APPENDIX 1 - Tests of Organic Soil Materials

(source Soil Survey Staff 1975)

The following tests of organic soil material can be used in the field or in field offices. Determinations include sodium pyrophosphate extract colour, fibre percentages, and pH in 0.01 M CaCl₂.

Since it is not practicable in the field to base determinations on a dry sample weight, moist soil material is always used. The specific volume of moist soil depends on how it is packed. It is, therefore, important that packing be standardized in order to obtain comparable results by different soil scientists.

Materials

a.

Water; tap water is satisfactory unless so coloured that it affects the pyrophosphate colour test.

b.

Solid sodium pyrophosphate (Na₄P₂O₇ 10H₂O) ground to 2 mm. Air drying makes grinding easier.

c.

CaCl₂ solution, 0.015 M.

d.

Sample containers; plastic bags (optional) and 30 cc plastic cups with plastic lids.

e.

Liquid dispenser; automatic constant volume pipettor, 3 ml capacity, modified to 4 ml by inserting a piece of plastic tubing into the end of the inlet stem. Cap with a rubber bulb from a medicine dropper to reduce leakage in transit.

f.

Chromatographic paper strips, 0.5 cm by 3 cm or larger.

g.

pH paper or pH meter.

h.

Absorbent paper; thick soft chromatographic paper cut into 10 cm squares, or any absorbent colour-fast blotting paper or heavy towelling is satisfactory for a field kit.

i.

15, 2.5 and 0.5 ml measures,

j.

Half-syringe; a 6 ml plastic hypodermic syringe with the body cut longitudinally to make a half-cylinder measuring device. Volume is one-half that indicated on the syringe.

k.

Sieve, 100 mesh.

l.

Spatula, 10 cm, plain. Shape the end to conform with the contour of the 2.5 ml measure of half-syringe.

m.

Wooden coffee stirrers.

n.

Marking pencil.

o.

10YR and 7.5YR pages of Munsell soil colour book.

p.

Tweezers.

Test Procedure

Field samples collected for use in field tests should be representative of the dominant kind of organic material within the layer described and from the same layer on which field estimates of rubbed and unrubbed fibre percentages are determined. The field sample can be collected in plastic bags and the subsample transferred into a cup after thorough mixing, or the mixing can be done at the sample site and only a small subsample placed in a cup for transport to the field office. If protracted storage is likely, the living tissues should be removed before the sample is transferred to a cup because their identifications become progressively more difficult with time.

Preparation of sample

Place about 25 cc of wetted field sample on a piece of absorbent paper. Smear the soil material over the absorbent paper to mix and bring it in contact with the paper. Or roll the absorbent paper around the sample and remove the water by squeezing with a second piece of absorbent paper. Continue desorption until the soil material no longer glistens but is still very moist. Remove coarse fragments and plant parts that were living at the time of sampling. If not done previously, visually estimate the rubbed and unrubbed fibre percentage.

Pyrophosphate colour test

Place 1 g of pyrophosphate and 4 ml of water into a 30 ml plastic container. Allow to equilibrate for a few minutes. Pack a half-syringe adjusted to 5 cc mark (2.5 cc volume) level full with moist soil material. In filling, pack firmly with a spatula but avoid expressing water. Transfer soil material cleanly to the container that holds the pyrophosphate solution. Mix thoroughly using a wooden coffee stirrer. Cover and let it stand overnight unless the colour of the liquid changes rapidly and is too dark or has too high a chroma for hemic materials. The temperature should be between 15 and 32°C. Mix again next morning. Insert vertically, by about 1 cm, a strip of chromatographic paper with tweezers. Leave it there until the paper test strip has wetted 2 cm above the slurry surface (about 5 minutes but this may be longer if the cover is closed). Remove the paper test strip with tweezers and tear off and discard the portion to which soil adheres. Place the strip on a piece of blotting paper and press gently with tweezers to make firm contact. Remove the paper strip with tweezers and compare the colour on the side next to the blotting paper with Munsell soil colour charts (Fig. 35). If successive samples are blotted systematically a comparative record is obtained on the sheet of blotting paper.

Figure 35. Value and chroma of pyrophosphate solution of fibric and sapric materials

Unrubbed fibre percentage (approximate)

Adjust a half-syringe to the 5 cc mark (2.5 cc volume) and pack it level full with moist soil. Transfer all the soil material from the syringe to a 100 mesh sieve and wash under a stream of cold tap water run at the rate of 200-300 ml per 5 seconds until the water passing through the sieve appears clean. Wash the residue to one side of the screen and blot from underneath with absorbent tissue to remove excess water. Remove any plant remains that were living at the time of sampling. If a measurable volume is removed, return it to the half-syringe, blot, and record the scale reading (cc) and subtract from the original 5 cc reading. Repack the washed fibres into the half-syringe. Blot further with absorbent tissue. The moisture content should be approximately the same as that of the 2.5 cc subsample previously measured. Record the scale reading (cc) on the repacked syringe. Unrubbed fibre percentage is equal to the reading (cc) times 20 if there is no correction for live tissues. If there is a correction, the percentage equals the reading (cc) divided by the corrected pre-washed volume times 100.

Rubbed fibre percentage

Transfer the residue from the unrubbed fibre test back to the 100 mesh sieve. Rub the sample between thumb and fingers under a stream of cold tap water running at the rate of 150-200 ml per 5 seconds. Continue until water passing through the sieve appears clean. Clean rubbed fibres will roll between the thumb and fingers rather than slide or smear. Measure the relative volume with the half-syringe and calculate percentage as described in the previous paragraph.

Place 2.5 cc of the prepared sample in a 30 ml plastic container and add 4 ml of 0.015 M CaCl₂ (gives a final concentration of roughly 0.01 M CaCl₂ with most packed, moist organic materials). Mix, cover, and allow to equilibrate at least 1 hour. Uncover and measure its pH with either pH paper or a meter.

a.	Water; tap water is satisfactory unless so coloured that it affects the pyrophosphate colour test.

b.	Solid sodium pyrophosphate (Na₄P₂O₇ 10H₂O) ground to 2 mm. Air drying makes grinding easier.

c.	CaCl₂ solution, 0.015 M.

d.	Sample containers; plastic bags (optional) and 30 cc plastic cups with plastic lids.

e.	Liquid dispenser; automatic constant volume pipettor, 3 ml capacity, modified to 4 ml by inserting a piece of plastic tubing into the end of the inlet stem. Cap with a rubber bulb from a medicine dropper to reduce leakage in transit.

f.	Chromatographic paper strips, 0.5 cm by 3 cm or larger.

g.	pH paper or pH meter.

h.	Absorbent paper; thick soft chromatographic paper cut into 10 cm squares, or any absorbent colour-fast blotting paper or heavy towelling is satisfactory for a field kit.

i.	15, 2.5 and 0.5 ml measures,

j.	Half-syringe; a 6 ml plastic hypodermic syringe with the body cut longitudinally to make a half-cylinder measuring device. Volume is one-half that indicated on the syringe.

k.	Sieve, 100 mesh.

l.	Spatula, 10 cm, plain. Shape the end to conform with the contour of the 2.5 ml measure of half-syringe.

m.	Wooden coffee stirrers.

n.	Marking pencil.

o.	10YR and 7.5YR pages of Munsell soil colour book.

p.	Tweezers.