Fundamentals of Soil Science (1+1)

Exercise: 1

Aim: Collection and preparation of Soil Samples
Importance:

• Soil sampling is the most vital step for analysis. As a very small fraction of the huge soil mass in the field is used for analysis.
• One of the most important aspects in soil testing is the matter of obtaining a soil sample that is representative of the area. If the samples do not represent the field, it is impossible to make the good recommendations.
• It is said that sample is filth or dirt, if it has been not drawn properly by adopting standard methods.
• Soil sampling is done in order to find out the quantity of essential nutrients available to plants and other relevant physico-chemical properties which influence plant growth such as water retention, acidity, alkalinity etc.

Time and depth of Soil Sampling :

• The soil sampling is done when there is no crop in the field i.e., before the growing the next crop and also prior to application of manures and fertilizers. Rainy season should be avoided for sampling work.
• The root penetration is important for crop growth, therefore, for cearls, vegetables and another seasonal crops the sample should be drawn from 0-15cm i,e., plough layer.
• For deep rooted crops like sugarcane, cotton or under dry farming conditions, obtain samples from different depths. For plantation crops or fruit trees samples should be collected from 0-30, 30-60, and 60-100 cm depth.
• In case of saline alkali soils, salt crust if visible on the soil surface should be sampled separately and record the depth of sampling. Generally, the sample may be drawn up to 15 cm depth from surface for testing.
• Ten-fifteen soil samples collected from homogeneous sampling units are mixed and smaller sample is drawn which represents the entire sampling unit is known as composite sample. A field can be treated as a single unit if it is appreciably uniform and 0.5 ha is taken as one sampling unit.

Tools and Materials:

• Sampling tools: Soil auger/soil tube/spade/pick-axe/khurpi
• All sampling tools and storage bags (Fig-1.1) should be cleaned to avoid contamination.
• A bucket or tray for collecting and mixing the composite sample.
• Cloth bags or polythene bags , Paper tags, Information sheet, Ball point pen or copying pencil, Pestle and mortar and 2 mm sieve.

Procedure:

• Divide the field (Fig1.2) on the basis of variations in slope, colour, texture, crop growth and management practices.
• Scrap away the surface litter without much disturbing soil.
• Take 15 soil samples randomly distributed over each area using a soil auger/tube and place them in clean bucket(Fig.1.3A &B). A khurpi or spade can be also used if auger is not available. In this case, make a V shaped cut up to 15 cm thick slice of soil from top to bottom, moving in a zig-zag manner from each sampling unit and place the samples in a clean bucket.
• From fields having standing crops in rows draw samples in between rows(Fig.1.4)
• Mix thoroughly the soil sample taken from each area on a clean piece of cloth or polythene sheet by hand(Fig.1.5)
• Reduce the bulk of soil sample to about 500g by quartering process in which the entire soil is spread, divided in four equal parts; two opposite ones are discarded and remaining two are remixed. Repeat this procedure until about half kilogram of the soil is left. Put the soil in a clean cloth/ polythene bag free from any contamination of fertilizer, salt etc.
• Prepare two labels, one to be put inside the bag (Fig1.6)
• Fill out an information sheet for each composite sample. The information should be as complete and accurate as possible.
• Pack the soil sample bags along with information sheet in a clean and dry gunny bags.

Processing of Soil Samples for Analysis:

• Air dry the soil samples in shade.
• Crush the soil clods lightly and grind with the help of wooden pestle and mortar.
• Pass the entire quantity through 2 mm stainless steel sieve.
• Discard the plant residues, gravels and other material retained on the sieve.
• If the gravel content is substantial, record as per cent of the sample(w/w)
• For certain type of analysis (organic carbon), grind the soil further so as to pass it through 0.2 to 0.5 mm sieves.
• Remix the entire quantity of sieved soil thoroughly before analysis

Precautions:

1. Draw the soil samples from uniform piece of land. Sample should never be collected either from hills or depressions in the field as these do not represent the natural conditions of the field.
2. Do not draw any sample from extreme corners of the field, areas recently manured or fertilized, old bunds, marshy spots, manure piles and non-representative areas.
3. Avoid contact of samples with chemicals, fertilizers and manures.
4. Use steel augers instead of rusted iron khurpi or kassi for sampling of micro-nutrient analysis.
5. Do not take less than 0.5 kg of composite sample
6. Do not dry the soil directly under sunlight or by artificial heating. Air dry the sample under shade.
7. Store the samples preferably in a clean cloth or polythene bags.
8. Use glass, porcelain or polythene jar for long duration storage

Information sheet
The following information should be collected and send with the sample:

1. Date of sampling
2. Address of the farmer
3. Field number
4. Depth of sampling.
5. Purpose of sampling. (fertility appraisal, garden plantation, etc.)
6. Soil name
7. Slope
8. Irrigated/ Rain fed
9. Natural drainage
10. Water table
11. Land use and rotation followed
12. Details of manures or soil amendments applied

Fig.1. Procedure for collecting a representative soil sample