D. Quantitatively transfer the suspension to a 1000 mL settling cylinder and bring the volume to 1000 mL.

Hydrometer Measurements

A. Allow time for the mixture to equilibrate thermally (10 min is sufficient) and record temperature.

B. Insert the plunger and while holding the bottom of the cylinder, mix the solution thoroughly. Stirring should be done with rapid upstrokes and gentle downstokes such that all the sediments are dislodged from the bottom of the cylinder. Mix in this fashion for about 5 to 10 seconds, finish with two or three slow smooth strokes.

C. When mixing is completed, insert the hydrometer slowly into the solution and take a reading (at the top of the meniscus)at 0.5 min and 1 min. Record as R0.5 and R1. Read to the nearest 0.5 g. Rinse and dry the hydrometer between samples. Record temperature after the R1 reading.

D. Take a hydrometer reading at 90 min and 1440 min (24 h). Record as R90 and R1440. Record the temperature at the time of each reading.

E. Prepare a blank solution (100 mL HMP, 900 mL distilled water, no soil) and read and the record the temperature at the same times as the soil solution above. Record as R_L 0.5, R_L1, R_L90, and R_L1440 .

3. Calculations

Clay Fraction

A. Use the 90 min and 1440 min readings (both R and R_L values)

B. Compute the mean particle size diameter in suspension (X) and the summation percentage (P) for the 90 and 1440 min readings. Use the equations below:

X equals qt^-½

Where t is time and q is the sedimentation parameter and is a function of hydrometer settling depth, solution viscosity, particle density and solution density.

The sedimentation parameter, q, can be determined as follows:

q equals (18nh ' /[g(rs - rl)])^½

Where:

n = solution viscosity in poise, g cm^-1s^-1

(viscosity of H2O at degreeC *1.02125)

h ' is effective hydrometer depth, cm

(-0.164R+16.3) R is the uncorrected hydrometer reading. This assumes ASTM 152H hydrometer and standard sedimentation cylinder. (There is a formula to calculate h¢ for other hydrometers and cylinders we need to discuss this prior to analysis)

g is gravitational constant, cm s^-2 (980 cm s^-2 )

rs is soil particle density, g cm-3 (2.65 g cm-3)

rl is solution density, g cm-3 (density of H2O† at oC*1.00315)

^† This can be obtained form the Handbook of Chemistry and Physics

The summation percentage for a given time interval can be determined as follows:

P is (C/C_o)*100

Where:

C is concentration of soil in suspension in g L^-1.

(determined C is R – R_L)

R ishydrometer reading of sample

R_L ishydrometer reading in blank

(R and R_L taken at each time interval)

C_o isoven dry weight of the sample

C. Compute the summation percentage at 2 um. This is determined from the equation below: (This is the percent clay)

P_2um is m ln (2/X₁₄₄₀) + P₁₄₄₀

Where:

m is (P₉₀ – P₁₄₄₀)/ln(X₉₀/X₁₄₄₀)

X₁₄₄₀ ismean particle diameter in suspension at 1440 min

P₁₄₄₀ is summation percentage at 1440 min

X₉₀ is mean particle diameter in suspension at 90 min

P₉₀ is summation percentage at 90 min

Sand Fraction

Compute the 50 um summation percentage using the same procedure as above, but using 0.5 and 1 min hydrometer readings. The equation for summation percentage at 50 um differs as follows: (note the bold)

P_50um equals m ln (50/X₁) + P₁

Silt Fraction

Determine the silt fraction by subtraction as follows:

% silt equals 100 - (% sand + % clay)

Comments

Assuming weighing of the samples is accurate, two sources of error exist:

1) Flocculation of clays by soluble salts or gypsum during sedimentation. This can be dealt with in two ways. a) Increasing the concentration of HMP to levels high enough to maintain dispersion (be sure to use same solution in blank), and b) treatment with an acid most commonly 1 M HCl adding enough so the sample is acidified to pH 3.5 to 4.0. I would recommend increasing the concentration of HMP first since this is less troublesome than working with HCl.

2) Hydrometer reading can be a major source of error. Studies have shown that a 1% error in reading will result in a 2% error in clay calculations. (Read at the top of the meniscus since capillary action wicks the H2O up the hydrometer stem)

Soil Moisture Correction Factor

Typically, air dry soil is used for analysis as oven drying can change the nutrient content of a soil by volatilizing nutrients, or causing changes in shape of clay humus particles. Results of soil analyses are expressed on an oven dry basis as soil moisture changes with time. When analysis is performed on a soil that will have results expressed a dry weight, a soil moisture correction factor (smcf) must be calculated on a subsample.

Procedure

The weight of the sample should be about 10 grams and should be taken at the time you weigh the samples intended for analysis. The smcf should be taken from the bulk sample in such a way as to be representative of the bulk sample. This means avoid the edges of the container or stray chunks of soil which are of a differing moisture content that the bulk sample. Samples are usually dried for 24 hours at 105oC.

After removal of the smcf sample from the drying oven, place in a desiccators for about 30 to 60 minutes. This will prevent the sample from adsorbing moisture from the air as it cools.

The smcf is calculated as follows:

smcf equals (dry weight/moist weight)

(where dry weight is the oven dry weight of the soil dried for 24 hours at 105 degreeC)

To convert the moist soil weight in your lab analysis to dry weight:

Dry Weight equals (Moist weight)(smcf)

The formula for smcf is a modification of the formula for percent dry matter.