The methods and procedures for obtaining soil samples vary according to the purpose of the sampling. Analysis of soil samples may be needed for engineering and agricultural purposes. In our case describes soil sampling for agricultural purposes, i.e. for soil fertility evaluation and fertilizer recommendations for crops.
The results of even very carefully conducted soil analyses can only be as good as the soil samples themselves. Thus, the efficiency of a soil testing service depends on the care and skill with which soil samples are collected. Non-representative samples constitute the largest single source of error in a soil fertility programme.
The most important phase of soil analysis takes place not in the laboratory but in the field where the soil is sampled.
Soils vary from place to place. In view of this, efforts should be made to take the samples in such a way that they are fully representative of the field. Only
1–10 g of soil is used for each chemical determination and this sample needs to represent as accurately as possible the entire surface 0–22 cm of soil, weighing about 2 million kg/ha.
Sampling tools and accessories
Depending on the purpose and precision required, the following tools may be needed for taking soil samples:
- A soil auger – it may be a tube, post-hole or screw-type auger or even a spade for taking sample;
- A clean bucket or a tray or a clean cloth – for mixing the soil and subsampling;
- Cloth bags of a specific size;
- A copying pencil for markings, and tags for tying cloth bags;
- Soil sample information sheet.
Selection of a sampling unit
A visual survey of the field should precede the actual sampling. Note the variation in slope, colour, texture, management and cropping pattern by traversing the field. Demarcate the field into uniform portions, each of which must be sampled separately. Where all these conditions are similar, one field can be treated as a single sampling unit. Such a unit should not exceed 1–2 ha, and it must be an area to which a farmer is willing to give separate attention. The sampling unit is a compromise between expenditure, labour and time on the one hand, and precision on the other.
1. Sampling procedurePrepare a map of the area to be covered in a survey showing different sampling unit boundaries. Enter a plan of the number of samples and manner of composite sampling on the map, designating different fields by numbers (1, 2,3 , etc.).
Traverse each area separately.
2. Cut a slice of the plough layer at intervals of 15–20 steps or according to the area to be covered. Generally, depending on the size of the field, 10–20 spots must be taken for one composite sample. Scrape away surface litter to obtain a uniformly thick slice of soil from the surface to the plough depth from each spot.Make a V-shaped cut with a spade to remove a 1–2‑cm slice of soil. Collect the sample on the blade of the spade and put it in a clean bucket. In this way, collect samples from all the spots marked for one sampling unit. In the case of hard soil, take samples with the help of an auger from the plough depth and collect them in the bucket.
3. Pour the soil from the bucket onto a piece of clean paper or cloth, and mix it thoroughly. Spread the soil evenly and divide it into quarters. Reject two opposite quarters and mix the rest of the soil again. Repeat the process until left with about 0.5 kg of the soil.
4. Collect it and put in a clean cloth bag. Mark each bag clearly in order to identify the sample. The bag used for sampling must always be clean and free from any contamination. If the same bag is to be used a second time, turn it inside out and remove the soil particles.
5. Write the details of the sample on the information sheet. Put a copy of this information sheet in the bag. Tie the mouth of the bag carefully.
When sampling a soil, bear in mind the following:
- Do not sample unusual areas, such as unevenly fertilized areas, marshy areas, old paths, old channels, old bunds, areas near trees, sites of previous compost piles, and other unrepresentative sites.
- For a soft and moist soil, the tube auger or spade is considered satisfactory.
- For harder soil, a screw auger may be more convenient.
- Where crops have been planted in rows, collect samples from the middle of the rows in order to avoid the area where fertilizer has been band placed.
- Avoid any type of contamination at all stages. Soil samples should never be stored with fertilizer materials and detergents. Contamination is likely when the soil samples are spread out to dry in the vicinity of stored fertilizers or on floor where fertilizers were stored previously.
- Before putting soil samples in bags, they should be examined for cleanliness as well as for strength.
- The information sheet should be filled in clearly with a copying pencil.
Dispatch of soil samples to the laboratory
Before sending soil samples to the testing laboratory, it is necessary to ensure that proper identification marks are present on the sample bags and labels placed in the bags. It is essential to use a copying pencil and not ink because ink can smudge and become illegible. The best system is to obtain soil sampling bags from the soil testing laboratory with most of the information printed or stenciled on them in indelible ink.
Compare the number and details on the bag with the dispatch list. The serial numbers of different places should be distinguished by putting the identification mark specific for each Centre. This may be in letters, e.g. one for the district, another for the block/county, and a third for the village.
Pack the samples properly. Wooden boxes are most suitable for long transport.
Sample bags should be packed only in clean bags never used for fertilizer or detergent packing.
Farmers may bring soil samples directly to the laboratory. However, most samples are sent to the laboratories through field extension staff. An organized assembly–processing–dispatch system is required in order to ensure prompt delivery of samples to the laboratory.
Preparation of soil samples for analysis
Handling in the laboratory
As soon as the samples arrive at the soil testing laboratory, they should be checked against the accompanying information list. If the laboratory personnel have collected the samples themselves, then adequate field notes should have been kept.
All unidentifiable samples should be discarded. Information regarding samples should be recorded in a register, and each sample should be given a laboratory number, in addition to the sample number, to help to distinguish it where more than one source of samples is involved.
Drying of samples
Samples received in the laboratory may be moist. They should be dried in wooden or enameled trays. Care should be taken to maintain the identity of each sample at all stages of preparation. During drying, the trays can be numbered or a plastic tag could be attached. The samples are allowed to dry in the air.
Drying has a negligible effect on total N content, but the nitrate content in the soil changes with time and temperature. Drying at a high temperature affects the microbial population.
After drying, the samples are taken to the preparation room. Air-dried samples are ground with a wooden pestle and mortar so that the soil aggregate is crushed but the soil particles do not break down. Samples of heavy clay soils may have to be ground with an end-runner grinding mill fitted with a pestle of hard wood and rubber lining to the mortar. Pebbles, concretions and stones should not be broken during grinding.
After grinding, the soil is screened through a 2‑mm sieve. The practice of passing only a portion of the ground sample through the sieve and discarding the remainder is erroneous. This introduces a positive bias in the sample as the rejected part may include soil elements with differential fertility. Therefore, the entire sample should be passed through the sieve except for concretions and pebbles of more than 2 mm. The coarse portion on the sieve should be returned to the mortar for further grinding. Repeat sieving and grinding until all aggregate particles are fine enough to pass the sieve and only pebbles, organic residues and concretions remain.
Soil Profile Description Procedure
- Clean off the pit face to expose fresh soil. Dig out the bottom of the pit to expose parent material.
- Describe soil horizons from top to bottom, as follows:
- Identify O horizons, if present; measure depth (O horizons are measured down to the surface of the mineral soil; e.g. Oi 3-0 cm); identify composition (e.g. Oe partially decomposed sugar maple litter); and determine boundary (e.g. abrupt smooth boundary).
- Locate preliminary horizon boundaries based upon differences in texture, structure, and color, and mark with nails. Assign each horizon a master horizon designation (e.g. A, E, B, C).
- For each preliminary mineral horizon, perform the following determinations:
- Break out a handful of soil aggregates and describe structure, including grade, size and shape (p. 6-8, e.g. weak medium granular structure).
- Determine soil texture following the flow chart on the last page of the field guide (e.g. sandy loam).
- iii. Determine the soil color using the Munsell color chart (e.g. dark yellowish brown 10YR 3/4).
- Estimate % coarse fragments in each soil horizon and apply appropriate modifier for texture (e.g. gravelly sandy loam).
- Measure pH of each mineral horizon using soil pH test kit and guide, record reaction (e.g. slightly acid pH 6.2).
- Note presence of mottling (if observed in any horizon, under special features.
- Apply master and subordinate horizon designations, as appropriate, based upon soil description just completed (e.g. Oi, Ap, Bt, Bhs...).
- Determine upper boundary of the C horizon using the HCl test for carbonates. Note degree of effervescence, if present ( 9), under special features.
- Record depths of each mineral horizon using the surface of mineral soil as a zero point. (e.g. A 0-10 cm, B 10-30 cm...).
- Describe the (lower) boundary of each mineral horizon, including distinctness and topography (e.g. gradual wavy boundary).
- Complete the site description form, making notes on landform, topography and vegetation, nutrient regime, drainage, and ecological moisture regime.