9. Sampling Tools and Sample Preparation

A sampling tool should be:

uncontaminated
approximately uniform in cross section to the desired depth
provide reproducible sampling units

Tapered cores or slices may bias the analysis result if systematic variations with depth are significant. Commonly used sampling tools are (Figure 5):

blades: trowel, spade, shovel, spoon, knife, cutlass
tubes: open-sided and plain cylinders
augers: wood-bit, post-hole, sheathed auger

Figure 5. Blade, tube, and auger (left to right).

For comparison over periods of time, take soil samples at approximately the same time of the year (e.g. before planting). When making comparisons, consider other factors like weather conditions, crops, treatments, seasonal fluctuations.

Select sampling depth according to the purpose of sampling. For soil fertility evaluations in annual crops, sample at a depth of 0 to 15 or 0 to 20 cm. For perennial crops (e.g. trees), take deeper samples since tree roots often grow deep into the soil.

Soil samples usually need preparation before laboratory analysis:

Air-drying. Crush large soil clods to facilitate drying. Do not dry at high temperature. During air-drying, avoid contamination (i.e. from dust, gases, rain, etc.). Air-drying usually takes one week.
Crushing. Crush the sample in a mortar using a rubber or porcelain-capped pestle (Figure 6). The mortar is usually made from porcelain.

Figure 6. Mortar and pestle, sieve.

Pulverizing. Not all samples require pulverizing. The subsampling error is a function of the ratio between the average weight of the largest particles and the weight of the subsample. If the subsample being analyzed is small (i.e. for total nitrogen and organic carbon analysis), the sample has to be pulverized to a fine powder (less than 0.5 mm).
Sieving. Sieve the soil through a 2 mm sieve made of brass, stainless steel, or plastic. Use plastic sieves when micronutrients are to be analyzed.
Mixing and storage. Mix samples thoroughly, then store in clean closed containers (i.e. polyethylene bags or bottles). Label the containers.